FXUS63 KLOT 230038
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
638 PM CST TUE DEC 22 2009
.DISCUSSION...
350 PM CST
.NEAR TERM (THIS EVENING THROUGH WEDNESDAY MORNING)...
AFTERNOON SNOW CONTINUES TO FALL IN A LIGHT FASHION ACROSS PORTIONS
OF THE AREA PRIMARILY ALONG AND NORTHEAST OF A ROCKFORD TO JOLIET TO
KOUTS LINE. THIS SHOULD CONTINUE TO DRIFT NORTHEAST AND GRADUALLY
DECREASE IN INTENSITY THIS EVENING.
ATTENTION THEN TURNS TO CHANCES FOR FREEZING DRIZZLE TONIGHT ACROSS
MAINLY AREAS ALONG AND TO THE SOUTH OF I-80. MODEL SOUNDINGS
INDICATE SHALLOW SATURATED LAYER AND LAYER OF WEAK RISING MOTION IN
ASSOCIATION WITH WARM AIR ADVECTION AT AND AROUND 850 MB ACROSS THIS
AREA TONIGHT. SOUNDINGS ALSO INDICATE LAYER OF VERY DRY AIR ABOVE
THIS SATURATED LAYER...LIKELY THE RESULT OF LARGE SCALE SUBSIDENCE
IN ASSOCIATION WITH RISING HEIGHTS IN THE MID LEVELS OF THE
ATMOSPHERE. WITH THIS PROFILE IN MIND....ALONG WITH SURFACE TEMPS
AT OR BELOW FREEZING LATE THIS EVENING...THINK AREAS OF FREEZING
DRIZZLE ARE A PRETTY GOOD BET ACROSS THIS AREA. AFTER COORDINATION
WITH NEIGHBORING OFFICES...WINTER WEATHER ADVISORY FOR FREEZING
DRIZZLE WILL BE IN EFFECT FROM 03Z TONIGHT UNTIL 18Z ON WEDNESDAY.
AT THIS POINT...I CERTAINLY COULDNT RULE OUT FREEZING DRIZZLE OR
LIGHT
FREEZING RAIN LASTING BEYOND 18Z FOR THE NORTHERN THIRD OR SO OF THE
ADVISORY...BUT CONSIDERING THAT WINTER WEATHER WATCH GOES INTO
EFFECT FOR THOSE AREAS STARTING AT 18Z...SEEMS LIKE PRUDENT THING TO
DO IS SAVE THE FINE TUNING FOR THIS EVENING AND TONIGHTS
SHIFTS...AND NOT COMPLICATE HEADLINES ANY FURTHER.
BOXELL
.WINTER STORM (WEDNESDAY THROUGH FRIDAY)...
HAVE ISSUED WINTER STORM WATCH FOR ALL OF CWA EXCEPT SOUTHERN TIER
OF COUNTIES FROM 18Z WED THROUGH 18Z THURSDAY FOR POTENTIAL FOR ICE
ACCUMULATION FROM FREEZING RAIN.
MODELS HAVE CONTINUED TO TREND FARTHER TO THE WEST OVER THE PAST
SEVERAL RUNS...TAKING SURFACE LOW UP MISSISSIPPI VALLEY AND ACROSS
EASTERN IOWA THROUGH CHRISTMAS MORNING. WHILE THIS KEEPS THE THREAT
OF HEAVY SNOW WELL WEST OF THE CWA...IT DOES PRESENT POTENTIAL FOR
MIXED PRECIPITATION EVENT WEDNESDAY INTO EARLY THURSDAY AS VERY WARM
MOIST AIR SPREADS NORTHWARD ATOP SFC BASED COOL AIR MASS ASSOCIATED
WITH RETREATING HIGH PRESSURE OVER ONTARIO/GREAT LAKES REGION.
AS NOTED IN NEAR TERM DISCUSSION ABOVE...SPOTTY LIGHT PRECIP MAY BE
ONGOING WEDNESDAY MORNING IN WEAK BUT PERSISTENT WARM ADVECTION
REGIME ACROSS LOW LEVEL BAROCLINIC ZONE CURRENTLY DRAPED ACROSS THE
REGION ALONG THE SOUTHERN BOUNDARY OF COOLER AIRMASS. FORECAST
SOUNDINGS ACROSS THE FORECAST AREA INDICATE SATURATED LOW LEVELS
WITH SURFACE TEMPS AT OR BELOW FREEZING WEDNESDAY MORNING...WITH
LACK OF MID LEVEL MOISTURE SUGGESTING FREEZING DRIZZLE SETUP EARLY
ON. AS DEEPER SATURATION DEVELOPS DURING THE DAY...EXPECT PRECIP TO
SHIFT TO A MIX OF SNOW...SLEET AND FREEZING RAIN...WITH MORE SNOW
AND SLEET NORTH...AND MORE FREEZING RAIN SOUTH. APPEARS THAT FAR
SOUTHERN COUNTIES OF FORECAST AREA WILL WARM ABOVE FREEZING DURING
THE LATE MORNING/AFTERNOON...THUS LEAVING THEM FROM INCLUSION IN
WATCH.
