Winter Storm Warning 2, NWS Chicago, IL

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO IL
336 AM CST THU JAN 31 2008

.DISCUSSION...

MAIN FORECAST CONCERN CONTINUES TO CENTER ON WINTER STORM TAKING
SHAPE ACROSS THE SOUTHERN PLAINS THAT WILL IMPACT THE AREA TONIGHT
THROUGH FRIDAY.

WATER VAPOR IMAGERY EARLY THIS MORNING INDICATES A VIGOROUS UPPER
TROUGH DIGGING INTO THE SOUTHERN PLAINS WITH A 45 KNOT LOW LEVEL JET
IN ADVANCE OF THE MID AND UPPER LEVEL TROUGH. AS THIS UPPER TROUGH
TRACKS INTO EASTERN OKLAHOMA TODAY...ISENTROPIC ASCENT WILL
GRADUALLY INCREASE WITH STRENGTHENING LOW LEVEL FLOW. WRF 280K
CONDENSATIONAL PRESSURE DEFICITS LOWER TO AROUND 10 MB THIS
AFTERNOON IN RESPONSE TO THIS INCREASING LIFT AND CURRENT THINKING
IS THAT PROSPECTS FOR GOOD COVERAGE LIGHT SNOW WILL INCREASE BY MID
AFTERNOON. STRONGEST ISENTROPIC FORCING SHOULD BE CONFINED TO
SOUTHERN PORTIONS OF THE CWA WHERE DID INDICATE SOME HALF INCH SNOW
ACCUMS FROM MID TO LATE AFTERNOON. DEEPER LIFT/FORCING EXPECTED BY
THIS EVENING AS UPPER DIVERGENCE INCREASES IN RESPONSE TO INCREASING
DIFFLUENCE ALOFT WITH UPPER LEVEL HEIGHT MINIMUM MOVING INTO
SOUTHEAST MISSOURI. AN UPPER LEVEL SPEED MAX PROGRESSING THROUGH
BASE OF THE SOUTHERN PLAINS TROUGH WILL LIFT INTO THE MID/UPPER MS
VALLEY TONIGHT WITH MODEL GUIDANCE STRENGTHENING THIS JET IN UPPER
LEVEL CONFLUENCE ZONE AND ENHANCING UPPER DVG IN THE RIGHT ENTRANCE
REGION OF THIS JET. INCREASING UPPER DYNAMICS COUPLED WITH STRONG
LOW LEVEL ISENTROPIC ASCENT WILL LEAD TO DEEP UPWARD VERTICAL MOTION
PROFILES THIS EVENING AND RAPIDLY EXPANDING SNOW COVERAGE AND
INTENSITY. BASED ON TRACK OF THIS SYSTEM...IT STILL APPEARS AS
THOUGH SOUTHEAST PORTIONS OF THE FORECAST AREA WHICH WERE UNDER THE
WINTER STORM WATCH WILL SEE THE GREATEST POTENTIAL OF HEAVY SNOW
WITH THIS SYSTEM. HAVE UPGRADED WINTER STORM WATCH TO A WINTER STORM
WARNING AND ADDED A SNOW ADVISORY JUST TO THE WEST OF THE WARNING
AREA WHICH INCLUDES MCHENRY...DEKALB...KANE...KENDALL...AND LA SALLE
COUNTIES. DID ALSO INCLUDE LAKE COUNTY ILLINOIS IN THE SNOW
ADVISORY.

TOWARD SUNRISE ON FRIDAY...STRONGEST LOWER LEVEL ISENTROPIC ASCENT
WILL HAVE SHIFTED EAST OF THE AREA...BUT CONCERN WILL BE ON
CONTINUED HEAVY SNOW THREAT WITH DEFORMATION FORCING/MID LEVEL
FRONTOGENESIS. 00Z WRF INDICATES A FAVORABLE JUXTAPOSITION OF
ELEVATED INSTABILITY JUST ATOP STRONG MID LEVEL FORCING SUGGESTING
POTENTIAL FOR INTENSE BANDED SNOW...ESPECIALLY ACROSS NORTHWEST
INDIANA COUNTIES AND INTO FORD...IROQUOIS...KANKAKEE IN NORTHEAST
ILLINOIS EARLY FRIDAY MORNING. SNOW RATES OF 1 TO 2 INCHES PER HOUR
WILL BE POSSIBLE FOR A TIME LATE TONIGHT INTO EARLY FRIDAY MORNING
ACROSS THESE LOCATIONS. OTHERWISE...SNOW WILL TAPER OFF ACROSS THE
REMAINDER OF NORTHEAST AND NORTH CENTRAL ILLINOIS THROUGH THE
MORNING FRIDAY AS THIS SYSTEM PULLS OUT. HAVE GONE WITH STORM TOTAL
SNOWFALL FROM 1 TO 4 INCHES ACROSS THE EXTREME NORTHWEST TOWARD
RFD...TO AROUND 6 INCHES FROM NORTHERN COOK COUNTY TO PONTIAC...WITH
MAXIMUM AMOUNTS IN THE 8 TO 11 INCH RANGE ACROSS THE FAR SOUTHEAST
INTO BENTON AND IROQUOIS COUNTIES...WITH A POSSIBILITY OF LOCALLY
HIGHER AMOUNTS CLOSER TO A FOOT GIVEN GREATER POTENTIAL FOR BANDING
IN THIS AREA. IN ADDITION...GUSTY WINDS ON THE BACK SIDE WILL CREATE
SOME BLOWING AND DRIFTING OF THE SNOW INTO FRIDAY MORNING.

