Issued by NWS
000 FOUS11 KWBC 222004 QPFHSD PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD 302 PM EST SUN NOV 22 2009 VALID 00Z MON NOV 23 2009 - 00Z THU NOV 26 2009 DAY 1... NORTHERN ROCKIES... UPPER-LEVEL TROUGH AND WEAKENING SURFACE WAVE MIGRATES THROUGH THE PACIFIC NORTHWEST THIS EVENING...AND INTO THE NORTHERN DIVIDE TONIGHT. BEST SNOWFALL POTENTIAL WILL BE ALONG WEST EXPOSURES ABOVE 3500 FT MSL WITH MOIST LOW AND MID-LEVEL FLOW COMBINING WITH STEEP LAPSE RATES TO PRODUCE SCATTERED SNOW SHOWERS AT LOWER ELEVATION WITH MINOR ACCUMULATIONS. BEST OROGRAPHIC PRECIPITATION PREVAILS TONIGHT ALONG THE IDAHO/MONTANA BORDER AND WESTERN SLOPES OF THE TETONS/WASATCH...WITH THE SURFACE COLD FRONT SPREADING OUT ACROSS THE NORTHERN GREAT BASIN...REACHING THE DIVIDE OF WY/CO BY MONDAY MORNING/AFTERNOON. MANUAL SNOW GRAPHICS LEANED UPON THE PRELIM HPC DAY1 QPFS FOR THE SNOWFALL AMOUNTS ACROSS THE NORTHWEST US. THE PROBABILITY OF SIGNIFICANT ICING AND/OR SNOW IS LESS THAN 10 PERCENT. DAY 2... CENTRAL PLAINS... UPPER-LEVEL TROUGH AND H5 LOW CENTER WILL REINFORCE THE COLD FRONT MOVING OUT OF THE NORTHERN ROCKIES AND INTO THE NORTHERN HIGH PLAINS ON MONDAY NIGHT. THE MODELS HAVE BEEN IN GENERAL AGREEMENT WITH THE CLOSING OFF OF A H5-H7 CIRCULATION OVER WEST CENTRAL NEBRASKA...THEN SLOWLY ARCING THE TRACK OF THIS MID-LEVEL CIRCULATION ACROSS SOUTHERN NEBRASKA AND ALONG THE IOWA/MISSOURI BORDER THROUGH TUESDAY NIGHT WITH THE SYSTEM BECOMING VERTICALLY STACKED FROM THE SFC TO H5 OVER SERN NEBRASKA. VORTICITY AND JET-LEVEL DYNAMICS DEVELOPING OVER THE NEBRASKA PANHANDLE WILL BE STRONG...BUT NEGATED BY EQUALLY IMPRESSIVE SUBSIDENCE/DOWNWARD MOTION AND DRY NORTHWEST FLOW PUSHING OFF THE FRONT RANGE BEHIND THE COLD FRONT. WITH ISOLATED ELEVATED CONVECTION AND SNOWFALL POTENTIAL BRIEFLY LIMITED TO THE NORTHEAST QUADRANT OF THE DEVELOPING MID-LEVEL CIRCULATION...THE MANUAL GRAPHICS LINED UP WITH THE PRELIM DAY 1 HPC QPF. DOWNSTREAM ACROSS CENTRAL NEBRASKA TO THE MISSOURI RIVER...GOOD UP GLIDE AND LOW-LEVEL MOISTURE WRAPPING INTO THIS STACKED SYSTEM WITH A WELL-DEFINED SFC FEATURE...WILL ALLOW FOR ISOLATED BURSTS OF SNOWFALL WITHIN THE COLD SECTOR. HOWEVER...FROM THE SYNOPTIC SCALE PERSPECTIVE...THE STORM TRACK...LACK OF A GOOD LLJ AND DRY NORTHWEST FLOW SURGING QUICKLY IN BEHIND THE SFC COLD FRONT WOULD NOT FAVOR HIGH QPFS AND HEAVY SNOWS ACROSS WESTERN NEBRASKA. THE BRIEF APPEARANCE OF A MID-LEVEL DRY SLOT ENTRAINING INTO THE SYSTEM WILL ALSO SEVERELY LIMIT THE POTENTIAL FOR WIDESPREAD HEAVY SNOWFALL WITHIN THE DEFORMATION PRECIPITATION SHIELD/COMMA HEAD...NORTHWEST OF THE SFC LOW TRACK ACROSS THE MID MISSOURI VALLEY. THE MANUAL SNOW FORECAST WILL CARRY A SLIGHT RISK OF 4 INCHES TO RESPECT THE STRONG DYNAMICS AND COLD POOL ALOFT. IN A NARROW BAND...ARCING SW TO NE...BETWEEN 24/06Z-15Z...SNOW WILL LIKELY DEVELOP ACROSS EASTERN NEBRASKA ALONG THE NORTHWEST QUADRANT OF THE STACKED SYSTEM WHERE H7 VV PROGS AND BEST DENDRITIC GROWTH HAS HIGHEST POTENTIAL. BUT CONFIDENCE IS LOW THAT THE DYNAMICS...UP GLIDE AND MOISTURE AXIS WILL BE SUSTAINED AND COMPLETELY OVERWHELM THE DRIER AIR FOR MORE THAN A 6-9 HOUR PERIOD. PARTICULARLY DURING THE DAYLIGHT HOURS OF DAY 2...WHEN THE COLUMN/THERMAL PROFILES WILL BE MARGINAL FOR SNOW. THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT. DAY 3... THE PROBABILITY OF SIGNIFICANT ICING AND/OR HEAVY SNOW IS LESS THAN 10 PERCENT. VOJTESAK $$
