FXUS63 KLOT 261812
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1212 PM CST SAT DEC 26 2009

.DISCUSSION...
1011 AM CST

LARGE CUT-OFF UPPER LOW CONTINUES TO SPIN OVER THE WESTERN GREAT
LAKES AND UPPER MISSISSIPPI VALLEY WITH SMALLER SCALE VORTICITY
MAXIMA PIVOTING AROUND THE LARGER SCALE CIRCULATION. ONE SUCH
FEATURE WAS NOTED ON WATER VAPOR IMAGERY THIS MORNING OVER
SOUTHERN IOWA WITH BACKING MID LEVEL FLOW IN ADVANCE OF THIS
FEATURE RESULTING IN ENHANCED ISENTROPIC ASCENT AND MID-LEVEL
FRONTOGENESIS OVER NORTHERN ILLINOIS. ALREADY HAVE A FAIRLY WELL
DEVELOPED EAST-WEST SWATH OF LIGHT TO OCCASIONALLY MODERATE SNOW
ACROSS NORTHERN ILLINOIS AND SHORT RANGE NUMERICAL GUIDANCE WOULD
SUGGEST THAT THIS SHOULD PERSIST THROUGH MUCH OF THE
AFTERNOON...POSSIBLY PIVOTING A BIT AND BECOMING MORE ENE-WSW
ORIENTED WITH TIME. 

FORECAST AND OBSERVED SOUNDINGS SHOW THE PRESENCE OF A DEEP
DENDRITIC GROWTH ZONE WITH TEMPS NEARLY ISOTHERMAL AROUND -18C
FROM 800MB ALL THE WAY UP TO 550MB. ALREADY HAVE BEEN OBSERVING
DENDRITES HERE AT KLOT WITH LIGHT AND FLUFFY SNOW WITH HIGH
SNOW:LIQUID RATIOS EXPECTED TODAY. SO DESPITE QPF OF ONLY AROUND
0.15" THOUGH THIS EVENING...THEN EXPECTED 20-30:1 SNOW:LIQUID
RATIOS SHOULD STILL RESULT IN THIS SNOW FLUFFING UP TO 3-5 INCHES
ACROSS MUCH OF THE ADVISORY AREA. 

STRONGEST LIFT WITH THIS VORT IS EXPECTED TO MOVE EAST/NORTHEAST
OF THE AREA BY LATE AFTERNOON OR EARLY EVENING RESULTING IN A
TAPERING OFF OF THE SNOWFALL. 

ANOTHER VORT IS FORECAST TO PIVOT AROUND THE HUGE ATMOSPHERIC
GYRE TOMORROW AND COULD BRING ANOTHER ROUND OF ENHANCED
SNOWFALL...WITH TOMORROW'S SNOW LOOKING TO TAKE AIM MORE ON OUR
SOUTHERN COUNTIES. MORE ON THAT LATER...

UPDATED GRIDS/DERIVED TEXT PRODUCTS HAVE BEEN SHIPPED ALREADY.

IZZI

&&

.AVIATION...
1210 PM CST

1800 UTC TAFS...WATER VAPOR SATELLITE LOOP SHOWS MAIN UPPER
CIRCULATION CENTER OVER SOUTHERN WI WITH MULTIPLE IMPULSES
ROTATING AROUND IT. ONE SIGNIFICANT VORT MAX OVER THE IA-MO BORDER
IN UPPER TROF THE EXTENDS FROM THE MAIN LOW CENTER SW TO THE TX
PANHANDLE. AXIS OF THIS TROF TO SWING E OVER IL DURING THE
AFTERNOON SO EXPECT SN TO CONTINUE THRU THE AFTERNOON.
HOWEVER...AS SFC AND H8 LOWS OVER SE IA CONTINUE MOVING TO THE NE
AND REACH SE WI BY 00Z THE LOW LEVEL WAA AND UVV MOVES N AND NE OF
THE TERMINALS AND INTENSITY OF SN TO DIMINISH AS A RESULT. AFTER
TROF AXIS PASSES BY SHOULD SEE ONLY NON-ACCUMULATING -SN THIS EVE
AND INTO FIRST PART OF OVERNIGHT.

