FXUS63 KILX 242100
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
300 PM CST THU DEC 24 2009
.DISCUSSION...
ISSUED 300 PM CST THU DEC 24 2009
MAIN FORECAST CONCERNS TODAY REVOLVE AROUND ONGOING MAJOR WINTER
STORM SYSTEM. THESE CONCERNS INCLUDE HOW MUCH RAIN WILL STILL FALL
IN OUR AREA...WHEN PRECIPITATION WILL CHANGE OVER TO SNOW...HOW
MUCH SNOW WILL FALL AS THE SYSTEM VERY SLOWLY PULLS AWAY HEADING
INTO THE WEEKEND...AND HOW WINDY IT WILL BE. 12Z MODELS IN GOOD
OVERALL AGREEMENT WITH ONGOING SYSTEM...SO DO NOT REALLY HAVE A
PREFERRED SOLUTION.
SHORT TERM...TONIGHT THROUGH SUNDAY...WE ARE CURRENTLY SEEING THE
MAIN EVENT WITH RESPECT TO THE STORM IMPACTS OVER CENTRAL AND
SOUTHEAST ILLINOIS. A PLUME OF RICH MOISTURE IS STREAMING NORTH
FROM THE GULF OF MEXICO TO THE EAST OF THE MAIN SYSTEM.
STRONG/DEEP ISENTROPIC ASCENT AND UPPER DIFFLUENCE ARE BOTH
HELPING TO EFFICIENTLY WRING THIS MOISTURE OUT IN THE FORM OF
MODERATE TO HEAVY RAIN. THE SURFACE LOW...CURRENTLY CENTERED OVER
ARKANSAS...WILL LIFT INTO MIDWEST/GREAT LAKES TONIGHT AND LINGER
THROUGH MUCH OF THE WEEKEND. THE STRONG UPPER LOW...CURRENTLY
DRIVING THE SURFACE LOW...WILL LIFT NORTHEAST FROM THE SOUTHERN
PLAINS TONIGHT. THEN...THIS UPPER LOW WILL DUMBBELL AROUND ANOTHER
VIGOROUS WAVE THAT IS STARTING TO DIVE INTO THE NORTHERN PLAINS.
THESE TWO UPPER FEATURES WILL DEVELOP INTO A MUCH LARGER UPPER LOW
THAT WILL BE VERTICALLY STACKED OVER THE SURFACE LOW.
EXPECT RAINS TO CONTINUE THROUGH THE NIGHT...THOUGH THEY SHOULD
BEGIN TO TAPER OFF OVERNIGHT AS THE RICHER MOISTURE PUSHES OFF TO
OUR EAST. FOR THIS REASON...WILL CONTINUE TO LET THE FLOOD WATCH
EXPIRE AT MIDNIGHT. THE DRY SLOT ASSOCIATED WITH THE SOUTHERN
PLAINS WAVE WILL ARRIVE BY CHRISTMAS MORNING AND GREATLY REDUCE
THE MEASURABLE PRECIPITATION. THE COLD AIR THAT IS ASSOCIATED WITH
THIS FEATURE WILL ALSO CHANGE ANY LINGERING RAINFALL OVER TO SNOW
CHRISTMAS MORNING.
ONCE THE SNOW SHOWERS BEGIN CHRISTMAS DAY...THEY WILL BE WITH US
PERIODICALLY THROUGH THE WEEKEND. THE VERTICALLY STACKED SYSTEM
WILL KEEP US IN DEEP CYCLONIC FLOW...WITH OCCASIONAL ENHANCEMENTS
IN LIFT/MOISTURE DEPTH AS IMPULSES ROTATE AROUND THE SYSTEM.
HOWEVER...THERE ARE A COUPLE THINGS GOING AGAINST MUCH IN THE WAY
OF SNOW ACCUMULATION FRIDAY INTO SUNDAY. FIRST...THE MOISTURE DEPTH
IS EXPECTED TO BE SHALLOW FOR MUCH OF THE TIME AND SHOULD NOT
SUPPORT MORE THAN FLURRIES. SECOND...THE AIR MASS OVERHEAD DURING
THE PERIOD WILL BE VERY COLD AND SHOULD RESULT IN SMALL SNOW
FLAKES THAT WILL NOT ACCUMULATE MUCH. SO...HAVE KEPT SNOW CHANCES
NO HIGHER THAN THE CHANCE CATEGORY. HOWEVER...USED SCATTERED
WORDING RATHER THAN CHANCE SINCE IT WILL SNOW...JUST NOT VERY
MUCH.
