Severe Thunderstorm Warning 99, NWS Quad Cities, IA

AREA FORECAST DISCUSSION...UPDATED TO INCLUDE WATCH
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
228 PM CDT FRI JUL 23 1999

MAIN CONCERNS ARE WITH SURFACE TROF/EXTENT OF DRYING AND EFFECTS
ON HEAT ADVISORY...AND POTENTIAL FOR AFTN TSRA/S NEXT FEW DAYS.

STRONG HEATING COUPLED WITH SURFACE DEWPTS IN THE M/U 70S HAVE
RESULTED IN CAPES OF 4000-6000 J/KG OVER MUCH OF E IOWA/NW ILLINOIS
PER 18Z MSAS. WDLY SCTD TSRA/S DVLPG AHEAD OF OUTFLOW BOUNDARY AND
WEAK SFC TROF. WATER VAPOR ALSO SHOWS VORT MAX OVER SE IOWA AIDING
LIFT. THUNDERSTORMS EXPECTED TO CONTINUE DEVELOPING THIS AFTN/EVE.
12Z KDVN SOUNDING SHOWS FAIRLY UNIDIRECTIONAL SHEAR PROFILE WITH
LITTLE SPEED SHEAR. THREAT FROM PULSE TYPE STORMS WITH TORRENTIAL
RAIN (PWAT/S IN OF AROUND 1.5 INCHES) AND STRONG WINDS. HAIL WILL
BE A THREAT THOUGH NOT MAIN THREAT WITH FREEZING LEVELS RUNNING
AROUND 15,000 FT. GOOD NEWS IS THAT STEERING WINDS A BIT STRONGER
THAN WHAT WE HAVE SEEN OF LATE SO THEY SHOULD MOVE ALONG FAST ENOUGH
LIMITING WIDESPREAD FLOODING PROBLEMS. OTHER ACTIVITY OVER SW IOWA
AND NW MISSOURI AHEAD OF SECONDARY SHORTWAVE SHOULD SCOOT SE AND
STAY MAINLY S OF CWA. ANTICIPATE BULK OF TSRA/S TO DISSIPATE WITH
LOSS OF HEATING THUS WILL LIKELY WORD EVENING POPS.

12Z MODELS ALL AGREE ON AMPLIFYING UPPER FLOW OVER NEXT 48 HRS
AS UPPER RIDGE RETROGRADES INTO PLAINS. MODELS ALL SHOWING MORE
AMPLIFICATION TO UPPER RIDGE FROM PLAINS INTO MIDWEST WHICH APPEARS
TO BE IN RESPONSE TO DEVELOPING H5 CLOSED LOW OVER QUEBEC CANADA
BY 12Z SUNDAY. DIFFERENCES ARISE AT SURFACE WITH MODELS OFFERING
VARYING SOLUTIONS ON TIMING/PLACEMENT OF WINDSHIFT LINE/TROF
AND EXTENT OF DRYING. NGM/AVN VERY AGGRESSIVE IN BRINGING TROF
THROUGH MUCH OF CWA BY 12Z SATURDAY WHILE ETA SLOWER BRINGING IT ONLY
INTO CNTRL ZNS. MODELS THEN EFFECTIVELY WASH WEAK BOUNDARY OUT WHILE
STRENGTHENING MID LEVEL CAP. SITUATION SIMILIAR TO RECENT DAYS IN
WHICH MODELS TRIED TO WASH OUT PREVIOUS BOUNDARY YET IT HUNG AROUND
AND WAS FOCUS FOR ISOLD/WDLY SCTD LATE AFTN/EARLY EVE TSRA/S.
BELIEVE ATLEAST THIS BOUNDARY AND PERHAPS OTHERS (CONVECTIVELY
INDUCED OUTLFOWS) WILL BE LINGERING AROUND ON SATURDAY AND COULD
SERVE AS FOCUSING MECHANISMS FOR ISOLD/WDLY SCTD AFTN/EVE TSRA/S.
NEG FACTORS FOR PCPN INCLUDE STRENGTHENING MID LEVEL CAP...AND LACK
OF UPPER SUPPORT. ETA FORECAST SOUNDINGS DO SHOW CAPES OF 3000-4000
J/KG AND LI/S -8 TO -11 OVER CWA SATURDAY AFTN...AND WITH ANY
BOUNDARIES PROVIDING ENOUGH LLVL CONVERGENCE NOT COMPLETELY OUT OF
QUESTION TO SEE A FEW TSRA/S. WILL INSERT LOW POPS FOR LATE AFTN/EVE
ACROSS ENTIRE CWA GIVEN UNCERTAINTY ON LOCATION OF BOUNDARIES.
EARLIER IN THE WEEK WE SAW SOME CONVECTION WITH STALLED BOUNDARY
WEDNESDAY THEN NOTHING YESTERDAY. WILL FOLLOW SIMILIAR TREND WITH
THIS BOUNDARY(S) PLACING POPS IN SATURDAY AND LEAVING FCST DRY
SUNDAY WITH WEAK SFC HIGH PROGGED OVER AREA.