GREATEST THREAT OF FREEZING RAIN LOOKS TO BE LATE
WEDNESDAY/WEDNESDAY NIGHT AS STRONG WARM/MOIST ADVECTION RESULTS IN
STRENGTHENING OF ELEVATED WARM LAYER...AND SIGNIFICANT QPF IN THE
0.75 TO 1.00 INCH RANGE ALONG LOW LEVEL FREEZING/NON-FREEZING
INTERFACE. WARMING OF LOW LEVELS OCCURRING WHERE QPF GREATEST...
MAKING SOUTHERN PERIPHERY OF FREEZING RAIN AREA SOMEWHAT
TRICKY...THOUGH POTENTIAL FOR 1/4 INCH OR MORE ICE ACCUMULATION
VERY REAL IN SOME AXIS FROM SQI/RPJ SOUTHEAST TO IKK AREA. GUIDANCE
IN VERY GOOD AGREEMENT WITH EVENTUALLY LIFTING ABOVE FREEZING
SURFACE TEMPS NORTHWARD ACROSS THE AREA THURSDAY MORNING...ALLOWING
CHANGE FROM FREEZING RAIN TO JUST RAIN. RAIN AND ABOVE FREEZING
TEMPS SHOULD MAINTAIN WET ROADS AND BETTER TRAVEL CONDITIONS ACROSS
NORTHERN IL AND NORTHWEST INDIANA CHRISTMAS EVE INTO CHRISTMAS
MORNING. DRY SLOT MAY WORK IN ESPECIALLY ACROSS EASTERN COUNTIES
LATE THURSDAY NIGHT/EARLY FRIDAY WITH A PERIOD OF DRIZZLE CHRISTMAS
MORNING. LOW THEN LIFTS TO THE NORTH FRIDAY...AND COLDER AIR STARTS
TO WRAP BACK IN FROM THE WEST DURING THE MORNING. THIS WILL HERALD
A SHIFT BACK OVER TO SOME LIGHT SNOW OR SNOW SHOWERS WITH PERHAPS A
LITTLE LIGHT ACCUMULATION ON CHRISTMAS AFTERNOON.
RATZER
.POST WINTER STORM (FRIDAY NIGHT THROUGH NEXT TUESDAY)...
FORECAST GETS QUITE A BIT EASIER BY FRIDAY EVENING...WITH PRECIP
HAVING CHANGED BACK TO ALL SNOW. APPEARS THAT REMAINS OF SURFACE
AND UPPER LOW WILL SPIN ON TOP OF US THROUGH THE WEEKEND...AND WITH
THAT IN MIND...DO THINK THAT SCATTERED SNOW SHOWERS ARE A DECENT BET
THROUGH SUNDAY. HIGH PRESSURE THEN BUILDS IN EAST FROM THE PLAINS
ON MONDAY...GIVING US WHAT SHOULD BE A COOL BUT NICE...AND CERTAINLY
MUCH MORE CALM...BEGINNING OF THE POST-CHRISTMAS WORKWEEK.
BOXELL
&&
.AVIATION...
638 PM CST
0000 UTC TAFS...OVERALL VERY LITTLE CHANGE FOR THE 00Z TAFS
COMPARED TO EARLIER THINKING WITH MAIN FORECAST FOCUS CONTINUING TO
CENTER ON COMPLEX PRECIPITATION TYPE SCENARIO OVER NEXT 36 HOURS
AND POTENTIAL OF IFR CIGS/VSBYS THROUGH THE PERIOD.
BROAD ISENTROPIC UPGLIDE CONTINUES TO SUPPORT LIGHT PRECIPITATION
ACROSS THE REGION WITH MAIN AREA OF -SN LIFTING ACROSS NORTHWEST
AND NORTHERN SUBURBS OF CHICAGO. MID LEVEL SATURATION COMES TO
QUESTION THIS EVENING INTRODUCING QUESTION OF ICE INTRODUCTION.
