Texas/New Mexico/Louisiana/Mexico Obs And Discussion Thread Part 4

[Srain]

The overnight guidance continues to advertise a wet pattern developing late week into the weekend. A stalling frontal boundary and deep tropical moisture pooling in the Western Gulf are classic heavy rainfall set ups in our part of the world. While the Euro suggests an open wave, the GFS has remained steadfast in developing a tropical cyclone slowly meandering N into the S Texas Coast. While it does not appear we are looking at a strong Hurricane, the GFS solution of a strong depression or Tropical Storm does raise an eyebrow. That model has earned a bit more respect as of late and cannot be dismissed, IMO. What is troubling is the amount of enhanced rainfall that may well be in the cards for the Southern half of Texas including our area. Stay Tuned. The forecast could be changing and rather quickly at that...

[DoctorMu]

looking a bit spicy early next week

[Ed Lizard]

Starting to lose some faith in anything but possibly moisture from ex-TD #7 for STX/SETX. But Canadian, for example, does a fine job raining on us this weekend without any TCs (suggestion on Canadian of some moisture of former Ernesto/soon to be former Hector coming in from the Pacific and helping the crawling front generate rain).

Looking at more detailed Euro on AccuWx PPV, between clouds, some rain, and at least somewhat of an airmass change (dewpoints in the low 60s in August aren't typical) high temps only mid-upper 80s Sunday and Monday.

Rain may be ending just in time for a near normal commute in the DFW Metroplex. Ballpark inch and a half to three inches of rain overnight...

[Srain]

Morning e-mail from Jeff:

Changes from hot and dry to wet and cooler on the way.

High pressure over the SW US will gradually shift further west and combine with building high pressure over the NW US to carve out a deep downstream eastern US trough. This pattern will unlock so fairly cool air by August standards and send it southward over the northern plains and then SE toward the Ohio Valley and east coast this weekend. Across TX the cold front will arrive into the state Thursday and slowly sag southward over the weekend. Main brunt of the cold air advection will be to our NE, but the front or its associated outflow boundaries from thunderstorms will likely sag into central TX and our northern counties by late Saturday.

The combination of the frontal boundary, increasing moisture from both the Pacific and the Gulf, and short wave riding down the backside of the trough and across the frontal boundary will support much better rain chances starting Friday and continuing into the weekend. Should see some decent organization of the storms both along the frontal boundary and the inland moving seabreeze front, so most locations should see some rainfall. Pattern does favor heavy rainfall and will need to keep an eye on this aspect as the boundary will linger into early next week providing several days of wet weather.

Big wild card is whether or not some sort of tropical system will try and spin up over the western Gulf of Mexico late this weekend into early next week. GFS has been consistent on moving the remains of TD #7 across central America and the Yucatan then into the Bay of Campeche where it slows the wave and closes it off into a weak surface low. The EURO and CMC continue the wave axis westward and into mainland Mexico with no development. The GFS has performed very well this year in the tropics with both Debby and Ernesto and TD #7, so it could be on the right track. With that said, shear looks to be increasing over the western Gulf in response to the tail end of the trough over the eastern US and I think the GFS is too strong with any surface low that may develop. That pattern fits a classic western Gulf of Mexico surface trough that may close off into a weak low, but is highly sheared and all the weather is N and NE of its surface center. If the EURO is correct the frontal boundary would stall across the northern Gulf and this would also help increase the surface gradient with lowering pressures to the south.

For now will keep the frontal boundary stalled near/over our area inland of the coast and any tropical development weak. Combination of these features will keep rain chances high into early next week and if low pressure does form in the western Gulf, rain chances would likely increase into the middle of next week.

[Quixotic1]

Amped about the FROPAs the last week or so and the one this upcoming weekend. 1.57" overnight brings us to the 25" range for the year. Not too bad. Still can't figure why we're still at stage 3 water restrictions. While the drought was bad last year, this area was one of the few in the state that was only in the 0 or 1 stage drought for the last couple of years.

[msp]

like jorge posted in the other thread, 12z gfs says ex-7 sits off tamaulipas and never comes north. looks to be moving inland at 168.

[Srain]

Morning e-mail from Jeff:

One more day of dry and hot before the pattern begins to change.

Large ridge of high pressure that has been anchored over W TX and NM for weeks has weakened and shifted westward allowing a downstream trough to develop over the central and eastern US. This unusually deep trough for mid August will allow a rare cold front to move into TX later today and approach SE TX over the weekend. Gulf moisture begins to increase later today and really on Friday with PW values increasing to 2.0 inches by Friday afternoon. With weakening low level winds and building moisture, expect the seabreeze to become active on Friday while thunderstorms from the north move southward during the afternoon and may clip our N and NE counties.

