My personal calls on Irene

[OHweather]

Hurricane Irene forecast/discussion #2:

As of mid-Monday afternoon—Irene is developing an inner core as it transverses the small gap between the Puerto Rico and northern Hispaniola coastlines. Irene is also exhibiting excellent outflow, especially on the northern half of the storm which is likely contributing—along with rather warm sea surface temperatures—to the continual firing of deep convection.

http://www.wundergro...f=9999&smooth=0

Radar out of Puerto Rico indicates that Irene at the very least has an open eyewall, with deep convection around mainly the eastern side of the circulation. An eye has occasionally tried popping out on visible imagery but currently there are no signs of an eye on IR imagery and no microwave images to clarify the structure of Irene’s inner core. However, over the past few hours the half eyewall visible on radar has tightened some which may be indicative of some strengthening of Irene.

Irene has been and is expected to remain under an upper level anti-cyclone. This is in tandem with an upper low well east of Irene providing for the excellent outflow noted on satellite imagery. There is some dry air in the vicinity of the hurricane, with arc clouds once in a while emitting out of the core, however this does not appear to be having a major impact on the inner core of Irene which is continuing to fire consistent deep convection.

Over the coming days global models generally agree on Irene remaining right under or very near the upper level anti-cyclone that is above it now. This will continue to provide for outstanding outflow/ventilation, which will promote continued deep convection within the inner core of the hurricane whenever the system is over water, which should be a good part of its remaining life. Water temperatures are very warm in the path of Irene and the warmth is somewhat deep over the far southwestern Atlantic, which along with good upper level divergence and low vertical wind shear may result in significant intensification:

The question becomes at what point does the intensification stop, and how strong is Irene when this occurs?

The global models (GFS used as an example) generally show Irene remaining under the upper level anti-cyclone until it gets to near the latitude of the Carolinas. North of here, Irene will begin to interact with the westerlies (which may briefly provide for a burst of intensification as they vastly improve upper level lift over the cyclone as it approaches them). However, up until the latitude of the Carolinas upper level wind shear should remain fairly light and Irene should be able to maintain good outflow, with the westerlies beginning to add to the upper level divergence in the vicinity of Irene as it approaches the Carolinas.

As Irene nears the US coastline continental dry air will become a possible issue. Shear may remain low enough from the Carolinas points south that significant dry air entrainment does not occur; however the 12z GFS (shown above) along with the 0z ECM indicate continental dry air edging right up to Irene’s moisture field, which may cause some disruptions within the inner core as Irene gets near the US mainland.

While Irene will move over very warm sea surface temperatures even up to the Carolina coast, the depth of the warm water becomes much shallower as one moves north of the Bahamas (shown above) which may also result in some decrease in instability available for Irene if her forward motion is slow enough off the southeast US coast.

So essentially, will forecast a steady state or slight weakening days 4-5 as Irene nears the coast, however based on outstanding upper level conditions and only minor issues otherwise I do not believe Irene will significantly fall apart upon landfall (assuming the system does make a landfall).

Irene will interact with Hispaniola over the next 24 or so hours, but then will have at least 3-4 days before possible interacting with the southeastern US. Irene as mentioned will be in a very favorable environment for intensification above sufficiently high TCHP waters to support a very strong cyclone. So at the very least will need to ramp up Irene to minimal category 3 status within 72 hours, with some rapid intensification being possible between 24-96 hours out given the very warm sea surface temperatures the hurricane will be transversing and the outstanding upper level pattern the hurricane should be under. Most models peak Irene somewhere between 72-96 hours out as a strong Cat 2 or low end Cat 3. Will peak at around 110 knots between hours 72-96, and will have to watch carefully for periods of rapid intensification. Will bring down to 105 knots by day 5 to reflect the slightly more hostile (but still relatively good) environment around Irene as discussed above.

A weakness in ridging has opened up north of Irene due to a trough moving off the US east coast. Initially this was thought to possibly not have a significant impact on Irene’s motion due to Irene being a good bit farther south, however center relocations and a deeper cyclone have allowed Irene to end up farther north than thought just 24-48 hours ago. Irene should continue to move WNW in the near term on the southwestern periphery of subtropical ridging centered to the east of the cyclone.