COULD HAVE SOME PATCHY FREEZING DRIZZLE MIX IN WITH THE SNOW DUE
TO MARGINAL THERMODYNAMIC PROFILES. WHILE CANNOT DISCOUNT
CONTINUATION OF SOME LIGHT PRECIP AFTER 02Z...NOT MUCH TO LATCH
ONTO IN TERMS OF CONTINUED PRECIP MENTION...ALTHOUGH BROAD
ISENTROPIC UPGLIDE WILL CONTINUE. DO HAVE A BIT MORE CONFIDENCE IN
MORE MENTIONABLE PRECIP LATE TONIGHT IN THE 08Z TO 12Z TIMEFRAME
AS STRONGER LOW FLOW IMPINGES ON PREEXISTING LOW LEVEL BOUNDARY.
AGAIN PRECIP TYPE IS QUESTIONABLE...BUT FORECAST SOUNDINGS SUGGEST
ENOUGH MID LEVEL SATURATION TO KEEP PRECIP TYPE AS -SN FOR NOW.
MAIN UPPER TROUGH WILL CONTINUE TO DIG INTO CENTRAL/SOUTHERN
PLAINS WEDNESDAY SETTING UP STRENGTHENING LOW LEVEL MOISTURE
TRANSPORT. AN UPPER JET STREAK ALSO EXPECTED TO ROUND BASE OF THIS
TROUGH AND NOSE INTO MID MS RIVER VALLEY LATE WEDNESDAY. WEAKER
LIFT WEDNESDAY MORNING MAY PROMOTE PERIOD OF FZDZ EARLY WEDNESDAY
BEFORE STEADIER MIX OF FREEZING RAIN...SLEET...AND SNOW MOVES INTO
THE REGION LATE IN THE AFTERNOON. ONLY CHANGE TO PREVIOUS TAFS WAS
TO INDICATE MORE OF A SNPL MIX AS OPPOSED TO FZRA THROUGH MID
AFTERNOON DUE TO LIMITED MAGNITUDE OF WARM LAYER. BY LATE
AFTERNOON...WARM LAYER SHOULD BE STRONG ENOUGH TO SUPPORT MORE OF
FZRA THREAT.
IN TERMS OF CIGS/VSBYS...DID CONTINUE IDEA OF IFR CIGS THROUGH
MUCH OF THE PERIOD WITH OCCASIONAL IFR VSBYS IN HEAVIER PRECIP
PERIODS LATE WEDNESDAY AFTERNOON/EVENING.
MARSILI
&&
.MARINE...
213 PM CST
SIGNIFICANT WINTER STORM SYSTEM COMING TOGETHER IN THE DESERT SW
DEEPEN AS IT CROSSES THE SOUTHERN ROCKIES OVERNIGHT AND MOVES OUT
OVER THE SOUTHERN PLAINS DURING THE DAY WED. MEANWHILE...HIGH
PRES CENTERED OVER ONTARIO WITH A RIDGE EXTENDING SW ACROSS THE
UPPER OH VALLEY WILL CONTINUE TO BUILD.
INCREASING PRES GRADIENT WILL RESULT IN GRADUAL INCREASE IN
EASTERLY WINDS TONIGHT THROUGH THU NIGHT...FIRST ACROSS THE S
PORTION AND THEN SPREADING UP THE LAKE AS THE LOW CONTINUES TO
DEEPEN AND TURN NE THEN NORTHWARD. GOOD POTENTIAL FOR GALES ON THE
S BY OVERNIGHT WED/PRE-DAWN THU AND ON THE N BY MID THU AFTERNOON.
WINDS WILL EVENTUALLY BACK TO OUT OF THE SE AND S AS THE LOW LIFTS
N TO THE UPPER MS VALLEY BY LATER FRI. LOW TO BE FILLING SOMEWHAT
BY THIS TIME AND WHILE CANADIAN ANTICYCLONE CONTINUES TO BUILD IT
MOVES OFF TO ENE ACROSS QUEBEC WITH THE RESULT BEING THE TIGHTER
PRE GRADIENT SHIFTS TO THE LOWER LAKES AND UPPER ST. LAWRENCE
VALLEY AND WINDS ON LK MI EXPECTED TO DIMINISH.
TRS
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ019-ILZ021-ILZ023-ILZ032-ILZ033-
ILZ039...9 PM TUESDAY TO NOON WEDNESDAY.
WINTER STORM WATCH...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
ILZ023...NOON WEDNESDAY TO NOON THURSDAY.
IN...WINTER STORM WATCH...INZ001-INZ002-INZ010-INZ011...NOON
WEDNESDAY TO NOON THURSDAY.
WINTER WEATHER ADVISORY...INZ010-INZ011-INZ019...9 PM TUESDAY TO
NOON WEDNESDAY.
LM...GALE WATCH...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 WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...4 PM THURSDAY TO NOON FRIDAY.
SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
LMZ745...MIDNIGHT WEDNESDAY TO 4 AM THURSDAY.
&&
$$
