Thursday, February 26, 2009

MESO: Mesoscale Discussion 0146

ACUS11 KWNS 261234
SWOMCD
SPC MCD 261233
WIZ000-MNZ000-IAZ000-NEZ000-SDZ000-NDZ000-261800-

MESOSCALE DISCUSSION 0146
NWS STORM PREDICTION CENTER NORMAN OK
0633 AM CST THU FEB 26 2009

AREAS AFFECTED...PORTIONS NERN NEB...NERN/N-CENTRAL IA...SERN
ND...ERN SD...CENTRAL/SRN MN...NWRN WI.

CONCERNING...WINTER MIXED PRECIPITATION

VALID 261233Z - 261800Z

COMPLEX AND OCCASIONALLY OVERLAPPING MESOSCALE WINTER PRECIP
SCENARIO WILL MOVE GENERALLY EWD ACROSS DISCUSSION AREA THROUGH 18Z
CHARACTERIZED PRIMARILY BY THESE REGIMES...

1. HEAVY SNOW GENERALLY N OF I-90 FROM ERN PORTIONS ND/SD LINE
ACROSS NERN SD...WRN/CENTRAL MN AND EVENTUALLY NW WI. GENERAL RATES
1-2 INCHES/HOUR...UPWARD TO 3 INCHES/HOUR IN BANDED SNOWFALL MAXIMA
-- ARE EXPECTED. MOST PROLONGED AREA OF HEAVY SNOW IS FCST TO SHIFT
FROM ND/SD BORDER AREA EWD-ESEWD OVER REGION BETWEEN ATV-FFM-MSP.
ELEVATED CONVEYOR OF STRONGEST LARGE-SCALE ASCENT -- JUXTAPOSING LOW
LEVEL WAA AND MIDLEVEL DPVA AHEAD OF EJECTING WRN SD SHORTWAVE
TROUGH -- WILL SPREAD/SHIFT EWD FROM ERN SD AND NERN NEB TOWARD
CENTRAL/SRN MN AND NRN IA. VERY DEEP LAYER OF ASCENT IS EVIDENT IN
THIS REGIME PER RUC CROSS-SECTION ANALYSIS AND PLANAR ISENTROPIC
ASCENT PROGS ON 288-300K SURFACES...WITH IDEAL DENDRITIC GROWTH
ZONES INCLUDED IN CONVEYOR OF STRONGEST LIFT.

2. MIXED PHASE TRANSITION WILL OCCUR ACROSS PORTIONS SERN SD...NERN
NEB...EXTREME SRN MN AND NRN IA...WITH POCKETS OF SLEET AND FREEZING
RAIN AT .05-.25 INCH/HOUR LIQUID EQUIVALENT WITHIN SRN PORTION OF
NEWD-RETREATING AREA OF 1-2 INCH/HOUR SNOWS. MOISTURE
ADVECTION/TRANSPORT IN WAA CONVEYOR WILL LEAD TO SLGT COOLING AND
NEAR-SATURATION OF WEDGE OF SUPERFREEZNG AIR ABOVE SFC THAT WILL
REMAIN WARM ENOUGH TO SUPPORT MIXED PRECIP TYPE. 12Z SFC ANALYSIS
SHOWED WAVY 32 F ISOTHERM JUST N OF I-80 IN IA THEN NWWD TO NEAR
ONL. SLGT/LOCALIZED NWD MOVEMENT IS POSSIBLE...THOUGH STG PRESSURE
FALLS AND RELATED NELY BACKING OF SFC WINDS N OF FREEZING LINE WILL
MAINTAIN WEAK CAA AND CONTRIBUTE TO WET BULBING EFFECTS THAT SLOW
NWD SHIFT OF MELTING LINE.

3. TSTMS PRODUCING HIGHLY LOCALIZED BUT SHORT-LIVED SLEET/FREEZING
RAIN RATES TO .25-.75 INCH/HOUR -- ALONG WITH HAIL UP TO NEAR SVR
LIMITS -- FROM NERN NEB ACROSS WRN IA. SVR HAIL POTENTIAL -- WHILE
NONZERO -- IS SECONDARY TO HAZARD FROM FREEZING/FROZEN PRECIP. DPVA
ALOFT AND LOW LEVEL WAA JUST AHEAD OF UPPER WAVE IS STEEPENING
LOW-MIDLEVEL LAPSE RATES. ISENTROPIC ASCENT TO LFC WILL CONTINUE TO
SUPPORT CONVECTIVE PRECIP GENERATION AMIDST ELEVATED MUCAPE 400-800
J/KG AND EFFECTIVE SHEAR 35-45 KT -- BASE ON MODIFIED 12Z RAOBS AND
RUC SOUNDINGS. THIS WILL SUPPORT STG CONVECTION AND POTENTIAL FOR
LOCALLY HEAVY MIXTURE OF PRECIP TYPES WITHIN DOWNDRAFT PLUMES.

..EDWARDS.. 02/26/2009

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

...NOTICE...
SPC WINTER WEATHER MESOSCALE DISCUSSIONS WILL BE ISSUED ON A
SCHEDULED BASIS FROM DECEMBER 1 THROUGH FEBRUARY 28...FOUR
TIMES A DAY...AROUND 00 UTC...06 UTC...12 UTC...AND 18 UTC
FOR SIGNIFICANT SHORT RANGE WINTER WEATHER EVENTS.

ATTN...WFO...DLH...ARX...MPX...DMX...FGF...FSD...OAX...ABR...
BIS...

LAT...LON 42219686 42799743 44299702 45459819 46009945 46439842
46489584 46069366 45579252 45009224 44429256 43089332
42319373 42119585 42219686

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