Saturday-Sunday:

Slow moving/stalling frontal boundary arrives into/near the area as moisture increases even more off the western Gulf. Expect a good round of thunderstorms on Saturday with heating both from the seabreeze and the stalling front. GFS and EURO are really hammering away at Sunday as being the wettest day. Once again widespread storms are likely at really any point from late Saturday night into Sunday evening. Weak short waves in the NW flow aloft could develop storms in a highly moist air mass and nearly anytime during that period. Starting to get slightly concerned on the rainfall totals with the potential for a surface boundary in the area and deep tropical moisture in place. Both of those factors point to a locally heavy rainfall threat.

Early Next Week:

Decent agreement in the model guidance through Sunday begins to diverge early next week. Tropical wave (old TD 7 remains) currently over the southern Yucatan and central America will lift WNW into the Bay of Campeche this weekend. GFS has backed away from its tropical cyclone development and has fallen in line closer to the EURO and CMC and keeping the system a tropical wave. Two main questions focus on early next week: 1) how much moisture is pulled northward from this tropical wave along the western Gulf coast and 2) how far south does the frontal boundary move. EURO and CMC try and push the front offshore early next week allowing a drier air mass the move into the region, while the GFS keeps the boundary over the area with deep tropical moisture in place. Convective outflows could push the front southward into the Gulf, but then again it is mid August. Will take a blend of the GFS and other guidance and show the front making slow progress toward the coast early next week. With several days of a stalling boundary and deep tropical moisture in place, many rounds of heavy rainfall appear likely. As the ground saturates over time with daily rains, will start to see increased run-off and the threat for flooding may increase especially if the boundary lingers into the middle of next week.

[Ed Lizard]

Locally, we have about six weeks for the pattern to change or the odds of a cyclone impact would seem below normal, which in any given year isn't that great.

6 weeks, because the Equinox is an unofficial end of the season in SETX, storms later than that, storms like the 1949 storm, probably a major, near Freeport and 1989's barelycane Jerry being awfully infrequent.

Being just West of the trough, or on the Eastern side of a big ridge, suggests generally storms will want to hit East or continue Westward and to the South.

However, six weeks is actually a pretty long time, and with the season starting to change, a pattern change seems possible.

FWIW, 8/15/12 CFS week 4 500 mb height anomalies and rainfall anomalies suggest the pattern could change in such a way Texas's chances of a TC would increase.

[Srain]

Morning e-mail from Jeff:

Weather to turn wet starting today and lasting through the weekend.

Rare mid August cold front extends from OK to north of Lubbock this morning with a pre-frontal trough along the Red River. Large complex of thunderstorms is moving ESE over NE TX and NW LA producing very heavy rainfall. Locally, the nearshore waters are active with showers already suggesting moisture is on the increase over the area.

Front will sag southward today, but remain well to our north. However, deep tropical moisture over the Gulf of Mexico will begin to move inland over the TX coast leading in increasing chances for showers and thunderstorms. Air mass will become unstable by mid to late morning and upper level winds are weakly divergent with no capping. Feel the wetter guidance may win out today given the presence of showers already along the coast currently…usually a good sign in the summer of an active seabreeze.

Factors come together over the weekend for organized thunderstorms with heavy rainfall. Slow moving front to the north this morning will sag into the area and combine with deep tropical moisture and disturbances rotating through the deep eastern US trough to produce a good chance of rainfall. Think Saturday will be much like today, with only more coverage of rainfall as moisture increases even more (PW surpassing 2.0 inches). Sunday continues to show the best potential for widespread rainfall…some will be very heavy. Frontal boundary moves slowly across the area from midday-early Monday and slow moving thunderstorms with heavy rainfall will be possible. Given the deep moist profile, storms will be efficient at producing excessive rainfall in a short period of time. Will broad brush 1-2 inches widespread, but suspect a few locations will get much more. Could see some flooding problems develop if storms train over the same area.

Early next week:

Front looks to slip off the coast Monday-Tuesday leading to drier conditions at least north of I-10. Front and moisture may linger close enough to the coast to keep rain chances in the forecast for those areas. Then all eyes turn to the south and what if anything develops tropically over the weekend into early next week.

