Severe Thunderstorm Warning 59, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
330 PM CDT MON JUN 18 2007

.DISCUSSION...

MID LEVEL VORTEX OVER DOWNSTATE ILLINOIS IS QUITE APPARENT ON WATER
VAPOR IMAGERY AND IN THE REGIONAL PROFILER NETWORK...WITH ISOLD TO
SCT CONVECTION FIRING IN THE VICINITY OF THE SHEAR AXIS WHERE
STRONGER HEATING HAS TAKEN PLACE. FARTHER NORTH ACROSS MOST OF OUR
CWA STRATUS HAS BEEN VERY SLOW TO BURN OFF KEEPING TEMPERATURES A
BIT COOLER AND PREVENTING BOUNDARY LAYER FROM DRYING OUT MUCH TODAY.

THE MOIST BOUNDARY LAYER AND LIGHT WINDS SHOULD SET THE STAGE FOR
FOG DEVELOPMENT TONIGHT. FORECAST SOUNDINGS SHOW A POSITIVE
HYDROLAPSE IN THE LOWEST 1000FT OVERNIGHT WITH POOR DISPERSION AND
TEMPS FALLING BELOW THE CROSSOVER TEMP...GROUND WORK SEEMS TO BE SET
FOR DENSE FOG TO DEVELOP TONIGHT. WILL ADVERTISE POTENTIAL IN THE
HWO AND GRIDS AND ADVISE LATER SHIFT TO MONITOR FOR POTENTIAL NPW.

FOG/STRATUS SHOULD GRADUALLY LIFT AND MIX OUT INTO CUMULUS DECK BY
MIDDAY MONDAY. WEAK MID LEVEL CIRCULATION/SHEAR AXIS SHOULD CONTINUE
TO PROVIDE FOCUS FOR ISOLD/WDLY SCT AFTERNOON CONVECTION...PRIMARILY
SOUTH OF OUR AREA HOWEVER FAR SOUTHERN PORTIONS OF THE CWA COULD BE
CLOSE ENOUGH TO SEE SOME DEVELOPMENT DURING THE AFTERNOON. ITS
POSSIBLE LAKE BREEZE CIRCULATION COULD KICK OFF A STORM OR TWO AS
WELL...HOWEVER FARTHER NORTH FROM THE SHEAR AXIS MID LEVEL TEMPS ARE
WARMER AND SUPPORT FOR CONVECTION WANES.

REPEAT ON TAP FOR TUESDAY WITH REGARD TO STORM CHANCES. WITH MID
LEVEL SHEAR AXIS MEANDERING SOMEWHERE OVER CENTRAL ILLINOIS WILL
LIKELY SEE ADDITIONAL AFTERNOON CONVECTION DEVELOP...AGAIN CONFINED
MAINLY TO OUR SOUTHERN CWA. GFS/WRF BOTH SUGGEST LAPSE RATES WILL
STEEPEN UP A BIT ON TUESDAY AFTERNOON AS COOLER MID LEVEL TEMPS INCH
FARTHER NORTHWARD IN ASSOCIATION WITH THE WEAK MID LEVEL SYSTEM. THE
COOLER MID LEVEL TEMPS AND VERY WARM/MOIST BOUNDARY LAYER SHOULD
ALLOW MLCAPES TO CLIMB TO NEAR/ABOVE 2000 J/KG DURING THE AFTERNOON.
WHILE DEEP LAYER SHEAR IS VIRTUALLY NON-EXISTENT...PRESENCE OF SOME
MID LEVEL DRY AIR IN FORECAST SOUNDINGS LEAD TO THETA-E DIFFERENCES
APPROACHING 25C FROM SFC TO 700MB. GIVEN THIS AND MODERATELY STRONG
INSTABILITY ANY STORMS THAT DO FORM COULD PRODUCE WET MICROBURSTS.