APPEARS SOME BAD SOUNDING DATA FROM KABR MADE IT/S WAY INTO THE
MODELS. ETA DID BETTER JOB OF DAMPENING OUT DATA THAN NGM.
RESULT IS THAT NGM IS EXTREMELY AGGRESSIVE ON DRYING THINGS OUT
SATURDAY/SUNDAY WITH SFC DEWPTS DROPPING INTO THE 40S TO LOWER 50S
ACROSS MUCH OF CWA. NGM VERIFYING HORRIBLY AS RESULT AT 18Z
WITH SFC DEWPTS PROGGED INTO U40S WHERE CURRENTLY IN THE 70S.
WILL FOLLOW ETA/AVN WHICH KEEPS CWA IN THE SOUP (DEWPTS GENERALLY
IN THE 70S). HEAT ADVISORY CRITERIA LIKELY TO BE MET CNTRL AND SRN
ZNS THROUGH WEEKEND WHILE CONDITIONS WILL BE MARGINAL ACROSS NRN ZNS
WHERE TEMPS IN M90S AND DEWPTS NEAR 70 DEGS YIELD HEAT INDICES OF
100 TO 105 SO WILL OPT TO LEAVE THEM IN.

TEMPS...FWC QUITE A BIT WARMER ON MAXES OVER WEEKEND (100/S) AS NGM
IS MORE AGGRESSIVE WITH STRENGTH OF UPPER HIGH. FAN MAXES COOLER
THOUGH NOT SURE WHY OTHER THAN BAD DATA AS H85 TEMPS PROGGED SIMILIAR
TO THOSE OF ETA/NGM. FEEL PERSISTANCE IS WAY TO GO FOR TEMPS WHICH IS
CLOSE TO SPLITTING DIFFERENCE BETWEEN FAN/FWC. LIKE WARMER FAN MINS.

...3 TO 5 DAY...
UPPER HIGH FROM ROCKIES INTO SOUTHEAST US FLATTENS AS VERTICALLY
STACKED SYSTEM OVER NORTHERN CANADA AND RESULTANT CYCLONIC FLOW/
TRAILING SHORTWAVE ENERGY/CARVES FAST ZONAL FLOW OVER NORTHERN TIER
OF US. NOGAPS/AVNX/ECMWF AGREE ON SWEEPING FRONTAL BOUNDARY THROUGH
AREA MONDAY/TUESDAY THOUGH DIFFER ON TIMING AND AMOUNT OF
COOLING/DRYING IN IT/S WAKE. MRF TAKES NORTHERN PORTION OF BOUNDARY
AND SWINGS IT THROUGH UPPER MIDWEST WHILE HOLDING UP SOUTHERN PORTION
OVER PLAINS. BELIEVE CYCLONIC FLOW/TRAILING ENERGY SHOULD BE ENOUGH
TO BRING FRONT THROUGH. AVNX A BIT MORE AGGRESSIVE WITH COOLER TEMPS
TUESDAY/WEDNESDAY. GIVEN EXPECTED AIRMASS MODIFICATION AND STRONG
LATE JULY SUNSHINE FAVOR GOING NORMAL/SLIGHTLY ABOVE ON TEMPS. MAIN
EMPHASIS WILL BE WITH LESS HUMIDITY. NOGAPS/AVNX/ECMWF SUPPORT WEAK
SFC HIGH OVER AREA ON WEDNESDAY SO PLAN ON LEAVING OUT POPS ATTIM.
SOME CONCERNS THOUGH WITH NOGAPS BRINGING IN ANOTHER BOUNDARY
LATE WEDNESDAY NIGHT/THURSDAY AND POTENTIAL FOR PLAINS MCS ACTIVITY
DROPPING INTO PORTIONS OF IOWA. STILL PLENTY OF TIME ON THIS AND FEEL
BEST THING IS TO START WEDNESDAY DRY AND WATCH LATER MODEL RUNS FOR
CONSISTENCY.

COORD WITH KILX...THANKS

.DVN...
IA...HEAT ADVISORY THROUGH SUNDAY FOR ZONES 063>068-076>078-087-
089-098-099.
tIL...SEVERE THUNDERSTORM WATCH UNTIL 9 PM CDT ZONES 001-002-007.
HEAT ADVISORY THROUGH SUNDAY FOR ZONES 001-002>007-009-015>018-
024>026-034-035.

MCCLURE