Gulf:

Remains of TD # 7 have moves over the warm waters of the Bay of Campeche and have shown signs of organization this overnight and this morning. Deep convection has developed over a possible low/mid level center with a few banding features. The system is moving toward the WNW and will be crossing inland along the eastern MX coast later today. Forecast models have come into slightly better agreement on trying to close off some sort of surface low out of a coastal trough along the eastern MX coast late this weekend into early next week. Not sure if they are picking up on the current system, or a trough that will be left behind near the coast. 00Z CMC has come in fairly aggressive showing development off of Brownsville, while the GFS lingers a broad trough/weak surface low along the NE MX coastline for days. Think the current system will move inland, but leave unsettled weather over the western Gulf well into next week and we will have to continue to monitor for any possible development.

With the front likely stalling over the waters on Monday and low pressure over the western Gulf, winds will gradually back and increase out of the NE to ENE early next week. For now will limit rain chances near the coast, but it is possible that the front/any tropical system may back northward and greatly increase mid week rain chances….stay tuned!

[Ed Lizard]

New NAM not all that happy for SETX, precip North, and a lot of precip offshore, but a mininimum along I-10 from SAT to HOU.

[Srain]

Update from Jeff:

Reports from the USAF aircraft and Mexican oil platforms over the SW Gulf of Mexico indicate that a small, but tight tropical storm has formed with 45mph winds. The center is located 80-90 miles off the eastern coast of Mexico and is moving toward the NW at 7mph. This system has slowed down greatly today as it has reached the western edge of the weak ridge over the northern Gulf coast.

Track:

Available track guidance moves the system NW and then NNW either just inland or right along the east MX coast for the next several days due to a break and erosion of the ridge to its north by a strong summer trough digging along the US east coast. Motion will be slow as the system is in a region of weak steering flow and this alone leads to the potential for some degree of uncertainty. On this track it is likely that the center will cross the coast and move inland over MX in the next 24 hours.

Intensity:

The intensity will be a factor of interaction with land. If the center remains offshore or near the coast then some slow but steady intensification will be possible, but if the center moves inland then weakening is likely. All other factors seem to point to good conditions for intensification including warm water temperatures, weak wind shear aloft, and a good envelope of moist air. Based on the current NHC forecast track, the system has about 18-24 hours to gain some modest intensity.

[DoctorMu]

Honest to goodness raindrops!...and the beginning of the end to Scorching Hot Season.

The front is really sagging South and wet times ahead!

[DoctorMu]

[Srain]

Morning e-mail from Jeff:

Very weak tropical storm nearing the eastern Mexico coast just north of Cabo Rojo this morning.

Since the USAF aircraft left the system yesterday late afternoon the cloud pattern of the storm has greatly weakened with little to no deep convection near/over the center all night long. Due to the small size of the center, it is likely that Helene has spun down overnight with the lack of convection and has likely weakened into a tropical depression. Helene has been moving slowly toward the NW overnight and is about 30 miles off the coast this morning. Given its current forward motion the center should cross the coast in the next 3-4 hours.

Track:

Model guidance is in good agreement for the next 24-72 hours with taking Helene just inland and NNW to N along the eastern coast of Mexico. Given the small and fragile low level center associated with this system it is likely that this track, even though very near the coast, will lead to dissipation by late Sunday. There are a handful of forecast models that do track Helene more toward the N and NNE past Sunday and back over the western Gulf, but it is highly questionable at this point if any low level center will be left by that time. It should be noted however that only a very small deviation of the center to the right (east) would possibly bring it back over the water.

The CMC and to a degree the GFS show low pressure forming over the western/northwestern Gulf of Mexico off the TX coast middle to late this week and it is possible that this will be the remains of Helene or possibly a new low that attempts to form along the old decaying frontal boundary that pushes into the Gulf on Monday. The GFS model has performed very well with both Helene and other systems this year and keeps a broad low pressure trough along the eastern MX coast through much of next week. It is possible that at some point the remains of Helene or a new surface low could spin up along this trough near the intersection with the weakening northern Gulf front.

Intensity:

With the center expected to cross the coast in the next few hours, weakening is likely until the system dissipates.

94L:

A well defined tropical wave over the eastern Atlantic continues to show signs of organization with banding features and modest deep convection near/around the center. This system has good model support for development including the GFS, CMC, HWRF, and NOGAPS, while the ECMWF races it westward and keeps it weak. Will lean toward the stronger solutions with this system as it is not moving as fast as past systems this year in the deep tropics and has plenty of moisture surrounding it unlike Ernesto and TD # 7. GFS brings this as a hurricane toward the eastern Caribbean late next week.