ATTENTION WILL THEN TURN TO THE STRONG SHORTWAVE WHICH IS NOW
APPROACHING THE PAC NW COAST. WHILE THIS SYSTEM WILL PASS WELL NORTH
OF THE AREA AND LARGELY STAY NORTH OF THE CANADIAN
BORDER...ASSOCIATED SURFACE FRONT SHOULD SAG SOUTHWARD INTO THE AREA
WEDNESDAY. AMPLE MOISTURE/INSTABILITY AHEAD FRONT SHOULD ALLOW FOR A
DECENT COVERAGE OF SHOWERS/THUNDERSTORMS WED AFTERNOON/EARLY
EVENING...BUT STRONGER MID LEVEL FLOW REMAINS CONFINED TO THE COOL
SECTOR LEAVING DEEP LAYER SHEAR QUITE WEAK AHEAD OF THE FRONT.
GIVEN WEAK SHEAR STORM ORGANIZATION SHOULD BE LARGELY BE
ABSENT...HOWEVER WITH SUFFICIENT DESTABILIZATION DURING PEAK HEATING
CANT RULE OUT SOME PULSY STRONGER STORMS WED PM. ACTIVITY WILL
LIKELY HAVE A HARD TIME HOLDING ON TOO MUCH PAST SUNSET GIVEN LACK
OF DEEP LAYER SHEAR...STABILIZING BOUNDARY LAYER...AND LESS THAN
STELLAR UPPER SUPPORT WITH PRECIP CHANCES DECREASING WITH
SOUTHEASTWARD EXTENT IN OUR CWA.

IN THE WAKE OF THIS SYSTEM...MEDIUM RANGE MODELS ARE IN EXCELLENT
AGREEMENT IN THE OVERALL PATTERN THURSDAY THROUGH SUNDAY...WITH HIGH
AMPLITUDE RIDGING FROM THE HIGH PLAINS OF THE CONUS NORTHWARD INTO
THE NORTHWEST TERRITORIES OF CANADA. BY WEEKS END MEDIUM RANGE
GUIDANCE STRONGLY POINTING TO THE DEVELOPMENT OF AN OMEGA BLOCK WITH
NO SIGNIFICANT DAY TO DAY CHANGES EXPECTED IN OUR WEATHER THURSDAY
THROUGH SUNDAY...AND DAY 6-10 MEANS FROM THE ECMWF SUGGEST THE
PATTERN COULD PERSIST EVEN BEYOND NEXT WEEKEND.

LOCALLY THIS PATTERN SHOULD SPELL AN EXTENDED PERIOD OF
EXCEPTIONALLY PLEASANT SUMMER WEATHER. TEMPERATURES THROUGH THE
PERIOD WILL RUN SLIGHTLY BELOW AVERAGE WITH SYNOPTIC FLOW EITHER OFF
THE LAKE OR FAVORING THE DEVELOPMENT OF LAKE BREEZES EACH DAY. WHILE
NORTHWEST FLOWS THIS TIME OF YEAR TEND TO YIELD SURPRISES...IT
APPEARS AS THOUGH MOISTURE WILL BE SUPPRESSED TOO FAR SOUTH AND WEST
TO PROVIDE MUCH CHANCE OF PRECIPITATION THROUGH THE PERIOD.

IZZI

&&

.AVIATION...

18Z TAFS...CEILINGS ACROSS THE REGION HAVE BEEN SLOW TO IMPROVE.
UNSEASONABLE DISMAL CONDITIONS SHOULD CONTINUE FOR A FEW MORE HOURS
AND HAD PREVIOUSLY AMENDED THE TAFS TO REFLECT A LATE AFTERNOON
IMPROVEMENT OF CEILINGS.  LATEST VISIBLE SATELLITE TRENDS SHOWING
SOME THINNING OF THE LOWER CLOUDS...SO FEEL THIS IS STILL ON TRACK.
ALL SITES SHOULD GO VFR FOR THE LATE AFTERNOON AND EVENING
HOURS...BUT CEILING/VISIBILITIES WILL TAKE A NOSE DIVE SHORTLY AFTER
MIDNIGHT.  WITH CLEARING SKIES...AMPLE SFC MOISTURE AND LIGHT
WINDS...EXPECTING WIDESPREAD IFR CONDITIONS TO DEVELOP OVERNIGHT.
SOME UNCERTAINTY AS TO WHETHER TO LIMIT THE IFR TO EITHER LOW
STRATUS OR FG/BR.  FEEL THAT THERE IS A BETTER CHANCE FOR IFR VIS
THAN IFR CEILINGS...BUT THERE ARE INDICATORS THAT EITHER ARE VERY
LIKELY.  WET GROUNDS AND LIGHT WINDS WOULD POINT MORE TOWARD
RESTRICTED VISIBILITY...WHILE PERSISTENT SELY FLOW OFF OF THE
LAKE...WARMER SOIL TEMPS AND SUBSIDENCE DRYING/INVERSION ABV 850MB
POINT MORE TOWARD STRATUS. WITH DAYTIME HEATING AND DRY AIR ABV
850MB...MIXING SHOULD STRENGTHEN BY LATE MORNING AND CONDITIONS
SHOULD QUICKLY IMPROVE TO VFR.