VERY LITTLE CHANGE WAS MADE TO THE REMAINDER OF THE FORECAST. MODELS
CONTINUE TO INDICATE SHORT WAVE MOVING ACROSS NORTHERN GREAT LAKES
FRIDAY NIGHT INTO SATURDAY WITH CHANCES OF LIGHT SNOW LIKELY
REMAINING NORTH OF THE AREA. NEXT SYSTEM OF SIGNIFICANCE STILL
APPEARS TO BE IN STORE FOR LATE SUNDAY INTO MONDAY AS NEXT AMPLIFIED
PACIFIC SYSTEM APPROACHES FROM THE CENTRAL PLAINS. STRONG LOW LEVEL
WARM/MOISTURE ADVECTION IS EXPECTED BY SUNDAY NIGHT IN ADVANCE THIS
SYSTEM WITH THERMAL PROFILES PERHAPS EVEN SUGGESTIVE OF A PERIOD OF
SLEET/FREEZING RAIN SUNDAY NIGHT BEFORE TRANSITIONING TO ALL RAIN
MONDAY. WILL NOT INCLUDE ANY SLEET/FREEZING RAIN MENTION AT THIS
POINT GIVEN LIKELY SHORT DURATION IF IT EVEN DOES OCCUR.

MARSILI

&&

.AVIATION...
1118 PM CST

PAIR OF PHASING SHORT WAVES CONTINUE TO MOVE EAST INTO THE REGION
LATE THIS EVENING BUT MODELS DAMP THESE OUT OVERNIGHT SO ONLY
EXPECT CONTINUED INVASION OF MAINLY HI AND MID CLOUDS ACROSS THE
TERMINALS. ANOTHER SHORT WAVE LIFTING NORTHEAST FROM OK AND KS
AHEAD OF THE UPPER LOW OVER THE EASTERN CO-NM BORDER. MODELS LIFT THIS
WAVE TO MO OVERNIGHT AND TO IL BY AFTERNOON AS THE UPPER LOW CONTINUES
TO DIG INTO THE SOUTHERN PLAINS. UVV DEPICTED ACROSS FAR SOUTHERN
WI...NORTHERN IL...NORTHERN IN AND SOUTHWEST LOWER MI BY MODELS
AS WELL AS LIGHT QPF DURING THU AFTERNOON AHEAD OF THE MAIN
SYSTEM. AS COLUMN MOISTENS EXPECT AT LEAST AREAS OF -SN TO BREAK
OUT IN THIS BROAD REGION OF ISENTROPIC LIFT BY LATER AFTERNOON BUT
CONDITIONS ONLY DROPPING DOWN TO MVFR THRU 00Z. RAPID DETERIORATION
EXPECTED DURING THU EVE AS DEEP STRONG UVV SPREADS OVER NORTHEAST
IL AND NW IN. LATEST MODELS STILL DEPICT HEAVIEST SNOWBAND WILL
ALIGN SW TO NE FROM E CENTRAL IL ACROSS N CENTRAL IN INTO
SOUTHERN LOWER MI SO HAVE KEPT CONDITIONS ABOVE VLIFR.

TRS

&&

.MARINE...

330 AM CST

HIGH PRESSURE RIDGE CONTINUES TO MOVE ACROSS SOUTHERN LAKE
MICHIGAN THIS MORNING...AND WILL MOVE ACROSS THE REMAINDER OF THE
LAKE DURING THE DAY ALLOWING WIND SPEEDS TO FURTHER DIMINISH.
STILL A FEW GUSTS INTO THE 25 KNOT RANGE BEING REPORTED ON THE FAR
NORTH END OF THE LAKE EARLY THIS MORNING...WITH AIR TEMPERATURES IN
THE SINGLE DIGITS MAINTAINING FREEZING SPRAY THREAT FOR A FEW MORE
HOURS UNTIL WINDS SUBSIDE THERE. THUS WILL CONTINUE HEAVY FREEZING
SPRAY WARNING TO RUN ITS COURSE THROUGH 7 AM CST. RIDGE THEN LIFTS
NORTHEAST OF THE LAKE TONIGHT...AS LOW PRESSURE DEVELOPS OVER
LOWER MISSISSIPPI VALLEY AND DEEPENS AS IT LIFTS NORTHEAST INTO
OHIO VALLEY OVERNIGHT. THIS WILL TIGHTEN GRADIENT AGAIN
TONIGHT...ESPECIALLY ON SOUTHERN HALF OF THE LAKE...WITH WINDS
EXPECTED TO INCREASE TO 30 KNOTS TONIGHT THROUGH FRIDAY MORNING.
SURFACE LOW PASSES OFF TO THE EAST OF THE LAKE BY LATE
FRIDAY...AS WEAK HIGH PRESSURE RIDGE MOVES IN FROM THE WEST
ALLOWING WINDS TO QUICKLY SUBSIDE. ALTHOUGH ILLINOIS AND INDIANA
NEARSHORE WATERS LARGELY ICE COVERED...VIA MODIS POLAR ORBITING
SATELLITE IMAGES FROM WEDNESDAY AFTERNOON...WITH HOIST SMALL CRAFT
ADVISORY FOR 20-30 KNOT WINDS 00Z-21Z FRIDAY.

RATZER

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...SNOW ADVISORY FROM 6 PM THIS EVENING TO 9 AM CST FRIDAY FOR
     ILZ005-006-011-012-019-020.
     WINTER STORM WARNING FROM 6 PM THIS EVENING TO 6 PM CST FRIDAY
     FOR ILZ013-014-021>023-032-033-039.
IN...WINTER STORM WARNING FROM 6 PM CST /7 PM EST/ THIS EVENING TO 6
     PM CST /7 PM EST/ FRIDAY FOR INZ001-002-010-011-019.
LM...SMALL CRAFT ADVISORY FROM 6 PM THIS EVENING TO 3 PM CST FRIDAY
     FOR LMZ740>745.

&&

WFO LOT

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