Helene Forecast Track:

[Srain]

Another update from Jeff:

Frontal boundary will move across SE TX on Sunday with periods heavy rainfall.

This morning a frontal boundary was stalled over N TX into W TX with a pre-frontal trough from north of Austin to just north of Huntsville. Radar shows numerous showers/thunderstorms just north of College Station to Huntsville on the tail end of a short wave moving into Louisiana and the lift from the pre-frontal trough. Visible satellite images shows a plume of deep tropical moisture moving NW from the southern and central Gulf of Mexico and within the next 30 minutes to hour a large cumulus field will rapidly develop over the area.

After a few hours of heating this morning, expect the tropical air mass to begin to bubble up and produce showers and thunderstorms along the coastal seabreeze and any southward moving outflow from storms to our north currently. Activity should peak in the early to mid afternoon hours with a break from late afternoon through around midnight.

Front over N TX will get a push southward tonight and begin to cross SE TX on Sunday. Expect showers/thunderstorms to begin to develop at really any point after midnight as the front begins to move into the region. Tropical air mass will continue to support very heavy rainfall rates under the stronger storms and while most areas will pick up 1-2 inches through late Sunday isolated totals of 4-5 inches will be possible.

Front should clear the coast early Monday allowing a rare intrusion of very dry air from the north with dewpoints falling into the lower 60’s to upper 50’s over the area by late Monday. Potential is there for overnight to bottom out in the 60’s for the first time in a long time.

Front will stall over the NW Gulf through the middle of next week and then begin to retreat northward around the latter part of next week. Forecast models show the possibility of the formation of either the remains of Helene or a new surface low near/along the tail end of the front toward the middle of next week. Decaying fronts over the warm Gulf of Mexico this time of year are always some cause for concern as tropical systems do like to form on such features. Model support is weak currently, but the potential is there and we will need to keep an eye on the tail end of the feature next week. For now will keep winds backed out of the NE into the middle of the week with high pressure to our NE sending dry air SW into the area and lower pressures to our south.

[msp]

picked up .84" here yesterday from a pop-up shower... still waiting on precip today. looks like showers are slowly creeping in from the west.

[ticka1]

had a storm yesterday and today... more forecasted for tomorrow!!

[DoctorMu]

Indeed sRain...once off the coast this boundary could stir up some tropical mischief

[Ed Lizard]

Showers well offshore and all, I was hoping for uppr 60s dewpoints. Low to mid 70s is a slight improvement on status quo, I can't complain too much

[Ed Lizard]

Nothing really to suggest future Isaac comes to Texas. Now, Louisiana could be a different story...

And if NOLA seems to have an ensemble member or three in the area, maybe in 10 days the area could creep farther West. Not likely, but did anyone think Texas would beat the semi-pro cheaters from USC in 2005-2006 to win the National Title?

[Srain]

Morning e-mail from Jeff:

New tropical depression forms in the Atlantic Ocean 645 miles East of the Leeward Islands.

Tropical Storm Warnings are issued for the Leeward Islands

Tropical Storm Watches are issued for the US/British Virgin Islands and Puerto Rico.

Discussion:

The well defined low pressure system that has been crossing the Atlantic for the past several days has developed enough deep convection overnight to be classified as at TD. The system clearly has a large circulation envelop on visible images with deep convection on its SW side this morning. The system appears to be undergoing a bit of northeasterly wind shear which is keeping thunderstorms removed to the SW side of the circulation. Additionally dry air is lurking to the NE and W of the system, but its large circulation should help to protect the developing inner core from much dry air penetration.

Track:

The depression is located south of a deep layer ridge of high pressure over the central Atlantic Ocean and this will result in a W to WNW track for the next 72 to 96 hours. This will bring the system across the Leeward and Windward Islands on Wednesday and then south of Puerto Rico and Hispaniola late this week. After 72 hours the system will begin to reach the western edge of the sub-tropical ridge over the Atlantic and a fairly decent short wave trough is forecast to dig SE from the Midwest into the central Gulf coast. This should induce a more WNW or NW motion in the days 4-5 time period toward Cuba. Model guidance is actually in very good agreement with the GFS, GFDL, and HWRF fairly close on track through 96 hours. The ECMWF is slower and more westward while the CMC is far to the right in the longer range. The GFS has been consistent over the past 24 hours with this general track reasoning. The current NHC track is a little to the west of the multi model consensus at 120 hours.

