000
FXUS63 KILX 272315
AFDILX
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LINCOLN IL
515 PM CST MON DEC 27 2010
.DISCUSSION...
ISSUED 249 PM CST MON DEC 27 2010
INITIAL CONCERN ONCE AGAIN ENCOMPASSES CLOUDS/TEMPS/FOG TONIGHT.
THEN FOCUS TURNS TO THE WARMING TREND/FOG AND THE EXTENDED PERIOD OF
RAIN CHANCES FOR THE LAST HALF OF THE WEEK...AS A LONG WAVE TROUGH
DEVELOPS ACROSS THE PLAINS. THE SUBSEQUENT COLD FRONTAL PASSAGE THIS
WEEKEND MAY BE PRECEDED BY SOME THUNDERSTORMS. THE RAPID MELTING OF
OUR SNOW AND SUBSEQUENT RAIN MAY CREATE SOME FLOODING ISSUES LATER
THIS WEEK.
SHORT TERM...TONIGHT THROUGH TUESDAY NIGHT.
CLOUDS LAST NIGHT KEPT LOWS WELL ABOVE FCST IN MOST AREAS...EXCEPT
FOR AROUND CMI WHERE LATE CLEARING ALLOWED TEMPS TO PLUMMET BELOW
FCST LOWS. SOME AREAS LAST NIGHT SAW LOCALLY DENSE FOG DEVELOP...
WITH A THIN LAYER OF ICE DEPOSITING ON SOME UNTREATED AND ELEVATED
SURFACES DUE TO THE FOG. THE FOG EVEN FORMED UNDER STRATUS
DECKS...INSTEAD OF ONLY IN AREAS THAT CLEARED.
WITH THAT IN MIND...AND THE RIDGE AXIS REMAINING CLOSE BY JUST EAST
OF OUR CWA BY TUES MORNING...WE MAY SEE FOG FORMING AGAIN TONIGHT...
IR-REGARDLESS OF HOW THE CLOUDS PROGRESS. SATELLITE TRENDS AND
FORECAST SOUNDINGS INDICATE WE SHOULD SEE PROGRESSIVE CLEARING BY
THIS EVENING. CLOUDS ARE ERODING AS THE AFTERNOON MIXING OF DRY AIR
FROM ALOFT INTENSIFIES.
IN CLEAR AREAS THIS EVENING...TEMPS WILL QUICKLY DROP OFF AFTER
SUNSET...VERY CLOSE TO CURRENT DEWPOINTS...PUSHING RH LEVELS UP
ABOVE 85-90%. WILL TRY TO PUT LOWS CLOSER TO COLDER GFS/MAV
GUIDANCE...WITH THE EXPECTED CLEARING THIS EVENING.
FOG MAY BEGIN DEVELOPING BY MIDNIGHT UNDER CLEAR SKIES. HAVE ADDED
FOG TO THE GRIDS AT LEAST AFTER MIDNIGHT...BUT THAT MAY NEED TO BE
ADJUSTED BASED ON TRENDS THIS EVENING. CAN NOT RULE OUT THE NEED FOR
A DENSE FOG ADVISORY TONIGHT AT SOME POINT. SKY GRIDS ARE ALWAYS A
TOUGH CALL IN SITUATIONS WHERE SKIES CLEAR OUT...BUT THEN FOG
DEVELOPS AND HIDES THE SKY. WILL INCLUDE SOME CLOUD COVER TO PRODUCE
AT LEAST PARTLY CLOUDY WORDING.
FOG SHOULD BURN OFF BY MID MORNING ON TUESDAY...AS SOUTHWEST WINDS
BEGIN TO MIX DOWN DRY MID LEVEL AIR. WAA FLOWS AT THE SURFACE AND
MID LEVELS SHOULD HELP TEMPS CLIMB ABOVE GUID. SOME OF THE SUNS
ENERGY SHOULD GO INTO MELTING THE SNOW...INSTEAD OF WARMING THE
AIR...BUT LESS SNOW IN OUR S CWA SHOULD HELP THEM WARM UP A BIT MORE
THAN THE NORTH. WILL CONTINUE WITH FORECAST HIGHS ABOUT 8-12 DEGREES
WARMER THAN TODAY.
LONG TERM...WEDNESDAY THROUGH MONDAY.
ON WEDNESDAY...A SHORT WAVE UPPER TROUGH WILL LIFT OUT OF THE
SOUTHERN PLAINS...WELL AHEAD OF THE LONG WAVE TROUGH DEEPENING ALONG
THE WEST COAST AT THAT TIME. THIS WILL INCREASE OUR RAIN CHANCES BY
WED AFTERNOON. THERE IS SOME QUESTION ABOUT IF WE WILL WARM ABOVE
FREEZING BEFORE ANY RAIN REACHES THE GROUND WED AFTERNOON. FOR
NOW...WE ARE LEAVING FREEZING RAIN OUT OF THE GRIDS...BECAUSE THE
INITIAL WAVE OF PRECIP WILL TAKE SOME TIME TO SATURATE THE LOWER
LEVELS...ALLOWING TEMPS TO CLIMB ABOVE FREEZING. HOWEVER...WE HAVE
TO REMAIN AWARE THAT RAIN CAN DROP THROUGH DRY LOWER LAYERS MUCH
BETTER THAN SNOW CAN...SO IT TYPICALLY CAN REACH THE GROUND BEFORE
LOW LEVEL SATURATION. THIS MAY PUT SOME RAIN ON THE GROUND DURING
FREEZING TEMPS.
