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Mesoscale Discussion 27 |
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MESOSCALE DISCUSSION 0027
NWS STORM PREDICTION CENTER NORMAN OK
1225 PM CST MON JAN 05 2015
AREAS AFFECTED...PORTIONS OF SOUTHEAST SOUTH DAKOTA...SOUTHERN
MINNESOTA...IOWA...FAR WESTERN ILLINOIS
CONCERNING...HEAVY SNOW
VALID 051825Z - 060030Z
SUMMARY...HEAVY SNOW WITH RATES OF 1.0 TO 1.5 INCHES PER HOUR WILL
SPREAD ESEWD ACROSS PORTIONS OF THE UPPER MIDWEST AND MIDDLE
MISSISSIPPI VALLEY THIS AFTERNOON INTO THE EARLY EVENING.
DISCUSSION...THE LATEST SFC ANALYSIS PLACES A 1014-MB LOW NEAR THE
MT/SD BORDER...WITH A WELL-DEFINED...QUASI-STATIONARY BOUNDARY
EXTENDING SEWD ACROSS SD AND FARTHER SE INTO THE MIDDLE MISSOURI
VALLEY. DIURNAL HEATING OF THE COLD/DRY AIR MASS S OF THE BOUNDARY
IS STEEPENING LAPSE RATES IN A SHALLOW...NEAR-SFC LAYER ON THE
RELATIVELY WARMER SIDE OF THE FRONT. WITH ADDITIONAL DIABATIC
HEATING DURING THE NEXT FEW HOURS FURTHER REDUCING LOW-LEVEL STATIC
STABILITY...THIS TRANSLATES TO /1/ INCREASED PROPORTION OF
WAA-RELATED RESPONSE DISTRIBUTED TO ISENTROPIC ASCENT ATOP THE
SLOPING FRONT...AND /2/ BOLSTERED SFC CYCLOGENESIS AS A
LOW-STATIC-STABILITY-ENHANCED GEOSTROPHIC-ADJUSTMENT RESPONSE TO
DCVA PRECEDING A MIDLEVEL SHORTWAVE IMPULSE NOW CROSSING THE NRN
HIGH PLAINS PER MOISTURE-CHANNEL IMAGERY. WITH A CORRESPONDING
INCREASE IN LOW/MID-LEVEL ASCENT...ESPECIALLY AS AN 850-700-MB
FRONTOGENETIC BAND BECOMES ESTABLISHED ACROSS THE DISCUSSION AREA
DURING THE NEXT FEW HOURS...SNOW WILL SPREAD ACROSS THE REGION THIS
AFTERNOON AND BECOME HEAVY.
THE 12Z OAX AND MPX RAOBS SUGGEST THAT THE BULK OF THE THERMODYNAMIC
PROFILE REMAINS BELOW -10C...YIELDING SHALLOW/LOW-LEVEL DENDRITIC
GROWTH ZONES BENEATH STRONGER ASCENT. REGARDLESS...SUCH COLD
PROFILES WILL SUPPORT HIGH SNOW-TO-LIQUID RATIOS...WITH RECENT SREF
OUTPUT SUGGESTING 15:1 TO PERHAPS AS HIGH AS 25:1 RATIOS ACROSS THE
REGION. THIS WILL TRANSLATE TO SEVERAL HOURS OF 1.0-1.5-INCH/HOUR
SNOWFALL RATES SPREADING ESEWD ACROSS THE AREA...ESPECIALLY AFTER
20Z/21Z.
THE HEAVIEST SNOW IS ANTICIPATED IN A 50-60-MILE-WIDE CORRIDOR
CENTERED ALONG A LINE FROM 30 SSE SIOUX FALLS SD TO 25 NNE DES
MOINES IA TO 15 NNW PEORIA ILLINOIS. THIS INCLUDES AREAS NEAR...AND
PARTICULARLY NORTH OF...THE DES MOINES IA AREA. BY EARLY
EVENING...THE HEAVIER SNOW WILL BE SHIFTING ESE OF THE MS
RIVER...WITH SNOWFALL DECREASING IN COVERAGE AND INTENSITY ACROSS
THE DISCUSSION AREA FROM WNW TO ESE THROUGH THE EVENING.
..COHEN.. 01/05/2015
ATTN...WFO...LOT...ILX...DVN...ARX...MPX...DMX...FSD...
LAT...LON 43889771 44329646 43489288 42419053 42038984 41478940
40858990 40659137 41229326 42919678 43889771
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