FXUS63 KLOT 221613
AFDLOT
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
1013 AM CST TUE DEC 22 2009
.DISCUSSION...
1015 AM CST
...NEAR TERM UPDATE...
JUST A FEW TWEAKS TO NEAR TERM FORECAST THIS MORNING TO ACCOUNT FOR
LATEST RADAR TRENDS REGARDING MORNING SNOW...AND CHANCES FOR LIGHT
FREEZING RAIN/DRIZZLE THIS AFTERNOON AND EVENING.
12Z UPPER AIR ANALYSIS INDICATES THAT SHORTWAVE RESPONSIBLE FOR
OVERNIGHT AND MORNING SNOW IS ON ITS WAY OUT OVER EASTERN INDIANA AT
THIS HOUR...WITH LOW LEVEL BAROCLINIC ZONE AND 850 WAA OVER THE CWA
AND ASSOCIATED ISENTROPIC LIFT BEING THE PRIMARY FORCING FOR
REMAINING SNOW THIS MORNING. SHORT TERM MODEL GUIDANCE INDICATES
THAT GOING IDEA OF LIGHT SNOW LIFTING NORTH ACROSS NORTHERN TWO OR
SO TIER OF COUNTIES LATE THIS MORNING AND AFTERNOON IS ON TRACK.
STILL EXPECT TO SEE A DOWNWARD TREND IN INTENSITY WITH THIS AREA OF
SNOW...BUT VERY MINOR AMPLITUDE SHORTWAVE ANALYZED OVER IOWA AT 12Z
WILL RIPPLE THROUGH THE FORECAST AREA THIS AFTERNOON...LIKELY ADDING
JUST A TOUCH OF RISING MOTION TO OVERALL PICTURE AND KEEPING SNOW
JUST ABOVE THE FLURRY LEVEL. AT ANY RATE...DON'T EXPECT ANYTHING
MORE THAN PERHAPS AN ADDITIONAL TENTH OF TWO OF SNOW AT THE MOST FOR
THOSE LOCATIONS THAT SEE SNOW LATE THIS MORNING AND AFTERNOON.
ATTENTION THIS AFTERNOON AND EVENING THEN TURNS TO LIGHT PRECIP AND
QUESTIONS OF PRECIP TYPE ACROSS THE FORECAST AREA. WARM AIR
ADVECTION LOOKS TO START CRANKING UP ACROSS THE SOUTHERN FORECAST
AREA THIS EVENING. MODELS ARE SPITTING OUT SEVERAL TENTHS OF AN
INCH OF QPF ACROSS MAINLY AREAS TO THE SOUTH OF I-88. FORECAST
SOUNDINGS INDICATE THAT FREEZING DRIZZLE OR LIGHT FREEZING RAIN ARE
THE PROBABLE PRECIP TYPE THIS EVENING AND OVERNIGHT...THOUGH A FEW
SNOWFLAKES ARE CERTAINLY POSSIBLE...ESPECIALLY BETWEEN I-80 AND
I-88.
UPDATED FORECAST PRODUCTS AND GRIDS OUT SHORTLY.
BOXELL
&&
.PREV DISCUSSION...
334 AM CST
HAVE DECIDED TO FORGO ANY HEADLINES FOR WINTER WX AT THIS TIME.
CURRENT WATER VAPOR IMAGERY SHOWS SHORT WAVE REMAINING FAIRLY
STEADY STATE AS IT APPROACHES MS RVR EARLY THIS MORNING. AREA OF
SNOW NOW OVR NRN IL CONTINUES TO SHIFT SLOWLY EWD...BUT IS
EXPECTED TO BE ONGOING AT START OF FCST PERIOD...SO HAVE GONE WITH
CATEGORICAL MENTION OVR NERN AND E CNTRL IL...AND LIKELY POPS EWD
INTO NWRN INDIANA. SFC REPORTS INDICATE AT SNOW HAS TAPERED OFF IN
STERLING/QUAD CITIES AREA...SO WILL JUST GO WITH CHC POPS OVR WRN
FA BASED ON RADAR TRENDS. SNOW SHOULD PERSIST A LITTLE LONGER INTO
THE MORNING OVR FAR NRN IL...SO WILL MENTION 2-3 IN AMTS
THERE...AND AROUND 2 IN MOST EVERYWHERE ELSE. THOUGH SNOW WILL
LIKELY IMPACT MORNING TRAVEL...AMOUNTS ARE EXPECTED TO REMAIN
BELOW ADVSRY CRITERIA.
