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000
FXUS63 KLOT 241118
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
518 AM CST TUE NOV 24 2009
.DISCUSSION...
333 AM CST
DENSE FOG THIS MORNING...RAIN AND ISOLATED THUNDER CHANCES THIS
EVENING...AND EVOLUTION OF PRECIP TYPE WEDNESDAY THROUGH FRIDAY
ARE MAIN CHALLENGES FOR THIS FORECAST PACKAGE.
FIRST THINGS FIRST...DENSE FOG HAS ONCE AGAIN FORMED ACROSS THE
AREA THIS MORNING. MANY OBS OUTSIDE OF URBAN AREAS DIPPED TO OR
VERY NEAR 1/4 MILE BY MIDNIGHT...WITH URBAN SITES STAYING IN THE
1-3 MILE RANGE SO FAR. EXPECT VERY LOW VIS TO MAINTAIN ITSELF
THROUGH MID MORNING FOR THOSE LOCATIONS THAT HAVE ALREADY DIPPED
TO 1 MILE OR LESS...AND WOULD ANTICIPATE AT LEAST INTERMITTENT
DROPS BELOW 1 MILE FOR URBAN SITES. LOW STRATUS DECK CONTINUES TO
BUILD IN ACROSS THE AREA IN A SIMILAR FASHION TO LAST NIGHT...AND
EXPECT EVOLUTION OF FOG TO ALSO BE SIMILAR TO YESTERDAY...WITH A
GENERAL LIFT INTO THE LOW STRATUS BY MID MORNING.
THIS AFTERNOON...RAIN BAND ASSOCIATED WITH OCCLUDED SURFACE FRONT
AND UPPER LOW CURRENTLY OVER FAR SOUTHWESTERN IOWA WILL BEGIN TO
MOVE ACROSS THE FORECAST AREA FROM SOUTHWEST TO NORTHEAST. AFTER
MUCH DEBATE...DECIDED TO ADD IN ISOLATED THUNDER MENTION AS AXIS
OF HEAVIEST RAIN AND UVV MOVES ACROSS THE AREA FROM 21-06Z
TONIGHT. THUNDER CURRENTLY OCCURRING OVER NORTH CENTRAL IOWA WITH
7-7.5 DEGREE MID LEVEL LAPSE RATES PER SPC MESOANALYSIS...AND
NAM/SREF/RUC IN GENERAL AGREEMENT IN BRINGING SIMILAR LAPSE RATES
ACROSS THE AREA. BY NO MEANS IS THUNDER EXPECTED TO BE
WIDESPREAD...AND MOST LOCATIONS WOULD LIKELY HEAR JUST A RUMBLE OR
TWO AT BEST.
AFTER MAIN BAND OF RAIN MOVES ACROSS THE AREA TONIGHT...MAY SEE A
SHORT BREAK IN WIDESPREAD PRECIP BY WEDNESDAY MORNING AS SHORTWAVE
RIDGING BEHIND FIRST UPPER TROUGH/LOW MOVES THROUGH. SECOND UPPER
WAVE WILL BE JUST ON ITS HEELS THOUGH... WITH POPS INCREASING FROM
THE NORTHWEST WEDNESDAY NIGHT. WITH COLDER AIR ALOFT SPREADING
INTO THE AREA WITH THIS SECOND WAVE...STILL ANTICIPATE A
CHANGEOVER TO A MIX OF RAIN AND SNOW LATE WEDNESDAY AND INTO
THURSDAY. WRAP AROUND MOISTURE AND ASSOCIATED PRECIP ON THE
WESTERN SIDE OF THE SECOND UPPER LOW WILL BEGIN TO MOVE INTO THE
AREA THURSDAY. HAVE MAINTAINED THE RAIN/SNOW MIX...AND FORECAST
SOUNDINGS CONTINUE TO INDICATE THAT WHILE MUCH OF THE COLUMN WILL
BE BELOW FREEZING...IT MAY NOT BE COLD ENOUGH TO SUPPORT
WIDESPREAD ICE CRYSTAL GENERATION. IF THIS ENDS UP BEING THE
CASE...COLD DRIZZLE MIXED WITH LIGHT SNOW AT TIMES WOULD BE THE
LIKELY P-TYPE. UPPER TROUGH FINALLY BEGINS TO LIFT OUT ON
FRIDAY...WITH JUST A FEW RESIDUAL SNOW SHOWERS REMAINING UNTIL
UPPER FLOW TURNS ANTICYCLONIC.
TEMPERATURES WILL ALSO BE TRICKY...WITH MOS GUIDANCE SEEMINGLY TOO
WARM WEDNESDAY THROUGH FRIDAY. INSTEAD GENERALLY FOLLOWED
DETERMINISTIC GFS OUTPUT...WITH UPPER 40S DURING THE DAY WEDNESDAY
DROPPING TO UPPER 30S TO AROUND 40 FOR HIGHS THURSDAY...AND RIGHT
AROUND 30 BOTH WEDNESDAY AND THURSDAY NIGHT.
NO CHANGES MADE TO PREVIOUS FORECAST SATURDAY THROUGH NEXT MONDAY.
BOXELL
&&
.AVIATION...
