.PREV DISCUSSION... /ISSUED 528 AM CDT SAT SEP 12 2009/
PRIMARY FORECAST CONCERN IS FLASH FLOODING POTENTIAL THIS
AFTERNOON THROUGH THIS EVENING. 00Z MODELS PICK UP ON SOME
INTERESTING FEATURES THIS AFTERNOON THAT COULD COME TOGETHER TO
PROVIDE INGREDIENTS FAVORABLE FOR FLASH FLOODING. IT IS DIFFICULT
TO DISCERN HOW MUCH OF THIS IS DUE TO CONVECTIVE FEEDBACK DUE TO
ALL 00Z GUIDANCE SHOWING WIDESPREAD CONVECTION OVER NORTH TX.
NAM/GFS/ECMWF ALL AGREE ON LOW PRESSURE SYSTEM BECOMING STRONGER
ALONG THE GULF COAST THIS AFTERNOON. THIS SEEMS LIKELY TO OCCUR AS
08Z WATER VAPOR IMAGERY AND 08Z RUC TROPOPAUSE ANALYSIS INDICATE A
STRONG SHORTWAVE TROUGH DIVING SOUTHEAST ALONG THE TX/MEXICO
BORDER.
AS THIS LOW PRESSURE SYSTEM DEEPENS ALL MODELS PICK UP ON TWO
INTERESTING FEATURES...THE FIRST IS AN AREA OF WARM AIR ADVECTION
IN THE H800 TO H600 LAYER OVER EASTERN PORTIONS OF THE CWA. THIS
CAUSES THE MID LVL THERMAL GRADIENT TO TIGHTEN UP OVER THE
FORECAST AREA...AND BY THIS AFTERNOON MODELS ARE INDICATING A
FAIRLY STRONG ZONE OF MID LVL FRONTOGENESIS SETTING UP FROM NEAR
PARIS SOUTHWEST TOWARDS COMANCHE. FORECAST SOUNDINGS INDICATE A
NEARLY SATURATED TROPOSPHERE OVER NORTH TX...SO UNDER THE
INFLUENCE OF FRONTOGENETIC FORCING...WOULD EXPECT SOME ENHANCED
PRECIPITATION TO DEVELOP ALONG AND SOUTHEAST OF THIS FORCING AXIS
WITH UPWARD VERTICAL MOTION TO THE WARM SIDE OF THE AXIS. THE
SECOND FEATURE OF INTEREST IS THE MODELS INDICATING A FAIRLY
STRONG EASTERLY LOW LVL JET AROUND THE H800 LVL THIS AFTERNOON.
09Z WIND PROFILERS AND RADAR DERIVED VAD WIND PROFILES INDICATE
EASTERLY FLOW OF 15 TO 20 KTS OVER THE FORECAST AREA. MODELS
INDICATE THIS WILL INCREASE TO 35 TO 40 KTS BY LATE THIS
AFTERNOON. 12/00Z SHV SOUNDING WAS NEARLY SATURATED THROUGH THE
TROPOSPHERE...AND IF STRONG EASTERLY LLJ PANS OUT...LOW LVL
TRAJECTORIES WOULD KEEP AN AMPLE SUPPLY OF RICH MOISTURE FLOWING
INTO NORTH TX THIS AFTERNOON AND THIS EVENING. WITH A COUPLE OF
ENHANCEMENTS LOOKING LIKELY FOR MESOSCALE FORCING THIS AFTERNOON A
LOOK AT TYPICAL PARAMETERS USED TO DIAGNOSE POTENTIAL FOR HEAVY
RAINFALL BECOME IMPORTANT. FIRST...PRECIPITABLE WATER VALUES LOOK
TO REMAIN ABOVE 2 INCHES FOR THE NEXT 24 HRS OVER NORTH TX.