As the trough currently along the east coast lifts to the northeast, it will leave a small weakness in the ridging over the southeastern US. With good ridging still extending just about to the Carolina coastline and Irene being on the southwest side of it, a general WNW motion should continue for the next couple of days, taking it right into the Bahamas.

The next trough will move through on the rather zonal flow Thursday-Friday. How fast this trough moves through and how much it digs will likely determine if Irene makes a direct impact on the US southeast coast or not.

There are two main factors that will determine how much this trough digs:

• –NAO ridging over Greenland and the Davis strait.
• The +PNA troughing over the Aluetians/Alaska.

The NAO and PNA have generally been modeled to weaken some by the end of the week into the weekend. Since both the –NAO and +PNA support troughs along the east coast, the stronger these teleconnections are the more the trough digs along the east coast.

As you can see above, the 12z ECM does show the –NAO weakening noticeably and the +PNA remaining about the same. The ensemble NAO forecasts have come into slightly better consensus on the NAO bouncing back towards neutral by mid-week, which is crucial to where Irene tracks:

The GFS ensembles are in fairly good agreement in moving the first trough out relatively quickly and clearly leave Irene behind off the southeast coast:

The next trough comes through the flow roughly 36 hours later (Saturday night) and will most likely be the trough that accelerates Irene to the N and then NE.

At this point most global models have Irene just off the southeast coast, which would make escaping without landfall difficult. Out of the 12z model suite the Euro remained most consistent and indicated a landfall near the SC/NC boarder. The CMC/GFS indicated a brush/possible direct hit on the outer banks. The NOGAPS indicated a likely NC landfall and the UKMET remains well left of all guidance and brings Irene north along the Florida coast and into either Georgia or SC. The 12z GFS ensemble mean appeared to be in agreement with the op GFS/CMC. The 0z Ensemble mean appeared to indicate a landfall somewhere near the NC/SC boarder, but the spread out pressure fields indicated that some members had landfall in Georgia and some missed the outer banks to the east.

Given the above information and likelihood of the trough on Friday leaving Irene behind off the southeast coast, will forecast a landfall near the NC/SC boarder and will continue moving Irene NE just off the Mid Atlantic coastline.

Track forecast (black dots are on 24 hour interval from 18z Monday):

Intensity forecast:

Initial (18z Monday): 70 knots/Cat 1

12 hours (6z Tuesday): 75 knots/Cat 1

24 hours (18z Tuesday): 80 knots/Cat 1

36 hours (6z Wednesday): 85 knots/Cat 2

48 hours (18z Wednesday): 95 knots/Cat 2

72 hours (18z Thursday): 105 knots/Cat 3

96 hours (18z Friday): 110 knots/Cat 3

120 hours (18z Satursday): 105 knots/Cat 3

[HurricaneJosh]

I don't know why people feel the need to troll here. If you're not interested in his thoughts, don't open the thread.

Back on topic...

The forecast looks reality-based to me and I found it interesting. Thanks for posting it-- I'll be sure to come back here to see how your thoughts evolve.

[Bluescat1]

Nice job.

[Ian]

I don't know why people feel the need to troll here. If you're not interested in his thoughts, don't open the thread.

Back on topic...

The forecast looks reality-based to me and I found it interesting. Thanks for posting it-- I'll be sure to come back here to see how your thoughts evolve.

agreed.. several posts deleted

[winterymix]

Nice work, OHweather.

Good luck and please update if conditions change.

[buckeye]

folks live in sunset beach NC...

nice analysis, just hoping you're wrong

[OHweather]

Thanks all.

Finally got some time to sit back down at the computer for a little while.

Hurricane Irene continues to move NW with outstanding outflow in all quadrants. However, microwave imagery and reports from recon show that a double eyewall structure is occurring with a much larger eye forming, along with conventional IR satellite imagery clearly showing the original, smaller inner eyewall collapsing.

