November 22-23 Severe Weather Thread

[1900hurricane]

Texas has not had a severe report since the morning of October 13th, which is the timespan of over a month, but this long stretch of tranquil weather appears to be coming to an end. Model guidance has for the past several days been honing in on a southern stream trough ejecting into the southern plains while beginning to acquire a negative tilt this Saturday. While several of the smaller scale details are yet to be known, above average confidence exists in the overall synoptic setup.

At the surface, a lee-side low is expected to develop near the Big Bend and move northeast while deepening as our trough ejects from the west. This will put almost the entirety of the eastern half of Texas in the system's warm sector with notable low-level backing of winds. Above that, the low level jet should be enthusiastically be ripping at 40 kt or more at 850 mb and above while veering with height, which will lead to impressive shear parameters and enlarged hodographs. Capping all of that off, the warm sector will lie in the left exit region of a stacked 500-250 mb jet streak rounding the base of the sharp trough.

Perhaps the biggest question at the moment regarding this future event is the quality of the warm sector following the most recent cold air intrusion. Models indicate that the eventual warm sector boundary layer air will originate from the airmass that existed over the Florida/Georgia area this morning. Needless to say, this airmass is going to need a great deal of modification. However, with the impressive dynamics currently forecasted, only a modest 500 J/kg or so of CAPE may be needed for thunderstorms to become severe.

The SPC had put Southeast Texas and some adjoining areas in a Day 5 risk area yesterday, and that area remains largely unchanged with today's Day 4 Convective Outlook. As I mentioned above, many details are yet to be known, but chances look quite good that an impactful event will be arriving in the not so distant future.

[Srain]

Afternoon briefing from Jeff:

***Severe weather outbreak including tornadoes possible Saturday across much of SE TX.***

 

Strong storm system off the west coast currently will dive rapidly SSE and into the AZ/NM region on Friday and then sweep across TX on Saturday. Low level southerly winds have already returned to the region today ushering in much warmer temperatures and starting to increase moisture levels. Stronger moisture advection will begin on Thursday and stronger yet by Friday with increasing rain chances starting Thursday night. Lead short wave ejecting across on Friday with a chances of showers and thunderstorms in the warm air advection regime.

 

Concerning period is starting to key on Saturday afternoon as a powerful upper level system sweeps across central TX. Unstable warm sector air mass will likely have spread far inland by Saturday morning with dewpoints in the mid 60’s as maritime tropical air mass makes a return to the region. Impressive wind energy will come to bear on this unstable atmosphere as the low level jet is overrun by a strong mid level jet streak. This will result in strong low level turning with height and favorable low level shear for updraft rotation. Instability looks a little better today (800-1500 J/kg) which appears possible if not likely given the timing of the system in the mid to late afternoon allowing surface temperatures to rise into the mid to upper 70’s. I am concerned with the forecast low level shear values and long curved forecasted hodographs suggesting significant supercell potential in the warm sector ahead of the main band of thunderstorms. Tornado outbreaks almost always have these leading supercell storms in the warm sector that tend to produce the vast amount of tornadoes and damage. The GFS does attempt to develop warm sector supercells in a convergent band ahead of the main line of storms Saturday afternoon. We are still talking about 72 hours out which is far out in severe weather time frames, but the confidence in the event and model agreement is fairly high at this time range. With that, SPC has already placed a Day 4 slight risk outlook for all SE TX and if parameters continue to look favorable into Thursday and Friday an upgrade to a moderate risk is certainly possible.

 

Main threats appear to be tornadoes and wind damage in the noon to midnight time period on Saturday. I am tempted to favor the tornado threat higher in the southern 2/3rds of the area where instability is forecasted to be strongest, but since we are 72 hours out I think a general widespread threat is best at the moment.

 

This system and its severe weather impacts bears close watch especially on the tornado threat.

 

Note: record low of 30 degrees was established this morning at BUSH IAH breaking the 111 year old record of 31.   

 

SPC Day 4 (Saturday) Severe Weather Outlook:

[andyhb]

I'm actually a intrigued at the potential around the I-35 corridor in between DFW and San Antonio earlier on Saturday afternoon as it sits currently, which might also be the best time for both stronger instability to be available and also for more discrete storms. Directional shear will be favorable throughout the day and storm motions should be to the NE/NNE with SSW 700 mb winds.

[1900hurricane]

HGX looks like they are beginning to buy in to more discrete storms based on the wording of this afternoon's forecast discussion. They sound fairly impressed.

