How Severe will the storms be this weekend? 3/19-20/05

[ticka1]

An email from Jeff and the local radio reports are saying we could have hail and severe storms either on Saturday or Sunday....

Parameters appear to be coming together for strong to severe
thunderstorms Saturday afternoon into the evening hours.

Short wave within northern brach of the polar jet over the upper
Midwest will move ESE into the mid-MS valley this evening pushing a
weak cold front into TX. A potent short wave off the S CA coast is
moving east and is already creating a low pressure trough over SW TX.
This has caused winds to swing around to the S this morning and start
the moisture increase.

Cold front should stall just north of the region tonight allowing
moisture to pool over the area. Dewpoints on the ETA and GFS reach the
low 60's by midday Saturday. Surface heating and continued moisture
advection push SBCAPE values to 2500-3000 J/kg on the ETA forecast
soundings by mid-afternoon over the western 1/2 of SE TX. Approaching
lead short wave from S CA low will supply large scale ascent during
max heating. LI's drop to -4 to -6 and lapse rates become increasingly
steep. 30-40kts of SW shear support storm organization into clusters
and possibly an MCS.

Storms should fire along the C TX dry line by mid to late afternoon
after surface heating removes the convective inhibition (Cap). Not
sure if we will have storms going over W LA as the ETA seems to think
during the morning, although if we do they could build westward into
the unstable air mass over our region.

It should be noted that the ETA is wetter with this event than the GFS
and it may be suffering from convective feedback problems. Anyhow, SPC
has the entire area outlooked for severe potential Saturday evening,
so we will probably see some form of adverse conditions.

Thunderstorms chances continue into Sunday and Monday as various
disturbance move across the area with a warm and humid air mass
remaining in place.

Jeff Lindner

[jeffl]

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.

[ticka1]

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.

[ticka1]

SPC has issued a Severe Thunderstorm Watch for portions of C and SW TX until 900pm.

Discussion:

Air mass over SE and SC TX is becoming very unstable as mid level temps. cool and surface temps warm into the low 80's. SBCAPE values are now exceeding 3000 J/kg over SC and Sw SE TX along and south of a stationary cold front.

Thunderstorms have developed W of Austin within the last hour and are moving E along and south of the stalled frontal boundary. Visible imagery also shows Tcu across the coastal bend and it appears the cap may break shortly from Victoria to N of Corpus Christi.

Strom will increase in intensity and coverage through the afternoon and spread E this evening

Severe Thunderstorm Watch Outline with radar overlay: