[MCD] SWOMCD

[Mesoscale Convective Discussions]

ACUS11 KWNS 291829
SWOMCD
SPC MCD 291829 
SDZ000-NDZ000-300030-

MESOSCALE DISCUSSION 2599
NWS STORM PREDICTION CENTER NORMAN OK
1229 PM CST THU DEC 29 2005

AREAS AFFECTED...NORTHERN SOUTH DAKOTA...SOUTHERN NORTH DAKOTA

CONCERNING...WINTER MIXED PRECIPITATION 

VALID 291829Z - 300030Z

A SWATH OF MODERATE TO HEAVY SNOW LOCALLY PRECEEDED BY MIXED
PRECIPITATION IS EXPECTED ACROSS NORTHERN SOUTH DAKOTA AND SOUTHERN
NORTH DAKOTA THIS AFTERNOON WITH LOCALIZED SNOWFALL RATES
APPROACHING ONE INCH PER HOUR.

A UPPER LEVEL STRONG SHORT WAVE TROUGH EVIDENT IN WATER VAPOR
IMAGERY AND RUC ANALYSES OVER CENTRAL COLORADO WILL PROGRESS
EASTWARD AND COMBINE WITH LOW LEVEL CYCLOGENESIS TO PRODUCE DEEP
LAYER ASCENT OVER THE THREAT AREA THIS AFTERNOON. STRONG 800-700 MB
LAYER FRONTOGENESIS IN RUC FCST ALSO HELPS FOCUS ASCENT NEAR ND...SD
BORDER AREA THIS AFTERNOON.

AT 14Z... 105 KNOT WINDS AT 9KM WERE OBSERVED AT THE PLATTEVILLE CO
WIND PROFILER AS THE JET STREAK AXIS SLID EASTWARD ACROSS COLORADO.
SITUATION IS COMPLICATED BY A WEAKER LEAD SHORTWAVE CURRENTLY
EVIDENT IN WV AND RUC ANALYSES OVER SW SD CURRENTLY CONTRIBUTING TO
MODERATE SNOWFALL OVER SOUTHERN ND. BOTH RUC AND SREF APPEAR TO HAVE
A POOR HANDLE ON THIS FEATURE.

MODIFIED PACIFIC AIRMASS IS EVIDENT OVER PORTIONS OF NORTHWEST
NEBRASKA AND EASTERN WY AND MT WITH GPS PW OF NEARLY THREE QUARTER
INCH CURRENTLY OBSERVED NEAR CDR EARLIER TODAY. RUC AND NCEP SREF
AGREE THAT LOCALIZED INCH PER HOUR SNOWFALL RATES ARE POSSIBLE...IN
PARTICULAR LATE IN THE PERIOD. TYPICAL POLAR AIRMASS IS ABSENT
ACROSS THE EASTERN US SO COOLING OF THE LAYER THROUGH DEEP ASCENT
AND EVAPORATION WILL BE NECESSARY IN PORTIONS OF THE THREAT AREA
BEFORE THE ONSET OF FROZEN PRECIPITATION. THE PRESENCE OF THE POORLY
HANDLED LEAD SHORTWAVE COMBINED WITH OBSERVED MODERATE RAIN WITH
ABOVE FREEZING TEMPERATURE AT MBG SD AT 18Z SHIFTED FOCUS OF WINTER
PRECIPITATION SLIGHTLY NORTHWARD BUT CONFIRMS POTENTIAL FOR INCH AN
HOUR RATES IN SNOW AREAS. 12Z ABR SOUNDING HAD ONLY A SHALLOW
ABSOLUTELY STABLE LAYER WITH WET BULB TEMPERATURES ABOVE ZERO
DESPITE 150MB DEEP LAYER WITH A DRY BULB TEMPERATURE ABOVE FREEZING.
MODEL SOUNDINGS SUGGEST THAT PRECIPITATION WILL TRANSITION FROM
FREEZING RAIN TO MIXED PRECIPITATION AND MAINLY SNOW OVER NORTHEAST
SD BETWEEN 21 AND 03Z. HEAVIEST PRECIPITATION RATES WILL SPREAD FROM
WEST TO EAST ACROSS THE THEAT AREA THIS AFTERNOON AND EVENING.

..SCHNEIDER.. 12/29/2005

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

ATTN...WFO...FGF...FSD...ABR...BIS...UNR...

45980223 46640089 46709924 46209760 45419681 43999689
45099843 45559949 45510117 

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