1200 UTC TAFS...AVIATION FORECAST ISSUES INCLUDE IFR/LIFR
CONDITIONS WITH FOG AND LOW STRATUS THIS MORNING...AND
PRECIPITATION TRENDS ALONG WITH CONTINUED CEILING AND
VISIBILITY CONCERNS LATER TODAY AND TONIGHT.
DEEPENING LOW PRESSURE VICINITY KMCI THIS MORNING...WITH WARM
FRONT EXTENDING ACROSS CENTRAL IL/SOUTHERN IN. INCREASING LIFT
ACROSS SHALLOW MOIST AIR MASS NORTH OF WARM FRONT HAS LED TO
EXTENSIVE DEVELOPMENT OF IFR/LIFR STRATUS OVERNIGHT...WITH
VISIBILITIES FALLING TO LESS THEN A MILE IN AREAS MAINLY SOUTH
AND SOUTHWEST OF CHICAGO WHERE SKIES HAD CLEARED FOR A TIME LAST
EVENING. OVER PAST HOUR OR SO...HAVE ACTUALLY SCATTERED OUT LIFR/IFR
LAYER ACROSS PORTIONS OF IMMEDIATE CHICAGO AREA...THOUGH IFR
CEILINGS JUST OFF TO THE SOUTH ARE EXPECTED TO MOVE BACK INTO
THESE TERMINALS THIS MORNING AS WARM FRONT LIFTS NORTH AND WINDS
BECOME MORE SOUTHEASTERLY. WOULD THEN EXPECT A BIT OF DIURNAL
IMPROVEMENT BACK INTO MVFR RANGE THIS AFTERNOON.
RAIN ASSOCIATED WITH PLAINS LOW PRESSURE SYSTEM WILL SPREAD INTO
AREA LATER THIS AFTERNOON...WITH MODEL GUIDANCE IN GOOD AGREEMENT
IN DEPICTING PERIOD OF FAIRLY STEADY RAIN DURING EVENING AND EARLY
OVERNIGHT HOURS. FAIRLY STEEP MID-LEVEL LAPSE RATES SUGGEST
PERHAPS ENOUGH ELEVATED INSTABILITY TO SUPPORT A FEW ISOLATED
THUNDERSTORMS DURING THIS PERIOD...THOUGH CONFIDENCE/COVERAGE TOO
LOW FOR TAF INCLUSION. THOUGH LOW CLOUDS MAY SCATTER
BRIEFLY...WOULD EXPECT RAIN AND MOIST LOW LEVELS IN LIGHT WIND
REGIME IN VICINITY OF LOW CENTER WILL GENERALLY MAINTAIN IFR
VISIBILITY/CEILINGS. PRECIP INTENSITY EXPECTED TO DIMINISH IN
HOURS AFTER MIDNIGHT WITH PASSAGE OF MID LEVEL CIRCULATION AND
SURFACE LOW/OCCLUDED FRONT...ALONG WITH WIND SHIFT FROM SOUTHEAST
TO SOUTHWEST.
RATZER
&&
.MARINE...
158 AM CST
MAIN CONCERN FOR MARINE FORECAST IS PERIOD OF STRONG NORTHWEST WINDS
LATE THURSDAY INTO EARLY FRIDAY BEHIND LOW PRESSURE SYSTEM AND
ASSOCIATED PUSH OF COLD AIR ACROSS THE WESTERN LAKES.
LOW PRESSURE CURRENTLY ANALYZED OVER NORTHEASTERN KANSAS TO MOVE
NORTHEAST ACROSS WISCONSIN AND NORTHERN/CENTRAL LAKE MICHIGAN THOUGH
WEDNESDAY EVENING. WITH LOW MOVING ACROSS CENTRAL/NORTHERN LAKE
MICHIGAN...MAY SEE A BRIEF PERIOD OF GUSTY 20-25 KNOT
SOUTHWEST/WEST WINDS ON FAR SOUTH END LATE WEDNESDAY/WEDNESDAY
NIGHT. MORE IMPORTANTLY...SECOND UPPER DISTURBANCE DIGGING INTO
TROUGH ALOFT WILL MAINTAIN SLIGHT DEEPENING OF TRAILING
TROUGH/COLD FRONT WEDNESDAY NIGHT INTO THURSDAY. THIS TRAILING
TROUGH WILL MOVE ACROSS THE LAKE THURSDAY...WITH TIGHTENING OF
PRESSURE GRADIENT AND SURGE OF LOW LEVEL COLD AIR EXPECTED TO
PRODUCE NORTHWEST WINDS AROUND 30 KNOTS LATE THURSDAY/THURSDAY
NIGHT. MODEL FORECAST SOUNDINGS OVER THE LAKE DURING THIS PERIOD
INDICATE GUSTS JUST BELOW GALE FORCE...THOUGH A FEW GALE FORCE
GUSTS ARE POSSIBLE. PRESSURE GRADIENT AND ASSOCIATED NORTHWEST
WINDS WILL THEN GRADUALLY DECREASE FRIDAY...AS LOW CONTINUES TO
PULL AWAY TO THE NORTHEAST AND WEAK HIGH PRESSURE RIDGE SPREADS
ACROSS THE MISSISSIPPI VALLEY AND WESTERN LAKES FRIDAY NIGHT.
RATZER
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...DENSE FOG ADVISORY...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-ILZ022-
ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 10 AM TUESDAY.
IN...DENSE FOG ADVISORY...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 10
AM TUESDAY.
LM...NONE.
&&
$$
