Tornado Warning 28, NWS Norman, OK

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE NORMAN OK
330 PM CDT WED APR 21 1999

SPECIAL 18Z OUN SOUNDING ILLUSTRATES MAIN LIMITING FACTOR FOR
CONVECTION.  STRONG CAP REMAINS IN PLACE EVEN THOUGH ONE SHORT WAVE
IS ROTATING INTO THE CENTRAL PLAINS THIS AFTERNOON. WEAK COLD FRONT
TRAILS FROM LOW IN NORTHERN KANSAS SOUTHWEST INTO EXTREME NORTHWEST
OKLAHOMA AND NORTHERN TEXAS PANHANDLE....WHILE DRYLINE LIES FROM
INTERSECTION WITH FRONT NEAR KANSAS LINE THROUGH WESTERN OKLAHOMA
AND WESTERN NORTH TEXAS.

BEST CHANCES FOR STORMS FIRST PERIOD WILL BE IN NORTH WHERE CAPPING
IS WEAKER AND LOW-LEVEL BOUNDARIES CAN PROVIDE CONVERGENCE.  THOUGH
IN WINDS IN MOIST SECTOR ARE VEERED THIS AFTERNOON...EXPECT WINDS TO
BACK DURING THE EVENING AS MIXING DECREASES WITH LOSS OF HEATING AND
NEXT SHORT WAVE DIGS INTO WESTERN U.S. TROUGH. THIS SCENARIO SEEMS
WELL DEPICTED BY OU/CAPS ARPS MODEL.  ARPS AND OTHER MODELS DO
FORECAST SOME COOLING (LIKELY OVERDONE) OF CAP TONIGHT WHICH IS IN
PART SUPPORTED BY COOLER MID-LEVEL TEMPERATURES IN UPSTREAM RAOB
DATA.  THIS COOLING COMBINED WITH ANOTHER SHORT WAVE MOVING INTO THE
REGION TOWARD MORNING SUGGESTS OVERNIGHT CONVECTIVE POTENTIAL WILL
BE NON-ZERO...BUT NOT HIGH.  SOME POTENTIAL ALSO EXISTS FOR ELEVATED
CONVECTION GIVEN VERY STEEP MID-LEVEL LAPSE RATES. HOWEVER...STILL
BELIEVE BEST EVIDENCE ARGUES AGAINST MANY STORMS FIRST ONE TO TWO
PERIODS SO WILL LOWER POPS TO THE 10-30 PERCENT RANGE.

STEEP LAPSE RATES AND SLOWLY INCREASING DEW POINTS WILL YIELD STRONG
INSTABILITY THE NEXT COUPLE OF DAYS. WIND SHEAR WILL ALSO BE
SUFFICIENT FOR SEVERE STORMS WITH THE CAP REMAINING THE MAIN
LIMITING FACTOR.  EXPECT FRONT TO THE NORTH TO GRADUALLY EASE
SOUTHWARD WITH EACH WAVE PASSAGE WHILE DRYLINE ALSO REMAINS IN THE
VICINITY. THIS SCENARIO ARGUES FOR GRADUALLY INCREASING
POPS...ESPECIALLY FRIDAY INTO WEEKEND AS FRONT MOVES THROUGH AREA
THEN HANGS UP SOUTH OF RED RIVER.  ..DA..

PRELIM NUMBERS FOLLOW:

FCSTID = 16
OKC   63  80  61  72 /  20  20  30  40
HBR   58  83  55  73 /  20  30  30  40
SPS   62  87  62  78 /  20  20  20  40
GAG   55  75  47  63 /  30  20  30  40
PNC   60  77  55  69 /  30  30  40  50
2DUA   64  80  64  79 /  10  10  20  30

.OUN...
OK...NONE.
TX...NONE.

OKZ004>048-050>052-TXZ083>090-220100-

THUNDERSTORM OUTLOOK...UPDATED...
NATIONAL WEATHER SERVICE NORMAN OK
350 PM CDT WED APR 21 1999

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING AND
OVERNIGHT TONIGHT ACROSS CENTRAL AND WESTERN OKLAHOMA AND WESTERN
NORTH TEXAS.  THE SLIGHT RISK AREA IS NORTH AND WEST OF A LINE FROM
CHANDLER...TO OKLAHOMA CITY...TO WICHITA FALLS...TO ARCHER
CITY.

DISCUSSION...A DRYLINE REMAINED NEARLY STATIONARY AT MID AFTERNOON
FROM ALVA...TO CLINTON...TO ALTUS...TO SEYMOUR.  TO THE EAST OF THE
DRYLINE...THE ATMOSPHERE WAS VERY UNSTABLE OVER MUCH OF OKLAHOMA AND
WESTERN NORTH TEXAS WITH CAPE VALUES IN EXCESS OF 3000 J/KG.
HOWEVER...THE ATMOSPHERE ALSO REMAINS STRONGLY CAPPED ACROSS THE
AREA AND LOW LEVEL CONVERGENCE ALONG THE DRYLINE HAS NOT INCREASED
MUCH DURING THE AFTERNOON.  WITH THE LACK OF STRONGER LOW LEVEL
CONVERGENCE NEAR THE DRYLINE...IT APPEARS THAT THE CAP WILL BE VERY
DIFFICULT...IF NOT IMPOSSIBLE...TO BREAK ACROSS THE AREA THIS
EVENING AND TONIGHT.

