Severe Thunderstorm Warning 153, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
328 AM CDT FRI SEP 7 2007

.DISCUSSION...
328 AM CDT

PLENTY TO PONDER THIS MORNING...NOT THE LEAST OF WHICH IS THE MCS
TAKING AIM ON OUR CWA

LARGE MCS OVER IOWA INTO NORTHERN MISSOURI CONTINUES TO MARCH
STEADILY EASTWARD EARLY THIS MORNING. WATER VAPOR IMAGERY AND
REGIONAL PROFILER NETWORK BOTH SHOW VIGOROUS SHORT WAVE OVER WESTERN
IOWA EARLY THIS MORNING PUSHING ENE TOWARD THE WESTERN LAKES REGION.
PROFILERS SHOW 40-50KT 500MB JET ASSOCIATED WITH THIS SHORTWAVE
PROVIDING MODERATELY STRONG DEEP LAYER SHEAR...WHICH WHEN COMBINED
WITH STRONG LLJ/LL WAA HAS BEEN SUCCESSFUL IN MAINTAINING MCS.
AIRMASS AHEAD OF THE MCS IS WEAKLY UNSTABLE...HOWEVER NO LESS
UNSTABLE THAN THE AIRMASS THE CONVECTION HAS BEEN ABLE TO SURVIVE IN
THUS FAR. BIGGEST POTENTIAL NEGATIVE FOR PRECIP SURVIVING INTO THE
MORNING HOURS IS RECENT SATELLITE/RADAR TRENDS WHICH SUGGEST THE
SOUTHERN PORTION OF THE MCS MAY BE BREAKING OFF AND COULD INTERCEPT
THE LLJ AND THUS ALLOW WAA AND LL CONVERGENCE TO WEAK IN ADVANCE OF
THE NORTHERN PIECE OF THE MCS. IN ADDITION...STRONG FORCING WITH THE
SHORTWAVE APPEARS TO BE TAKING AIM ON SOUTHERN WISCONSIN...SO WHILE
CATEGORICAL POPS APPEAR JUSTIFIED AM EXPECTING A SLOW WEAKENING
TREND.

ALL INDICATIONS ARE THAT THIS SHORTWAVE WILL LARGELY BE EAST OF OUR
CWA BY MIDDAY WITH SUBSEQUENT SUBSIDENCE NOT PARTICULARLY SUPPORTIVE
FOR ADDITIONAL CONVECTIVE DEVELOPMENT ALONG THE FRONT...AND WITH
FRONT LIKELY TO BE THROUGH ABOUT THE NW HALF OF OUR CWA BY 18Z FELT
COMFORTABLE TAPERING POPS OFF CONSIDERABLY FROM SE TO NW THIS
AFTERNOON. WITH BRUNT OF THIS MORNINGS SHORTWAVE PASSING TO THE
NORTH OF THE AREA OUR SOUTHERN COUNTIES MAY ONLY GET A GLANCING BLOW
AND WITH SLOWER FROPA DECIDED TO HANG ONTO HIGH POPS INTO THE
AFTERNOON THERE. DEEP LAYER SHEAR CERTAINLY LOOKS AS THOUGH IT
SHOULD BE SUPPORTIVE FOR ORGANIZED IF NOT SEVERE STORMS THIS
AFTERNOON...HOWEVER PLENTY OF QUESTIONS LINGER REGARDING THE AMOUNT
OF DESTABILIZATION AND STORM COVERAGE/MODE SO HARD TO GET TOO
EXCITED ABOUT SEVERE POTENTIAL JUST YET.

FRONT SHOULD QUICKLY BEGIN TO LOSE STEAM AS SHORTWAVE AND SURFACE
LOW BOTH ZIP RAPIDLY NE INTO CANADA LEAVING THE BAROCLINIC ZONE HUNG
UP JUST TO OUR SOUTH. LOW AMPLITUDE WAVE IS THEN PROGGED TO
TRANSLATE EASTWARD ACROSS THE NORTHERN PLAINS AND TOWARD THE
MISSISSIPPI VALLEY THROUGH THE LARGELY ZONAL FLOW. AS THIS WAVE
APPROACHES MAY SEE THE SURFACE BOUNDARY ATTEMPT TO CREEP NORTHWARD
WITH INCREASED ASCENT/LARGE SCALE SUPPORT LEADING TO INCREASED
CONVECTION NEAR/NORTH OF THE FRONT OVER THE WEEKEND. WHILE MOST OF
THIS ACTIVITY SHOULD REMAIN TO THE SOUTH OF OUR CWA THE CHANCE IS
THERE THAT SOME COULD CREEP UP INTO OUR SOUTHERN COUNTIES OVER THE
WEEKEND.

