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000
FXUS63 KLOT 131650
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1150 AM 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
&&
.PREV DISCUSSION...
445 AM CDT
FORECAST FOCUS ON SEVERE WEATHER POTENTIAL FOR THIS AFTERNOON AND
THIS EVENING. LITTLE CHANCE IN PREVIOUS FORECAST THOUGH MAIN
THREAT AREA APPEASES TO HAVE SHIFTED BACK A LITTLE FURTHER AS
COMPARED TO LOCATION POINTED TO YESTERDAY.
MOST OPERATIONAL MODELS AND ENSEMBLE MEMBERS IN GOOD AGREEMENT IN
SHARPENING AND DEEPENING THE UPPER TROUGH AS IT MOVES EAST FROM
THE NORTHERN ROCKIES TO THE UPPER MS AND MID MO VALLEYS BY LATE
AFTERNOON/EARLY EVENING AND ON OVER THE WESTERN GREAT LAKES
TONIGHT. COLD FRONT TRAILING FROM PARENT LOW PRESSURE TRACKING
ACROSS ONTARIO THIS AFTERNOON AND TONIGHT TO SWEEP ACROSS THE
LOCAL FORECAST AREA DURING THE EVENING.
AREA OF RAIN MOVING ACROSS THE REGION EARLY THIS MORNING WITH
INITIAL SURGE OF WARM AIR ADVECTION...H8 TEMP +19C AT KSGF VS +8C
AT KILX AND KDVN PER 13/00Z RAOBS. NEW THUNDERSTORM DEVELOPMENT
HAS BEEN OCCURRING RECENTLY ACROSS NORTHEAST MO INTO SOUTHERN AND
WEST CENTRAL IL ALONG AND NORTH OF THE SURFACE WARM FRONT AND MID
LEVEL LAPSE RATES QUICKLY INCREASING. MODELS SHOW MID LEVEL TROUGH
AND SURFACE WARM FRONT LIFTING INTO NORTHEAST IL BY LATE MORNING
AND ANTICIPATE A TEMPORARY LULL IN CONVECTIVE ACTIVITY MIDDAY
THROUGH MID AFTERNOON.
WHILE SEVERAL INGREDIENTS SHOULD BE IN PLACE FOR POTENTIAL STRONG
TO SEVERE THUNDERSTORM DEVELOPMENT INCLUDING FAVORABLE UPPER
DIVERGENT FLOW...APPROACHING STRONG SHORT WAVE TROUGH...STEEPENING
MID LEVEL LAPSE RATES...STRONG LOW LEVEL THETA-E ADVECTION...AND
SIGNIFICANT SPEED SHEAR AND LESSER DIRECTIONAL SHEAR IN LOWEST
1-3KM LAYER. BIG QUESTION MARK IS IF LOW LEVEL DESTABILIZATION
WILL OCCUR. MODEL FORECAST SOUNDINGS AND MOS GUIDANCE TEMPERATURES
SAY NO...AT LEAST AS FAR NORTH AS NORTHERN IL AS MODELS KEEP
STRATOCU ACROSS THE AREA THROUGH THE AFTERNOON AND LIMIT SURFACE
HEATING AND RESULTING DESTABILIZATION OF LOW LEVELS. FEEL SPC
ADJUSTMENT OF MODERATE RISK AREA A LITTLE FURTHER BACK TO THE
SOUTHWEST APPROPRIATE GIVEN QUESTIONABLE DESTABILIZATION INTO FAR
NORTHERN IL. HOWEVER...IF CLOUDS DO BREAK THEN ELEVATED THREAT
WILL ONCE AGAIN BE BE PRESENT ACROSS MOST IF NOT ALL OF THE LOCAL
FORECAST AREA.
MODELS HAVE THE COLD FRONT THROUGH ALL BUT THE FAR EASTERN AND
SOUTHEASTERN PORTIONS OF THE FORECAST AREA BY 14/06Z AND EXPECT
SHOWERS AND THUNDERSTORMS TO BE ENDING WEST NORTHWEST TO EAST
SOUTHEAST ACROSS THE FA DURING THE EVENING.
ANOTHER SHORT WAVE MOVING ACROSS SOUTH CENTRAL CANADA AND THE
NORTHERN AND CENTRAL PLAINS FRIDAY BRINGS THE THREAT OF MORE
SHOWERS AND THUNDERSTORMS TO THE LOCAL AREA FOR FRIDAY AND FRIDAY
NIGHT. MODELS HAVE THE COLD FRONT THROUGH THE AREA BY LATE
SATURDAY MORNING SO ONLY LINGERING CHANCES IN THE MORNING AS COLD
FRONT PROGGED TO THE OH RIVER BY SUNDAY EVENING WITH STRONG LOW
LEVEL DRYING AS A SURFACE RIDGE BUILDS SOUTHEASTWARD INTO THE
AREA. THOUGH A SECONDARY SHORT WAVE TROUGH INDICATED ROTATING
ACROSS THE UPPER MIDWEST SATURDAY AFTERNOON FEEL STRONG DRYING
WILL PRECLUDE ANY SIGNIFICANT CHANCE OF SHOWERS.
TRS
&&
.AVIATION...
1200 UTC TAFS...
ACARS SOUNDINGS SHOW SOME LOW LEVEL WIND SHEAR THIS MORNING AT ORD
AND RFD. AVERAGE HEIGHT OF THE SHEAR IS AROUND 2000 TO 2100 FEET.
AND THE WIND SPEED IS 40 KNOTS. WILL ADD LOW LEVEL WIND SHEAR TO
THE FORECAST UNTIL 15 UTC. BY THEN THERE MAY BE ENOUGH MIXING TO
GIVE GUSTINESS AT THE SURFACE. THUNDERSTORMS WERE FROM GIBSON CITY
ILLINOIS TO MENDOTA ILLINOIS. THIS LARGE AREA OF THUNDERSTORMS
WILL MOVE NORTHEAST INTO THE CHICAGO AREA AND NORTHWEST INDIANA.
WILL HAVE TEMPO GROUPS OF THUNDER IN THE FORECASTS FOR THIS
MORNING.
INCLUDED IN THESE FORECAST IS A BETTER TIMING OF THE FRONTAL
PASSAGE TONIGHT. WE HAVE KEPT THE FORECAST FOR THUNDERSTORMS THIS
EVENING AND POSSIBLE FOG FOR LATE TONIGHT. THIS FORECAST WAS
BASED ON THE PRECIPITATION THAT WAS OCCURRING THIS MORNING.
SURFACE AND PROFILER DATA AND THE NEW MODEL RUNS OF THE LOCAL WRF
AND SREF MODELS. THE WIND WILL BE SOUTHEAST TODAY UNTIL THE FRONT
PASSES THROUGH TONIGHT. THE WIND WILL BECOME WEST AT 10 TO 15
KNOTS AFTER THE FRONT PASSES THROUGH NORTHERN ILLINOIS AND
NORTHWEST INDIANA.
&&
.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.
&&
$$