UPSTREAM...AREA OF STRONG ENERGY ASSD WITH IMPRESSIVE DIGGING
SHORT WAVE IS NOW INDICATED ON WATER VAPOR IMAGERY OVER SRN CA.
THIS IS THE FEATURE THAT WILL BRING VERY UNPLEASANT CONDITIONS TO
THE MIDWEST...ESPECIALLY FROM WED AFTN THROUGH THURSDAY NIGHT.
THIS SYS WILL BEGIN TO IMPACT AREA LATER TODAY AND TONIGHT BY
INDUCING MID AND UPR LVL RIDGING OVR MS VLY REGION...INHIBITING
THE DVLPMT OF WARM ADVECTION PRECIPITATION TUESDAY NIGHT INTO
WED...SO WILL MAINTAIN ONLY LO CHC POPS DURG THIS PERIOD.
THOUGH MODELS HAVE SETTLED ON TRACK OF DEEP STORM CENTER GOING
WELL W OF AREA...EVENTUALLY PLACING ENTIRE FA IN MILD
AIR...TRANSITION TO THIS POINT IS COMPLICATED. PRECIPITATION IS
EXPECTED TO OVERSPREAD REGION FROM S WED-WED EVENING...AND
CONTINUE IN PHASES THROUGH THURSDAY NIGHT...CULMINATING IN PERIOD
OF POTENTIALLY HIGH QPF AHEAD OF OCCLUDED FRONT DU TO PASS THROUGH
FA FROM SW-NE ERLY FRIDAY MRNG. IN QUESTION IS HOW MUCH FREEZING
PRECIP WILL OCCUR BEFORE EVENTUAL CHG OVER TO ALL RAIN ON
THURSDAY. HIGH PRESSURE WILL BE BUILDING N AND NE OF LK SUPERIOR
THROUGH WED NIGHT...ADDING TO STG ELY WIND THAT SHOULD MAINTAIN
COLD LLVL FLOW THROUGH WED NIGHT. IN ADDITIONS...GFS HAS PICKED UP
ON A PERIOD OF STRONG DYNAMIC COOLING WED NIGHT THAT MAY CAUSE
PRECIP TO MIX WITH SNOW AT TIMES OVER MUCH OF NRN IL AND EXTREME
NWRN INDIANA. ROUGHLY ALONG AND N OF I-80...MODELS DON'T RAISE
TEMPS ABOVE FREEZING TIL AFTER 12Z THURSDAY...AND HPC WINTER WX
FCSTS CRANK OUT 1/4 TO 1/2 INCH OF ICE ACROSS NRN IL BY 12Z
THURSDAY. HOWEVER...GIVEN MARGINAL TEMPS JUST BELOW
FREEZING...SUSPECT THAT ACTUAL ICE ACCUMULATION MAY NOT BE TOO
SEVERE...AND FEEL MORE CONFIDENT THAT COMBO OF MIXED PRECIP WILL
REMAIN BELOW WARNING CRITERIA WHEN ALL IS SAID AND DONE AND THAT
WINTER WX ADVSRYS WILL BE THE WAY TO GO. BY THURSDAY
AFTERNOON...GUIDANCE IS UNANIMOUS IN ADVERTISING ANOTHER ALL OUT
SWILLFEST AS PCPN BECOMES A WIND DRIVEN COLD RAIN OVER ENTIRE FA
UNTIL COLDER AIR WRAPS IN FROM SW CHRISTMAS MORNING.
AREA SHOULD BECOME DRY SLOTTED BY 12Z FRI SO WILL REDUCE PRECIP
MENTION TO OCNL LGT RAIN OR DRIZZLE OVERNIGHT THURSDAY.
COLDER AIR THEN TO BECOME REINFORCED THROUGH WEEKEND AS LARGER
SCALE TROFFING EVOLVES OVR ERN CANADA AND GRTLKS. THUS...LTL OR NO
CHG TO FCST BEYOND FRIDAY.
MERZLOCK
&&
.AVIATION...
12Z TAFS...TWO PRIMARY CONCERNS THIS FORECAST...SNOW THIS MORNING
AND CIGS THRU THE PERIOD.
AREA OF SNOW CONTINUES TO PUSH EAST ACROSS NORTHERN IL WITH THE
BACK EDGE MAKING STEADY PROGRESS ACROSS WEST CENTRAL IL...BUT
STILL ANOTHER 2 TO 4 HRS OF SNOW ACROSS THE CHICAGO AREA
TERMINALS. PREVAILING IFR CIGS/VIS UNTIL THE SNOW ENDS...THOUGH
CIGS HAVE BEEN BOUNCING AROUND 1KFT AT TIMES THIS MORNING AND
DPA/RFD HAVE ACTUALLY SCATTERED OUT ALL LOW CIGS. DON/T EXPECT
THIS TO LAST HOWEVER BUT WILL MONITOR TRENDS.
