Severe Thunderstorm Warning 151, NWS Quad Cities, IA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
145 AM CDT MON AUG 24 1998

THE 06Z SURFACE ANALYSIS SHOWED A BOUNDARY FROM MI TO KDBQ TO KDSM TO
SOUTHERN NE TO WY. UPPER CIRCULATION OVER ONTARIO DRIVING EASTERN
PORTION OF THIS BOUNDARY...BUT WESTERN PORTION HUNG UP AS FLOW IS
MAINLY PARALLEL TO FRONT. PROFILERS SHOW CWA IN RIGHT EXIT REGION OF
JET...A MOST UNFAVOURABLE POSITION FOR CONVECTION. SURFACE BASED
PARCELS CAPPED. THIS IS PROHIBITING TSRA SO FAR...ALTHOUGH 30KT LOW
LEVEL JET SUPPORTING SHALLOW MID-LEVEL CLOUDS SEEN ON IR LOOP. ABOVE
850MB LAPSE RATES CONDITIONALLY UNSTABLE...SO NOCTURNAL CONVECTION
STILL POSSIBLE...BUT DON/T BET ON IT. WEAK TROUGH ACROSS MN WAS
NOTED...MAINLY A DEW POINT DISCONTINUITY. WEAK SHORTWAVE NOTED ON
WATER VAPOUR OVER NE HELPING BROKEN TSRA THERE. SHORTWAVE OF INTEREST
FOR TODAY WAS RACING EAST OUT OF THE NORTHERN ROCKIES. THE EFFECTS OF
THE MT SYSTEM IS TODAY/S CONCERN.

THE 00Z ETA/NGM INITIALIZED OKAY AND AT 06Z REPRESENTED REALITY.
SPEEDY WESTERLY FLOW ACROSS THE NORTHERN US WILL CONTINUE TODAY. MT
SHORTWAVE WILL ZIP INTO WESTERN MN BY EVENING. FORCING AHEAD OF
SHORTWAVE WILL INDUCE WEAK LOW PRESSURE ALONG MAIN SURFACE
BOUNDARY...ENDING UP AS A WEAK LOW/INVERTED TROUGH OVER IA BY
00Z...GIVING US INCREASED SURFACE CONVERGENCE...SOMETHING WE DIDN/T
HAVE MUCH OF YESTERDAY...WHICH MADE MY FORECAST BUST.

FAVOURABLE RIGHT ENTRANCE REGION OF 85KT UPPER LEVEL JET STREAK WILL
MOVE TO MN/NORTHERN IA DESTABLIZING MID-LEVELS OF ATMOSPHERE WITH
7C/KM LAPSE RATES. 65KT 500MB JET STREAK FROM THE WEST COMBINED WITH
LOW LEVEL WINDS TURNING MORE SOUTHEAST AHEAD OF SURFACE LOW WILL
INCREASE LOW LEVEL TURNING BRINGING HELICITIES UP TO 300M2/S2. SOLAR
INSOLATION INCREASES CAPES TO 3KJ/KG. COMBINATION OF THESE FACTORS
SUGGEST SPIRITED CONVECTION LATER THIS AFTERNOON INTO THE EVENING
HOURS. NEGATIVE FACTOR WILL BE CAP...BUT MODEL SOUNDINGS SUGGEST CAP
WEAKENING TO 1C OR LESS BY 00Z. I/M HESITANT TO GET TOO EXCITED ABOUT
SEVERE PROSPECTS GIVEN YESTERDAY/S BUST /ONCE BITTEN...TWICE
SHY/...BUT GIVEN EXPECTED SURFACE DEVELOPMENT...IT SEEMS SEVERE MORE
POSSIBLE TODAY.

TEMPERATURES TODAY WILL BE NEAR YESTERDAY/S HIGHS WITH LITTLE CHANGE
IN LOWER TROPOSPHERE THERMAL PROFILE. THIS IS CLOSER TO WARMER FWC
READINGS.

WE NEED TO ADD POPS TO IA TODAY WITH REST OF FORECAST REASONABLE.

CID 90/30   DBQ 89/30   MLI 92/30   BRL 95/30

HELGESON

IAZ040>042-051>054-063>068-076>078-087>089-098-099-ILZ001-002-
007-009-015>018-024>026-034-035-242155-

SIGNIFICANT WEATHER OUTLOOK FOR EASTERN IOWA AND NORTHWEST ILLINOIS
NATIONAL WEATHER SERVICE QUAD CITIES IA IL
457 AM CDT MON AUG 24 1998

...ACTIVE WEATHER...

ALTHOUGH YESTERDAYS SIGNIFICANT WEATHER OUTLOOK DID NOT PAN OUT...IT
APPEARS TODAY/S FORECAST VERIFY BETTER.

LOW PRESSURE WILL CONTINUE TO DEVELOP OVER THE CENTRAL PLAINS TODAY
IN RESPONSE TO AN UPPER LEVEL JET STREAK ZIPPING ACROSS THE NORTHERN
PLAINS. THE LOW WILL MOVE ALONG A FRONTAL BOUNDARY DRAPED ACROSS
EASTERN IOWA AND NORTHWEST ILLINOIS. SOUTHERLY FLOW AHEAD OF THE LOW
WILL FOCUS MOISTURE CONVERGENCE ALONG THE FRONT. COOLER AIR ALOFT
AND HEATING NEAR THE GROUND WILL DESTABILIZE THE ATMOSPHERE.

SURFACE BASED INSTABILITY WILL BECOME MODERATE BY LATE AFTERNOON. A
65KT 500MB SPEED MAX WILL DRIVE INTO NORTHERN IOWA BY EVENING. WINDS
AT THE SURFACE WILL TURN SOUTH OR JUST EAST OF SOUTH. THIS SETS UP
WHAT WE CALL WIND SHEAR...WHICH IS THE CHANGE OF WIND DIRECTION AND
SPEED WITH HEIGHT. WIND SHEAR WILL BE SUFFICIENT TO SUPPORT ROTATING
UPDRAFTS AND POSSIBLE TORNADOES.

ALL OF EASTERN IOWA AND NORTHWEST ILLINOIS IS UNDER THE GUN FOR
SEVERE THUNDERSTORMS LATE THIS AFTERNOON AND EVENING. THE BEST
CHANCE FOR SEVERE WEATHER WILL BE NORTH OF INTERSTATE 80 AND WEST OF
THE MISSISSIPPI RIVER BETWEEN 400 PM AND 900 PM AND NORTH OF
INTERSTATE 80 AND EAST OF THE MISSISSIPPI RIVER BETWEEN 700 PM AND
MIDNIGHT. FURTHER SOUTH THERE IS LESSER CHANCE OF SEVERE WEATHER.
WIND GUSTS UP TO 80 MPH...1 INCH DIAMETER HAIL...AND ISOLATED
TORNADOES ARE POSSIBLE.

IF YOU LIVE IN EASTERN IOWA AND NORTHWEST ILLINOIS BE PREPARED FOR
FUTURE SEVERE WEATHER WATCHES LATE THIS AFTERNOON AND TONIGHT.

OVERNIGHT...THE MAIN THREAT WILL BE LOCALLY HEAVY RAIN WITH AN
IMPROVING TREND EXPECTED ON TUESDAY.

SPOTTER ACTIVATION MAY BE REQUIRED ACROSS PORTIONS OF EASTERN IOWA
AND NORTHWEST ILLINOIS THIS AFTERNOON AND TONIGHT.

HELGESON