ON FRIDAY A POTENT SHORTWAVE WILL DIVE TO THE SOUTH TOWARDS
SOUTHERN CALIFORNIA AND THEN BEGIN TO PUSH TO THE EAST. THE
SHORTWAVE WILL THEN CROSS TEXAS SATURDAY BRINGING A CHANCE OF
SEVERE WEATHER TO THE AREA. THE SHORTWAVE WILL BEGIN TO TAKE A
NEGATIVE TILT TO IT SATURDAY AFTERNOON AS IT APPROACHES SOUTHEAST
TEXAS. PWATS WILL ALSO CLIMB TO AROUND 1.70" WHICH IS AROUND 2SD
ABOVE NORMAL FOR THIS TIME OF YEAR. CAPE VALUES SATURDAY AFTERNOON
ARE FORECASTED TO BE BETWEEN 800 AND 1500 J/ KG K. GREATEST
POTENTIAL INSTABILITY LOOKS TO BE TOWARDS THE SOUTH BUT SHEAR
VALUES REMAIN IMPRESSIVE ACROSS THE CWA. SFC TO 1KM SHEAR IS
FORECASTED TO BE AROUND 20 KTS WITH SFC TO 6 KM SHEAR AROUND 45
KTS. UPPER LEVELS WILL ALSO PROVIDE DIFFLUENCE ACROSS THE AREA
SATURDAY. THE ESPECIALLY TRICKY PART WILL BE WHERE THE BEST UPPER
LEVEL DIFFLUENCE SETS UP. THE GFS PLACES THE BEST UPPER LEVEL
SUPPORT TOWARDS THE DALLAS/ FORT WORTH AREA WHILE THE EURO HAS THE
BEST UPPER LEVEL SUPPORT THROUGH OUR AREA. THE OVERALL PATTERN
SUPPORTS HEAVIER RAIN TOWARDS THE NORTH WITH THE STRONGER MORE
DISCRETE CELLS TOWARDS THE SOUTH IN THE HIGHER INSTABILITY. THE
MAIN THREATS WITH THIS WILL BE THE POTENTIAL FOR TORNADOES AND
DAMAGING WINDS. SECONDARY THREAT OF HEAVY RAINFALL WILL BE

POSSIBLE. 

 

*I think I finally got this post fixed.

[1900hurricane]

I'm actually a intrigued at the potential around the I-35 corridor in between DFW and San Antonio earlier on Saturday afternoon as it sits currently, which might also be the best time for both stronger instability to be available and also for more discrete storms. Directional shear will be favorable throughout the day and storm motions should be to the NE/NNE with SSW 700 mb winds.

 

I'm keeping an eye on that area too, mainly because that is a much nicer region to chase, minus the cities along there of course. I am free this Saturday to chase and I might end up doing so, but I also want to stay out of the dense forests that are further to the east if at all possible.

[andyhb]

Areas further south into CRP's CWA might get into the action as well if a more southerly track verifies, which would further enhance the risk of discrete cells interacting with stronger buoyancy early on. They mentioned the Euro's stronger surface low leading to very prominent backing of the low level winds, which certainly appears to be the case. The GFS tends to have problems with this type of thing in the first place, a good example being 4/28 this year in Dixie Alley.

 

Oh and purely anecdotal, but interesting that this is literally falling directly on November 21st/22nd again like in 1992 although November 21st was the big day in TX in that event and in this case, the latter day will have the strongest potential.

[Srain]

I also believe the severe threat will be expanded N and E into portions of Arkansas, Louisiana and possibly into Mississippi with the overnight Day 4-8 Update.

[andyhb]

I also believe the severe threat will be expanded N and E into portions of Arkansas, Louisiana and possibly into Mississippi with the overnight Day 4-8 Update.

 

While the shear does seriously explode there after dark, instability is going to be limited with eastward extent given the late moisture return after this cold shot, not that it would take a lot to get a notable wind/tornado threat.

 

You don't see all of the CWAs over central/east TX noting the same severe potential with vigour very often.

[1900hurricane]

Also worth noting is the intensity of the jet streak rounding the base of such a sharp trough axis. Who needs Geostrophic wind anyway!

 

[1900hurricane]

Forgot to mention this in the original post, but Friday (11/21) also has somewhat of a severe risk. The SPC already has a slight risk out for western Texas on their Day 3 Outlook, with the text highlighting a hail risk. Still though, Friday will likely be small potatoes to the potential Saturday holds.

 

[NTXYankee]

Maybe some of the experts on here can shed some light, but reading the FWD discussions regarding Saturday the last couple of days, they talk about the severe potential, but don't mention specifically what area of their coverage zone. Is the threat much of a concern for those of us in DFW, it seems like it is further south of here based on model data and of course the SPC outlook. Would be nice to see some action up this way, but if it doesn't happen the heavy rain will be welcome.

[andyhb]

00z GFS appears to be slowing down the Saturday wave significantly, with the trough axis still west of San Antonio by 00z Sunday.

