FXUS63 KLOT 120238
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
838 PM CST FRI DEC 11 2009
.DISCUSSION...
837 PM CST
FOR EVENING UPDATE...
GOING FORECAST IN PRETTY GOOD SHAPE THIS EVENING...WITH ONLY A
FEW MINOR TWEAKS MADE TO HOURLY TEMP AND SKY COVER GRIDS.
SERIES OF WEAK MID LEVEL RIPPLES CONTINUE TO RACE ACROSS THE MIDWEST
ALONG CYCLONIC SHEAR SIDE OF POLAR JET AXIS. EVENING ANALYSIS OF
RAOB/PROFILER AND SATELLITE DATA INDICATE ONE SUCH LITTLE RIPPLE
MOVING ACROSS FORECAST AREA AT THIS TIME...MAINLY DIAGNOSED BY
SUBTLE BACKING/VEERING OF MID LEVEL WINDS IN PROFILER TIME/HEIGHT
DATA FROM SLATER IOWA...AND IR SATELLITE DEPICTION OF PATCHY AREA OF
8-10 KFT CLOUDS PROPAGATING ACROSS NORTHERN IL/NORTHWEST IND THIS
EVENING. LITTLE SURFACE REFLECTION OF THIS FEATURE NOTED HOWEVER AS
WEAK HIGH PRESSURE RIDGE AXIS EXTENDS INTO FORECAST AREA FROM STRONG
HIGH ACROSS SOUTHERN APPALACHIANS/MID ATLANTIC.
PATCHY CLOUD COVER SHOULD DECREASE WEST TO EAST AFTER MIDNIGHT AS
MID LEVEL WAVE HURRIES OFF TO THE EAST. WITH LIGHT WINDS AND DEEP
SNOW COVER ACROSS NORTH CENTRAL IL HAVE NOTED TEMPS FALLING TO
SINGLE DIGITS IN A FEW ISOLATED SPOTS THIS EVENING...THOUGH THIS
BEING MODULATED BY CURRENT PATCHY CLOUD COVER. HAVE ADJUSTED HOURLY
TEMP GRIDS PER RECENT OBS TRENDS...AND WENT A DEGREE OR TWO COOLER
FOR OVERNIGHT MINS ACROSS THOSE PARTS OF FAR NORTH CENTRAL IL. SFC
RIDGE ALSO TO MOVE OFF TO THE EAST LATE TONIGHT...ALLOWING LIGHT
SOUTH WINDS TO DEVELOP WHICH SHOULD HELP THE BOTTOM FROM FALLING
OUT...ESPECIALLY WITH MODEST WARM ADVECTION OCCURRING ALOFT.
ONLY OTHER SHORT TERM CHANGES WERE TO REMOVE POPS/SNOW SHOWERS
ACROSS SOUTHERN LAKE MICHIGAN...AS GRADUAL WARMING OF COLUMN HAS
LOWERED INVERSION HEIGHTS ENOUGH TO END LAKE INDUCED PRECIPITATION.
RATZER
&&
.PREV DISCUSSION...
331 PM CST
OVERALL HARD TO JUSTIFY MANY CHANGES TO GOING FORECAST WITH NEW
DATA SUPPORTING TRENDS OF PAST FEW DAYS. MAIN FORECAST CONCERNS
WILL CONTINUE TO BE MODERATING TEMPERATURE TRENDS...AND RESOLVING
PRECIP TYPE TRENDS SATURDAY NIGHT THROUGH EARLY MONDAY.
BROAD SURFACE HIGH PRESSURE TO THE SOUTHEAST OF THE FORECAST AREA
WILL MOVE INTO MID ATLANTIC STATES TONIGHT SETTING UP SLIGHTLY
STRONGER SOUTHERLY RETURN FLOW FOR THE OVERNIGHT HOURS. THIS WILL
ALLOW FOR CONTINUATION OF MUCH WELCOMED WARM ADVECTION AND
NORTHWARD DISPLACEMENT OF ARCTIC AIR MASS THAT HAS GRIPPED AREA
PAST FEW DAYS. MID LEVEL PROFILER DATA DOES DEPICT MID LEVEL SPEED
MAX TRACKING ACROSS IOWA THAT APPEARS TO BE ENHANCING MID LEVEL
WAA AND HAS PROMOTED INCREASING MID CLOUD COVERAGE ACROSS EASTERN
IOWA/WEST CENTRAL ILLINOIS THIS AFTERNOON. WITH EASTWARD
EXTRAPOLATION OF THIS SPEED MAX THROUGH THE SOUTHERN GREAT LAKES
REGION THIS EVENING...WOULD EXPECT MID LEVEL WAA CLOUDS WILL
AFFECT NORTHERN ILLINOIS EARLY THIS EVENING...AND THEN SHIFT OFF
TO THE EAST LATE IN THE EVENING. SNOW COVER OBVIOUSLY STILL HAVING
A LOT TO SAY IN TERMS OF SFC TEMPS THIS AFTERNOON WITH SNOW
COVERED AREAS ACROSS NORTH CENTRAL ILLINOIS STILL IN THE MID TEENS
WHILE BARE FAR SOUTHERN AREAS RISING INTO THE LOWER 30S LATE THIS
AFTERNOON. TRIED TO DEPICT TEMP DROPS TONIGHT MAINLY DURING THE
EVENING HOURS WITH THE SUSPICION TEMPS WILL BECOME STEADY OR EVEN
SLOWLY RISE LATE EVENING INTO OVERNIGHT HOURS AS WAA CONTINUES AND
GRADIENT SLOWLY BEGINS TO STRENGTHEN DUE TO SFC HIGH DEPARTING TO
THE EAST.