WE DID INCLUDE SOME FZRA IN THE GRIDS FOR WED NIGHT IN OUR N
CWA...WHEN TEMPS BEGIN TO DROP OFF AGAIN. THE SOUTHERN CWA SHOULD
REMAIN ABOVE FREEZING...AS WAA CONTINUES THROUGH A DEEP LAYER OF THE
ATMOSPHERE. TEMPS MAY REMAIN NEARLY STEADY WED NIGHT...INSTEAD OF
DROPPING OFF MUCH...DUE TO THE WAA. FOG WILL BE LIKELY...DUE TO AN
ABUNDANCE OF MOISTURE FROM DAYTIME MELTING...AND SUBSEQUENT RAIN.
THE INITIAL SHORTWAVE IS EXPECTED TO LIFT NE OF ILLINOIS BY 12Z
THURSDAY...SO WE EXPECT A BREAK IN THE RAIN FOR THURSDAY INTO
THURSDAY NIGHT. AFTER MIDNIGHT THURSDAY NIGHT...DIFFLUENT FLOWS
ALOFT AHEAD OF THE UPPER TROUGH WILL HELP GENERATE RISING MOTION
LIFTING THE INCREASING MOISTURE AT LOW LEVELS. THE SOUTHERLY WINDS
SHOULD BE ABLE TO TAP INTO MOISTURE FROM THE GULF OF MEXICO TO
RAPIDLY INCREASE RH NUMBERS AT ALL LEVELS.
ON NEW YEARS EVE MORNING...THE COLD FRONT WILL BEGIN TO INCREASE
EASTWARD MOTION OUT OF THE PLAINS...AS THE SURFACE LOW LIFTS INTO
CENTRAL MINNESOTA BY 18Z. A 45-55KT SW LLJ WILL HELP TO DESTABILIZE
THE LOWER TROPOSPHERE. LI/S DROP TO NEAR ZERO IN OUR S CWA...WITH
TOTAL TOTALS AROUND 52. THUNDER MAY DEVELOP AHEAD OF THE
FRONT...ESPECIALLY FRIDAY AFTERNOON AND EVENING. HAVE CONTINUED WITH
THE SLIGHT CHANCE OF THUNDER IN THE GRIDS.
THE EXTENDED GUIDANCE CONTINUES TO SHOW THE MAIN COLD FRONTAL
PASSAGE BEGINNING NEW YEARS EVE EVENING...WITH THE FRONT REACHING
INDIANA BEFORE 12Z NEW YEARS DAY. COLD AIR ARRIVING FROM WEST TO
EAST WILL CHANGE OVER ANY LINGERING PRECIPITATION TO SNOW.
HOWEVER...THE COLDER AIR WILL ALSO BE MUCH DRIER...AND SYNOPTIC
FLOWS WILL ALREADY HAVE BEGUN SUBSIDENT ISENTROPIC MOTIONS...HELPING
TO SHUT DOWN THE POST FRONTAL PRECIPITATION. WE DECREASED POPS TO
CHANCE AFTER MIDNIGHT NEW YEARS EVE IN THE WESTERN CWA. NEW YEARS
DAY MORNING WE WENT WITH DRY CONDITIONS EVERYWHERE BUT EAST OF
I-57...WHERE WE ONLY KEPT SLIGHT CHANCE POPS FOR RAIN OR SNOW.
THE ECMWF NO LONGER HAS ANY SIGNS OF THE SECONDARY LOW LIFTING FROM
LOUISIANA TO THE OHIO RIVER VALLEY ON NEW YEARS DAY AND SATURDAY
NIGHT. THE GFS AND CANADIAN NEVER SHOWED IT...NOW THE ECMWF HAS COME
INTO AGREEMENT WITH THAT.
THEREFORE WE REMOVED ALL POPS FROM SATURDAY AFTERNOON THROUGH SUNDAY
NIGHT. THEY WERE MAINLY SLIGHT CHANCES...SO THAT WILL NOT HAVE A
MAJOR AFFECT ON THE WORDED FORECAST.
WE LOWERED TEMPS FRIDAY NIGHT AND SATURDAY IN RESPONSE TO THE MORE
PROGRESSIVE COLD FRONTAL MOVEMENT. WE ALSO EXPANDED THE MENTION OF
SNOW CHANCES FRIDAY NIGHT FARTHER EASTWARD. HOWEVER...WE DO NOT
EXPECT MUCH IF ANY SNOW ACCUMULATION BEHIND THE COLD FRONT.
SHIMON
&&
.AVIATION...
ISSUED 515 PM CST MON DEC 27 2010
LOW CLDS AND FOG WL CONT TO BE THE PRIMARY WX CHALLENGE THIS TAF PD.
STRATUS DECK BLO 1K FT OVR ERN IL IS MOVG SLOWLY EWD...AND WL CLR
BMI/DEC SHORTLY. ANOTHER AREA OF LOW CLDS OVR WRN IL APPEARS TO BE
SHRINKING WITH TIME. FOR NOW WL COUNT ON THE CURRENT CLD TRENDS
CONTG THIS EVENING. WL INDICATE IFR CIGS AND 5SM BR FOR CMI THRU
05Z...AS THERE WILL BE A CHC THAT THE MSTR OVR ERN IL WL GET TRAPPED
UNDER THE NOCTURNAL INVERSION ASSOCIATED WITH THE SFC HI PRES RDG.
WITH THE SFC HI CNTRD OVR WRN TN...SFC WNDS WL BE LGT SWLY. EXPCT
FOG TO DVLP TNGT ESP OVR AREAS THAT SEE CLR SKIES. SINCE WNDS WL
STAY UP SOMEWHAT...WL KEEP THE VSBYS A LTL HIER THAN THOSE SEEN
EARLIER TDY. WTR VPR STLT LOOPS SHOW A SMALL VORT MAX APCHG FM SWRN
IA...BUT THIS FEATURE SHOULD ONLY RESULT IN SCT CLDS FOR CNTL IL ON
TUE.
04
&&
.ILX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
