FXUS63 KLOT 230451
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1051 PM CST TUE DEC 22 2009
.DISCUSSION...
747 PM CST
THERE IS SOME RADAR ECHOES SHOWING PRECIPITATION. SOME SURFACE REPORTS
ARE ISOLATED FREEZING DRIZZLE AND SNOW SHOWERS. WE WILL UPDATE THE
FORECAST FOR PATCHY FREEZING DRIZZLE AND SNOW SHOWERS. WE WILL
EXAMINE THE DATA DURING THE NEXT TWO HOURS FOR UPDATES ON THE
ADVISORIES.
&&
.PREV 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...
1049 PM CST
0600 UTC TAFS...NO MAJOR CHANGES ANTICIPATED FOR THE 06Z TAFS
WITH CONTINUED MAIN FORECAST ISSUES BEING MIXED PRECIPITATION
THROUGH THE PERIOD...IMPACTS OF APPROACHING STORM SYSTEM ON
CIGS/VSBYS...AND STRONG EAST WINDS.
00Z RAOB FROM DVN SHOWING A RATHER MESSY SOUNDING IN TERMS OF
PRECIPITATION TYPE WITH DEEP SATURATED LAYER UP TO ABOUT 700 HPA
WHERE TEMPS OF AROUND -8 DEG CELSIUS WERE OBSERVED WHICH
INTRODUCES QUESTION OF ICE AVAILABILITY. THROUGH TIME
TONIGHT...SHOULD SEE CONTINUED MID LEVEL DRY AIR ADVECTION WITH IN
SOUTHWESTERLY MID LEVEL FLOW. THIS SHOULD PROGRESSIVELY TRANSITION
TO WHAT HAS BEEN MAINLY LIGHT SNOW WITH OCCASIONAL FREEZING
DRIZZLE TO MAINLY MORE IN THE WAY FREEZING DRIZZLE OR LIGHT
FREEZING RAIN. WITH MOISTURE BECOMING PROGRESSIVELY MORE SHALLOW
PRECIPITATION AMOUNTS SHOULD BE LIGHT BUT SHOULD FAVOR -FZDZ OR
-FZRA. FAIRLY STRONG DRY EASTERLY LOW LEVEL FLOW DOES APPEAR TO BE
LIMITING POTENTIAL FOR IFR CIGS THIS EVENING...AND FEEL THAT THIS
TREND SHOULD CONTINUE THROUGH THE OVERNIGHT HOURS AND WILL LIKELY
STAY WITH MORE MVFR CIGS OVERNIGHT.
MAIN UPPER LOW WILL APPROACH THE SOUTHERN PLAINS TOMORROW EVENING
WITH SFC CYCLONGENESIS BEGINNING TO RAMP UP. AS THIS OCCURS SHOULD
BEGIN TO SEE LOW LEVEL WINDS VEER SLIGHTLY TO THE SOUTHEAST THAT
SHOULD ALLOW FOR MORE SUBSTANTIAL LOW LEVEL MOISTURE ADVECTION.
FORECAST SOUNDINGS CONTINUE TO SUGGEST A POTENTIAL FOR FREEZING
RAIN WEDNESDAY MORNING WITH BEST CHANCE WEDNESDAY MORNING ACROSS
RFD WHERE MOISTURE ADVECTION WILL BE STRONGER. BETTER CHANCE OF
PRECIP WILL BE WEDNESDAY AFTERNOON/WEDNESDAY NIGHT AND FOR
REMAINDER OF TERMINALS...00Z DATA STILL SUGGESTS THAT A FREEZING
RAIN/SLEET MIX STILL THE MOST LIKELY. SOME CONCERNS ABOUT
LIQUID/FREEZING PRECIP TYPE THAT WILL NEED TO BE CONSIDERED BUT
FOR NOW AM HEDGING ON IDEA OF RECENT NEW SNOW COVER HOLDING SFC
TEMPS BELOW FREEZING. DID TRANSITION BACK TO IFR CONDITIONS BY
LATE AFTERNOON WITH BETTER MOISTURE ADVECTION. EAST WINDS GUSTING
TO 20 KNOTS TONIGHT WILL INCREASE TO AROUND 25 KNOTS TOMORROW WITH
STRENGTHENING PRESSURE GRADIENT NORTH OF DEVELOPING SOUTHERN
PLAINS SFC TROUGH/LOW.
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 UNTIL 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 UNTIL 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...ILLINOIS NEARSHORE WATERS UNTIL 4 AM THURSDAY.
SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...3 AM WEDNESDAY TO
4 AM THURSDAY.
&&
$$
