FXUS63 KLOT 102341
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
541 PM CST THU DEC 10 2009
.DISCUSSION...
314 PM CST
VERY COLD TEMPERATURES TO CONTINUE TONIGHT WITH SLOWLY MODERATING
TEMPERATURES OVER NEXT FEW DAYS. THE NEXT CHANCE OF LIGHT
PRECIPITATION TO ARRIVE SATURDAY NIGHT...ALTHOUGH RESOLVING
PRECIPITATION TYPE APPEARS TO BE SOMEWHAT PROBLEMATIC AT THIS
FORECAST DISTANCE.
VISIBLE SATELLITE IMAGERY SHOWS WELL DEFINED SOUTHERN EDGE TO
SNOW COVER FROM NORTHERN LA SALLE COUNTY EASTWARD THROUGH
INTERSTATE 80 CORRIDOR. SFC PARCEL TRAJECTORIES HAVE QUITE AN
EXTENDED FETCH OVER SNOW COVER AND TEMPERATURES HAVE STRUGGLED TO
AROUND 5 ABOVE ACROSS FAR NORTHERN ILLINOIS THIS AFTERNOON. CORE
OF COLD LOW LEVEL AIR MASS HAS BEGUN TO SHIFT TO EASTERN GREAT
LAKES ALTHOUGH THERMAL ADVECTIONS TONIGHT WILL BE QUITE WEAK.
GIVEN MODERATION IN AIR MASS SHOULD NOT SEE TEMPS DROP OFF THE
TABLE TONIGHT...BUT WITH MAINLY CLEAR SKIES AND WINDS BEGINNING TO
DROP OFF MORE SUBSTANTIALLY THIS EVENING DO HAVE FAIRLY HIGH
CONFIDENCE THAT MUCH OF NORTH CENTRAL ILLINOIS WILL DROP BELOW
ZERO ONCE AGAIN AND POSSIBLY 5 BELOW OR COLDER IN FAVORED LOW
LYING COLD SPOTS. TAKING INTO ACCOUNT WINDS DROPPING INTO THE 10
TO 15 MPH RANGE WILL ONCE AGAIN FLIRT WITH REACHING WIND
CHILL ADVISORY CRITERIA ONCE AGAIN BUT AT THIS TIME EXPECTING A
VERY MARGINAL RISK...AND WILL BE HOLDING OFF ON ANY WIND CHILL
ADVISORY HEADLINES AT THIS TIME. THE ONE FACTOR THAT COULD
MITIGATE TEMP DROP TONIGHT IS EXTENT OF CLOUDINESS ACROSS FAR
NORTHERN ILLINOIS. SATELLITE IMAGERY INDICATING SOME INCREASE IN
STRATOCU ALONG AXIS FROM WEST CENTRAL MINNESOTA INTO NORTHEAST
IOWA...THAT MAY TEND TO PUSH EASTWARD INTO NORTHERN ILLINOIS THIS
EVENING IN ASSOCIATION WITH FAIRLY TIGHT LOW LEVEL THERMAL
RIBBON/SHEARED UPPER LEVEL WAVE MOVING INTO THE WESTERN GREAT
LAKES FROM THE NORTHERN PLAINS. NOT ANTICIPATING COVERAGE TO BE
ALL THAT SIGNIFICANT AT THIS POINT...AND THUS NOT A HUGE IMPACT ON
TEMPERATURES IS EXPECTED. ALSO CANNOT RULE OUT SOME FLURRIES ACROSS
THE FAR NORTH IF SOME OF THIS STRATOCU DOES MOVE INTO THE AREA.
NOT MUCH CHANGE EXPECTED FOR FRIDAY WITH BROAD LONGWAVE TROUGH
PERSISTING FROM SOUTH CENTRAL CANADA THROUGH NORTHEAST UNITED
STATES. STILL DOES APPEAR MID/UPPER LEVEL FLOW WILL BEGIN TO
DEAMPLIFY SETTING UP MORE OF ZONAL FLOW FOR SATURDAY THAT SHOULD
EVENTUALLY YIELD MORE SIGNIFICANT MODERATION IN TEMPS FOR THE
WEEKEND. FRIDAY HOWEVER SHOULD STILL BE COLD AS SFC ANTICYCLONE
ACROSS LOWER/MID MS RIVER VALLEY BEGINS TO SHIFT OFF TO THE EAST
SETTING UP ONLY WEAK SOUTHERLY FLOW. DID KEEP IDEA OF UPPER TEENS
GOING ACROSS FAR NORTHERN ILLINOIS BUT HAVE SOME RESERVATIONS THAT
THIS MAY EVEN BE A BIT TOO OPTIMISTIC FOR HIGHS
TOMORROW...ESPECIALLY ACROSS FAR NORTHERN ILLINOIS IN DEEPER SNOW
PACK AND WHERE FETCH OF SOUTH SOUTHWEST WINDS OVER SNOW COVERED
GROUND IS MAXIMIZED. GIVEN ABOVE FACTORS DID GO SLIGHTLY COLDER THAN
COLDEST MET GUIDANCE ONCE AGAIN.
