FXUS63 KLOT 231904
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
104 PM CST WED DEC 23 2009
.DISCUSSION...
932 AM CST
.NEAR TERM UPDATE...
VERY COMPLEX SCENARIO BEGINNING TO PLAY OUT ACROSS THE CWA THIS
MORNING...WITH CONTINUED LOW CONFIDENCE REGARDING EVOLUTION OF
PRECIPITATION TYPE...AND ASSOCIATED ICING POTENTIAL...TODAY.
HOURLY SURFACE ANALYSIS FROM 12-14Z INDICATE THAT FREEZING LINE
SITUATED ACROSS FAR SOUTHERN CWA...ROUGHLY ACROSS SOUTHERN THIRD OF
LASALLE...LIVINGSTON...FORD...AND IROQUOIS COUNTIES IN
ILLINOIS...AND FAR SOUTHERN NEWTON AND JASPER COUNTIES IN INDIANA.
THIS LINE HAS BEEN ESSENTIALLY STATIONARY SINCE ABOUT 03Z LAST
NIGHT...AND REAL QUESTION IS EVOLUTION NORTH TODAY. SREF AND NAM12
SHORT TERM MODEL GUIDANCE IS IN GENERAL AGREEMENT IN A VERY SLOW
TRANSITION NORTH TOWARD THE I-80 AREA AND CITY OF CHICAGO BY LATE THIS
AFTERNOON...WITH THE RUC HOLDING IT NEAR ITS CURRENT LOCATION
THROUGH 00Z TONIGHT. CONTINUED WARM AIR ADVECTION ALOFT AND PERHAPS
SLIGHT DIURNAL INCREASE IN TEMPERATURE WOULD FAVOR THIS TRANSITION
TOWARD THE NORTH...BUT REAL FLY IN THE OINTMENT IS DEWPOINT
DEPRESSIONS NORTH OF THE FREEZING LINE...WITH VALUES ANYWHERE FROM
2-4 DEGREES. WITH EAST FLOW IN PLACE...NO REAL CHANCE OF ADVECTING
IN ANY MOISTURE...WITH SATURATION HAVING TO OCCUR THROUGH
EVAPORATION OF PRECIP AS IT MOVES NORTH...AND THUS THE POTENTIAL OR
A DEGREE OR TWO OF EVAPORATIVE COOLING AS THIS TAKES PLACE.
ALSO OF NOTE FOR LAKE AND COOK COUNTIES IN ILLINOIS...LAKE INFLUENCE
APPEARS TO BE TAKING PLACE ACROSS GENERALLY EASTERN THIRD OF BOTH
COUNTIES...WITH SURFACE TEMPERATURES RISING ABOVE FREEZING WITH
SLIGHTLY WARMER AIR ADVECTING INLAND FROM THE LAKE.
UNFORTUNATELY...APPEARS THAT LAKE INFLUENCE NOT MAKING IT ACROSS
ENTIRE PORTION OF LAKE AND COOK COUNTIES...LEAVING SOME CONCERN
ABOUT COOL TEMPERATURES AND ASSOCIATED PRECIP TYPE ACROSS WESTERN
PORTIONS OF THESE TWO COUNTIES.
WITH ALL THIS IN MIND...HAVE ELECTED TO ISSUE WINTER WEATHER
ADVISORY FOR AREAS THAT AREN'T UNDER ONE ALREADY. EXP TIME IS FOR 3
PM...THOUGH ANTICIPATE BEING ABLE TO TRIM OFF COUNTIES TO THE SOUTH
AS TEMPS GRADUALLY WARM ABOVE FREEZING.
BOXELL
&&
.AVIATION...
103 PM CST
EXPECT .15 TO .20 QPF (LIQUID EQUIVALENT) THROUGH 00Z AT ORD AND
MDW TERMINALS. ICE ACCUMULATION ON UNTREATED SURFACES OF ONE TENTH
OF AN INCH.
PREVS DISCUSSION...
1800 UTC TAFS...LEADING EDGE OF PRIMARY PRECIPITATION AREA FINALLY
SPREADING NORTH ACROSS ORD AND MDW AND RFD TERMINALS PAST HOUR
PRETTY MUCH RIGHT ON TARGET WITH GOING PREVS FCST TIMING. PRECIP
TYPE WILL REMAIN A VERY CHALLENGING FCST THIS AFTERNOON AS ACCARS
AND MODEL FORECAST SOUNDINGS DEPICT COMPLEX LOW-MID LEVEL THERMAL
PROFILES THAT WILL BE IMPACTED THROUGHOUT THE AFTERNOON BOTH
THERMODYNAMICALLY AS A RESULT OF FALLING PRECIPITATION AS WELL AS
WARMER AND MORE MOIST AIR VERY SLOWLY ADVECTING NORTHWARD INTO THE
REGION.
INITIALLY EXPECT PRIMARILY LIGHT PL WITH SOME FZRA TO PERSIST
WITH SURFACE BASED COLD LAYER BELOW FREEZING AND WARM LAYER ALOFT
NOT SUFFICIENTLY DEEP TO COMPLETELY MELT MOST HYDRO-METEORS. AS
HIGHER INTENSITY PRECIPITATION SPREADS NORTHWARD INTO THE REGION
THIS AFTERNOON... LOW LEVELS OF THE ATMOSPHERE MAY DYNAMICALLY
COOL ENOUGH TO SWITCH PRECIP OVER TO MORE SNOW AS EVIDENCED BY SFC
OB FROM PNT AT 1705Z. THUS COULD SEE MINOR ACCUMULATIONS OF A HALF
INCH OF SLUSHY SNOW AND SLEET THROUGH THE AFTERNOON INTO THE
EVENING ESPECIALLY FARTHER NORTH (ORD AND DPA).
