FXUS63 KLOT 120937
AFDLOT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
337 AM CST SAT DEC 12 2009

.DISCUSSION...
335 AM CST

DIFFERENT DAY BUT SAME FORECAST CHALLENGES...PRECIP CHANCES/TYPE 
TONIGHT AND TEMPERATURES THE NEXT SEVERAL DAYS. 

WATER VAPOR IMAGERY DOES A NICE JOB THIS MORNING IN SHOWING THE 
DE-AMPLIFICATION OF THE POLAR JET ALONG THE CANADIAN BORDER AS THE 
POLAR VORTEX CONTINUES TO RETREAT NORTHWARDS. MEANWHILE...THE 
SUBTROPICAL JET IS BEGINNING TO BUCKLE A BIT WITH A COUPLE 
SHORTWAVES EVIDENT IN WATER VAPOR IMAGERY THIS MORNING...THE FIRST A 
LEAD SHORTWAVE TROUGH EMERGING OUT OVER THE HIGH PLAINS OF COLORADO 
WITH A SECOND SHORTWAVE TROUGH MOVING ACROSS THE FOUR CORNERS 
REGION. THE LEAD SHORTWAVE IS SYNOPTICALLY ENHANCING THE NOCTURNAL 
LOW LEVEL WIND MAXIMUM WHICH IS ALREADY RESULTING IN A NORTHWARD 
SURGE IN MODIFIED GULF MOISTURE. 

MODELS SUGGEST THAT MOISTURE WILL BE TOO LACKING FOR THIS LEAD WAVE 
TO HAVE ANY FANFARE AS IT MOVES THRU THIS AFTERNOON...HOWEVER AS 
COLUMN WORKS TOWARD SATURATION DO EXPECT CLOUD COVER TO REALLY
RAMP UP TOWARD MIDDAY ONWARD FOLLOWING A FAIRLY SUNNY START TO
THE DAY. THE SECOND SHORTWAVE MOVING INTO THE FOUR CORNERS REGION
ATTM WILL BE OUR WEATHER MAKER TONIGHT. STRONG LOW LEVEL JET WILL
LEAD TO FAIRLY IMPRESSIVE WARM AIR ADVECTION/ISENTROPIC ASCENT
WITH ATMOSPHERE FORECAST TO SATURATE FROM TOP DOWNWARD. DEEPEST
MOISTURE AND THEREFORE HIGHEST QPF/PRECIP PROBS APPEAR TO BE
LINING UP OVER OUR EASTERN/SOUTHEASTERN COUNTIES. HAVING SAID
THAT...IT DOES APPEAR AS THOUGH MUCH OF THE CWA WILL SEE A HIGH
PROBABILITY OF A LOW QPF EVENT TONIGHT AND HAVE CONTINUED TO TREND
POPS UPWARD ACROSS THE CWA.

PRECIP TYPE MAY BE TRICKY...THOUGH THICKENING/LOWERING CLOUD
COVER THIS EVENING ALONG WITH MODERATE SOUTHERLY WINDS SHOULD
PREVENT ANY DIURNAL FALL IN TEMPERATURES TONIGHT WITH ANY VERY
MODEST TEMP DROP TONIGHT LIKELY ASSOCIATED WITH WET BULB PROCESSES
AS THE ATMOSPHERE SATURATES. WITH MAJORITY OF THE CWA EXPECTED TO
START OUT AND GENERALLY REMAIN ABOVE FREEZING EXPECT TO SEE MOSTLY
LIGHT RAIN...THOUGH PRECIP COULD START OUT AS SOME GRAUPEL OR
SLEET AS COLUMN SATURATES FROM TOP DOWN. OVER AREAS WITH DEEPER
SNOWPACK COULD SEE TEMPS STRUGGLE MORE TO GET ABOVE FREEZING AND
COULD SEE SOME LIGHT FREEZING RAIN FOR A TIME...THOUGH LUCKILY IT
APPEARS AS THOUGH THE BRUNT OF THE QPF WILL REMAIN EAST OF THE
AREA MOST PRONE TO FREEZING RAIN. IT TAKES VERY LITTLE FREEZING
RAIN TO PRODUCE TRAVEL PROBLEMS AND GIVEN SUB-FREEZING GROUND
TEMPS ANY FRZ RAIN THAT DOES OCCUR COULD REALLY TURN ROADWAYS INTO
SKATING RINKS...SO NOT OUT OF THE QUESTION THAT SOME AREAS MAY
NEED A FREEZING RAIN ADVISORY TONIGHT...BUT THAT WILL DEFINITELY
BE SOMETHING THAT WILL NEED TO BE MORE OF A LAST MINUTE TYPE
DECISION FOR SUBSEQUENT SHIFTS TO CONSIDER.