CURRENTLY CLR TO SCT CLOUDS ACROSS MUCH OF WI...SE MN AND IA WITH
MODELS TRYING TO BRING THIS DRYING FURTHER EASTWARD THIS EVENING
THO WITH UPPER LOW PROGGED TO MAKE A CYCLONIC LOOP OVER SOUTHERN
AND CENTRAL WI BETWEEN THIS AFTERNOON AND SUN MORNING AND EXPECT
AT LEAST BKN STRATOCU TO REMAIN ACROSS THE LOCAL TERMINALS INTO
THE NIGHT BEFORE STRONG PVA PUSHES SOUTHEASTWARD OVER NORTHERN IL DURING
THE EARLY MORNING HRS OF SUN. MODELS SHOW MID LEVELS SOMEWHAT
DRY AT THAT TIME...AND THEN TO HAVE DRIED MORE ACROSS THE
TERMINALS BY 15Z SUN AS UPPER TROF AXIS ONCE AGAIN MOVES ACROSS
NORTHERN IL. EXPECT PD OF LIGHTER ACCUMULATING SNOW AS PVA WILL
ONLY HAVE LIMITED MOISTURE IN MID LEVELS TO WORK WITH. 

TRS

&&

.MARINE...
327 AM CST

EVOLUTION OF WEAKENING SFC LOW ACROSS IOWA OVER NEXT 36 
HOURS AND ARCTIC FRONT APPROACHING THE REGION EARLY MONDAY BRINGING 
STRONGER WINDS WILL BE THE MAIN FORECAST ISSUES THIS MORNING.

SURFACE LOW ACROSS SOUTHERN IOWA CONTINUES TO FILL THIS MORNING AND 
WILL BE APPROACHING CENTRAL PORTIONS OF LAKE MICHIGAN LATE THIS 
EVENING. SOUTH WINDS WILL CONTINUE AND OBS INDICATING GUSTS TO 
AROUND 25 KNOTS CONTINUE ACROSS SOUTHERN LAKE MICHIGAN THIS MORNING. 
AS LOW CONTINUES TO FILL AND SFC PRESSURE GRADIENT SLACKENS OFF 
LATER THIS MORNING SHOULD SEE GUSTS OF THIS MAGNITUDE BEGIN TO 
SUBSIDE BACK INTO THE 15 TO 20 KT RANGE ACROSS SOUTHERN PART OF THE 
LAKE. LOW VERY SLOW TO DEPART CENTRAL/NORTHERN LAKE MICHIGAN 
SUNDAY...AND FOCUS FOR SUNDAY NIGHT WILL BE APPROACH OF ANOTHER 
TROUGH ROTATING THROUGH BACKSIDE OF THIS LOW BRINGING SHOT OF STRONG 
COLD AIR ADVECTION. SOME CONCERN FOR GALES DURING THIS PERIOD GIVEN 
DEEP MIXING AND PRESENCE OF 40 KNOT 850 HPA FLOW. AT THIS TIME 
BELIEVE NORTHWEST GALES WOULD REPRESENT JUST THE HIGHER END GUSTS 
AND DO NOT HAVE ENOUGH CONFIDENCE TO GO WITH A GALE WATCH AT THIS 
TIME FOR MONDAY...BUT WILL CONTINUE TO MENTION IN GLF MAINLY ACROSS 
NORTHERN HALF OF LAKE WHERE MIXING HEIGHTS WILL BE DEEPER. WINDS 
THEN BEGIN TO SUBSIDE MONDAY NIGHT AND TUESDAY AS STRONG HIGH 
PRESSURE BEGINS TO BUILD IN FROM THE WEST.

MARSILI

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...WINTER WEATHER ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-
     ILZ010-ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022 UNTIL 9 PM SATURDAY.

IN...WINTER WEATHER ADVISORY...INZ001 UNTIL 9 PM SATURDAY.

LM...NONE.
&&

$$