Hurricane Irene is still under an upper level anti-cyclone which is providing for excellent outflow in essentially all quadrants. However, CIMSS shear analysis shows 10-20 knots of shear over Irene, which may limit intensification some. Given Irene is over warm waters and has outstanding outflow but is in the midst of an eyewall replacement cycle, will bring the intensity down to 100 knots through much of Thursday.

As time wears on, Irene will remain in a similar environment, characterized by good outflow and the warm waters of the Gulf stream fueling convection but light to moderate southwesterly shear on the western edge of an upper level anti-cyclone perhaps limiting intensification somewhat. Given Irene is currently undergoing an eyewall replacement cycle and will have some shear to contend with, will not show appreciable strengthening for the rest of the forecast but will maintain Irene as a category 3 hurricane, albeit a minimal one, for about 48 hours. Beyond this Irene will begin moving over slightly lower TCHP waters, and will begin to feel increasing southwesterly shear:

This forecast intensity is lower than the majority if not almost all of the 0z intensity guidance through about 72 hours. However, like Earl and Igor last year, I feel as though the models are over estimating how much re-intensification will occur after an eyewall replacement cycle at a fairly high latitude with some small environment negatives against intensification.

Between 72-96 hours out the intensity forecast becomes more uncertain due to possible land interaction and interaction with a trough.

There is little doubt that once Irene nears the latitude of the Carolinas that it will begin feeling rather strong upper level winds, which should result in gradual weakening. Note increasing upper level southwesterly winds surrounding the hurricane by this point. In addition, water temperatures north of the Delmarva off the east coast are generally sub 80 degrees:

However, one interesting variable will be interactive with a baroclinic jet streak to the northeast of the hurricane as it comes up with the coast an the beginnings of an extra-tropical transition which may allow Irene to retain a rather low pressure and broad/strong wind field even as the inner core likely gradually deteriorates due to increasing southwesterly shear/cooler than favorable water temperatures.

Note how (off the NJ coast at this time) Irene will be under the right-entrance portion of an upper level jet streak, which promotes large scale ascent. In addition to this factor favoring a very strong low pressure system, as Irene begins the extra-tropical transition off the Mid Atlantic coast baroclinic processes such as cold air advection to the west and warm air advection to the east will likely favor enough low level convergence to support a continued deep low pressure system as the extra-tropical transition occurs:

So, even with possible land interaction I do expect Irene to remain a very power system, likely at least a minimal hurricane until it moves completely inland over New England, with only a gradual decrease in winds days 2.5-4 as Irene moves up the eastern sea board, with a broadening of the wind field likely occurring as the inner core structure gradually degrades but as a large area of intense pressure gradient remains.

The intensity forecast follows the general downward trend of the 0z ATCF intensity models and is in fairly good agreement with the 0z GFS/12z ECM, which hint at hurricane force winds attempting to be maintained until the system moves completely inland over New England. This would cause significant disruptions along the east coast, although this is more of a track/intensity forecast rather than impact forecast.

Irene is continuing to be steering to the NW by a ridge centered over the eastern-central subtropical Atlantic ocean. A trough currently over the Great Lakes will beat down the ridge some and allow Irene to turn more due north over the coming 24-48 hours. Irene’s motion will remain slow over the next two days or so as the hurricane is caught in the weakness between two ridges with the westerlies remaining well north of the cyclone. This scenario is well agreed upon by most hurricane track models and skillful global models (namely GFS/Euro). A gradual right turn should continue through about 3 days out as Irene rounds the ridge within rather weak steering currents. This will take Irene close to or over the eastern Carolinas as a large and possibly major hurricane.

The next shortwave will begin to dig a little bit farther west (due to some ridging along the west coast an short wavelengths), over the upper Midwest/Great Lakes Friday-Saturday. Given the –NAO and large scale subsidence to the north of Irene, this shortwave could well slow down and perhaps take on a neutral tilt over the Great Lakes/Ohio Valley. This would allow for higher heights to occur north of Irene and draw Irene back westward. This is not the classic pattern for an east coast hurricane hit given the +PNA and –NAO, however timing may allow for the trough to dig west of Irene and amplify enough to draw the system westward or at least due north after dancing with the Outer Banks.