Intensity:

Except for the current shear and potential for dry air intrusion, conditions appear favorable for intensification. Most of the models bring this to a tropical storm in the next 12 -24 hours, if it is already not one currently. Suspect the large circulation will take a little bit of time to consolidate and build an inner core so will follow a slow but steady intensification in the next 48 hours and then more rapid increases beyond. One big caveat to the intensity is the potential for land interaction in the 72-120 hour time period with the islands of Hispaniola and Cuba. Any slight deviation of the forecast track could bring the system deeper across these islands which would slow intensification and likely result in some weakening. The current track brings the system just south of these islands and continues over warm waters…so it makes sense currently to follow the stronger intensity guidance at the longer range (the LGEM and SHIPS). Both make the system a major hurricane, while the GFDL, HWRF, GFS, and ECMWF do not show anywhere near as much intensification, but this is likely due to the models indicating more land interaction. Current NHC forecast brings the system to just below major hurricane status at 96 and 120 hours. Forecast intensity in the longer range is fairly low confidence.

96L:

Another area of well defined low pressure has developed about 450 miles SW of the Cape Verde Islands. Thunderstorms activity has increased with this system overnight and conditions appear favorable for development into a tropical cyclone in the next 48 hours. NHC currently gives the system at 60% chance of formation in the next 48 hours.

Forecast Track Guidance:

[Ed Lizard]

The Euro solution would affect the Louisiana members of this forum.

[Srain]

Morning e-mail from Jeff:

Isaac becomes a tropical storm and approaches the Leeward Islands

Weak ENE wind shear has relaxed overnight allowing a large ball of deep convection to develop directly over the low level circulation. The last recon mission early this morning did not indicate that this organization had translated into intensification yet with flight level winds of around 40-45kts so the intensity is held at 45mph. Since that time however the inner structure of the system appears to be getting better organized with deep convection maintaining itself and good banding to the west and SE of the system. There still appears to be a bit of dry air on the NE flank of the storm, but it is likely that the inner core is keeping this dry air out of the center of the system. Outflow is excellent to the W and S of the system and a little restricted to the E and NE of the system.

Track:

Global and hurricane forecasting models are in good agreement that Isaac will move nearly due west for the next 72 hours under the influence of a deep layer ridge over the central and SW Atlantic. This will take the storm across the Leeward Islands today and then south of Puerto Rico on Thursday. After 72 hours, Isaac will reach the western edge of the deep layer ridge as a trough digs southward over the SE US and the eastern Gulf of Mexico. This steering pattern will induce a turn toward the NW across the western part of Hispaniola or eastern Cuba and then toward FL or the Bahamas. While the meteorological reasoning is sound, how quick and hard of a right turn Isaac takes in the 4-5 day period will determine what impacts the storm will have on the US coast. Due to the orientation of the FL peninsula and the positioning of the storm and forecast track, only a slight shift to the right of left could/will have a fairly big impact on any landfall locations. The dynamical consensus have been shifting ever so slightly eastward over the past 36 hours and it is possible that some additional eastward shifts will be needed. Now is a good time to remind everyone that the average forecast track error at Days 4 and 5 is around 225 miles.

Intensity:

Thus far the well organized pattern on satellite has not translated into intensification, but it is likely only a matter of time until the winds and central pressure begin to respond to the increased organization. Upper level outflow is increasing across the system as noted by the high level cirrus clouds peeling off to the west and south of the center which is helping to vent the inner core convection. Convection overnight has maintained itself very well compared to the up-down eruptions of the past few nights. Once the inner core consolidates there appears to be little to prevent intensification over the next 48 hours and Isaac will likely become a hurricane after passing the Leeward Islands. After 48 hours the system will begin to interact with the mountainous areas of the island of Hispaniola and it then becomes very important how much interaction the system has with this landmass. A track directly across the island will take a heavy toll on the system and organization of even major hurricanes in the past have struggled. The exact track of the inner core will be key to intensity in the longer range and with the error being still upwards of 100-150 miles at 72 hours there is several possibilities. After getting past Hispaniola and eastern Cuba the system will emerge into the very warm waters between the northern Lesser Antilles and SE FL. Upper level conditions look very favorable for intensification in this region with the potential for very favorable outflow with a trough digging down to the west of the system. Main question is what condition does the storm exit the islands in and how long does it take the inner core to rebuild if it is heavily disrupted. Current NHC forecast brings Isaac to a 90mph category 2 hurricane as it nears the SE FL coast. It should be noted that the intensity forecast is fairly low confidence.