 

I figured we might see some changes since the lead energy associated with this has finally begun to enter the upper air network.

[1900hurricane]

00z GFS appears to be slowing down the Saturday wave significantly, with the trough axis still west of San Antonio by 00z Sunday.

 

I figured we might see some changes since the lead energy associated with this has finally begun to enter the upper air network.

 

I'm not sure it really slowed down that much. Both today's 12Z and 18Z GFS runs kept the 500 mb trough axis west of I 35 at 00Z Sunday. Today's 12Z ECMWF did too.

 

 

Maybe some of the experts on here can shed some light, but reading the FWD discussions regarding Saturday the last couple of days, they talk about the severe potential, but don't mention specifically what area of their coverage zone. Is the threat much of a concern for those of us in DFW, it seems like it is further south of here based on model data and of course the SPC outlook. Would be nice to see some action up this way, but if it doesn't happen the heavy rain will be welcome.

 

There's plenty of time for things to change and wiggle around a little bit yet, but it looks like the severe threat will probably be confined to areas south of I 20 due to issues with instability. Heavy rain still looks like a pretty good bet though due to extensive isentropic ascent.

[andyhb]

I'm not sure it really slowed down that much. Both today's 12Z and 18Z GFS runs kept the 500 mb trough axis west of I 35 at 00Z Sunday. Today's 12Z ECMWF did too.

 

Yeah on second thought I think my eyes played tricks on me, it seemed to be a bit slower coming ashore on the West Coast.

[1900hurricane]

Yeah on second thought I think my eyes played tricks on me, it seemed to be a bit slower coming ashore on the West Coast.

 

Yeah, no worries, I kinda saw the same thing when I first saw the 00Z data, but then went back to compare. I don't know what was so deceiving about that run.

 

Anyway, HGX dropped a little knowledge about November tornado climatology in this evening's forecast discussion update.

 

LASTLY...SEVERE WEATHER IN NOVEMBER DOES OCCUR. SOME OF THE MORE

NOTORIOUS TORNADO OUTBREAKS THAT HAVE AFFECTED SE TX OCCURRED

DURING NOVEMBER. HERE ARE A FEW REMINDERS...

DATE EF SCALE LOCATION

NOV 8 2011 EF-1 HOUSTON AND TEXAS CITY

NOV 17 2003 EF-2 24 TORNADOES - 72 INJURIES

NOV 6 2000 EF-1 CONROE AND SHEPHERD

NOV 16 1993 EF-1 DOWNTOWN HOUSTON - 26 MINOR INJ

NOV 21 1992 EF-4 KATY...HOUSTON...CHANNELVIEW

LAST EF-4 TO AFFECT SE TX

NOV 15 1987 EF-2 4 TORNADOES

CALDWELL AND NORMANGEE

5 KILLED AND 35 INJURED

[Quixotic1]

Maybe some of the experts on here can shed some light, but reading the FWD discussions regarding Saturday the last couple of days, they talk about the severe potential, but don't mention specifically what area of their coverage zone. Is the threat much of a concern for those of us in DFW, it seems like it is further south of here based on model data and of course the SPC outlook. Would be nice to see some action up this way, but if it doesn't happen the heavy rain will be welcome.

Hail would be the biggest threat for DFW IMO due to the cold air aloft. Think the tornado threat will be further south.

For DFW, the threat is almost always straight line winds and hail.

[1900hurricane]

I'm not sure how the inner floater nest on the NAM is run on the Penn State site or if it's any good, but its solution is very impressive. That's some big time surface wind backing beneath already nicely veering wind profiles. You can even see the tracks of the modeled discrete cells in the QPF fields.

 

 

[andyhb]

Looks like the first ever proper enhanced risk in the D3 for S TX. Includes Austin, San Antonio and Houston, Corpus Christi on the south end.

 

DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0230 AM CST THU NOV 20 2014

VALID 221200Z - 231200Z

...THERE IS AN ENH RISK OF SVR TSTMS OVER A PORTION OF CNTRL THROUGH
SERN TX...

...THERE IS A SLGT RISK OF SVR TSTMS FROM CNTRL AND ERN TX INTO THE
LOWER MS VALLEY...

...THERE IS A MRGL RISK OF SVR TSTMS OVER A LARGE PART OF TX INTO
THE LOWER MS VALLEY...

...SUMMARY...
NUMEROUS STRONG TO SEVERE STORMS ARE EXPECTED TO DEVELOP ACROSS
CENTRAL THROUGH EASTERN TEXAS SATURDAY. OTHER SEVERE STORMS ARE
POSSIBLE FROM SOUTHERN LOUISIANA INTO SOUTHERN MISSISSIPPI SATURDAY
NIGHT. PRIMARY THREATS WILL BE DAMAGING WIND AND A FEW
TORNADOES...BUT LARGE HAIL WILL ALSO BE POSSIBLE MAINLY OVER TEXAS.