RETURN SOUTHERLY FLOW WILL STRENGTHEN A BIT FRIDAY NIGHT/SATURDAY AS
RIDGE
OF HIGH PRESSURE MOVES OFF TO THE EAST AND NEXT WEAKER UPPER LEVEL
PACIFIC SHORT WAVE TROUGH APPROACHES WHICH IS CURRENTLY EVIDENT ON
WATER VAPOR IMAGERY ENTERING SOUTHERN CA COAST. DETERMINISTIC
MODEL SOLUTIONS ARE PRETTY MUCH ALL ON BOARD WITH INCREASING
PRECIP CHANCES SATURDAY NIGHT WITH ISENTROPIC ASCENT STRENGTHENING
IN ADVANCE OF THIS WAVE...WITH SUPPORT ALSO FROM SREF INCREASE TO
HIGH CHANCE POPS SATURDAY NIGHT. DID NUDGE UP CHANCE POPS IN
ACCORDANCE TO THESE TRENDS...BUT MAIN FORECAST ISSUE IS WHAT TYPE
OF LIGHT PRECIPITATION WILL FALL. FORECAST SOUNDINGS AT LEAST FOR
SOUTHERN PORTIONS OF THE FORECAST AREA ARE ON THE FRINGE OF
SUPPORTING SEVERAL PRECIP TYPES. QUESTION AS TO ICE AVAILABILITY
IS RAISED WITH MARGINAL MID LEVEL RH PROFILES...AND A FAIRLY DEEP
ISOTHERMAL TEMP PROFILE HOVERING AROUND THE FREEZING MARK NOTED
THROUGH THE LOWEST 3K FT ON GFS BUFFER SOUNDINGS. STILL A BIT TOO
FAR OUT IN FORECAST DISTANCE TO TRY AND GET TOO FANCY WITH NAILING
DOWN PRECIP TYPES BUT DID INCLUDE MENTION OF LIGHT FREEZING RAIN
OR LIGHT SNOW ACROSS SOUTHERN HALF OF FORECAST AREA AND CHANCE OF
LIGHT SNOW ACROSS THE NORTH. DID KEEP MOST OF SUNDAY DRY...BUT
ANOTHER FAST MOVING SHORT WAVE IS EXPECTED SUNDAY NIGHT THAT COULD
BRING SOME MORE LIGHT FREEZING DRIZZLE OR LIGHT SNOW BACK TO THE
AREA.
AFTER MODERATION IN TEMPS THIS WEEKEND INTO EARLY NEXT
WEEK...FOCUS WILL THEN BE ON EVOLUTION OF PACIFIC SHORT WAVE
DIVING DOWN EASTERN PACIFIC UPPER RIDGE THAT EVENTUALLY WILL HELP
PUSH POLAR LOW ACROSS NORTHERN SASKATCHEWAN EASTWARD. WHILE
COLDEST CORE OF THIS ARCTIC AIR MASS MAY PASS JUST TO THE NORTH OF
THE WFO LOT FORECAST AREA...SHOULD SEE SIGNIFICANTLY COLDER TEMPS
AGAIN FOR THE TUESDAY NIGHT THROUGH THURSDAY TIME PERIOD WITH WELL
BELOW NORMAL TEMPS AGAIN.
MARSILI
&&
.AVIATION...
00Z TAFS...NO SIGNIFICANT FORECAST CONCERNS THIS PERIOD. WEST
WINDS HAVE TURNED MORE SOUTHWEST EARLY THIS EVENING. STILL A FEW
GUSTS IN THE 18-20KT RANGE BUT EXPECT PREVAILING WIND SPEEDS 10-15
KTS THRU MOST OF THE NIGHT...POSSIBLY LOWER...WITH AN OCCASIONAL
HIGHER GUST HERE AND THERE. WINDS REMAIN WEST/SOUTHWEST FRIDAY
WITH SPEEDS SLOWLY DIMINISHING UNDER 10 KTS BY LATE AFTERNOON.
A WEAK SHORTWAVE IS MOVING EAST ACROSS SOUTHEAST MINNESOTA INTO
WISCONSIN THIS EVENING. A FEW LOCATIONS BRIEFLY DIPPING INTO LOW
MVFR AND EVEN IFR IN SNOW. EXPECT ANY PRECIP WITH THIS SYSTEM TO
REMAIN NORTH OF THE TERMINALS...BUT SCT/BKN CLOUDS 4-5KFT WILL
LIKELY DEVELOP/SPREAD ACROSS THE TERMINALS THIS EVENING. WILL NEED
TO WATCH TRENDS AND UPDATE ACCORDINGLY WITH THE 3 HR AMENDMENTS.
OTHERWISE...MID CLOUDS POSSIBLE FRIDAY AS WARM AIR ADVECTION
BEGINS...WITH CLOUDS EXPECTED TO SCATTER OUT BY FRIDAY EVENING.
CMS
&&
.MARINE...
1227 PM CST
A DEEP LOW PRESSURE OVER WESTERN QUEBEC...IN COMBINATION WITH A
SPRAWLING HIGH PRESSURE AREA OVER THE LOWER MISSISSIPPI VALLEY
WILL CONTINUE TO PRODUCE STRONG WEST WINDS ACROSS THE GREAT LAKES
THROUGH THIS EVENING. AS THE LOW CENTER TRACK OFF TOWARD LABRADOR
TONIGHT AND FRIDAY...WINDS WILL SLOWLY ABATE AND BACK SLIGHTLY
INTO THE SOUTHWEST. THE HIGH PRESSURE CENTER WILL SHIFT EASTWARD
TO THE MID ATLANTIC REGION BY LATE FRIDAY...AS A RIDGE EXTENDS TO
THE NORTHWEST...ACROSS THE WESTERN GREAT LAKES. WITH THE PASSAGE
OF THIS RIDGE AXIS FRIDAY EVENING...WINDS WILL DIMINISH MORE
NOTICEABLY...AND BACK INTO THE SOUTH.
MERZLOCK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...NEARSHORE WATERS UNTIL MIDNIGHT SATURDAY.
GALE WARNING...LMZ080-LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-
LMZ567-LMZ669-LMZ671-LMZ673-LMZ675-LMZ777-LMZ779-LMZ868-
LMZ870-LMZ872-LMZ874-LMZ876-LMZ878 UNTIL MIDNIGHT SATURDAY.
&&
$$