AS THIS SHORTWAVE PASSES EAST OF THE AREA BY EARLY SUNDAY MORNING 
FORECAST SOUNDINGS SUGGEST THAT A FAIRLY DEEP LAYER OF LOW LEVEL 
MOISTURE WILL BE LEFT BEHIND AND TRAPPED BENEATH AN INVERSION. 
MODELS DO TEND TO MOISTEN LOW LEVELS TOO RAPIDLY IN THESE TYPE OF 
SCENARIOS...HOWEVER WITH PRECIPITATION LIKELY HELPING SATURATE 
THINGS SUSPECT THAT A LOW STRATUS DECK MAY INDEED EVENTUALLY 
DEVELOP. GIVEN THE PRESENCE OF A LOW STRATUS DECK AS FORECAST 
SOUNDINGS SUGGEST THEN DRIZZLE (POSSIBLE FREEZING DRIZZLE OVER
DEEPEST SNOW PACKED AREAS) WOULD BE POSSIBLE SUNDAY MORNING.

FAST ON THE HEELS OF THESE WAVES WILL BE A NORTHERN STREAM TROUGH 
WHICH WILL ALREADY BE INDUCING LEE CYCLOGENESIS DURING THE DAY 
SUNDAY. MODELS IN PRETTY GOOD AGREEMENT ON TAKING THIS SURFACE LOW 
RATHER EXPEDITIOUSLY EASTWARDS ACROSS THE REGION WITH MOST OF OUR 
CWA LIKELY MAKING IT INTO THE WARM SECTOR MONDAY AFTERNOON...WITH 
THE ONE POSSIBLE EXCEPTION BEING AREAS NEAREST TO THE WI BORDER
OVER NORTH CENTRAL ILLINOIS. WEEKEND SYSTEM SHOULD TAKE ITS
MOISTURE WITH IT...SO ONLY EXPECTING A SMALL CHANCE OF SOME SCT
LIGHT SHOWERS IN THE WARM SECTOR...WITH BETTER PRECIP CHANCES
CLOSER TO THE TIGHTER THERMAL GRADIENT AND SFC LOW TRACK NEAR THE
WI STATE LINE.

SOUTHERLY WINDS IN ADVANCE OF MONDAYS SYSTEM SHOULD SEND TEMPS TO 
NOTICEABLY ABOVE AVERAGE LEVELS...WITH FAR SOUTHERN CWA CLIMBING 
WELL INTO THE 40S. SNOWPACK UP NORTH WILL LIKELY KEEP TEMPS MOSTLY 
IN THE 30S...THOUGH SOUTHERN EXTENT OF THE SNOW COVER WHICH WILL BE 
MORE INFLUENCED BY ACTUAL WINDS SHOULD CRACK 40F. MONDAY'S WARM-UP 
LOOKS TO BE SHORT-LIVED WITH A SHOT OF COLD AIR PROGGED TO ARRIVE 
TUESDAY. MODELS HAVE BEEN FLIP-FLOPPING A BIT WITH RESPECT TO THE 
INTENSITY OF THIS COLD SNAP...THOUGH LAST SEVERAL GFS RUNS ARE NOW 
IN LINE WITH 11/00Z ECMWF RUN AS WELL AS THE 12/00Z RUN OF THE ECMWF 
WITH A MUCH MORE TEMPERATE SHOT OF COLDER AIR. HAVE NUDGED TEMPS 
UPWARD JUST A BIT TUES THRU WED TO REFLECT THE LATEST MODEL 
TRENDS...THOUGH DID REMAIN BELOW OBJECTIVE GUIDANCE 
NUMBERS...ESPECIALLY OVER THE SNOW COVERED AREAS.

IZZI

&&

.AVIATION...

06Z TAFS...MAIN FORECAST CONCERNS THIS PERIOD ARE PRECIP TIMING
AND TYPE SATURDAY EVENING INTO SATURDAY NIGHT.

