MESO: Mesoscale Discussion 0120

ACUS11 KWNS 071941
SWOMCD
SPC MCD 071940
ILZ000-WIZ000-072315-

MESOSCALE DISCUSSION 0120
NWS STORM PREDICTION CENTER NORMAN OK
0140 PM CST THU FEB 07 2013

AREAS AFFECTED...SERN WI...N-CNTRL/NERN IL

CONCERNING...HEAVY SNOW

VALID 071940Z - 072315Z

SUMMARY...PRECIPITATION WILL CONTINUE TO TRANSITION TO SNOW FROM N
TO S THROUGH THE AFTERNOON. SNOW RATES OF 1 IN/HR WILL BE
LIKELY...IMPACTING METRO CHICAGO...ROCKFORD...AND MILWAUKEE.

DISCUSSION...REGIONAL MOSAIC RADAR IMAGERY DEPICTS A BROAD SHIELD OF
PRECIPITATION FROM THE MID-MS VALLEY TO SWRN LOWER MI. THIS ACTIVITY
LIES WITHIN THE WARM ADVECTION REGIME LEADING AN APPROACHING
SHORTWAVE TROUGH. THE BACK EDGE OF THIS ACTIVITY THAT ARCS FROM SWRN
WI INTO S-CNTRL IA ALIGNS WITH AN 850-MB DEFORMATION ZONE WNW OF A
NRN-IL LOW-LEVEL CYCLONE. AS THE CYCLONE GRADUALLY TRACKS ENEWD
DURING THE NEXT SEVERAL HOURS...AND FORCING FOR ASCENT PERSISTS
WHILE BEING AUGMENTED BY STRENGTHENING FRONTOGENETIC
CIRCULATIONS...PRECIPITATION WILL LIKELY CONTINUE INTO THE EVENING
HOURS.

WHERE PRECIPITATION IS CURRENTLY FALLING AS RAIN -- I.E. CNTRL/SRN
PARTS OF THE MCD AREA -- SOUNDINGS FROM AMDAR FOR APPROACHES TO/FROM
CHICAGO-OHARE/MIDWAY/MILWAUKEE AIRPORTS SUGGEST THAT IT WILL TAKE
LITTLE ADDITIONAL COOLING ALOFT FOR SNOW TO BECOME THE PREDOMINANT
PRECIPITATION TYPE. IN THESE AREAS...SHALLOW...1-2C WARM LAYERS ARE
NOTED BELOW THE 850-MB LEVEL. ADIABATIC COOLING ASSOCIATED WITH
FORCED ASCENT ACCOMPANYING THE FRONTOGENETIC RESPONSE SHOULD ALLOW
FOR WET-BULB-ZERO HEIGHTS TO BECOME SUFFICIENTLY LOW FOR SNOW TO
BECOME THE PREDOMINANT PRECIP TYPE FROM N TO S ACROSS THE MCD AREA.

SPECIFICALLY...SFC OBSERVATIONS AND REPORTS FROM THE NSSL
PRECIPITATION IDENTIFICATION NEAR THE GROUND /PING/ PROJECT SUGGEST
THAT MODERATE TO HEAVY SNOW IS ONGOING ALONG AND N OF A ROCK COUNTY
WI TO LAKE COUNTY IL LINE. BY 22Z...SNOW IS FORECAST TO BE THE
PREDOMINANT PRECIP TYPE ALONG AND N OF A LINE FROM DOWNTOWN CHICAGO
TO ROCKFORD. BY 00Z...THE RAIN/SNOW LINE SHOULD EXTEND FROM E OF
STERLING IL TO THE S SUBURBS OF CHICAGO.

GIVEN THE MAGNITUDE OF THE AFOREMENTIONED ASCENT OVERLAPPING WITH
THE NNEWD-EXTENSION OF A 0.5-0.7-INCH PW PLUME PER GPS DATA...AND A
DEEP ISOTHERMAL LAYER AROUND/JUST BELOW 0C AOB THE 725-MB LEVEL PER
SOUNDINGS -- AMDAR AND FORECAST -- HEAVY SNOW FROM EFFICIENT
AGGREGATION PROCESSES WILL BE LIKELY FOLLOWING THE RAIN-TO-SNOW
TRANSITION. SNOW RATES OF 1 IN/HR ARE ANTICIPATED...AS PER
COORDINATION WITH THE ROMEOVILLE IL AND MILWAUKEE/SULLIVAN WI
WFO/S...WITH IMPACTS ANTICIPATED ACROSS METRO
CHICAGO...ROCKFORD...AND MILWAUKEE.

..COHEN.. 02/07/2013

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...LOT...MKX...

LAT...LON 41648894 42168913 42788873 42998796 42878782 42418776
41908765 41568774 41438833 41648894

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