Note how by Saturday the ECM ensembles show one trough digging over the Lakes, and the lower heights associated by Irene being forced almost due northward due to higher heights ahead of the trough over the eastern US. This scenario was well agreed upon by the 12z model suite and brought the hurricane essentially due north along or just off the Mid Atlantic coast and into New England. So far, the 0z GFS, UKMET and CMC all also bring Irene essentially up the east coast.

However, since timing and amplitude of this second trough are still uncertain will follow a blend of the 12z/0z global and ATCF models rather closely and move Irene up the east coast in NNErly direction, and keep her off the coast until landfall over Long Island and then in New England. If more certainty regarding the timing, location and amplitude of the trough that digs over the Lakes towards the end of the week occurs, a shift west may be in order at that time. This forecast is a slight shift east through the Carolina’s compared to Monday’s forecast and a slight shift west north of there, due to changes in how the shortwave troughs are expected to play out, as discussed above.

Track forecast (black dots represent 24 hour intervals from 0z Thursday):

Intensity forecast:

Initial: 100 knots/Cat 3

12 hours (12z Thrusday): 100 knots/Cat 3

24 hours (0z Friday): 100 knots/Cat 3

36 hours (12z Friday): 105 knots/Cat 3

48 hours (0z Saturday): 100 knots/Cat 3

72 hours (0z Sunday): 95 knots/Cat 2

96 hours (0z Monday): 75 knots/Cat 1 (becoming extra-tropical)

120 hours (0z Tuesday): 50 knots/TS (extra-tropical).

[OHweather]

Hurricane Irene has turned more towards the NNW to due north this evening, and appears to be attempting to close and consolidate an eyewall per conventional IR satellite imagery. Irene is continuing to exhibit outstanding outflow in all quadrants and is over fairly warm SSTs. Given this and the fact that RECON is finding a gradually falling pressure, Irene may be attempting to intensify.

Irene continues to reside under an upper level anti-cyclone. A TUTT to the east of the cyclone is however resulting in about 15 knots of shear over Irene. Given Irene’s eyewall is attempting to close off and contract, along with a gradually lowering minimum central pressure, think that Irene could gain some intensity over the next 12-24 hours. Given the large size of Irene’s inner core, fairly warm SSTs, and outstanding outflow, light to moderate shear should not prohibit some modest strengthening during the next day or so.

As Irene begins approaching NC however, lower TCHP waters near the coast along with the hurricane encountering gradually increasing shear as it nears the westerlies should result in the commencement of a gradual weakening. Note the increasing southwest upper level winds surrounding Irene in the GFS image above.

As Irene moves up the coast however, it will begin to slowly loose tropical characteristics due to increasingly cooler SSTs and increasing wind shear. However, Irene’s large size (currently TC force winds extend up to 290 miles from the center, with hurricane force winds up to 80 miles from the center, these distances may increase as the system moves up the coast) in combination with the influence of baroclinic processes (note how while off or along the NJ coast in the image above, Irene is in the right-entrance portion of an upper level jet streak) that will begin to affect the system, especially as it approaches and passes 40N, should allow Irene to remain a large and fairly deep cyclone as it moves up the Mid Atlantic coast.

In fact, no global model appears to weaken Irene to a higher central pressure of about 970MB before landfall in/passage through the Long Island/southern New England coastlines. This, in combination with a high pressure to the east increasing the pressure gradient and increasing forward speed of Irene should result in sustained hurricane force winds being associated with Irene until it is well into New England, barring a track well inland through the Mid Atlantic which would result in a more rapid weakening.

Irene will most likely move completely inland and well inland over New England which should result in a weakening to below hurricane status. In addition, as shown above (noting the significant CAA on the backside of the cyclone, and WAA on the east/front side of the cyclone), baroclinic processes will take over by the time Irene moves over New England. This combined with the likely collapse of the inner core over land will likely result in Irene becoming an extra-tropical, but still potent cyclone as it moves into eastern Canada.