AS A RESULT...THE RISK OF SEVERE THUNDERSTORMS HAS DECREASED ACROSS
OKLAHOMA AND WESTERN NORTH TEXAS.  DESPITE THE NEGATIVE FACTORS
REGARDING THE POTENTIAL FOR THUNDERSTORM DEVELOPMENT...AREAS OF
LOCALIZED STRONGER CONVERGENCE MAY STILL LEAD TO THUNDERSTORM DEVELOPMENT
ALONG THE DRYLINE AROUND THE TIME OF MAXIMUM HEATING THIS
AFTERNOON...BETWEEN ABOUT 5 AND 7 PM.  THE AREAS MOST AT RISK FOR
THIS ARE IN SOUTHWEST OKLAHOMA AND WESTERN NORTH TEXAS...AND IN
NORTH CENTRAL OKLAHOMA WHERE THE CAP WILL BE SLIGHTLY WEAKER.

IF THUNDERSTORMS CAN DEVELOP...THEY WILL LIKELY BECOME SEVERE
RAPIDLY WITH THE STORNG INSTABILITY PRESENT AND ADEQUATE VERTICAL
WIND SHEAR PROFILES.  THE MAIN THREAT WITH ANY STORMS WHICH DO
DEVELOP WILL BE VERY LARGE HAIL AND DAMAGING WIND.  HOWEVER...AN
ISOLATED TORNADO CANNOT BE RULED OUT.  AREAL COVERAGE OF ANY STORMS
WILL ALSO LIKELY BE QUITE SMALL.

ADDITIONAL THUNDERSTORMS MAY ALSO DEVELOP AFTER MIDNIGHT TONIGHT IN
WESTERN OKLAHOMA AS ANOTHER UPPER LEVEL DISTURBANCE MOVES TOWARD TO
AREA.

SPOTTER ACTIVATION MAY BE REQUIRED ACROSS NORTHERN OKLAHOMA THIS
AFTERNOON AND TONIGHT...AND LATE THIS AFTERNOON AND TONIGHT ACROSS
CENTRAL AND SOUTHERN OKLAHOMA AND WESTERN NORTH TEXAS.

 MILLER

OKZ004>048-050>052-TXZ083>090-220100-

THUNDERSTORM OUTLOOK...UPDATED...
NATIONAL WEATHER SERVICE NORMAN OK
350 PM CDT WED APR 21 1999

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING AND
OVERNIGHT TONIGHT ACROSS CENTRAL AND WESTERN OKLAHOMA AND WESTERN
NORTH TEXAS.  THE SLIGHT RISK AREA IS NORTH AND WEST OF A LINE FROM
CHANDLER...TO OKLAHOMA CITY...TO WICHITA FALLS...TO ARCHER
CITY.

DISCUSSION...A DRYLINE REMAINED NEARLY STATIONARY AT MID AFTERNOON
FROM ALVA...TO CLINTON...TO ALTUS...TO SEYMOUR.  TO THE EAST OF THE
DRYLINE...THE ATMOSPHERE WAS VERY UNSTABLE OVER MUCH OF OKLAHOMA AND
WESTERN NORTH TEXAS WITH CAPE VALUES IN EXCESS OF 3000 J/KG.
HOWEVER...THE ATMOSPHERE ALSO REMAINS STRONGLY CAPPED ACROSS THE
AREA AND LOW LEVEL CONVERGENCE ALONG THE DRYLINE HAS NOT INCREASED
MUCH DURING THE AFTERNOON.  WITH THE LACK OF STRONGER LOW LEVEL
CONVERGENCE NEAR THE DRYLINE...IT APPEARS THAT THE CAP WILL BE VERY
DIFFICULT...IF NOT IMPOSSIBLE...TO BREAK ACROSS THE AREA THIS
EVENING AND TONIGHT.

AS A RESULT...THE RISK OF SEVERE THUNDERSTORMS HAS DECREASED ACROSS
OKLAHOMA AND WESTERN NORTH TEXAS.  DESPITE THE NEGATIVE FACTORS
REGARDING THE POTENTIAL FOR THUNDERSTORM DEVELOPMENT...AREAS OF
LOCALIZED STRONGER CONVERGENCE MAY STILL LEAD TO THUNDERSTORM DEVELOPMENT
ALONG THE DRYLINE AROUND THE TIME OF MAXIMUM HEATING THIS
AFTERNOON...BETWEEN ABOUT 5 AND 7 PM.  THE AREAS MOST AT RISK FOR
THIS ARE IN SOUTHWEST OKLAHOMA AND WESTERN NORTH TEXAS...AND IN
NORTH CENTRAL OKLAHOMA WHERE THE CAP WILL BE SLIGHTLY WEAKER.

IF THUNDERSTORMS CAN DEVELOP...THEY WILL LIKELY BECOME SEVERE
RAPIDLY WITH THE STORNG INSTABILITY PRESENT AND ADEQUATE VERTICAL
WIND SHEAR PROFILES.  THE MAIN THREAT WITH ANY STORMS WHICH DO
DEVELOP WILL BE VERY LARGE HAIL AND DAMAGING WIND.  HOWEVER...AN
ISOLATED TORNADO CANNOT BE RULED OUT.  AREAL COVERAGE OF ANY STORMS
WILL ALSO LIKELY BE QUITE SMALL.

ADDITIONAL THUNDERSTORMS MAY ALSO DEVELOP AFTER MIDNIGHT TONIGHT IN
WESTERN OKLAHOMA AS ANOTHER UPPER LEVEL DISTURBANCE MOVES TOWARD TO
AREA.

SPOTTER ACTIVATION MAY BE REQUIRED ACROSS NORTHERN OKLAHOMA THIS
AFTERNOON AND TONIGHT...AND LATE THIS AFTERNOON AND TONIGHT ACROSS
CENTRAL AND SOUTHERN OKLAHOMA AND WESTERN NORTH TEXAS.

 MILLER