FXUS63 KLOT 021825
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1225 PM CST WED DEC 2 2009
.DISCUSSION...
330 AM CST
MAIN CONCERN FOR THE MORNING UPDATE REMAINS TIMING THE ONSET OF OF
THE WINTRY PRECIPITATION EXPECTED TO BEGIN TO IMPACT THE AREA LATER
TODAY.
AT ISSUANCE TIME...THE SRN STREAM UPPER LOW HAS BEGUN TO LIFT NEWD
THROUGH CNTRL TX...AND LATEST SATELLITE IMAGERY INDICATES THAT THE
UPPER SYSTEM IS BEGINNING TO INTENSIFY AS IT DRAWS COOLER AIR IN
FROM IT'S NORTH AND WEST. THE ASSOCIATED SFC LOW CENTER IS OVER THE
CNTRL LOUISIANA GULF COAST AND IS ALSO SHOWING SIGNS OF MODEST
INTENSIFICATION. MEANWHILE...A STRONG NORTHERN STREAM SHORT WAVE IS
TRACKING OVER THE UPPER GREAT LAKES.
THE BIG QUESTION REMAINS...HOW MUCH PHASING OF THE NRN STREAM AND
AND SRN STREAM SYSTEM WILL TAKE PLACE AS THE SRN STREAM SYSTEM LIFTS
NEWD. THE LATEST MODEL RUNS ARE RATHER CONSISTENT IN HOW THEY
HANDLE THE SYNOPTIC SCALE FEATURES...LIFTING THE SRN STREAM SYSTEM
VERY QUICKLY NEWD...BEFORE IT CAN TAP THE NRN STREAM ENERGY AND
COOLER AIR...WHICH WOULD AID THE INTENSIFICATION OF THE SFC LOW.
THUS THE MODELS ARE IN RELATIVELY GOOD AGREEMENT ON THE TRACK OF SRN
STREAM SFC LOW...TAKING IT THROUGH CNTRL MISSISSIPPI BY 12Z THIS
MORNING...TO CNTRL KENTUCKY BY 00Z THURSDAY AND THEN ZIPPING ALONG
TO LAKE ONTARIO BY 12Z THURSDAY. THIS SFC LOW TRACK IS NOT VERY
CONDUCIVE TO SIGNIFICANT SNOWFALL FOR THE LOCAL AREA. HOWEVER...RUN
TO RUN CONSISTENCY IN THE MODEL SOLUTIONS FOR THIS SYSTEM HAVE BEEN
SLIGHTLY LACKING AS LAST NIGHT'S MODEL RUNS WERE TAKING A MORE
EASTERLY PATH THROUGH ERN KENTUCKY...WHILE PREVIOUS RUNS WERE TAKING
A MORE WESTERLY TRACK THROUGH WRN KENTUCKY. THE LATEST RUNS ARE
SORT OF A MIDDLE OF THE ROAD SOLUTION AND SINCE THERE IS GOOD
AGREEMENT...FEEL RATHER CONFIDENT ON THE SENSIBLE WEATHER FEATURES
ASSOCIATED WITH THIS EARLY SEASON WINTRY PRECIPITATION.
BASED ON THE TIMING OF THE SYSTEM...THE ONSET OF PCPN WILL OCCUR
THIS AFTERNOON OVER THE SERN PORTIONS OF THE CWA AND WILL BE
RAIN...AS THE COOLER AIR WILL NOT HAVE HAD TIME TO REACH THAT AREA.
BY EARLY THIS EVENING...LOW LEVEL WINDS WILL BEGIN TO BACK FROM NELY
TO NLY AND NWLY...AND COLD ADVECTION WILL INCREASE RAPIDLY.
SO...WILL KEEP THE NON-DIURNAL TEMPERATURE TREND FOR TODAY WITH THE
MAX TEMPS OCCURRING AT AROUND NOON. AS THE COOLER AIR FILTERS IN
THIS AFTERNOON...THE PCPN SHIELD IS EXPECTED TO INCREASE OVER THE
CWA. SO BY THE TIME PCPN BEGINS OVER THE NWRN LOCATIONS...