THE MODERATION IN TEMPERATURES TO CONTINUE TOMORROW WITH 850 HPA
WINDS INCREASING TO 30 TO 40 KNOTS AND LIKELIHOOD OF ACTUALLY
ADVECTING ABOVE FREEZING TEMPERATURES AT 850 HPA DURING THE
AFTERNOON. EXPECTING CONDITIONS TO START WITH SUN TOMORROW BUT
SHOULD GRADUALLY SEE STRATUS DECK ACROSS SOUTHERN PLAINS ADVECT
NORTHWARD. MAIN QUESTION IN TERMS OF MAXIMUM TEMPS TOMORROW WILL
BE HOW MUCH WARMING CAN OCCUR BEFORE LOW CLOUDS MOVE IN. STRENGTH
OF WARM ADVECTION ALSO FAIRLY SUBSTANTIAL HOWEVER SO FEEL FAIRLY
CONFIDENT IN GOING FOR HIGHS NEAR 40 ACROSS THE FAR SOUTH.
ATTENTION THEN TURNS TO FAST MOVING SYSTEM THAT WILL EJECT INTO
MID MS RIVER VALLEY FROM THE FOUR CORNERS REGION SATURDAY NIGHT.
TRENDS OVER PAST FEW RUNS HAVE BEEN TOWARD A WARMER SOLUTION AND
PERHAPS EVEN MORE OF LIQUID PRECIP TYPE ACROSS THE FAR SOUTH.
COULD BE A TRANSITION ZONE TO LIGHT SLEET/LIGHT FREEZING RAIN
BETWEEN INTERSTATE 80/INTERSTATE 88 CORRIDORS BUT EVEN HAVE SOME
CONCERN PORTIONS OF CHICAGO METRO AREA WITH LESS SNOW COVER COULD
EVEN GO OVER TO LIQUID LATE IN THE EVENING WITH SFC TEMPS POSSIBLY
REACHING FREEZING. EVENT STILL HOWEVER APPEARS TO BE QUITE LOW IN
TERMS OF QPF AND NOT EXPECTING ANY HEADLINES AT THIS TIME. GIVEN
HIGH COVERAGE/LOW QPF NATURE...DID INCREASE TO LIKELY POPS FOR
SOUTHERN PORTIONS OF CWA TOMORROW NIGHT. COULD BE SOME LINGERING
LIGHT DRIZZLE/FREEZING DRIZZLE AS MID LEVELS DRY OUT WITH PASSAGE OF
MID LEVEL TROUGH...BUT SUNDAY AFTERNOON STILL IS LOOKING TO BE
DRY.
ANOTHER FAST MOVING PACIFIC UPPER LEVEL SHORT WAVE TO APPROACH FOR
MONDAY WHICH STILL APPEARS TO BE THE MILDEST DAY OF THE PERIOD AND
PREVIOUS ADJUSTMENT TO INCREASE HIGHS FOR MONDAY STILL APPEAR TO
BE REASONABLE. MOST OF FORECAST SHOULD SUPPORT -RA WITH ASSOCIATED
PRECIP ON MONDAY WITH POSSIBLE EXCEPTION ACROSS FAR NORTHERN
ILLINOIS WHERE SNOW COVER COULD STILL ALLOW FOR SOME TYPE OF MIX.