...SYNOPSIS...

MODEL CONSENSUS IS THAT A STRONG VORTICITY MAXIMUM AND ATTENDANT
UPPER JET NOW APPROACHING THE NRN CA COAST WILL MOVE THROUGH TX
SATURDAY BEFORE EJECTING NEGATIVELY TILTED INTO THE LOWER MS VALLEY
SATURDAY NIGHT ACCOMPANIED BY A PACIFIC COLD FRONT. A WARM FRONT
WILL EXTEND ACROSS THE NRN GULF INTO SWRN TX EARLY SATURDAY. THIS
BOUNDARY WILL LIFT NWD INTO SRN PORTIONS OF THE LOWER MS VALLEY
REGION.

...CNTRL THROUGH SERN TX...

PARTIALLY MODIFIED GULF AIR WITH DEWPOINTS GENERALLY IN THE MID 60S
EXCEPT UPPER 60S CLOSER TO THE COAST WILL ADVECT NWD THROUGH TX IN
ADVANCE OF THE SRN-STREAM SHORTWAVE TROUGH. MLCAPE COULD APPROACH
1500 J/KG OVER PARTS OF SCNTRL TX WHERE MORE DIABATIC HEATING IS
EXPECTED...BUT SHOULD REMAIN AOB 1000 J/KG FARTHER NORTH. STRONG TO
SEVERE STORMS MAY BE ONGOING OVER WCNTRL TX EARLY SATURDAY.
SURFACE-BASED STORMS ARE EXPECTED TO DEVELOP IN PRE-FRONTAL
CONFLUENCE ZONE OVER SCNTRL TX WARM SECTOR DURING THE DAY. SOME OF
THE INITIAL STORMS MAY BE DISCRETE IN CHARACTER...AND VERTICAL WIND
AND SHEAR PROFILES WILL SUPPORT ORGANIZED STORMS INCLUDING
SUPERCELLS. HOWEVER...STORMS WILL EVENTUALLY GROW UPSCALE INTO AN
MCS...AND CONTINUE TOWARD THE LOWER MS VALLEY REGION DURING THE
EVENING WHERE THE SEVERE THREAT SHOULD PERSIST. THREAT WILL EXIST
FOR LARGE HAIL...DAMAGING WIND AND A FEW TORNADOES.

...LOWER MS VALLEY AREA...

INLAND ADVANCE OF THE MORE UNSTABLE WARM SECTOR OVER THE LOWER MS
VALLEY MAY BE LIMITED BY DEVELOPMENT OF WIDESPREAD CONVECTION OVER
THE NWRN GULF. NEVERTHELESS...THE BOUNDARY LAYER IS EXPECTED TO
MOISTEN AS THE SFC RIDGE SHIFTS EWD AND WINDS VEER TO SLY OVER THE
WRN GULF ALLOWING THE WARM FRONT TO DEVELOP INLAND. AS THIS OCCURS
SOME DESTABILIZATION OF THE BOUNDARY LAYER WILL EXIST...AND STORMS
MAY BECOME ROOTED NEAR THE SFC WHERE LARGE LOW LEVEL HODOGRAPHS WILL
SUPPORT A CONDITIONAL THREAT FOR LOW-LEVEL MESOCYCLONES AND POSSIBLE
TORNADOES. ALSO ANY SQUALL LINE THAT DEVELOPS OVER TX MAY CONTINUE
INTO THIS REGION OVERNIGHT WITH A THREAT OF DAMAGING WIND GUSTS.

..DIAL.. 11/20/2014

[Srain]

With the new changes to SPC classification and the potential for a Multi Regional threat, the first late season severe Topic has been moved to the Main Page from the Central/Western sub forum. From a synoptic perspective, HGX offered an interesting discussion regarding cool season severe episodes ~vs~ warm season...