NHC Forecast Track and Error Cone:

[Ed Lizard]

Assuming, since the Euro is an outlier, that a Central Gulf landfall is unlikely, Louisiana should only see peripheral affects, maybe some swells if Isaac does get into the Gulf. Wind would tend to be offshore, if anything, mean tides might run a tad low.

[Srain]

Afternoon e-mail from Jeff:

Upper level trough and old stationary frontal boundary over the NW Gulf combined with short waves in the NW flow aloft helping to produce a round thunderstorms off and along the coast this morning.

Outflow boundaries from the offshore convection are moving back toward the coast at this time and will likely combine/help form the seabreeze which is starting to develop along the coast early this afternoon.

With weak short waves aloft moving over the area and boundaries/moisture approaching from the south expect a decent shot at thunderstorms south of I-10 this afternoon.

Should see a similar pattern Thursday and Friday with even better moisture likely resulting in a bit more rainfall coverage. Could see some heavy rainfall and brief gusty winds under storms over the next 2 days.

Forecast for the weekend is fairly summer like with highs in the 90’s and lows in the upper 70’s as the area comes back under the influence of the Gulf air mass. Afternoon thunderstorms on the seabreeze will be possible.

Into next week, will need to keep at least one eye on the track of Isaac as ECMWF model is fairly far to the left and would bring impacts this way, while the GFS and other global guidance keep the system over FL/east Gulf/west Atlantic. GFS has been performing very well this season in the tropics so hard to go against its thinking, but the ECMWF has been the best model over the past few years.

[Ed Lizard]

Nice looking thunderstorm East of downtown. Well lit by the setting sun. Nowhere near my lawn, but still a nice looking storm.

[Srain]

Morning e-mail from Jeff:

Disorganized tropical storm moving across the eastern Caribbean Sea.

Discussion:

USAF plane overnight investigating Isaac has found multiple centers rotating around a large mean circulation…a disorganized tropical system. Convection continues to struggle to consolidate and maintain itself and this is likely due to dry air shown in the Leeward Island soundings in the mid to upper levels occasionally entering into the fragile inner core of the system. This along with the large size and the multiple centers has prevented Isaac from much intensification. Isaac continues toward the W, but has slowed in the last several hours.

Track:

I have spent a lot amount of time this morning reviewing the various global models and their handling of the upstream features over the US that will ultimately determine the final track and landfall of Isaac on the US coast. For the next 24-48 hours it appears the system will continue to move toward the W to WNW on the south side of the sub-tropical high over the central Atlantic. This will take the system toward the southern coast of Haiti, but if Isaac does not starting gaining some latitude it is very possible the system will pass south of Haiti.

In the longer term (after 48 hours) the guidance spread increases with the ECMWF on the left (west) edge of the clustering and the CMC on the right (east) edge of the clustering. The ECMWF has been shifting eastward in its last few runs while the CMC has been shifting westward and the GFS has been holding strong for the past several runs with very little shifting. It is interesting that at the 120-144 hr points the GFS shows a classic recurve over Isaac over the SE US while the CMC and ECMWF drive the system inland and do not show a hard recurve.

The differences are in the handling of the troughing that currently extends from the NE US into the Gulf of Mexico. This trough will begin to lift over the next 24 hours, but a weakness or break in the ridge over the SW Atlantic and a building high over the southern plains is left behind over the SE US. This weakness appears to be enough to induce a NW turn of Isaac near the eastern or central Cuban coast. At this point the weakness begins to fill some as a zonal flow briefly develops over the US. However a strong shortwave digs down from Alaska which helps pump up (build) the southern plains ridge resulting in troughing digging across the SE US toward early next week in a weak omega pattern. This pattern allows Isaac to be captured or tugged by the trough and brought NNW and then N across the eastern Gulf toward the FL panhandle. Each of the global models has the same solution, but each differ some on the intensity and amplification of the southern plains ridging and the resultant downstream trough. The ECMWF has a weaker trough which allows Isaac to move further west while the GFS has a deeper trough and curves the system nearly up the FL west coast and then into SC. The GFS has been very consistent on its track and the ECMWF has been slowly trending toward its solution over the past two model runs although the ECMWF ensembles are well west of the operational track toward the LA coast.

There is also the potential for short term center relocations to the south of the current position as supported by the aircraft fixes, and this could have some longer term track implications.

With all that said, the consensus is still in very good agreement through the Day 4-5 period with the system track toward the FL Keys and then NW along the west coast of FL.