VIS AND/OR CIGS MAY DIP INTO LIFR BUT NOT EXPECTING THOSE TO
BECOME PREVAILING. AS THE SNOW ENDS EXPECT BOTH VIS AND CIGS TO
IMPROVE TO LOW MVFR BUT FLURRIES OR EVEN A FEW SNOW SHOWERS WILL
LIKELY CONTINUE THRU THE DAY. AND WITH MOIST LOW LEVELS THRU MUCH
OF THE NIGHT...THINK GUIDANCE TREND OF IFR CIGS OVERNIGHT IS
REASONABLE. THOUGH AS EASTERLY FLOW INCREASES OVERNIGHT...
ADVECTING IN SOME DRIER AIR...WOULD EXPECT SOME IMPROVEMENT
TOWARD THE END OF THE PERIOD AND CERTAINLY INTO WEDNESDAY
MORNING...WHICH IS INDICATED WITH THE 30HR ORD TAF.
OTHERWISE EASTERLY WINDS UNDER 10KTS THIS MORNING WILL INCREASE
TO 10-15KTS BY AFTERNOON WITH GUSTS TO 18-20KTS DEVELOPING DURING
THE EVENING. WIND SPEEDS CONTINUE TO INCREASE WEDNESDAY MORNING
WITH GUSTS TO 25 KTS POSSIBLE. CMS
&&
.MARINE...
215 AM...MAIN FORECAST CONCERN REMAINS A STRONG WINTER STORM TO
AFFECT THE LAKES REGION FROM WEDNESDAY THROUGH FRIDAY WITH GALES
EXPECTED TO DEVELOP THURSDAY.
HIGH PRESSURE OVER CENTRAL ONTARIO WILL REMAIN NEARLY STATIONARY
THROUGH THURSDAY MORNING WHILE IT CONTINUES TO STRENGTHEN. NEW
LOW PRESSURE DEVELOPING OVER THE SOUTHERN PLAINS TODAY WILL
SLOWLY LIFT NORTH TO MISSOURI BY THURSDAY EVENING AS IT CONTINUES
TO DEEPEN. THE GRADIENT BETWEEN THE HIGH OVER ONTARIO AND THIS
LOW LIFTING NORTH WILL CONTINUE TO TIGHTEN TONIGHT THRU THURSDAY
MORNING. TRENDS HAVE BEEN IN FAIR AGREEMENT OVER THE PAST FEW DAYS
AND EXPECT GUSTS TO 30KTS TO DEVELOP BY WEDNESDAY AFTERNOON OVER
SOUTHERN LAKE MICHIGAN WITH GALES TO 40KTS BY MIDDAY THURSDAY AND
WILL BE GOING WITH A GALE WATCH STARTING THURSDAY MORNING. FURTHER
NORTH...LOW END GALES OVER NORTHERN LAKE MICHIGAN SEEM REASONABLE
THOUGH TIMING IS A BIT LATER...PROBABLY NOT DEVELOPING UNTIL
THURSDAY EVENING...BUT WILL STILL HOIST A GALE WATCH WITH THIS
FORECAST. AS THE LOW CONTINUES TO LIFT NORTH ACROSS IOWA BY
FRIDAY MORNING AND WISCONSIN FRIDAY NIGHT...IT BEGINS TO SLOWLY
WEAKEN...AND EXPECT WIND SPEEDS TO GRADUALLY RELAX FRIDAY INTO
FRIDAY NIGHT WITH GALES LIKELY ENDING BY MID/LATE MORNING FRIDAY.
CMS
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...NONE.
IN...NONE.
LM...SMALL CRAFT ADVISORY...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
LMZ745...MIDNIGHT WEDNESDAY TO 6 AM THURSDAY.
GALE WATCH...LMZ080-LMZ669-LMZ671-LMZ673-LMZ675-LMZ740-LMZ741-
LMZ742-LMZ743-LMZ744-LMZ745-LMZ777-LMZ779-LMZ870-LMZ872-
LMZ874-LMZ876-LMZ878...6 AM THURSDAY TO NOON FRIDAY.
GALE WATCH...LMZ261-LMZ362-LMZ364-LMZ366-LMZ563-LMZ565-LMZ567-
LMZ868...6 PM THURSDAY TO NOON FRIDAY.
&&
$$