 

AREA FORECAST DISCUSSION...COR
NATIONAL WEATHER SERVICE HOUSTON/GALVESTON TX
507 AM CST THU NOV 20 2014

.DISCUSSION...
THE WARM AIR ADVECTION PATTERN HAS BEGUN WITH THE REGION FALLING
UNDER A PRONOUNCED SOUTHEASTERLY FETCH OVER THE NEXT FEW DAYS. MID
TO UPPER 40 INTERIOR DEW POINTS WILL INCREASE INTO THE MEAN UPPER
50S BY THE CLOSE OF THE DAY...THICKENING CLOUD COVER AND A HIGHER
CHANCE FOR SCATTERED SHOWERS. THE HIGHEST PROBABILITY OF SHOWERS
WILL BE FOCUSED ALONG THE WESTERN CORRIDOR OF THE CWA...ESPECIALLY
OVER THE SOUTHWESTERN FORECAST AREA OR THOSE COUNTIES SURROUNDING
MATAGORDA BAY. HIGHER REZ MODELING IS FOCUSING QPF IN THIS AREA AS
IT DEVELOPS A WEAK CYCLONIC CIRCULATION NEAR MATAGORDA BAY.
ISENTROPIC UPGLIDE WILL BECOME MORE EFFICIENT OVER THE NEXT 36
HOURS AS SW-S WINDS ALONG THE APPLICABLE THETA SURFACE PRODUCES
SUB 20 CONDENSATION PRESSURE DEFICITS AND FURTHER SATURATES A
DEEPER COLUMN = MORE AREAL SHOWERS. 15 TO 20 KNOT NEAR SURFACE
WINDS MAKING IT ASHORE AND CREATING FRICTIONAL SPEED CONVERGENCE
WILL SATURATE THE LOWER LAYER TO PRODUCE STREAMER-LIKE SHOWERS...
TRANSITIONING FURTHER NORTH DURING THE WARMTH OF THE DAY. AS THE
COLUMN SATURATES UP THROUGH FRIDAY...THERE IS A HIGHER CHANCE FOR
MORE WIDESPREAD AREAL SHRA COVERAGE TOMORROW. VERY MILD AND WARM
CONDITIONS WITH AN AMPED UP SOUTH WIND...60S IN THE MORNING WARMING
INTO THE LOWER TO MIDDLE 70S. QUITE THE CHANGE IN A VERY SHORT
TIME! FOR PERSPECTIVE...PLEASE REFER TO THE CLIMATE SECTION BELOW.

SATURDAY IS STILL THE TARGET DAY FOR STRONG TO SEVERE WEATHER.
ALL OF THE INGREDIENTS ARE IN PLACE FOR A POSSIBLE ACTIVE WEATHER
DAY...FROM THE HIGH SHEAR TO THE IMPRESSIVE UPPER LEVEL DIVERGENCE
CREATED BY A DEEP SHORTWAVE TROUGH PASSAGE ACROSS CENTRAL AND NE
TEXAS. THE REGION DISPLAYS TWO PEAKS IN TORNADO FREQUENCY...ONE IN
THE MONTH OF MAY AND THE OTHER IN NOVEMBER. THE NATURAL TENDENCY
TO DESIRE HIGH THERMODYNAMIC INDICES NEEDS TO BE TEMPERED DUE TO
THE SEASON...COOL SEASON SEVERE WEATHER CAN RUN ON MUCH LOWER
THERMO DRIVERS. LOW TOPPED TORNADOES CAN AND WILL OCCUR DURING
THESE COOLER MONTHS UNDER LOWER CONVECTIVE TEMPERATURES...CAPE...
L.I.`S...ETC...COLD SEASON TORNADO CLIMATOLOGY SHOWS THAT A RANGE
OF 500-1500 ML CAPE OCCURS DURING TORNADIC EVENTS WITH THE 50TH
QUARTILE FALLING AROUND 1200....EFFECTIVE SHEAR IN THE LOW TO MID
40S...SRH IN THE 100 TO 300 RANGE. ALL OF THESE VALUES FALL IN
LINE WITH THE LATEST SOUNDINGS PROGS. IF THE REGION CAN SEE SOME SUN
BEHIND A LIFTING WARM FRONT EARLY SATURDAY...THERE MAY BE A HIGHER
THREAT FOR SEVERE TO BE ACHIEVED. HIGH TOTAL SHEAR VALUES WITH FAST
MOVING STORM MOTION(S) LEANS THE PRIMARILY THREAT TOWARDS WIND
DAMAGE. LOWER WBZ HEIGHTS MAY INTRODUCE A HAIL THREAT...AS WELL.
REGARDLESS OF THE THERMO AND DYNAMIC INDICES THE EXTREMELY HIGH PVA
PERPENDICULAR TO THE FLOW OF SUCH A SHARP MID-UPPER TROUGH SHOULD
KICK OFF AT LEAST STRONG DISCRETE CELLS. THESE CELLS MAY EVOLVE INTO
A MORE ORGANIZED MCS OR QCLS THAT ADVERSELY IMPACTS THE AREA FROM AS
EARLY AS NOON THROUGH AS LATE AS SATURDAY EVENING.

 

 

[1900hurricane]

From WPC:

 

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
148 AM EST THU NOV 20 2014

VALID NOV 20/0000 UTC THRU NOV 23/1200 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR
INGEST...