Intensity:

Isaac is continuing to suffer from dry air intruding into the inner core and the lack of a well defined center. While the potential has been and continues to be there for intensification, the internal dynamics of the inner core continue to be lacking which is preventing organized deep convection. We need a well defined center to take shape and to flush the dry air out of the inner areas of the system allowing deep convection to develop and maintain itself. Another factor is the large size of the system as larger systems tend to take time to consolidate. Isaac has a well defined outflow pattern established with high level cirrus expanding outward in all directions helping to vent the circulation. When the inner core finally does consolidate then the system should begin to intensify given warm waters and good upper level outflow. The next item of concern with intensity will be how much the system interacts with the islands of Hispaniola and Cuba. While the track takes the system across western Haiti and eastern Cuba, I am starting to consider that the system may pass south of both of these land areas and over the water which would have the potential to produce some significant intensity differences compared to the current forecast. Due to the uncertainty of the consolidation of the inner core and how long it is going to take and the possibility of land interaction will follow along with the NHC on the intensity, but if Isaac stays a little more to the south it could be stronger in the middle and latter part of the forecast. Also there is data suggesting Isaac is currently suffering from the passage of an upper level Kelvin wave which is helping to induce unfavorable conditions across this part of the basin in the face of otherwise favorable conditions. Isaac should come out from under the influence of this feature in the next 48 hours or so.

NHC Forecast Track and Error Cone:

[Srain]

Morning e-mail from Jeff:

Isaac becoming slightly better organized over the central Caribbean Sea

Discussion:

An onslaught of aircraft have been sampling the atmosphere in and around Isaac over the past 24 hours including a high attitude GIV mission over the Bahamas and several recon missions. Latest mission is in progress at this time and continues to suggest a disorganized tropical system with low level and mid level centers not aligned. There is a low level center around 16.7N that has been tracked for the advisory position for the past 36 hours, but a well defined mid level position (700-850mb) well to the S/SSW of the northern low level center. The most recent passes suggest that a new low level center may be forming under the mid level center which could result in a significant near term shift to the SW of the center location. One thing is for sure, convection has been vigorous overnight near the mid level center.

Track:

After some really big westward track shifts by nearly all the models yesterday afternoon most have swung back eastward overnight on their 00Z runs including the ECMWF. In fact the 00Z ECMWF and GFS and almost identical in their track out through 5 days. This is the first time that there has been an overall consensus with the track and some of the GIV data did get into these runs. Would like to at least see 1-2 more runs (12Z today and 00Z tonight) to see if this trend holds and what if any impact the relocation of the center may have.

Based on the potential for the center relocation and looking at IR images this morning, it would appear Isaac is going to miss Hispaniola to the south unless it takes a hard right turn and starting gaining latitude. I think there has been some uncertain motion over the past 24 hours based on tracking and merging of multiple centers and NHC trying to hold some continuity between advisory packages. We need to get a well defined low level center with good fixes to have some confidence in the motion.

Based on all of this and including the potential for a center relocation, the guidance is still in very good agreement out through 72 hours on Isaac rounding the western side of the ridge over the Atlantic and turning NW toward the weakness over the SE US. This tracks the system over east or center Cuba and then across the FL Straits and Keys and then into the eastern Gulf of Mexico. The guidance spread enlarges some beyond 72 hours after Isaac reaches the eastern Gulf, but there is still decent agreement amongst the major models showing a track toward the central/east US Gulf coast from MS to the FL panhandle. I still do not see a major trough capture pattern in most of the model solutions with several showing a continued NW/NNW motion after landfall and a significant slowing of forward speed suggesting high pressure is going to be building in north and over the top of the system. This has continued to be the controlling factor on track in the long range period and has to do with the movement and intensity of short waves moving onshore over the PAC NW and the downstream ridge intensity and position over the southern plains. Models tend to under-estimate the ridging at times which is why I would like to see the current consensus hold for the next two guidance cycles.

Based on everything and the current guidance/NHC track a hurricane should be nearing the US Gulf coast late Tuesday into early Wednesday morning. It is a good time to remind everyone that the error in track at 4-5 days in around 200 miles and this system will be large in size and have far reaching impacts.

Intensity:

Isaac has failed to intensify much to this point mainly due to the lack of organization of its inner core. There continues to be dry air entangled with the system which has limited the convection for most of its life to the SW semi-circle. Additionally large circulations, such as Isaac has, usually take time to consolidate. Until Isaac can consolidate its inner core any intensification will be slow. The interaction of land continues to also offer some challenges to the intensity forecast, but the mountains of Hispaniola may not play as big of a role as before. Once into the Gulf of Mexico the system should be in an environment favorable for intensification and most of the models support that although none are overly aggressive. We will have to see how long it takes the inner core to re-form if it is highly disrupted with its passage over any of the islands in the next 72 hours. Based on the available intensity guidance, Isaac should reach hurricane intensity in the SE Gulf of Mexico and approach the US Gulf coast as a 800-95mph hurricane.