AS FAR AS WIND CONCERNS...WHILE IT WILL BE WINDY THE NEXT FEW
DAYS...WE SHOULD STAY BELOW ADVISORY CRITERIA. THE STRONGEST WINDS
AT THE MOMENT ARE JUST WEST AND EAST OF THE MAIN SURFACE LOW. BY
THE TIME THE SURFACE LOW GETS HERE BY EARLY FRIDAY...IT WILL BE
STARTING TO OCCLUDE AND RESULT IN WEAKER WINDS ACROSS THE MIDWEST.
LONG TERM...SUNDAY NIGHT THROUGH THURSDAY...MODELS/ENSEMBLES IN
REASONABLE AGREEMENT THROUGH THE PERIOD. THE LARGE UPPER/SURFACE
LOW THAT WILL BE WITH US THROUGH MUCH OF THE WEEKEND WILL FINALLY
DEPART... RESULTING IN MAINLY DRY WEATHER AND MODERATING
TEMPERATURES. THE MEAN UPPER FLOW WILL SPLIT AGAIN AFTER THE
CURRENT SYSTEM DEPARTS...WITH THE MAIN ENERGY IN THE FLOW STAYING
TO OUR NORTH AND SOUTH. HOWEVER...A WEAK IMPULSE...AND ASSOCIATED
RISK OF SNOW...IS EXPECTED TO ARRIVE BY WEDNESDAY/THURSDAY. HAVE
ADDED A SLIGHT CHANCE OF SNOW TO COVER THIS POTENTIAL...THOUGH THE
BETTER PRECIPITATION CHANCES WILL STAY MORE CLOSELY TIED TO TO
THE SOUTHERN STREAM OF THE JET NEAR THE GULF COAST.
BAK
&&
.AVIATION...
ISSUED 1155 AM CST THU DEC 24 2009
EXTENDED LOW LEVEL WIND SHEAR IN THE CENTRAL IL TERMINALS THROUGH
TONIGHT WITH SE WINDS 40 KTS THIS AFTERNOON AT 2K FT AND
INCREASING TO AROUND 50 KTS TONIGHT. SURFACE WINDS ARE ESE 10 TO
20 KTS AND GUSTS OF 20 TO 30 KTS THIS AFTERNOON AND INCREASE TO 20
TO 25 KTS WITH GUSTS OF 25 TO 35 KTS TONIGHT. PERIODS OF LIGHT TO
MODERATE RAIN WITH PERIODS OF HEAVIER RAIN SHOWERS WILL REACH PIA
IN THE NEXT HOUR OR TWO AND CONTINUE THROUGH TONIGHT ACROSS
CENTRAL IL. ISOLATED THUNDERSTORMS POSSIBLE LATE THIS AFTERNOON
AND EVENING BUT COVERAGE TOO LIMITED TO INCLUDE IN THE TAFS. IFR
TO MVFR CEILINGS AND VISIBILITIES WILL PREVAIL THROUGH TONIGHT AND
CEILINGS COULD GET TO LIFR AT TIMES. RAIN TO MIX WITH AND CHANGE
TO LIGHT SNOW LATE FRI MORNING FROM 15Z-18Z (9 AM TO NOON) FROM SW
TO NE. MIDDAY SURFACE MAP SHOWS 994 MB LOW PRESSURE DEEPENING
ALONG THE TX/AR/OK BORDER WITH A FRONTAL BOUNDARY EXTENDING
NORTHWARD ACROSS CENTRAL MO/IA/MN. MODELS DEEPEN LOW PRESSURE NNE
TO 988 MB NEAR THE IL/MO/IA BORDER BY DAWN FRI AND THEN TURNS NW
INTO CENTRAL IA BY FRI AFTERNOON. ITS COLD FRONT WILL PIVOT NE
ACROSS CENTRAL IL DURING FRI MORNING WITH COLDER AIR FILTER IN
FROM THE SW AND CHANGING LIGHT RAIN TO LIGHT SNOW. THE HEAVIER
PRECIPITATION BY MIDDAY FRI WILL BE NE OF CENTRAL IL.
HUETTL
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
FLOOD WATCH UNTIL MIDNIGHT CST TONIGHT FOR ILZ027>031-036>038-
040>057-061>063-066>068-071>073.
&&
$$
