TX Severe Threat (Weekend)

[jeff]

Potentail for a stormy late Saturday into Sunday over SC, SE, E TX and LA.

Parameters appears to be coming together for a severe weather episode late Saturday evening through the overnight into Sunday morning from C TX to C LA.

S/W currently over W MT will move SSE into the mid MS valley by late Friday driving a cold front southward into TX. Models forecast this front to stall 50 miles either side of a line from Fort Polk, LA to College Station TX. Upper low currently over the EPAC will drop into the SW US and eject a lead S/W across TX by midday Saturday. Low level flow will back to the SE Friday and increase by early Saturday pooling moisture along and south of the stalled frontal boundary. S/W lift along with surface convergence along frontal bounday and C TX dry line should fire deep convection by mid to late afternoon. 40-50 kts of mid level shear will help organized storms into clusters and lines, especially after dark as the low level jet increases.

ETA and GFS solutions indicate 1500-2500 J/kg of CAPE over the Matagorda Bay region by 00Z Sun with LIS of -4 to -6 and steep lapse rates. Forecast soundings from KLCH and KCRP also show a decent cap early Saturday weakening due to moisture advection and sustained weak ascent by late Saturday.

I will follow the ETA guidance as the GFS appears to be suffering from convective feedback problems. Thunderstorms should be ongoing Saturday morning over N LA in an area of enhanced convergence and dynamics from the mid MS valley S/W. Thunderstorms should develop southward into W LA and E TX by early afternoon driven by heating and outflow propagation. Strong moisture advection and heating will result in a very unstable air mass over SC and SE TX by late afternoon with thunderstorms rapidly developing once inhibition is removed along the dry line and W along the cold frontal boundry. Storms should congeal into one or two clusters or MCS's and track E to ENE overnight across SE TX and LA. Main threats right now will be damaging winds and large hail, however if instability is being underforecast a higher tornadic threat can be expected.

[CaptinCrunch]

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE FORT WORTH TX
415 AM CST THU MAR 17 2005

.DISCUSSION...
COLD START THIS MORNING WITH FREEZING TEMPS ACROSS THE NORTH AND
WEST CWA...BUT 850 TEMPS WARM SUBSTANTIALLY BY 0Z COMPLEMENTS OF
SUBSIDENCE BEHIND THE DEPARTING SHORTWAVE TROUGH AS WELL AS
WESTERLY/DOWNSLOPING FLOW. WITH FULL SUNSHINE TODAY TEMPS SHOULD
WARM INTO THE LOW-MID 60S. LOW PRESSURE DEVELOPING IN THE PLAINS
THIS EVENING WILL CAUSE WINDS TO INCREASE WHICH WILL HOLD MIN TEMPS
UP. BREEZY WINDS ARE LIKELY FRIDAY MORNING AS LOW LEVEL JET MIXES
DOWN BUT THE WINDS RELAX BY THE AFTERNOON AS A WEAK FRONT SLIDES
INTO THE AREA. FRONT WILL STALL ACROSS THE SOUTHERN AREAS FRIDAY
NIGHT...AND DRIER AIR OVER THE NORTH WILL MAKE FOR COOLER LOW TEMPS.

MOISTURE POOLING ALONG AND SOUTH OF THE FRONT WILL CREATE A VERY
UNSTABLE AIRMASS IN SE TX ON SATURDAY. ECMWF/GFS/UKMET INDICATE
LEAD SHORTWAVE AND A PRONOUNCED 700 MB TROUGH EJECT OUT OF MEXICO
LATE SATURDAY AND PULL THE OLD FRONT AND UNSTABLE GULF AIRMASS NORTH
INTO THE AREA SATURDAY NIGHT. HIGHEST POPS WILL BE OVER THE SE
WHERE BEST TRAJECTORY OF GULF AIRMASS OCCURS. SPC HAS HIGHLIGHTED
THE EXTREME SOUTHERN AREA IN SLIGHT RISK ON SATURDAY...BUT GIVEN THE
UNCERTAINTY OF TIMING/DYNAMICS IN FAST SWLY FLOW ALOFT WE WILL NOT
MENTION SEVERE IN THE HAZARDOUS WEATHER OUTLOOK AT THIS TIME.

