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000
FXUS63 KLOT 131849
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
149 PM CDT WED MAY 13 2009
.DISCUSSION...
MESOSCALE UPDATE
1113 AM CDT
COMPLEX SCENARIO FOR THIS AFTERNOON AND EVENING AS A COUPLE MESOSCALE
CONVECTIVE COMPLEXES TRACK EASTWARD ACROSS THE REGION EFFECTIVELY
RETARDING THE NORTHWARD ADVANCE OF THE WARM FRONT...AT LEAST FOR
NOW. DESPITE THE SLOWER THAN ANTICIPATED NORTHWARD ADVANCE OF THE
WARM FRONT...THERE IS STILL A SEVERE THREAT ACROSS THE CWA THIS
EVENING.
AT 11 AM CDT...THE SOMEWHAT DIFFUSE WARM FRONT STRETCHED FROM NEAR
OMA ESE ACROSS NORTHERN MO...THEN EAST ACROSS CENTRAL ILLINOIS.
ONGOING ELEVATED THUNDERSTORMS NORTH OF THE WARM FRONT LIKELY
BEING FED BY CONTINUED WARM ADVECTION ASSOCIATED WITH 40-50KT LOW
LEVEL JET. REGIONAL PROFILERS AND WSR-88D VWPS INDICATE THAT THE
LOW LEVEL JET HAS BEGUN ITS TYPICAL DIURNAL VEERING WHICH SHOULD
LESSEN ISENTROPIC ASCENT AND LOW LEVEL MOISTURE CONVERGENCE WHICH
SHOULD ALLOW FOR GRADUAL WEAKENING OF THE ELEVATED CONVECTION THIS
AFTERNOON. GIVEN THE PRESENCE OF THE VERY STEEP LAPSE RATES JUST
UPSTREAM...CANNOT RULE OUT ONE OF THESE ELEVATED STORMS BRIEFLY
PULSING UP AND PRODUCING SOME HAIL.
OF GREATER CONCERN WILL BE ADDITIONAL CONVECTIVE DEVELOPMENT
TAKING PLACE LATER THIS AFTERNOON OVER MISSOURI AND IOWA ALONG AN
ADVANCING COLD FRONT. INCREASINGLY STRONG ASCENT IN ADVANCE OF THE
AMPLIFYING AND DEEPENING UPPER TROUGH HAS BEEN RESULTING IN PRESSURE
FALLS STRENGTHENING OVER EASTERN IOWA WITH WARM FRONT BEGINNING TO
LIFT NORTHWARD. STEEP LAPSE RATES OVER TOP INCREASINGLY MOIST
BOUNDARY LAYER SHOULD SUPPORT THE DEVELOPMENT OF THUNDERSTORMS
ALONG THE ADVANCING COLD FRONT THIS AFTERNOON AS SYNOPTIC SCALE
ASCENT HELPS LIFT THROUGH WHAT INVERSION THERE IS AT THE BASE OF
THE ELEVATED MIXED LAYER.
GIVEN THE UNLIKELIHOOD OF ANY APPRECIABLE INSOLATION AND RESULTANT
STRONG DESTABILIZATION THIS AFTERNOON...WOULD EXPECT THAT WE WILL
BE LOOKING AT A LOW CAPE/HIGH SHEAR ENVIRONMENT WITH STRONGLY
FORCED LINEAR CONVECTION. ASSUMING THIS SCENARIO PANS OUT...THEN
WE WOULD PRIMARILY BE LOOKING AT A DAMAGING WIND/SHORT LIVED
TORNADO THREAT THIS EVENING AS THE CONVECTION SWEEPS RAPIDLY EAST
ACROSS THE AREA.
GIVEN THE EXPECTATION FOR RATHER WEAKER INSTABILITY AND LOWER
CONFIDENCE IN ANY MORE SIGNIFICANT SEVERE WEATHER EVENT
OCCURRING...WILL HOLD OFF ON ANY NOAA WEATHER RADIO LIVE
BRIEFING/EM CONFERENCE CALL AT THIS TIME. SINCE ANY SEVERE THREAT
WILL LIKELY HOLD OFF UNTIL THIS EVENING...WE HAVE TIME TO CONTINUE
TO MONITORING MESOSCALE ENVIRONMENT EARLY THIS AFTERNOON...AND IF
SCENARIO LOOKS MORE OMINOUS THAN IT CURRENTLY DOES MAY CONSIDER
CONF CALL/LIVE BRIEFING AROUND 20Z.
IZZI
&&
.AVIATION...
1800 UTC TAFS...PERSISTENT AREAS OF RAIN HAVE SATURATED SFC LAYERS
AND RESULTED IN CIGS AND VSBY LOWERING TO IFR. SEVERAL BREAKS IN
PCPN MOVING THRU NRN IL NOW MAY RESULT IN BRIEF LIFTING TO MVFR OR
EVEN VFR AT TIMES. RETURN OF RAIN WILL LOWER US BACK TO IFR. TSTMS
IS THE MAIN CONCERN HOWEVER. ADDED SUNSHINE WILL AID IN SPARKING
AFTN TSTMS. THIS IS NOT CLEAR CUT NOW IN IL BUT CAN SEE DRY TONGUE
PUSHING UP THRU CENTRAL IOWA. NATIONAL RADAR MOSAIC INDICATING
SOME CONVECTION BUILDING IN THIS CLEARING SLOT. THIS LIKELY TO
MOVE EAST THIS EVENING AND BRING ACTIVE WX SCENARIO HERE BY
EVENING. WARM FRONT TO THE SOUTH HAS SLOWED ITS NWD PROGRESS AND
MAY NOT BE MOVG THRU TERMINALS UNTIL 20-22Z. THIS WILL SWITCH
WINDS MORE TO SW AND RAISE TEMPS FURTHER THRU THE CONVECTIVE
TRIGGER VALUES IN THE LOW 60S. COLD FROPA LIKELY AFTER MIDNGT...
PUTTING AN END TO TSTMS AND DRYING AND CLEARING BY DAYBREAK.
RLB
&&
.MARINE...
320 AM CDT
A STRONG LOW WITH A PRESSURE OVER SOUTHERN MANITOBA WILL MOVE
NORTHEAST TO NORTHERN ONTARIO BY THURSDAY MORNING. THIS LOW WILL
DRAG A FRONT INTO MICHIGAN AND INDIANA BY MIDNIGHT THURSDAY
MORNING. A LARGE HIGH WILL MOVE INTO MISSOURI BY THURSDAY
AFTERNOON. THE COOL WATER HAS SET UP AN INVERSION WHICH PREVENTS
STRONG WIND SPEED ALOFT TO REACH THE LAKE SURFACE. BECAUSE OF THE
INVERSION WE DO NOT EXPECT GALES OR HIGHER WIND SPEEDS AT THIS
TIME. WILL KEEP THE SMALL CRAFT ADVISORY IN EFFECT THROUGH
THURSDAY MORNING.
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 7 AM THURSDAY.
&&
$$