==============================================
00Z MODEL EVALUATION AND PREFERENCES
==============================================
NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO
SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT.
==============================================

 

...SOUTHWEST U.S. AND SOUTHERN PLAINS...

PREFERENCE: BLEND OF 20/00Z GFS/ECMWF
CONFIDENCE: AVERAGE

A LOW AMPLITUDE LEAD WAVE WILL MOVE INTO THE SOUTHERN PLAINS BY
FRIDAY. THIS WILL BE FOLLOWED BY A MUCH STRONGER TROUGH DIVING
FROM CALIFORNIA INTO THE RIO GRANDE BASIN AND CROSSING TEXAS ON
DAY 3. WITH SHORT WAVELENGTH AND STRONG GRADIENTS...THIS IS A
DYNAMIC SYSTEM. SO THE VERY DEEP SOLUTION FROM THE UKMET AND ONLY
MARGINALLY WEAKER NAM RUNS CANNOT BE DISCOUNTED...BUT THE
ENSEMBLES SIMPLY DO NOT OFFER ANY SUPPORT.

 

[Srain]

Morning briefing from Meteorologist Jeff Lindner (Harris County Flood Control District):

 

***Severe thunderstorms including tornadoes increasingly likely this Saturday.***

 

Powerful upper level storm system off the NW US coast currently will move rapidly SE and into W TX by early Saturday and sweep across the state late Saturday.

 

Moisture return clearly underway this morning with scattered showers developing in the corridor from Matagorda Bay to near I-10 west of Houston. Expect this activity to increase in coverage today and spread inland as a warm front moves toward the upper TX coast. Warm front will move inland tonight and progress across SE TX on Friday with additional scattered showers and a few thunderstorms especially along and north of the boundary. Warm sector air mass south of the boundary will support mid to upper 60 degree dewpoint maritime tropical air mass moving onshore. A few showers will be possible in the increasing onshore flow south of the warm front. Temperatures will be running about 20-30 degrees warmer than the first part of this week tomorrow and Saturday.

 

Saturday:
Powerful storm system sweeps across TX with impressively deep short wave rotating far south across S TX Saturday afternoon in the base of the upper level trough. Trough axis takes on a slight negative tilt pattern (SE to NW) which supports an overall increase in directional wind shear along the eastern flank of the trough axis (or in the region over the coastal bend and SE TX). Forecast models have shown a slight tendency to slow to eastward progression of the storm system slightly over the last 24 hours which places the greatest weather threat/impacts in the afternoon and evening hours of Saturday. A 35-45kt low level jet develops off the western Gulf of Mexico late Friday evening and continues into Saturday with a 100kt mid level jet streak carving into the region from the SW Saturday afternoon. The result is significant low level wind shear and wind energy in the lowest several thousand feet of the atmosphere. Latest guidance suggests breaks in the cloud cover are possible Saturday morning and early afternoon over SC/SE TX helping to boast surface based energy values (CAPE) and resulting in an increasingly unstable warm sector air mass.

 

Usually in this part of the state the warm sector is capped off and we await the actual strong forcing from the upper level trough or the cold front to develop a line of strong to severe thunderstorms. Saturday appears to be a possible exception where the warm sector air mass is only weakly capped and this could allow for the development of discrete supercell thunderstorms prior to the arrival of the main forcing toward early evening. This is important due to the fact that warm sector discrete supercells tend to produce a majority of the tornadoes in such severe weather outbreaks.

Model guidance is trending to point toward some development of discrete cells by early Saturday afternoon with an enhanced tornado threat which would be certainly supported by the sheared low level air mass resulting in updraft rotation. An example of the potential wind shear in the lowest part of the atmosphere being forecasted at noon on Saturday for IAH is a helicity value of around 260 m^2/s^2 on the GFS forecast sounding. Additionally the GFS shows a significant increase in PWS values between 06Z Saturday and 18Z Saturday from around 1.1 inches to nearly 1.70 inches.

The greatest severe weather threat currently appears across the southern 2/3rds of the area into the coastal bend and SC TX where the air mass becomes the most unstable Saturday afternoon and the favorable wind profiles and dynamics aloft align. (See SPC graphic outlines below).

 

The following threat analysis discusses the severe potential for Saturday:

 

Wind Damage:
Strong wind energy will be present both in the form of the low level jet overhead and mid level winds. Strong to severe thunderstorms will be capable of transporting some of this wind energy to the surface in the form of damaging winds of 60-70mph. This would be especially true in the event of an ESE or E moving squall line/MCS which guidance does develop out of C TX during the afternoon hours and sweeps across SE TX during the late afternoon and evening hours. The pattern supports bowing segments with the potential for larger than average corridors of wind damage. 