Irene is currently in between two subtropical ridges and is being steered by the western side of a large ridge located between Bermuda and the Azores. This is resulting in a northward turn for Irene. A shortwave trough passing to the north of Irene at this time is aiding in weakening the subtropical ridging to the east/north of Irene, which is resulting in the gradual turn to the right.

However, after the trough currently along the east coast shifts out, rising heights ahead of the next trough north of the Great Lakes and due to the large scale subsidence surrounding Irene should result in the subtropical ridging building back west a tad, allowing Irene to continue moving strait NNE, and causing the system to stop turning right once she approaches the Carolinas. Given Irene is currently due south of central NC, is not gaining longitude at this time and will likely turn a little more to the right before the current shortwave moves off to the northeast of the east coast, believe a track over extreme eastern mainland North Carolina (possibly just east of Morehead City, NC) and over the northern/central Outer Banks is the best way to go at this point.

After passage through the Carolinas, it appears as though the next shortwave will remain west enough long enough for ridging to basically push Irene due north or slightly east of due north up the east coast. Note the ridging impinging on Irene from the east, preventing an OTS escape to the east. The 0z ATCF and global models (excluding the ECM which I have not seen yet) are in very close agreement on this, and bring Irene up essentially along the Mid Atlantic coastline. Given this is supported well be the positioning of the incoming trough and ridging to the east, will show a track NNE just off the Mid Atlantic coastline north of the Carolinas. This is a slight nudge west from last night’s forecast. Will show a western Long Island landfall.

By the time Irene gets into New England it will get caught in a fast SWrly flow on the eastern side of a shortwave that appears as though it will be neutrally to positively tilted digging over the lakes (given a weak PNA and neutral to slightly positive NAO forecasted to be in place this weekend) and will really begin to accelerate to the NNE or NE. This is also well agreed upon by the 0z guidance.

Forecast track/intensity (points are on a 24 hour interval from 0z Friday):

Intensity:

Initial: 100 knots/Cat 3

12 hours (12z Friday): 105 knots/Cat 3

24 hours (0z Saturday): 105 knots/Cat 3

36 hours(12z Saturday): 100 knots/Cat 3

48 hours (0z Sunday): 95 knots/Cat 2

72 hours (0z Monday): 65 knots/Cat 1 (becoming extra-tropical)

96 hours (0z Tuesday): 45 knots/TS (extra-tropical)

As I said yesterday these are supposed to be my personal track/intensity guestimates, however given the potential number of people affected, will throw some potential wind/water impacts out...

Winds: Hurricane force winds likely over much of eastern North Carolina including the Outer Banks, most likely from Wilmington to the VA/NC boarder points SE. Widespread power outages likely with some structural damage likely near the coast.

Hurricane force winds possible, with tropical storm force winds likely over eastern VA, DE, and eastern NJ. Tropical storm force winds may extend into eastern MD, the bulk of NJ and extreme western PA if Irene deviates well west of the forecast track. Scattered to widespread power outages possible. Minor structural damage may occur along the immediate coast if Irene can track farther west than currently shown in my forecast.

Hurricane force winds will be possible on Long Island mainly east of NYC, with tropical storm force winds a very good possibility from extreme NE NJ points east along Long Island. Tropical storm force winds may get farther into NY if Irene deviates well west of the forecast track. Widespread power outages possible.

Tropical storm force winds with hurricane force gusts possible over much of southern and eastern New England. Tropical storm force gusts likely as far north as northern NH and northern Maine. Widespread power outages possible.

The right-entrance portion to a jet streak and mid level frontogenesis to the north of Irene will create the potential for an up to 24 hour period of heavy rain from eastern SC up through much of NC, VA, MD, eastern PA, eastern NY and New England with many areas seeing over 6" of rain. Widesperad freshwater flooding appears likely.

A storm surge of 5-10 feet appears possible over eastern NC. Sounds/Bays on the mainland along with the Outer Banks appear most susceptible given a period of E-NE winds ahead of Irene.