AS MOISTURE BEGINS TO LIFT NORTH SATURDAY AFTERNOON INTO SATURDAY
EVENING...AN UPPER LEVEL DISTURBANCE SWINGS ACROSS THE REGION.
MODELS REMAIN IN FAIR AGREEMENT WITH PRECIP DEVELOPING OVER EAST
CENTRAL IL BETWEEN 00Z AND 03Z THEN LIFTING/SPREADING NORTHEAST
OVERNIGHT INTO EARLY SUNDAY MORNING. HEAVIEST AXIS OF QPF APPEARS
TO REMAIN SOUTH OF THE TERMINALS. WHILE PRECIP WILL LIKELY REMAIN
LIGHT...WITH QPF AMOUNTS LIKELY UNDER A TENTH OF AN INCH...
SOUNDINGS/THERMAL PROFILES MAKE PRECIP TYPE A DIFFICULT FORECAST.
NOT TO MENTION WHAT THE SFC TEMPS ARE WHILE PRECIP IS FALLING.
HOWEVER...TRENDS AMONG THE MODELS ARE EMERGING AND CONFIDENCE ON
HOW THIS ALL PANS OUT IS INCREASING.

WITH HIGH TEMPS LIKELY REACHING THE MID TO UPPER 30S SATURDAY WITH
AN OVC DECK BY SUNSET...EXPECT TEMPS TO ONLY DROP INTO THE LOWER
30S DURING THE EVENING. AS SOUTHERLY FLOW CONTINUES AND WARM AIR
CONTINUES TO SPREAD NORTH...WHILE TEMPS MAY DIP BELOW FREEZING FOR
A PERIOD DURING THE MID/LATE EVENING HRS...EXPECT TEMPS TO RISE
BACK TO FREEZING PERHAPS A FEW DEGREES ABOVE FREEZING OVERNIGHT
INTO SUNDAY MORNING. ONLY EXCEPTION TO THIS WOULD BE OVER THE
HEAVIER SNOW PACK ACROSS NORTHWEST IL...GENERALLY NORTH/WEST OF A
LINE FROM PWK/DPA/ARR/VYS...WHERE TEMPS MAY REMAIN AT OR BELOW
FREEZING...INCLUDING AT RFD.

SO AS PRECIP BEGINS SATURDAY EVENING...LIKELY IN THE 02Z-04Z OVER
THE TERMINALS...EXPECT TEMPS THRU THE COLUMN TO SUPPORT LIGHT
SNOW. WARM AIR CONTINUES TO LIFT NORTH ALOFT...WHICH SHOULD ALLOW
SNOW TO MIX WITH OR CHANGE TO LIGHT SLEET. EVENTUALLY...AFTER 06Z
OR SO...SOUNDINGS SUPPORT PRECIP TO CHANGE OVER TO EITHER ALL
RAIN OR DRIZZLE...DEPENDING ON WHERE THE HEAVIEST QPF SETS UP.
WITH SFC TEMPS EXPECTED TO BE ABOVE FREEZING...PER ABOVE...HAVE
CHANGED PRECIP OVER TO LIQUID DRIZZLE AT ORD WITH THE 30HR TAF BY
08Z. SHOULD SFC TEMPS REMAIN BELOW FREEZING...THERE COULD BE A
PERIOD OF LIGHT FREEZING RAIN OR FREEZING DRIZZLE. AT THIS POINT
HAVE GONE WITH A WARMER TREND WHICH WOULD LIMIT ANY FREEZING
PRECIP.

OTHERWISE...LIGHT SOUTHEAST WINDS WILL SHIFT BACK SOUTHERLY BY
MORNING AND INCREASE TO 10-15KTS. GRADIENT CONTINUES TO TIGHTEN
THRU THE DAY WITH GUSTS 20+KTS DEVELOPING BY MID/LATE AFTERNOON
WHICH SHOULD DIMINISH SOME SATURDAY EVENING. CMS

&&

.MARINE...
308 AM CST

AFTER A PERIOD OF WEAKER WINDS UNDER A RIDGE OF HIGH PRESSURE
OVERNIGHT...WINDS ARE EXPECTED TO INCREASE AGAIN THIS AFTERNOON
AND TONIGHT AS A TROUGH OF LOW PRESSURE DEVELOPS TO THE NORTHWEST
OF THE GREAT LAKES AND BEGINS TO DROP SOUTHEAST AS A COLD FRONT.
LIGHT SOUTHWEST FLOW EARLY TODAY WILL TURN MORE SOUTHERLY AS
WINDS PICK UP...THEN VEER WEST BEHIND THE FRONTAL PASSAGE ON
SUNDAY. WINDS CONTINUE VEERING AHEAD OF THE NEXT LOW PRESSURE
SYSTEM WHICH APPROACHES THE CHICAGO AREA FROM THE SOUTHWEST AND
CROSSES THE LAKE ON MONDAY NIGHT.

LENNING

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...NOON SATURDAY TO 3 AM SUNDAY.

&&

$$