FXUS63 KLOT 112132
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
332 PM CST FRI DEC 11 2009
.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...
1150 AM CST
RE: 1800Z TAFS...AN EXTENSIVE RIDGE OF HIGH PRESSURE FROM THE TN
VLY...NWWD ACROSS UPR MS VLY WILL DOMINATE WX THROUGH TONIGHT. THIS
WILL MAINTAIN GENLY VFR CLR CONDITIONS. EXCEPTION WILL BE OVR
PORTIONS OF EXTRM NRN IL THIS AFTN AS SCT V BKN AREA OF MID LVL
CLOUDINESS NOW OVR IA AND NWRN IL SHIFTS EWD ACROSS REGION. VFR CIGS
TO DVLP SATURDAY MORNING AS WARM AIR ADVECTION KICKS IN ON BACK SIDE
OF RETREATING RIDGE AND WEAK SHORT WAVE APCHS FROM THE CNTRL PLAINS.
MAIN REASON FOR CHG GROUPS IN FCST THRU NXT 24 HRS WILL BE DUE TO
BACKING OF WIND AS RIDGE AXIS PASSES ACROSS REGION LATE THIS AFTN
AND THIS EVENING. SFC PRESSURE GRADIENT CONTINUES TO WEAKEN WITH
APCH OF THIS RIDGE...SO WSWLY FLOW IN 10-15 KT RANGE WILL BE IN THE
PROCESS OF TURNING SLY WITH SPEEDS BLO 10 KTS THIS EVENING. AS SYS
TO W APCHS ON SATURDAY...SLY WINDS WILL AGAIN BEGIN TO INCR...WITH
FQT GUSTS NR 20 KTS DVLPG AFT 18Z SAT.
MERZLOCK
&&
.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...SMALL CRAFT ADVISORY...NEARSHORE GARY TO MICHIGAN CITY IN UNTIL 4 PM FRIDAY.
&&
$$
