Chicago Discussion
Posted by JAC on 12/21/2009, 11:29 am
HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
429 AM CST MON DEC 21 2009

.DAYS TWO THROUGH SEVEN...TUESDAY THROUGH SUNDAY.

A MORE POTENT STORM SYSTEM IS EXPECTED TO AFFECT THE MIDWEST AS
EARLY AS WEDNESDAY AFTERNOON AND CONTINUE INTO CHRISTMAS DAY.
CURRENT INDICATIONS ARE THAT AN AREA OF LOW PRESSURE WILL DEVELOP
OVER THE SOUTHERN PLAINS WEDNESDAY...THEN LIFT NORTHWARD ALONG THE
MISSISSIPPI RIVER THURSDAY NIGHT INTO EARLY FRIDAY MORNING. IF THE
STORM WERE TO TAKE THIS TRACK...THIS COULD RESULT IN A PERIOD OF
MIXED PRECIPITATION INCLUDING SNOW...SLEET...AND FREEZING RAIN BY
WEDNESDAY NIGHT. SOME SIGNIFICANT SNOW AND ICE ACCUMULATIONS ARE
POSSIBLE OVER NORTH CENTRAL AND PERHAPS FAR NORTHERN ILLINOIS
BEFORE PRECIPITATION CHANGES TO ALL RAIN. THIS WOULD ALSO IMPLY AN
AREA OF HEAVY SNOW WEST OF THE FORECAST AREA...ACROSS PARTS OF
IOWA AND WISCONSIN.

COLD AIR WRAPPING AROUND THE BACK SIDE OF THE DEPARTING LOW IS
THEN EXPECTED TO CHANGE RAIN BACK TO SNOW OR SNOW SHOWERS OVER
MOST OF THE REGION BY DAYBREAK CHRISTMAS MORNING.



AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL
544 AM CST MON DEC 21 2009

.DISCUSSION...
345 AM CST

RUNNING A BIT BEHIND THIS MORNING...SO PARDON WORDING IF IT SEEMS
A BIT TERSE.

WILL GO CLOUDY THROUGH TODAY AS ACARS SOUNDINGS AND MODEL FCST
SOUNDINGS SHOW FAIRLY HIGH RH TRAPPED BELOW TOP OF INVERSION AT
AROUND 8 THSD FT...AND LITTLE IF ANY WIND IN LLVLS.

HAVE GONE WITH CATEGORICAL POPS FOR OVERNIGHT AS MODELS REMAIN
CONSISTENT IN DEVELOPING A BAND OF WARM ADVECTION SNOW ACROSS NRN
IL AND PERHAPS AS FAR E AS NWRN INDIANA...MAINLY AFTER MIDNIGHT.
THIS SEEMS PLAUSIBLE...NOT ONLY BASED ON MODEL CONSISTENCY...BUT
ALSO LOOKING AT WATER VAPOR IMAGERY THAT SHOWS FAIRLY WELL INTACT
SHORT WAVE TOPPING LARGER SCALE RIDGE OVER ROCKIES. THIS FEATURE
WILL HELP INDUCE MODEST LLVL WARM ADVECTION...WITH NOSE OF BETTER
ISENTROPIC UPGLIDE FOCUSED ON ERN IA/NWRN IL BY 06Z. THEREFORE
WILL GO WITH HIGHEST QPF OVR NWRN PORTIONS OF FA...AND TAPER POPS
AND SNOW AMTS HEADING OFF TO S AND E. RIGHT NOW LOOKS LIKE
RELATIVELY MINOR EVENT...WITH 2 TO 4 INCH ACCUMULATIONS POSSIBLE N
OF I-88...TAPERING TO AROUND AN INCH ALONG I-80 AND LESS THAN AN
INCH SWD FROM THERE.

AREA IS IN A LULL AS THIS SHORT WAVE MOVES OFF TO E AND UPR
RIDGING IS INDUCED W OF MS RVR IN RESPONSE TO MUCH STRONGER SYS
FCST TO MOVE ACROSS U.S./MEXICO BORDER AREA BY 12Z WED. THIS
SYSTEM IS EXPECTED TO EMERGE OVER SRN PLAINS DURG WEDNESDAY...AND
QUICKLY DVLP INTO A CLOSED SYS BY WED EVENING. GUIDANCE COMING
INTO MUCH BETTER AGREEMENT IN LIFTING OCCLUDING LOW NNEWD OVR OR
JUST TO W OF MS RVR WED NIGHT-THURSDAY NIGHT. THIS WILL CREATE A
REAL MESS OVR FA. INITIAL WARM ADVECTION PRECIP MAY BEGIN AS
SNOW...WITH LONGEST RESIDENCE TIME OF FROZEN PRECIP BEING OVR N
CNTRL IL...BUT OF ADDED CONCERN IS PROSPECT OF SOME SIGNIFICANT
ICING FROM FREEZING RAIN/SLEET COMBO OVR MUCH OF THE REGION WED
AND WED NIGHT AS COLD HIGH PRESSURE SLOWLY BUILDS OVER
ONTARIO...JUST TO NE OF LK SUP. THIS SHOULD MAINTAIN COLD LLVL ELY
FLOW INTO WED EVENING...BEFORE HIGH GETS DISPLACED FURTHER TO E
AND LLVL FLOW VEERS A LTL MORE INTO SE. BOTH 00Z RUNS OF GFS AND
NAM ADVERTISE SOUNDINGS THAT KEEP SFC TEMPS BLO FREEZING THROUGH
WED NIGHT OVR MUCH OF NRN IL ALONG AND N OF I-80.

EVENTUALLY...ENTIRE FA SHOULD CHG OVER TO ALL RAIN. THIS IS
EXPECTED TO TAKE PLACE WED EVENING OVR SERN FA...BUT MAY NOT
OCCUR UNTIL THURSDAY MORNING IN RFD AREA...WHERE SVRL INCHES OF
SNOW AND ICE MAY ACCUMULATE AS WAA CRANKS UP LATE WED-WED NIGHT.
COULD SEE ADVSRY CONDITIONS N CNTRL AND FAR NRN IL BEFORE CHANGE
OVER TO LIQUID.

WILL LIKELY BE FLIPPING BACK TO OCNL LIGHT SNOW OR FLURRIES BY
CHRISTMAS MORNING AS COLD AIR WRAPS IN FROM SW.

MERZLOCK

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A MAJOR HOLIDAY STORM BREWING - JAC, 12/21/2009, 10:00 am
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