WOULD EXPECT A BREAK IN THE PRECIP CHANCES ON SUNDAY AFTER THE
FIRST SHORTWAVE...BUT KEPT CHANCE POPS GOING...ONCE AGAIN DUE TO
TIMING UNCERTAINTIES. SECOND STRONGER SHORTWAVE WILL BEGIN TO MOVE
ACROSS OKLAHOMA SUNDAY NIGHT AND MONDAY...AND MODELS SUGGEST THE
CHANCES OF RAIN WILL CONTINUE MAINLY OVER THE EAST. GFS SOLUTION
LOOKS A LITTLE ODD IN HANDLING THE UPPER LOW BY CUTTING IT OFF AND
DROPPING IT SE INTO LOUISIANA TUE-WED. LATEST 17/0Z ECMWF SOLUTION
IS MORE BELIEVABLE WITH THE UPPER LOW CONTINUING A PROGRESSIVE
EASTWARD TRACK. THEREFORE DID NOT GO AS COOL AS MEX NUMBERS...BUT
DID SHOW TUESDAY COOLER DUE TO FRONTAL PASSAGE AND WRAP AROUND
CLOUDS. WARM/SUNNY WEATHER EXPECTED WED AS ZONAL FLOW CONTINUES.

[vbhoutex]

Jeff is this still a possiblity? Our local forecasts don't seem to indicate it and I am too lazy/busy to investigate myself.

[Johnny]

Here ya go VB.



http://www.spc.noaa.gov/products/outlook/day2otlk.html

[vbhoutex]

Thanks Johnny!!

[jeff]

Everything still looks good for strong to severe thunderstorms late Saturday into Saturday night, although I am not sure which model is handling the event the best....the wet ETA or dry GFS.

Anyhow 1500-2500 J/kg of SBCAPE by mid afternoon from Matagorda Bay to EC TX along and south of a stalled cold front and E of the C TX dry line, along with low LIS and a weakening cap should fire off something. Degree of heating will determine storm severity as well as amount of air mass modification.

[vbhoutex]

Thanks Jeff. Sounds like the right mixture to me for some rough weather. Guess I'll have to get the yard work done in the morning.

[Yankeegirl]

I have also noticed that the local weathermen arent really talking too much about the possible severe weather... Go figure.

[jeff]

Severe Thunderstorms possible this afternoon and evening over SE TX.

Air mass over SE TX has become moist and unstable overnight, although capped at 850mb. Shortwave that passed around midnight is currently producing subsidence over the region, however the next shortwave can be seen in the water vapor over C Mexico. A surface cold front extends from NW LA to NC TX and will sag southward today, while a dry line extends from W of Austin to W of Corpus Christi.

With surface heating and continued moisture advection, the atmosphere over SE TX will become very unstable by mid to late afternoon with forecasted SBCAPE of 2500-3000 J/kg from Houston to Matagorda Bay, LI of -4 to -6 and lapse rates (degree of cooling of the air column per every KM, so in this case the air cools 8C every KM) of 8.0 C/km. Approach of next shortwave trough will spread large scale lifting over the area with thunderstorms initiating along the C TX dry line, the cold front to the north, and possibly an inland moving seabreeze boundary. Given the high CAPE and dry 700mb level large hail and damaging winds will be the main threats. Current models and forecast soundings do not show the low level capping breaking until late afternoon or early evening when the shortwave lifting overspread the region. This should allow a great deal of surface heating for most of the day.

Once again main threats will be large hail and damaging winds. A plume of very steep lapse rates and high CAPE is expected from central Jackson County to SE Montgomery County by late afternoon and could support extremely large hail in this region.

Storms will push east into LA overnight, however air mass will remain moist and unstable and the C TX dry line will surge E on Sunday. With good surface heating once again we could set off a few severe thunderstorms by late afternoon.

MONDAY: Major Severe Weather Outbreak Expected, Including the possibility of large and very destructive tornadoes.

Surface low pressure will develop over W TX and move ENE into NE OK by late Monday with a strong dry line E of I-35 over TX and OK. A strong 140kts jet will be punching across NC and NE TX south of this low along with a good 850mb low level jet of 40-60kts. Storms will develop along the dry line by late afternoon as capping is broken and go quickly severe in a high CAPE high SHEAR environment. Tornadic supercells will be possible over the northern half of SE TX, but more likely over NE TX, LA and AR where a few violent events will be possible. Locally a severe squall line is possible with damaging winds and large hail. Convective developments today and Sunday will play a role in the Monday event.