KREIN

&&

.MARINE...

200 PM...BROAD HIGH PRESSURE OF 30.2 INCHES EXTENDS FROM THE EASTERN
GREAT LAKES THROUGH THE SOUTHEASTERN STATES. WEAK LOW PRESSURE OF
29.7 INCHES OVER WESTERN ONTARIO IS CONNECTED TO A SECOND LOW OF
29.5 INCHES OVER THE NORTHERN ROCKIES BY A WEAK COLD FRONT. THIS
SYSTEM WILL TRACK TO THE EAST TONIGHT WITH THE SOUTHERN LOW
DEEPENING SLIGHTLY TO 29.5 INCHES AS IT MOVES OVER THE WESTERN
DAKOTAS. THE HIGH PRESSURE OVER THE EASTERN SEABOARD WILL REMAIN
NEARLY STATIONARY OVER THE NEXT FEW DAYS. THE DAKOTAS LOW WILL TRACK
TO THE NORTHEAST...MOVING ACROSS SOUTHERN ONTARIO TUESDAY NIGHT AND
INTO QUEBEC BY WEDNESDAY...DRAGGING A TRAILING COLD FRONT SOUTH
ACROSS THE WESTERN GREAT LAKES WEDNESDAY. HIGH PRESSURE OF 30.4
INCHES WILL BUILD ACROSS THE NORTHERN PLAINS AND WESTERN GREAT LAKES
THURSDAY.

KREIN

&&

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

&&

HAZARDOUS WEATHER OUTLOOK...CORRECTION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
400 PM CDT MON JUN 18 2007

ILZ003>006-008-010>014-019>022-023-032-033-039-INZ001-002-010-011-
019-190930-
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-
400 PM CDT MON JUN 18 2007

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

.DAY ONE... TONIGHT

THUNDERSTORMS WILL MOVE THROUGH THE AREA OVERNIGHT. MAIN HAZARDS
WILL BE HEAVY RAINS CREATING PONDING OF ROADWAYS AND GUSTS TO 50
MPH OR GREATER. ADDITIONALLY WILL BE THE USUAL HAZARDOUS LIGHTNING
AND
POSSIBLY SOME SMALL HAIL. THERE HAVE BEEN SOME STORMS THIS EVENING
REACHING SEVERE WIND CRITERIA. ISOLATED SEVERE STORMS ARE STILL
POSSIBLE THROUGH THE EVENING.

.DAYS TWO THROUGH SEVEN... TUESDAY THROUGH SUNDAY

THE PROBABILITY FOR WIDESPREAD HAZARDOUS WEATHER IS LOW.

.SPOTTER INFORMATION STATEMENT...

SPOTTER REPORTS ARE REQUESTED AND APPRECIATED.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE
NWS CHICAGO WEBPAGE: WEATHER.GOV/CHICAGO (LOWER CASE)

LMZ740>745-766-190930-
WINTHROP HARBOR IL TO MICHIGAN CITY IN-
LAKE MICHIGAN-
400 PM CDT MON JUN 18 2007

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

.DAY ONE... TONIGHT

THUNDERSTORMS WILL MOVE OVER LAKE MICHIGAN OVERNIGHT. MAIN HAZARDS
WILL BE GUSTS TO 35 KNOTS OR MORE. ADDITIONALLY WILL BE THE USUAL
HAZARDOUS LIGHTNING AND POSSIBLY SOME SMALL HAIL.

A SMALL CRAFT ADVISORY FOR WAVES TO 3 FEET WILL REMAIN IN EFFECT
INTO THE MORNING HOURS TUESDAY FOR THE ILLINOIS AND INDIANA
NEARSHORE WATERS. WINDS AND WAVES MAY BE MUCH HIGHER IN AND AROUND
THUNDERSTORMS.

.DAYS TWO THROUGH SEVEN... TUESDAY THROUGH SUNDAY

THE PROBABILITY FOR WIDESPREAD HAZARDOUS WEATHER IS LOW.

&&

COMPLETE FORECAST INFORMATION IS AVAILABLE ON THE
NWS CHICAGO WEBPAGE: WEATHER.GOV/CHICAGO (LOWER CASE)

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