Severe Thunderstorm Warning 76, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
215 PM CDT WED MAY 17 2006

.DISCUSSION...
FOR AFTERNOON ZONES/GRIDS...
PRESENTLY...VIGOROUS UPR VORT N OF LAKE SUP SEEN ON WATER VAPOR
SATELLITE IMAGERY. MORNING ANALYSIS INDICATED HUGE HGT FALLS AND
VERY COLD AIR AT H5 ACROSS NRN MN/ONTARIO. H5-H7 DELTA T NEAR
20...INDICATING STEEP MID LEVEL LAPSE RATES ACROSS DAKOTAS...
MN...ONTARIO ADVECTING SSE. NE IL AND NW IN IN LEFT FRONT REGION OF
STRONG UPR LEVEL JET. ONLY THING MISSING IS LOW LEVEL MOISTURE.
DEWPOINTS IN LOWER 50S ARE MARGINAL FOR SEVERE WEATHER. AREA OF
SHOWERS AND THUNDERSTORMS WITH SMALL HAIL AND GUSTY WINDS MOVING
THROUGH THE REGION ATTM. MAIN THREAT WILL BE SMALL TO MODERATE
HAIL. CONVECTION SHOULD DIMINISH TO SHOWERS AFTER DARK AND END BY
MIDNIGHT.

LOOKS LIKE NRN IL/NW IN WILL BE IN SOMEWHAT SUBSIDENT REGIME THU.
ALSO DRIER AIR AT SURFACE. HOWEVER...SHOULD BE ENOUGH MORNING
INSOLATION...LINGERING LOW LEVEL MOISTURE...AND SUFFICIENTLY COLD
ALOFT FOR INSTABILITY CU AND PERHAPS A FEW SHOWERS.

NEXT CHANCE OF RAIN APPEARS TO BE SAT NIGHT AS BAROCLINIC ZONE SETS
UP BUT NOT MUCH IN THE WAY OF A MID LEVEL WAVE. WILL CONTINUE CHANCE
POPS FOR NOW.

STRONG UPPER LOW PUSHING INTO THE WEST COAST MAY FINALLY BREAK THE
PATTERN AND SHIFT FLATTENING WESTERN US RIDGE INTO MIDWEST NEXT
WEEK. IF THIS IS PANS OUT CHICAGO COULD GET FIRST TASTE OF SUMMER
WITH TEMPS WELL INTO THE 80S BY WED/THU. HOWEVER SINCE THIS IS THE
MODEL WORLD AND NOT THE REAL WORLD...WILL PLAY IT CONSERVATIVE AND
TEMPER WARM UP WITH FCST HIGHS AROUND 80 MID WEEK. COULD BE ANOTHER
CHANCE OF THUNDERSTORMS BY TUE NIGHT OR WED BUT WOULD LIKE TO SEE
ANOTHER MODEL RUN WITH SOME CONSISTENCY BEFORE JUMPING ON PRECIP
FORECAST THAT FAR OUT IN FORECAST.

ALLSOPP
&&

.AVIATION DISCUSSION REGARDING 18Z TAFS...

104 PM CDT

MAIN AVIATION CONCERNS ARE THUNDERSTORM POTENTIAL FOR NEXT SVRL
HOURS...ALONG WITH WINDS/WIND SHIFT WITH COLD FROPA THROUGH THIS
EVENING.

IMPRESSIVE MID LEVEL SHORT WAVE TROUGH DIGGING THROUGH UPPER MIDWEST
THIS AFTERNOON...AND WILL CONTINUE TO PROPAGATE SOUTH/SOUTHEASTWARD
AND ACROSS FCST AREA THROUGH THIS EVENING. STRONG SYNOPTIC SCALE
FORCING INDICATED BOTH IN SATELLITE/PROFILER OBS AND NUMERICAL MODEL
FCSTS. THUNDERSTORMS BECOMING INCREASINGLY LIKELY NEXT FEW HOURS AS
COMBINATION OF MID/UPPER LVL SPEED MAX APPROACHES...WITH LLVL WAA
AND COOLING ALOFT INCREASING INSTABILITY WITH RUC FCST MLCAPES
500-750 J/KG. 0-6 KM BULK SHEAR INCREASING INTO 40-50 KT RANGE
THROUGH AFTN SUPPORTS CONVECTIVE ORGANIZATION AND SVR
POTENTIAL...ALONG WITH FAIRLY HIGH CONFIDENCE IN TAFS. SVR TSTM
WATCH NOW IN EFFECT THROUGH 7 PM CDT FOR ALL TERMINAL SITES. TRW
ALREADY DEVELOPING RAPIDLY ACROSS WI VCNTY COLD POOL ALOFT AND
REGION OF UPPER DIVERGENCE IN LEFT EXIT REGION OF UPPER JET. THUS
HAVE MOVED TSRA TIMING UP AN HOUR OR TWO FROM PREVIOUS FCST INTO
19-20Z TIME FRAME. EXPECT MAIN TSRA THREAT AT TERMINALS TO BE
THROUGH LATE AFTN...DIMINISHING RAPIDLY WITH COLD FROPA 22-24Z. SHRA
MAY CONTINUE INTO EARLY EVENING AS MID LEVEL TROUGH AXIS MOVES
THROUGH...THOUGH SHOULD BE SOUTH OF TERMINALS BY MID EVENING AS
SUBSIDENCE BECOMES DOMINANT AND LOW LEVEL INSTABILITY DECREASES WITH
SUNSET AND LLVL COOL/DRY ADVECTION. SUBSIDENCE APPEARS STRONG ENOUGH
THAT STRAOCU DECK TRAILING FRONT SHOULD SCATTER OUT OVERNIGHT.
ANOTHER MORE SHEARED MID LEVEL VORT MAX APPROACHES MID DAY
THURSDAY...THOUGH INSTABILITY CONSIDERABLY LESS AND FOR NOW EXPECT
PERHAPS JUST BROKEN VFR STRATOCU DECK TO FORM MID/LATE MORNING.

FOR WINDS...WESTERLY GRADIENT AHEAD OF APPROACHING COLD FRONT THIS
AFTERNOON...WITH POTENTIAL FOR SOME GUSTINESS TO 20 KTS OR SO AT
TIMES. APPEARS FROPA WILL OCCUR 22-24Z ACROSS TERMINALS...WITH
WINDSHIFT TO NORTH. GRADUAL DECREASE AND SHIFT BACK TO MORE NW
EXPECTED LATER THIS EVENING WITH MAIN PRESSURE RISE/ISALLOBARIC
GRADIENT PASSING WEST OF FCST AREA ACROSS IOWA. SOME GUSTINESS AGAIN
MID/LATE THURS MORNING FROM NW AS LOW LEVELS MIX OUT.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
.IL...NONE.
.IN...NONE.
.LM...NONE.

&&

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