 

Tornado:
The concern is in the potential for discrete warm sector development of supercells early Saturday afternoon in a favorable 850mb convergent zone over SC/SE TX. If supercells do in fact initiate in such a highly sheared and unstable air mass tornado formation is certainly possible. Such tornadoes would tend to be stronger than what we would normally see in this area and could track for a longer period of time on the ground. Cell motions would likely be quick toward the NE and ENE reducing warning lead times. This enhanced tornado threat is strongly conditional on the development of supercells in the warm sector air mass which as stated above is a bit unusual for this area. A lack of warm sector discrete cell development would reduce the tornado threat across the area.

In addition there would also be a tornado risk along the main squall line/MCS toward the early evening hours with what we more commonly see in this area which is weaker rain wrapped tornadoes along the leading edge of squall lines where localized vorticity values are increased.

 

Large Hail:
Colder mid level temperatures will arrive with the cold pocket of the upper level system. This would support a large hail threat with both warm sector supercells and with the squall line. This threat is secondary to the wind and tornado threat at the moment.

 

Heavy Rainfall:
A significant increase in moisture will occur tonight-early Saturday with scattered showers for the next 24-36 hours. Heavy rains will certainly occur with the strong to severe thunderstorms on Saturday, but fast storm motions should help negate a serious flood threat. Of some concern is the model tendency to slow the system some which could result in a slightly longer duration of the heavy rainfall threat and the potential for high precipitation supercells to train in the warm sector. The heavy rainfall threat is secondary to the wind and tornado threat at the moment.

 

 

[1900hurricane]

I'm still concerned about low level lapse rates. Model fields are showing some improvement in the warm sector with them, but they're still certainly not a slam dunk. With the generally lowish CAPE values expected, storms could possibly fail to be rooted in the surface layer and instead tap into the elevated instability from the cold temps aloft. Breaks in the clouds during the day Saturday could quell this concern by raising surface temps, but overnight/morning precip from isentropic ascent could also make it a bigger issue than currently depicted.

[1900hurricane]

Model runs definitely slowed down with the trough today. Things are still fluid, but I'm starting to think about targeting Seguin for Saturday.

[1900hurricane]

Model slowdown continues, particularly with when the shortwave begins to take on its negative tilt. It still looks to be a potent system, but I'm much less impressed than I was 24 hours ago.

[Srain]

I'm not seeing anything that raises a big flag that changes the overall thinking regarding tomorrow. The warm front has moved well to the N of Central and SE Texas and as the upper low over Southern California begins its trek ESE across N Mexico and nears W Texas, rapid cyclogenesis should commence and a potent surface low will likely develop. If I recall correctly back in November 1992, we had a few early day storms and cloudiness before all heck broke loose and low topped rain wrapped tornado reports began coming in. The severe analogs for our cool season severe weather episodes are still showing up, so I encourage folks to not let their guard down tomorrow and into Sunday for our neighbors in Louisiana and Mississippi. After an extended cold snap, the last thing on many folks mind is a severe weather episode.

 

[Srain]

Morning brief from Jeff Lindner (Harris County Flood Control District):

 

***Severe Weather Outbreak likely Saturday afternoon and evening***

 

Warm air advection regime in place this morning with scattered showers and even a few pockets of heavy rain across the area. This will continue into the afternoon hours and then shift north and west with the core axis of the low level jet. Expect a fairly inactive overnight period. Main focus continues to be on Saturday which is covered from this point on.

 

Saturday:

Powerful upper level storm system currently over southern CA will move ESE today and tonight and sweep across TX on Saturday. Ingredients will come together to produce at least a couple of rounds of potentially severe thunderstorms Saturday afternoon and evening. Upper level system will move directly over SC TX Saturday evening taking on a slight SE to NW tilt (this indicates the trough axis is tilting negative compared to most of the time when they are positive or SW to NE). This tilt allows the cold pocket aloft associated with the storm system to overrun the warm sector ahead of the system to the SE allowing that warm sector air mass to become very unstable (cold air over warm air is unstable in the atmosphere). Additionally, very strong jet stream dynamics will come to bear across the region with 250mb high level winds splitting apart from the coastal bend through NE TX which allows general overall rising air motions. A 40kt low level jet will be in place from the coastal bend to NC TX which will be overtopped by a 100kt mid level jet streak out of the WSW. Wind profiles show strong low level veering of the wind field over the coastal bend and SE TX with 0-6km shear values of 60-70kts and 0-1km helicity values of 200-400 m^2/s^2. This all points to a lot of wind energy and certainly the potential for thunderstorm updrafts to rotate (turn with height). While the shear is there the instability is still lacking some. Forecasted CAPE values are in the 700-1200 J/kg range for Saturday afternoon which is not much surface based energy. The main reason for the lack of instability is the expected widespread cloud cover and showers/thunderstorms across the warm sector. The spreading of the cold pocket aloft could help increase these values some along with any breaks in the overcast. With that said, past historic severe weather outbreaks in November have not required significant amounts of CAPE and the forecasted energy values on Saturday fall very close in line with past analog outbreaks which produced both severe weather and tornadoes, so while I would like to see better values (say around 1500) what we currently are seeing forecasted can get the job done.