COOLER AND DRIER AIRMASS SHOULD FILTER INTO THE AREA IN THE WAKE OF
TODAYS FRONT WITH SKIES CLEARING OUT TONIGHT. WINDS SHOULD DROP OFF
QUITE QUICKLY TONIGHT AND GIVEN EXPECTED CLEARING SKIES AND DRIER
AIR ADVECTING IN THE STAGE SHOULD BE SET FOR A MUCH COOLER AND MORE
SEASONABLE NIGHT TEMP WISE TONIGHT...PARTICULARLY OVER THE NORTHERN
PORTIONS OF THE CWA. SOME POTENTIAL WILL EXIST FOR FOG WHERE SKIES
CLEAR AND WHERE DRY AIR HAS YET TO FULLY ADVECT IN...BUT THIS WILL
LIKELY BE MORE OF A NOWCASTABLE ISSUE VERSUS FORECASTABLE.

BEYOND THIS WEEKEND OUR METEOROLOGICAL CRYSTAL BALL BECOMES QUITE
MURKY AS RUN TO RUN AND MODEL TO MODEL CONSISTENCY HAS BEEN
SERIOUSLY LACKING. LARGE SPREAD IN GFS ENSEMBLES ISNT DOING MUCH TO
INCREASE CONFIDENCE IN THE FORECAST...SO GIVEN SUCH A HIGH DEGREE OF
UNCERTAINTY LARGELY LEFT GRIDS UNTOUCHED BEYOND SUNDAY. HOPEFULLY
MODELS WILL BEGIN TO SING A SIMILAR TURN SOONER RATHER THAN LATER
AND FORECAST CAN BE FINE TUNED THEN

IZZI

&&

.AVIATION...

06Z TAFS...SEVERAL CONCERNS THIS PERIOD INCLUDING TS AND FROPA
TIMING AS WELL AS LOW CLOUDS OVERNIGHT. LARGE AREA OF THUNDER OVER
IA TIMED INTO NW IL 09Z-10Z AND ACROSS NE IL 11Z-12Z AS MODELS SHOW
THIS ACTIVITY HOLDING TOGETHER AS IT MOVES EAST. ONCE THIS AREA
MOVES EAST MID/LATE MORNING...CONFIDENCE LOW ON FURTHER DEVELOPMENT
ALONG/AHEAD OF FROPA. FOR NOW HAVE JUST CARRIED PREVAILING SHOWERS
BUT THERE MAY ENDED UP BEING A THIN LINE OF TS AHEAD OF FROPA. WINDS
STAY SOUTHERLY OVERNIGHT WITH GUSTS TO 20KTS...THEN TURN MORE SW
DURING THE MORNING WITH GUSTS INCREASING TO 25KTS...TURNING WESTERLY
MID DAY THEN NORTHERLY WITH THE FRONT LATE THIS AFTERNOON. 1-2KFT
CIGS WIDESPREAD ACROSS THE AREA CURRENTLY...MAY SEE SOME BREAKS
AHEAD OF PRECIP TOWARD SUNRISE BUT CIGS/VIS GENERALLY MVFR WITH SOME
IFR POSSIBLE IN THE HEAVIER SHOWERS/STORMS. CIGS LIFT AND SCATTER
OUT LATE THIS AFTERNOON AND THIS EVENING. CMS

&&

.MARINE...