CONDITIONS SHOULD ALREADY BE COLD ENOUGH FOR THE PCPN TO BE ALL
SNOW AND THE PCPN IN A SWATH ROUGHLY FROM CHICAGO TO PONTIAC SHOULD
START OUT AS A RAIN/SNOW MIX. PCPN OVER THE NWRN INDIANA COUNTIES
AND FORD/IROQUOIS COUNTIES WILL REMAIN LIQUID FOR THE LONGEST AMOUNT
OF TIME...NOT CHANGING OVER TO SNOW UNTIL AFTER MIDNIGHT TONIGHT.
GIVEN THAT UPPER DYNAMICS WILL BE STRONGEST WHEN THE PCPN IS STILL
LARGELY LIQUID...SNOW AMOUNTS SHOULD BE SMALL AND ANY ACTUAL
ACCUMULATIONS SHOULD BE VERY MINIMAL AND CONFINED TO GRASSY AREAS.
BY THURSDAY...ANY REMAINING SNOWFALL WILL TAPER OFF TO OCNL
FLURRIES...WITH OCNL FLURRIES PERSISTING THROUGH FRIDAY AS A LARGER
SCALE UPPER TROUGH SWINGS THROUGH THE UPPER MIDWEST.
HOWEVER...THERE REMAINS SOME CONCERN REGARDING THE EXACT DEGREE OF
PHASING OF THE OF THE NRN AND SRN STREAMS AS TEMPERATURES ARE
MARGINAL AT THE TIME OF THE ONSET OF PCPN. THIS CAN BE PARTICULARLY
INTERESTING WHEN A SYSTEM ORIGINATING OVER THE GULF OF MEXICO IS
INVOLVED. SYSTEMS ORIGINATING OVER THE WARM GULF WATERS ALWAYS HAVE
A POTENTIAL TO TAP LARGE AMOUNTS OF POTENTIAL ENERGY...AND IF THEY
CAN INTERACT WITH THE COOLER AIR OF A STRONG NRN STREAM SHORT
WAVE...THE COMBINATION CAN LEAD TO RAPID INTENSIFICATION OF THE SFC
LOW...AND POSSIBLY A MORE WESTERLY TRACK OF THE SYSTEM. IF THIS
WERE TO OCCUR...SNOWFALL AMOUNTS COULD BE GREATER...WITH SOME
ACCUMULATING SNOW. WHILE POSSIBLE...THIS TYPE OF SCENARIO IS
UNLIKELY.
GIVEN THE FOCUS OF THE FORECAST MAINLY ON THE SHORT TERM...HAVE MADE
NO SGFNT CHANGES TO THE LONGER TERM FORECAST.
KREIN
&&
.AVIATION...
1800 UTC TAFS...MAIN CONCERN FOR THE FORECAST PERIOD WILL BE THE
ONSET OF PRECIP ALONG WITH PRECIP TYPE AND ASSOCIATED CEILINGS.
MID TO HIGH LEVEL CEILINGS HAVE BEEN EXPANDING NORTHWARD
THROUGHOUT THE MORNING AS UPPER LEVEL LOW CONTINUES TO PROGRESS
NORTHEAST THROUGH THE SOUTHERN PLAINS. THIS WILL BE THE MAIN AREA
OF INTEREST LATER THIS EVENING THROUGH EARLY THURSDAY AS ATTENDANT
SURFACE LOW TRACKS NORTHEAST THROUGH THE MIDWEST ALONG WITH UPPER
LEVEL FEATURE. OF CONCERN FOR THE NEAR TERM IS WITH A COLD
FRONT...WHICH HAS SINCE PUSHED THROUGH MUCH OF THE CWA...EXTENDING
FROM THE SOUTHERN PLAINS NORTHEAST THROUGH ILLINOIS INTO AN AREA
OF LOW PRESSURE NEAR LAKE SUPERIOR. THIS HAS INITIALLY PROVIDED A
WIND SHIFT TO MORE OF A NORTHERLY DIRECTION TODAY...BUT HAS SINCE
PROVIDED A MECHANISM FOR CLOUD FORMATION. LOW END MVFR CEILINGS
HAVE BEEN EXPANDING SOUTHWARD ALONG THIS FRONT OVER THE LAST
COUPLE OF HOURS. THIS HAS ACTUALLY EXPANDED FURTHER SOUTH EARLIER
THAN ANTICIPATED...WHICH HAS ALREADY PROMPTED AN AMENDMENT FOR
ORD. WITH COLDER AIR MAKING ITS WAY IN FROM THE NORTH AND MOISTURE
MOVING IN FROM THE SOUTH...EXPECT THIS CLOUD DECK TO CONTINUE TO
EXPAND THIS AFTERNOON INTO THE EVENING. ALSO...WITH AN INCREASING
PRESSURE GRADIENT ACROSS THE TERMINALS...WIND GUSTS WILL CONTINUE
TO INCREASE OUT OF THE NORTH THROUGH TONIGHT.