NHC Track and Error Cone:

[Srain]

Morning Update from Jeff:

Hurricane Warnings are issued for the SW FL coast from Bonita, FL south to Key West.

Hurricane Watch is in effect for the SE FL coast from Golden Beach southward.

Tropical Storm Warning is in effect for the SE FL coast south of Jupiter Inlet including metro Miami.

Discussion:

After being close to hurricane intensity last evening, Isaac crossed over the SW tip of Haiti with a large part of the circulation going across the mountains of Hispaniola. This has disrupted the overall circulation and once again Isaac is looking poorly organized on satellite images. The center appears to be near the western edge of the deep convection and the upper level outflow which has been very good the past several days has become restricted on the western side due to a trough over the SE Gulf of Mexico into FL. Isaac has been moving toward the NW and is nearing the eastern part of Cuba.

Track:

Forecast models are in good agreement that the trough currently just west of Isaac will lift NE allowing the sub-tropical ridge to build westward over the next 24-36 hours turning the system from the current NW heading to a more WNW heading. How fast this happens will determine exactly where Isaac crosses the FL coast the first time. Based on the current motion and speed, Isaac should cross over eastern Cuba this afternoon and then move toward the FL Keys tonight and Sunday. Isaac will then enter into the SE Gulf of Mexico and begin to turn toward the NW and N as a ridge of high pressure over the central plains amplifies due to a short wave moving into the Pac NW. This increasing ridging will result in the digging of a downstream trough over the SE US which should pull Isaac northward. How quickly this happens and where Isaac is when he begins to make the N turn will determine the final landfall on the US Gulf coast.

Forecast models remain in decent agreement with a landfall from eastern MS to the FL panhandle with a good clustering over the west/central FL panhandle. While the CMC has shifted way west in its 00Z run, it is currently being discounted. The GFDL and HWRF have also shifted west overnight, but the GFS and ECMWF (the two best models) are nearly on top of each other into the FL panhandle and the GFS has been extremely consistent. On this track Isaac will be approaching the US Gulf coast late Tuesday.

NOTE: it is important to understand that this is an above average size storm with Tropical Storm force winds extending outward 230 miles from the center. Adverse conditions will arrive well before the center and extend well away from the center.

Intensity:

While Isaac is not spending much time of the land areas of the Caribbean Islands, the inner core does appear to be somewhat disrupted this morning. With that said, the slight NE trend in the forecast track in the short term suggest Isaac will be firmly back over the warm waters of the FL Straits and the interaction with Cuba will be short. The upper levels currently do not look as favorable as they were forecast to be with the trough to the west restricting outflow and maybe even imparting a little bit of shear. This trough will lift out today and conditions should improve, but Isaac has a very large circulation and it will take time for the system to intensify. With that said, most of the guidance brings the system to hurricane intensity has in move into the FL Keys and then maintain category 1/low end category 2 intensity to landfall on the Gulf coast. The latest HRWF takes Isaac to a strong category 3 hurricane, but this is being discounted currently. Over the Gulf of Mexico Isaac will be crossing warm water, but the best heat potential (energy) is located over the central and western Gulf and not along the track of the system.

Impacts:

Preparations to protect life and property should be rushed to completion in the hurricane warning area. Storm surge values of 5-8 feet will be possible along the SW FL coast south of Cape Coral where strong south winds will pile the sea water against the SE/NE angle of the coast. Values may be locally higher in small inlets and bays. Near the final landfall location, some significant storm surge will be possible into Apalachicola Bay similar to Hurricane Dennis in 2005. Rainfall amounts of 5-10 inches will be possible across S FL spreading north across much of FL Sun-Tues. Hurricane conditions are likely over the FL Keys late Sunday and then spreading up the SW FL coast into Monday. Hurricane conditions will begin to impact the northern Gulf coast by midday Tuesday.

NHC forecast Track and Error Cone:

[Ed Lizard]

18Z GFS barely misses raining on my house from Isaac. Another sweet t-storm today, BTW, not mondo desparate for a storm as doing ok for rain w/o tropics..

We got an outer rainband from Katrina. Probably almost as much rain as I got at the house from Rita, which much closer.