Stay tuned to local TV, NOAA weather radio, for watch and warning information today through Monday.

[Yankeegirl]

Well its not even noon yet and its almost 80 ! I guess thats not good for the formation of the storms....

[jeff]

Current meso anaylsis as of noon

Weak cold front extends from KSHV to between Waco and Hearne and then WSW to near Georgetown. South of this boundary dewpoints have risen into the low to mid 60's. CAPE values are running in the 1500 J/kg range over most of SE and SC TX with a corridor of values in excess of 2500 J/kg from NW of KCRP to KVCT.

Visible images show a widespread Cu field over SC and SE TX where dewpoints are in the mid 60's. Some enhancment of the Cu is noted over the coastal bend and over SW TX where weak convection has develop on the leading edge of large scale ascent. A look out the window still shows fairly flat tops, so the cap continues to hold, but it is only a matter of time before heating and lifting erode the inhibition.

Storms should fire late this afternoon over SC TX and congeal into an MCS or line segments after dark as they move E to ENE across SE TX. Increasing low level inflow with surface pressure falls over C TX should help feed the convection and maintain the threat well after sunset, as well as 35-45kts of shear to help in storm organization.

[jeff]

SPC has just come out with a MCD for central TX.

Weather watchbox is expected in the next 2-3 hours.

[jkt21787]

Already have a severe tstm warning in Real County. Other cells in the surrounding area also starting to show hail potential.

Imagine a watch is not too far behind.

[Skywatch_NC]

http://www.spc.noaa.gov/products/md/mcd0309.gif

http://www.intellicast.com/WeatherImage ... t+bgwv.gif

[jeff]

Severe Thunderstorm Watch issued


http://www.spc.noaa.gov/products/watch/ww0053.html

[jeff]

Severe Thunderstorms developing along stalled frontal boundary in unstable air mass over C TX. Additionally visible images showing TCu along and east of diff. heating boundary over SC TX from W of KCRP to W of KVCT. 88D is showing a few blimps in this area, and with a little more heating storms should begin to fire over Live Oak and surrounding counties.

Also watching seabreeze boundary penetration around Houston and Matagorda Bay. Cumulus are showing some enhancement over SE Harris and Brazoria counties

Air mass continues to destabilize with approach of W TX/ N Mexico short wave and strong surface heating. CAPE values of 2500 J/kg are fairly common over much of SE and SC TX with surface dewpoints in the low to mid 60's. Thunderstorm cluster of C TX developed within an area of pressure falls which has increased the amount of WAA over SC TX and the coast bend. Storms will continue to develop and move E to ENE through the afternoon hours.

Suspect a new weather box may be required for areas south of the current watchbox over the Victoria and Corpus Christi areas.

[jeff]

Thunderstorms rapidly developing over SC TX and the coastal bend where capping is being broken.

Cells appear to be organizing into a cluster along with a cluster over C TX.

Also watching the sebreeze over metro Houston as we are getting TCu over the urban heat areas and large scale lifting is rapidly approaching from the WSW. In fact one cell has just fired over the chemical plants just west of Baytown as the seabreeze meets the heated urban air.

[KatDaddy]

Here go right with the cap breaking over S Central Tx.

Also note a rapidly developing cell over SE Harris County. Nice stair-step appearance of the cell off to my N.

[jeff]

New weather watchbox possible south of current watch across the coastal plains of TX. See new SPC MCD.

Also EWX has issued a Flash Flood Watch for their eastern counties to acount for slow storm motion and excessive hourly rainfall rates which may lead to flash flooding.

Storm clusters should gradually congeal into an MCS by early evening. Will be watching for development along the seabreeze boundary as storm motions over SE Harris have been very slow to nearly stationary.

[jeff]

New SVR watchbox for all of the TX coastal bend and SE TX until midnight.

Numerous severe thunderstorms are developing across C, SE, and SC TX ahead of approaching shortwave. Storm of downtown Houston is quite nasty producing hail and very heavy rainfall. Cluster over SC TX and C TX should evolve into an MCS and track E this evening.

I am becoming a little concerned with the heavy rainfall threat given the very slow storm motions.