WITH APPROACH OF SYSTEM FROM THE SOUTH AND COLDER AIR CONTINUING
TO FILTER IN FROM THE NORTH...LIKELIHOOD OF TERMINALS OBSERVING
PRECIP WILL INCREASE EARLY THIS EVENING. PREVIOUS FORECASTS HAVE
STARTED ONSET OF PRECIP WITH RAIN...BUT CURRENT GUIDANCE IS
INDICATING THAT COLDER AIR WILL HAVE ALREADY MOVED IN BY
THEN...WITH A BETTER CHANCE OF SEEING A RAIN SNOW MIX. SO HAVE
INCLUDED THAT IN THE FORECAST WITH A COMPLETE CHANGEOVER TO ALL
SNOW BY THE 07Z TIME FRAME. DURING THIS PERIOD...CEILINGS AND
VISIBILITY WILL COME DOWN...BUT AM STAYING CONSISTEN WITH PREVIOUS
FORECASTS OF LOW END MVFR CONDITIONS. PRECIP CHANCES AND WINDS
FINALLY DECREASE THURSDAY MORNING WITH THE SYSTEM DEPARTING TO THE
EAST...BUT CEILINGS WILL REMAIN AS NEXT WAVE MOVES IN FROM THE
NORTHWEST.
RODRIGUEZ
&&
.MARINE...
333 AM CST
A LULL BETWEEN LOW PRESSURE SYSTEMS WILL SHIFT RATHER WEAK SW WINDS
THIS MORNING TO GUSTY NORTH WINDS THIS AFTN AS THE DEEPENING SYSTEM
APPROACHING THE MID OHIO VLY FROM THE WESTERN GULF STATES TIGHTENS
THE PRESSURE GRADIENT OVER LAKE MICHIGAN AND PULLS DOWN COLD
CANADIAN AIR. WINDS WILL BE KICKING UP TO AROUND 30 KTS THIS AFTN
AND CONTINUE THRU TONIGHT AS THE STILL STRENGTHENING LOW KICKS OUT
ACROSS OHIO TO LK ONTARIO BY THURSDAY MORNING. FINALLY AS THIS LOW
CONTINUES NE INTO NEW ENGLAND...PRESSURE GRADIENTS LOOSEN OVER LK
MICHIGAN AND WINDS NOT ONLY START BACKING WEST BUT DIMINISH TO 20
KTS OR LESS AND PERMIT WAVES TO FLATTEN BELOW 8 FEET OVER EASTERN LK
MICH. WIND GUSTS MAY APPROACH GALE FORCE AT TIMES TONIGHT OVER THE
OPEN WATERS OF SOUTHERN LK MICHIGAN...BUT A GENERAL SUSTAINED GALE
WARNING IS PROBABLY NOT NECESSARY. A SMALL CRAFT ADVISORY SHUD GO
INTO AFFECT THIS AFTN AND CONTINUE THRU TONIGHT AND MOST OF THURSDAY
FOR THE IL AND INDIANA NEARSHORE WATERS.
RLB
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742...3 PM WEDNESDAY TO
3 PM THURSDAY.
SMALL CRAFT ADVISORY...LMZ743-LMZ744-LMZ745...3 PM WEDNESDAY TO
6 PM THURSDAY.
&&
$$
