Severe Thunderstorm Warning 49, NWS San Angelo, TX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN ANGELO TX
159 PM CDT TUE APR 13 1999

COMPLEX FORECAST SCENARIO LATE TODAY AND TONIGHT DUE TO MODEL
DIFFERENCES ON TIMING/POTENTIAL SEVERE PARAMETERS AND THE PERSISTENT
LOW CLOUD DECK ACROSS MUCH OF OUR COUNTY WARNING AREA. ETA MODEL
STILL APPEARS TOO FAST WITH SURFACE AND UPPER LEVEL FEATURES AND
PREFER THE SLOWER NGM/AVN SOLUTION. UPPER LEVEL LOW WILL MOVE
EASTWARD INTO NM THIS AFTERNOON AND INTO TX PANHANDLE LATE TONIGHT.
AN UPPER LEVEL RIDGE WILL AMPLIFY OVER EASTERN PACIFIC...AND THE
SOUTHERN STREAM SYSTEM IS PROGGED OPEN UP AS A CLOSED LOW/TROUGH OVER
NORTHERN PLAINS DIGS SOUTHEASTWARD. AT THE SURFACE...EXPECT DRYLINE
AND PACIFIC FRONT TO ADVANCE EASTWARD INTO WEST TX LATER THIS
AFTERNOON AND MOVING ACROSS OUR COUNTY WARNING AREA OVERNIGHT. HIGH
PRESSURE PROGGED TO BUILD DOWN INTO WEST TX LATER WEDNESDAY INTO
THURS.

FOR TONIGHT...AM EXPECTING WIDELY SCATTERED THUNDERSTORMS ALONG
DRYLINE OR PACIFIC FRONT TO ENTER OUR WESTERN CWA AND FILL IN AS A
SQUALL LINE AS IT CONTINUES EAST. COULD HAVE ADDITIONAL DEVELOPMENT
ALONG THE WARM FRONTAL BOUNDARY OVER OUR NORTHEASTERN CWA. WILL
ADJUST POPS TO RANGE FROM CHANCE OVER ABOUT WESTERN HALF OF CWA TO
LIKELY OVER EASTERN CWA. WILL CONTINUE WITH SEVERE WORDING. LATEST
DAY 1 SEVERE THUNDERSTORM OUTLOOK KEEPS OUR NORTHERN CWA IN MODERATE
RISK WITH SLIGHT RISK FURTHER SOUTH...WITH THE SETUP FAVORABLE FOR
SEVERE STORMS. ONLY POSSIBLE INHIBITING FACTORS ARE THE PERSISTENT
LOW CLOUDS WHICH ARE SIGNIFICANTLY RESTRICTING SURFACE HEATING...
AND THE MODELS (ESPECIALLY NGM) MEAN RELATIVE HUMIDITY FIELD BRINGS
IN A DRY SLOT ACROSS OUR AREA EARLY TONIGHT...SUGGESTING THAT
MOISTURE DEPTH WOULD BE SHALLOW. SATELLITE IS SHOWING SOME BREAKS
OVER OUR NORTHERN COUNTIES AND OVER FAR WESTERN CWA. APPEARS THAT
NORTHWEST AND WEST PARTS OF CWA WILL HAVE THE MOST HEATING THIS
AFTERNOON...AND SOME PARTIAL CLEARING MAY REACH OUR FAR
SOUTH/SOUTHEAST COUNTIES. WILL USE MOSTLY A 55 TO 60 RANGE FOR LOWS.
THE LATEST EXCESSIVE RAINFALL OUTLOOK HIGHLIGHTS OUR CENTRAL AND
EASTERN COUNTIES FOR HEAVY RAIN POTENTIAL TONIGHT.