MONDAY NIGHT INTO TUESDAY FOCUS SHIFTS TO EVOLUTION OF POLAR LOW
ACROSS NORTHERN SASKATCHEWAN. VORT LOBE ROTATING AROUND THIS
FEATURE WILL TAKE POLAR LOW SOUTHEASTWARD EVENTUALLY SWINGING COLD
FRONT THROUGH THE REGION MONDAY OR MONDAY NIGHT. HAVE GENERALLY
GONE WITH ECMWF/GFS IDEA OF TAKING SFC LOW THROUGH NORTHERN
ILLINOIS/SOUTHERN WISCONSIN MONDAY WHICH WOULD KEEP POTENTIAL OF
SNOW WELL TO THE NORTH OF THE AREA. WHILE MEDIUM RANGE MODELS HAVE
BEEN FAIRLY CONSISTENT IN THIS GENERALLY SCENARIO WITH ANOTHER
ARCTIC AIR MASS INTRUSION MONDAY NIGHT/TUESDAY...THERE HAVE BEEN
SOME FAIRLY SIGNIFICANT RUN TO RUN DIFFERENCES IN JUST HOW DEEP
INTO THIS ARCTIC AIR MASS NORTHERN ILLINOIS/NORTHWEST INDIANA
GETS. IT DOES APPEAR AS THOUGH HIGHS AROUND 20 AND SINGLE DIGIT
LOWS WILL BE POSSIBLE TUESDAY/WEDNESDAY. FORECAST HIGHS/LOWS FOR
THIS TIME PERIOD ARE UNDER THE ASSUMPTION THAT FORECAST AREA DOES
ESCAPE THE PERIOD WITH NO APPRECIABLE NEW SNOW HOWEVER. BROAD
UPPER RIDGING EXPECTED TO SLOWLY PUSH THROUGH CENTER PART OF THE
COUNTRY TOWARD END OF FORECAST PERIOD THAT SHOULD PROVIDE
MODERATING TEMPS ONCE AGAIN FOR DAYS 6 AND 7.
MARSILI
&&
.AVIATION...
00Z TAFS...MAIN FORECAST CONCERNS ARE THE WINDS TOWARD THE END OF
THE PERIOD...IN ADDITION TO PRECIP POTENTIAL/TIMING/TYPE AT AND
BEYOND THE END OF THIS PERIOD.
AREA OF BKN/OVC MID CLOUDS 8-10KFT PUSHING EAST ACROSS NORTHWEST
IL EARLY THIS EVENING. THIS CLOUD COVER SHOULD QUICKLY MOVE EAST
OF THE TAFS THIS EVENING WITH MAINLY SKC OVERNIGHT. MID/HIGH
CLOUDS WILL BEGIN INCREASING ACROSS THE AREA SATURDAY MORNING AND
EVENTUALLY LOWER TO 4-6KFT BY SUNSET.
WHILE WINDS AT SOME LOCATIONS HAVE ALREADY GONE LIGHT AND VRB AND
EVEN CALM...WITH THE CENTER OF THE HIGH OVER THE LOWER OHIO
VALLEY...EXPECT THE GRADIENT TO MAINTAIN A LIGHT SOUTHERLY WIND
THRU MOST OF THE NIGHT. AS THE GRADIENT TIGHTENS SATURDAY...WIND
SPEEDS WILL SLOWLY INCREASE THRU THE DAY WITH GUSTS 20-25KTS
POSSIBLE BY LATE AFTERNOON AND STILL GUSTY SATURDAY EVENING.
MOISTURE BEGINS TO RETURN ACROSS THE AREA SATURDAY EVENING AS A
WEAK UPPER LEVEL DISTURBANCE MOVES ACROSS THE REGION. EXPECT A
MIX OF LIGHT SNOW...LIGHT SLEET AND POSSIBLY LIGHT FREEZING RAIN
OR FREEZING DRIZZLE TO START SOMETIME LATE SATURDAY EVENING THEN
CONTINUE OVERNIGHT INTO SUNDAY MORNING. THIS WILL DEVELOP BEYOND
THE 24 HR PERIOD OF THE CURRENT TAFS AND DURING OR NEAR THE END
OF THE 30 HR ORD TAF. HAVE OPTED TO REMAIN DRY AT ORD WITH THIS
FORECAST AND HOPEFULLY GET A BETTER HANDLE ON PRECIP TYPE AND
ONSET TIMING WITH THE 06Z TAFS LATER TONIGHT. HOWEVER...DOES
APPEAR THAT BOTH VIS AND CIGS WILL BE SLOWLY LOWERING INTO MVFR
DURING THE EVENING...PERHAPS PRECIP RELATED...BUT FELT CONFIDENT
ENOUGH TO INCLUDE THIS WITH THE 30 HR TAF. CMS
&&
.MARINE...
155 PM CST
AN EXPANSIVE AREA OF HIGH PRESSURE OVER THE TENNESSEE
VALLEY...WILL SHIFT EAST TO THE MID ATLANTIC REGION BY EARLY
SATURDAY...AS A RIDGE EXTENDING TO THE NORTHWEST...ACROSS THE
WESTERN GREAT LAKES PASSES ACROSS LAKE MICHIGAN THIS EVENING. THIS
WILL ALLOW A MORE NOTICEABLE DECREASE IN WIND SPEED...ALONG WITH A
BACKING TO SOUTH. SOUTH TO SOUTHWEST WINDS WILL INCREASE AGAIN ON
SATURDAY AS A COLD FRONTAL TROUGH APPROACHES FROM THE NORTHWEST.
THIS FRONT WILL THEN SAG SOUTHWARD ACROSS NORTH AND CENTRAL LAKE
MICHIGAN ON SUNDAY.
MERZLOCK
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...NONE.
&&
$$