CIGS WILL GRADUALLY LOWER TO IFR BY 20-21Z AT ORD/MDW/RFD AS LOW
LEVELS CONTINUE TO SATURATE AND PCPN CONTINUES. MAY SEE
OCCASIONALLY LOWER VSBYS BRIEFLY WITH MORE HIGHER RADAR
REFLECTIVITY PCPN PATCHES (SN/PL/FZRA) LIFTING NORTHWARD.
TIMING OF SWITCH OVER TO RAIN WILL HINGE ON WARM LOW LEVELS
BECOMING SUFFICIENTLY DEEP ENOUGH ALONG WITH SURFACE TEMPS
CLIMBING ABOVE 32 DEG F. THIS WILL LIKELY FIRST OCCUR ALONG THE
I-55 AND I-57 CORRIDOR MOVING NORTHWARD... AS WELL AS THE
IMMEDIATE CITY OF CHICAGO AND AREAS DOWNSTREAM INCLUDING ORD AND
MDW. THIS DUE TO SLIGHT MODIFYING INFLUENCES OF BOTH LAKE MICHIGAN
(WITH SURFACE WATER TEMPS IN THE UPPER 30S OFF SHORE) AND THE
HIGHLY URBANIZED AREAS SERVING AS A HEAT ISLAND EFFECT (PERHAPS IF
EVEN A HALF TO THREE QUARTERS DEGREE) SHOULD BE JUST ENOUGH IN
THIS EVENT TO WARM SURFACE BASED TEMPS TO THE MAGIC 32 THRESHOLD
SOONER THAN OTHERWISE WOULD BE THE CASE AS THE WARMER AIR ALOFT
WORKS NORTH.
THUS EXPECT SWITCH OVER TO ALL RAIN TO OCCUR DURING THE 02-06Z
TIME FRAME BASED ON EXTRAPOLATION TECHNIQUES... MODEL FORECAST
SOUNDINGS AS WELL AS SHORT RANGE ENSEMBLE OUTPUT (SREF). INTERESTINGLY
THE 09Z SREF OUTPUT REFLECTS AFOREMENTIONED WARMING TO 32 SOONER
OVER CHICAGO AS A TONGUE OR BULLSEYE IN THE PRECIP TYPE DATA AS
WELL AS LOW LEVEL TEMPS.
ED F
&&
.MARINE...
220 AM...HIGH END GALES LIKELY THURSDAY AND THURSDAY NIGHT.
OVERALL...NO MAJOR CHANGES TO GOING GRIDS/FORECASTS. MAIN CHANGE
THIS MORNING IS TO UPGRADE THE CURRENT GALE WATCH TO A GALE
WARNING. EASTERLY GRADIENT IS BEGINNING TO TIGHTEN THIS MORNING AS
THE HIGH OVER ONTARIO STRENGTHENS AND THE LOW OVER THE SOUTHERN
PLAINS BEGINS TO DEEPEN. SEVERAL MARINE LOCATIONS NOW GUSTING
20-25KTS AND AS THE GRADIENT CONTINUES TO TIGHTEN...WINDS WILL
STEADILY INCREASE WITH GUSTS TO 30KTS TODAY WITH GALES DEVELOPING
LATE TONIGHT INTO THURSDAY MORNING. STRONGEST GRADIENT MOVES
ACROSS SOUTHERN LAKE MICHIGAN THURSDAY AFTERNOON INTO THURSDAY
EVENING AND CURRENT FORECAST OF GALES TO 45KTS DURING THIS TIME
LOOKS ON TRACK. WITH THE LOW OVER THE SOUTHERN PLAINS LIFTING
NORTH TO EASTERN IOWA AND DEEPENING TO AT LEAST 29.2 INCHES...
SOME STORM FORCE GUSTS CAN/T BE RULED OUT...BUT NOT EXPECTING
PREVAILING/FREQUENT STORM FORCE GUSTS AT THIS TIME. THE LOW WILL
BEGIN TO WEAKEN FRIDAY INTO FRIDAY NIGHT...BUT REMAIN NEARLY
STATIONARY OVER IOWA/NORTHERN ILLINOIS. CMS
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 3 PM WEDNESDAY.
FLOOD WATCH...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-
ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-ILZ023-
ILZ032-ILZ033-ILZ039...NOON THURSDAY TO 6 AM FRIDAY.
WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
ILZ010-ILZ011-ILZ012 UNTIL NOON THURSDAY.
IN...FLOOD WATCH...INZ001-INZ002-INZ010-INZ011-INZ019...NOON THURSDAY
TO 6 AM FRIDAY.
WINTER WEATHER ADVISORY...INZ001 UNTIL 3 PM WEDNESDAY.
WINTER WEATHER ADVISORY...INZ002-INZ010-INZ011 UNTIL 6 PM
WEDNESDAY.
LM...GALE WARNING...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
LMZ874-LMZ876-LMZ878...4 AM THURSDAY TO NOON FRIDAY.
GALE WARNING...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...4 PM THURSDAY TO NOON FRIDAY.
SMALL CRAFT ADVISORY...NEARSHORE WATERS
UNTIL 4 AM THURSDAY.
&&
$$