FOR WEDNESDAY MODELS SHOW A SECONDARY COLD FRONT SWEEPING SOUTH
ACROSS CWA WITH STRONG PRESSURE GRADIENT BEHIND IT. APPEARS WIND
ADVISORY CRITERIA WILL BE MET FOR AT LEAST EASTERN BIG COUNTRY AND
HEARTLAND...WITH WINDS LESS STRONG OVER NW HILL COUNTRY. DRY SLOT IN
MORNING WILL BE FOLLOWED BY SOME WRAP AROUND MOISTURE AFTERNOON. FOR
NOW WILL JUST BALANCE WORDING WITH PARTLY CLOUDY. DECENT COLD
ADVECTION WILL BE PARTIALLY OFFSET BY SUNSHINE AND WILL COMPOMISE
HIGHS BETWEEN FAN AND FWC NUMBERS. WEDNESDAY NIGHT WINDS WILL
GRADUALLY DECREASE WITH CLEAR SKIES AND COLDER TEMPS. PREFER A BLEND
BETWEEN COLDER FWC AND WARMER FAN NUMBERS FOR LOWS. FOR THURS HIGH
PRESSURE RIDGE WILL TRANSLATE ACROSS TX BRINGING SUNNY SKY AND COOL
TEMPS...WITH HIGHS GENERALLY IN THE 65 TO 70 RANGE.

ABI 058/066/040/065 5000
SJT 058/070/040/068 4000
JCT 060/072/040/069 4-00

19

TXZ049-053-054-064>066-071>073-076>078-098-099-113-114-127-128-
139-140-154-155-168>170-141200-

WEST CENTRAL TEXAS THUNDERSTORM OUTLOOK
NATIONAL WEATHER SERVICE SAN ANGELO TX
235 PM CDT TUE APR 13 1999

...TORNADO WATCH IS IN EFFECT FOR PORTIONS OF THE BIG COUNTRY UNTIL 800
PM CDT...

...MODERATE RISK OF SEVERE THUNDERSTORMS ALONG AND NORTH OF
INTERSTATE 20 THROUGH THE OVERNIGHT HOURS...

...SLIGHT RISK OF SEVERE THUNDERSTORMS SOUTH OF INTERSTATE 20
THROUGH THE OVERNIGHT HOURS...

AN UPPER LEVEL LOW PRESSURE SYSTEM CURRENTLY LOCATED OVER THE NM-AZ
BORDER IS EXPECTED TO TRACK TO THE EAST TOWARDS THE TX PANHANDLE
OVERNIGHT. AT THE SURFACE...A COLD FRONT WILL PUSH EAST TOWARDS THE
AREA AND ENCOUNTER A MOIST AND UNSTABLE AIRMASS OVER WEST CENTRAL TEXAS
WHERE CAPE VALUES IN THE 1500-2000 J/KG EXIST. ALONG THE FRONT...A
SURFACE LOW WILL TRACK ACROSS THE PANHANDLE IN ADVANCE OF THE UPPER
LOW. THIS FEATURE WILL ACT TO ENHANCE THE DIRECTIONAL SHEAR PROFILE
OVER THE NORTHERN PORTIONS OF THE CWA.

IT APPEARS THAT THUNDERSTORM DEVELOPMENT WILL TAKE THE FORM OF AN
EASTWARD MOVING SQUALL LINE CAPABLE OF PRODUCING LARGE HAIL AND DAMAGING
WINDS. IN ADDITION...HELICITY VALUES OF 200-400 M2/S2 AND MID LEVEL
SHEAR PROFILES INDICATE THAT THE FORMATION OF TORNADOES ARE ALSO
POSSIBLE...PRIMARILY OVER THE BIG COUNTRY AREA.

THE TIME FRAME FOR THE POTENTIALLY SEVERE THUNDERSTORMS WILL BEGIN OVER
THE WESTERN SECTIONS OF THE CWA BETWEEN 300 PM AND 500 PM. THE SQUALL
LINE WILL PUSH SLOWLY EASTWARD WITH THE THREAT FOR SEVERE THUNDERSTORMS
ENDING OVER EASTERN SECTIONS OF THE CWA SHORTLY AROUND MIDNIGHT.

SPOTTER ACTIVATION WILL LIKELY BE REQUIRED THIS AFTERNOON AND EVENING
ACROSS THE BIG COUNTRY AND MAY BE REQUIRED ELSEWHERE AS WELL.

FOR THE NORTH TEXAS OUTLOOK SEE PRODUCT FTWSPSFTW. FOR THE SOUTH CENTRAL
TEXAS OUTLOOK SEE PRODUCT SATSPSSAT. FOR THE CENTRAL OKLAHOMA OUTLOOK
SEE PRODUCT OKCSPSOKC.

21