 

Main threats will be wind damage and isolated to possibly scattered tornadoes.

 

Tornado:

Tornado threat remains conditional on favorable shear and instability to align and this looks possible early Saturday afternoon along a SE to NW orientated boundary from Matagorda Bay to Austin. TTU 3km WRF has shown this setup in the last few model runs of severe storms (some supercells) developing on this boundary and lifting NE across SE TX Saturday afternoon. This boundary appears to be the maritime front separating the truly tropical air to the SW over the western Gulf from the more modified tropical air mass in place over the region currently. Low level winds will likely be backed some along this boundary and this supports an increased tornado risk on storms crossing over the boundary. Would like to see the HRRR runs late today to see if that model suggests a similar setup to add confidence. Will focus the greatest tornado threat along and SW of a line from College Station to Houston to Galveston.

 

Secondary tornado threat may exists with incoming squall line/MCS Saturday evening. Linear mode of activity should reduce the threat, but notches and local enhanced vorticity in the line could result in weak tornadoes along the line. This is what we commonly see in this part of TX that results in trees blown down and some roof damage. This threat will cover the entire region.

 

Wind Damage:

Plenty of wind energy will be in place so it will not take much to produce strong winds. While the initial development of supercells with a tornado risks is possible Saturday afternoon strong upper air forcing arrives Saturday evening and this will likely result in a squall line developing and moving out of central TX. Potential is there for bowing segments in the line to transport strong winds aloft to the surface resulting in corridors of wind damage. Leading edge winds of 50-70mph will be possible along this line of thunderstorms.

 

Heavy Rainfall/Flooding:

PWS surge to +2SD above normal by Saturday afternoon and with lift maximized from multiple sources there is little doubt it is going to rain. Heavy rainfall will be possible under the stronger cells and would expected rainfall rates of 1-2 inches likely possibly in a short period of time. This amount of rain can be handled. Fast storm motions should negate any serious flood threat and boundary orientation compared to the upper flow on Saturday is perpendicular not parallel. Only concern is potential for any warm sector supercells to anchor or train as these storms would certainly be producing copious rainfall rates.

 

Large Hail:

Cold pocket aloft moving over top of the warm sector does increase the hail threat, but the freezing level is fairly high and the sounding moist so think the large hail threat is the lowest risk. Hail is possible, but compared to the wind and tornado risk it is the lowest of the three.

 

Timing:

Thunderstorms may erupt as early as 1000am around Matagorda Bay with these storms increasing and spreading across SE TX. Area will be under the highest threat from about noon to midnight.

 

Will update again this evening as additional meso scale model data become available and attempt to refine/define the greatest tornado/severe threat area.  

 

SPC Day 2 (Saturday) Severe Weather Outlook:

 

[andyhb]

Latest D2 removes the enhanced risk citing lack of instability/near moist adiabatic lapse rates.

[1900hurricane]

Not really surprised by that. With the energy aloft progged to hang back a little bit more than before, the cold air aloft associated with it won't overspread the warm sector until a bit later as well. Low level lapse rates will already probably be pretty sucky, so that could really hurt CAPE unless we can get some surface heating.

 

With that being said, I still plan to be out chasing tomorrow. Even with this negative, the potential is still there for chaseable severe weather with plenty of good shear. Still thinking about setting up in Seguin or the next town south (Stockdale).

[1900hurricane]

Looking a little more in depth at the main threat area tomorrow, it seems quite likely that most of the parameters should fall within those observed with past systems that have produced tornadoes. It is worth noting that the sample size is pretty small though, and most of the tornadoes observed were rather weak and short lived.

 

 

 

Worst case scenario I see is that this goes the way February 16th, 2008 went. There are some differences between 2/16/08 and tomorrow (as there are in any two events really), but that day also featured a very dynamic system with a clouded over warm sector. Low level lapse rates ended up being poo and all the storms that developed in the warm sector ahead of the main squall line were elevated and didn't produce severe weather. That one ended up being a bad Day 2 Moderate Risk bust.

 

http://www.spc.noaa.gov/exper/archive/event.php?date=20080216

 

 

[1900hurricane]

Ugh, so much isentropicrap. Not liking the looks of today at all.

Sent from my iPhone using Tapatalk