306 AM...LOW PRESSURE OVER WESTERN LAKE SUPERIOR WILL DEEPEN RAPIDLY
THIS MORNING AS IT LIFTS NORTHEAST ACROSS ONTARIO TODAY THEN ACROSS
QUEBEC TONIGHT AS IT CONTINUES TO DEEPEN. HIGH PRESSURE MOVES ACROSS
MANITOBA AND ONTARIO LATE TONIGHT AND ACROSS THE NORTHERN LAKES
SATURDAY. WITH THE LOW MOVING SO FAST...PRESSURE GRADIENT RELAXES
QUICKLY TONIGHT WITH WINDS DROPPING OFF TO 10-15KTS BY MORNING.
PERIOD OF GALES ONLY EXPECTED FROM MID MORNING THRU MID AFTERNOON.
WAVE ACTION WILL TAKE TIME TO SUBSIDE THROUGH THE NIGHT HOWEVER.
WEAK LOW PRESSURE DEVELOPS OVER THE PLAINS SATURDAY AND SHIFTS EAST
TO THE WESTERN LAKES SUNDAY THEN DISSIPATES. NET AFFECT OF THIS WILL
BE TO HAVE EASTERLY FLOW OVER THE LAKE THIS WEEKEND WITH ANOTHER
HIGH PRESSURE MOVING ACROSS THE PLAINS EARLY NEXT WEEK. CMS

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...GALE WARNING OPEN WATERS UNTIL 23Z TODAY.
     SMALL CRAFT ADVISORY NEARSHORE UNTIL 03Z SATURDAY.

&&

WFO LOT

HAZARDOUS WEATHER OUTLOOK...CORRECTED
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
430 PM CDT THU SEP 6 2007

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-
019-070930-
WINNEBAGO-BOONE-MCHENRY-LAKE IL-OGLE-LEE-DE KALB-KANE-DU PAGE-COOK-
LA SALLE-KENDALL-GRUNDY-WILL-KANKAKEE-LIVINGSTON-IROQUOIS-FORD-
LAKE IN-PORTER-NEWTON-JASPER-BENTON-
430 PM CDT THU SEP 6 2007

THIS PORTION OF THE HAZARDOUS WEATHER OUTLOOK IS FOR NORTH CENTRAL
AND NORTHEAST ILLINOIS...AND NORTHWEST INDIANA.

.DAY ONE...TONIGHT

THERE IS A CHANCE OF THUNDERSTORMS TONIGHT.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THUNDERSTORMS ARE LIKELY FRIDAY...WITH A CHANCE OF THUNDERSTORMS
LINGERING INTO FRIDAY EVENING FROM NORTHWEST INDIANA TO EAST CENTRAL
ILLINOIS. A FEW OF THESE STORMS COULD PRODUCE VERY STRONG GUSTY
WINDS.

THERE IS A CHANCE OF THUNDERSTORMS MONDAY AND MONDAY NIGHT.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION WILL NOT BE NECESSARY.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE NWS CHICAGO
WEBPAGE: WEATHER.GOV/CHICAGO

LMZ740>745-766-070930-
WINTHROP HARBOR IL TO MICHIGAN CITY IN-
LAKE MICHIGAN-
430 PM CDT THU SEP 6 2007

THIS PORTION OF THE HAZARDOUS WEATHER OUTLOOK IS FOR THE ILLINOIS
AND INDIANA WATERS OF LAKE MICHIGAN.

.DAY ONE...TONIGHT

THERE IS A CHANCE OF THUNDERSTORMS TONIGHT.

.DAYS TWO THROUGH SEVEN...FRIDAY THROUGH WEDNESDAY

THUNDERSTORMS ARE LIKELY FRIDAY...WITH A CHANCE OF THUNDERSTORMS
LINGERING INTO FRIDAY EVENING ON THE INDIANA WATERS. A FEW OF
THESE STORMS COULD PRODUCE VERY STRONG GUSTY WINDS.

GALE FORCE WINDS ARE EXPECTED ON THE OPEN WATERS OF SOUTHERN LAKE
MICHIGAN FRIDAY. HAZARDOUS WIND AND WAVE CONDITIONS FOR SMALL CRAFT
WILL EXIST FRIDAY INTO FRIDAY EVENING.

THERE IS A CHANCE OF THUNDERSTORMS MONDAY AND MONDAY NIGHT.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE NWS CHICAGO
WEBPAGE: WEATHER.GOV/CHICAGO