SECOND...MID LEVEL RH VALUES ARE FORECAST TO REMAIN AT OR ABOVE 90
PERCENT OVER THE SAME TIME PERIOD. LASTLY IS THE THERMODYNAMIC
PROFILE OVER THE CWA...FORECAST SOUNDINGS ARE NEARLY MOIST
ADIABATIC THROUGH THE ENTIRE TROPOSPHERE. THIS MEANS TALL AND
SKINNY TYPE CAPE WHICH HAS BEEN ASSOCIATED WITH SOUNDINGS
FAVORABLE FOR GOOD PRECIPITATION EFFICIENCY. THIS ALSO MEANS THE
THETA E DIFFERENCE WITH HEIGHT IS MINIMAL...MEANING THAT PERIODS
OF INTENSE CONVECTION WILL NOT QUICKLY STABILIZE THE ENVIRONMENT
ALLOWING FOR PROLONGED PERIODS OF CONVECTION.
ALL IN ALL THERE SEEMS TO BE A HIGH RISK FOR HEAVY RAINFALL WHICH
COULD LEAD TO FLASH FLOODING WHEREVER THE MOST INTENSE CONVECTION
SETS UP THIS AFTERNOON. BASED ON THE GENERAL AGREEMENT OF THE
PLACEMENT OF THE MID LVL BAROCLINIC ZONE IN THE NAM/GFS/ECMWF THIS
ZONE APPEARS TO MAINLY AFFECT AREAS IN THE FORT WORTH CWA ALONG
AND NORTH OF AN EMORY TO FORT WORTH TO COMANCHE LINE. A FLASH
FLOOD WATCH REMAINS IN EFFECT FOR THE ENTIRE CWA AS RAINFALL OVER
THE PAST 24 HRS HAS LED TO MOIST SOIL CONDITIONS ACROSS MUCH OF
NORTH TX. HEAVY RAINFALL OVER ANY PORTION OF THE AREA COULD LEAD
TO FLASH FLOODING. IN GENERAL...MOST LOCATIONS ARE EXPECTED TO
RECEIVE ANYWHERE FROM 1 TO 3 INCHES OF RAINFALL OVER THE NEXT 24
HRS...WITH ISOLATED AREAS RECEIVING TOTALS IN EXCESS OF 5 INCHES.
THE AREAS MOST LIKELY TO SEE ISOLATED HIGHER AMOUNTS WOULD BE
NORTHERN AND WESTERN PORTIONS OF THE FORECAST AREA.
SUNDAY AND SUNDAY NIGHT...AXIS OF HEAVIEST RAINFALL MAY SHIFT
NORTH AS MODELS INDICATE A DRY SLOT INTRUSION AT H500 SUNDAY
MORNING WHICH SHOULD TEMPORARILY INTERRUPT DEEP CONVECTION. WITH
EXPECTED RAINFALL OVER MUCH OF NORTH TX IN THE 1 TO 3 INCH RANGE
TODAY AND TONIGHT...ANY ADDITIONAL TSTM ACTIVITY COULD CAUSE FLASH
FLOODING...SO LEFT FLASH FLOOD WATCH CONTINUE THROUGH SUNDAY
AFTERNOON AT THIS TIME. IF DRY SLOT SHUTS THINGS DOWN...LATER
SHIFTS CAN REEVALUATE THE NEED FOR THE WATCH CONTINUING THRU 7 PM
SUNDAY.
REMAINDER OF THE FORECAST...THE GFS AND ECMWF DIVERGE ON HOW TO
HANDLE THIS UPPER LVL DISTURBANCE IN THE EXTENDED FORECAST. THE
GFS HAS THE UPPER LOW VACATING THE AREA BY TUESDAY NIGHT...WHILE
THE ECMWF HOLDS THE UPPER LOW OVER NORTHEASTERN TX THRU THURSDAY.
AT THIS TIME ONLY REAL CHANGES TO MEDIUM RANGE FORECAST WAS TO
BUMP UP POPS A BIT OVER THE NORTHEASTERN COUNTIES OF THE FORECAST
AREA. NO REAL CONFIDENCE WHETHER THE GFS OR ECMWF HAS THE RIGHT
SOLUTION...SO LEFT LOW CHC POPS IN THE FORECAST THRU THURSDAY
UNTIL A LARGE SCALE SOLUTION COMES INTO BETTER AGREEMENT.
I have already recieved over 3 inches or rain since yesterday morning here and looks as another 3 to 5 inches is on the way. This not the norm around here for this time of the year, it's more of a mid October pattern. Hope this can only be a sign of a awesome El NINO winter with lots of Snow.