FOUS11 KWBC 122039
QPFHSD
 
PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
338 PM EST THU FEB 12 2009
 
VALID 00Z FRI FEB 13 2009 - 00Z MON FEB 16 2009
 
DAY 1...
NRN CA...SRN OR..
DEEP LAYER TROUGH APPROACHING THE WEST COAST WILL PRODUCE A LARGE
AREA OF DIFLUENT FLOW ALOFT STARTING IN SOUTHWEST OR AND
CONTINUING SOUTH ACROSS CA.  AREAS OF DIVERGENCE ALOFT AND LOW-MID
LEVEL WARM ADVECTION COMBINE WITH UPSLOPE FLOW AND PW VALUES
PEAKING NEAR HALF AN INCH TO PRODUCE SNOW IN THE ELEVATED
LOCATIONS OF SWRN OREGON AND CA.  THE
UKMET/ECMWF/SREFMEAN/CANADIAN GLOBAL MODELS ARE IN GOOD AGREEMENT
ON THE TOTAL LIQUID EQUIVALENT PRECIPITATION AND A BLEND OF THESE
SOLUTIONS WAS USED FOR MANUAL PROGS.    
CNTRL ROCKIES TO HIGH PLAINS...
PROGRESSIVE DEEP LAYER SHRTWV WILL DEPART THE CENTRAL ROCKIES ACRS
THE CENTRAL PLAINS...WITH A CLOSED SFC-850 MB LOW DEVELOPING. 
SNOW DEVELOPS ACRS EASTERN CO/WY AND INCREASES IN COVERAGE ACROSS
NEBRASKA AS THE CLOSED 850 MB LOW GRADUALLY STRENGTHENS...ALONG
WITH AN INCREASE IN THE EASTERLY 850 MB WINDS IN THE CONVERGENCE
ZONE IN SOUTHERN NEBRASKA.  PW VALUES INCREASE TO NEAR 0.4 INCHES
IN SOUTHERN NEBRASKA TO SUPPORT SLIGHTLY HIGHER AMOUNTS FROM
EASTERN CO/WY.  MANUAL PROGS USED A BLEND OF THE 12Z
ECMWF/GFS/NAM/09Z SREFMEAN TO INDICATE THE RISK OF FOUR AND EIGHT
INCHES OF SNOW IN THE REGION. SYSTEM OVER THE HIGH PLAINS OVER NRN
CO...SERN WY...AND WRN NEB WHERE SEVERAL INCHES OF SNOW MAY
ACCUMULATE BY FRI MORNING.  
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY 2...
CA TO GREAT BASIN...
PAC TROUGH WILL PUSH INTO CA/NV DURING D2 WITH AREAS OF HVY SNOW
FROM THE CA MTNS TO THE GREAT BASIN AND WASATCH.  LOW SNOW LEVELS
WITH LOCALIZED 1 FOOT AMTS ARE INDICATED ACROSS THE NORTHERN
SIERRA/SISKIYOUS/SHASTAS. PERSISTENT MOIST SOUTHWEST TO WEST FLOW
OCCURS IN A WARM ADVECTION PROFILE AND LG AREAS OF 90-99 PERCENT
LAYER SFC-600 MB RH.    THIS SYSTEM WILL BRING SOME 4-6 INCH PLUS
TOTALS IN FAVORED UPSLOPE AREAS AS THE UPPER DIVERGENCE MAX
ACCOMPANIES THE UPPER TROUGH AS IT MOVE EAST ACRS THE GREAT BASIN
THE FIRST HALF OF THIS PERIOD BEFORE TRAVERSING THE WEST CO
ROCKIES AFTERWARD. 
CNTRL PLAINS TO MIDWEST...
SHRTWV AND ASSOCD SFC LOW CROSSING THE CNTRL PLAINS WILL PROGRESS
QUICKLY EWD AND FLATTEN WHILE REACHING INTO THE ZONAL FLOW.  THE
GFS/GFS PARALLEL/ECMWF/UKMET ARE THE PREFERRED MODELS WITH THE
SNOW EXPECTED TO DECREASE IN COVERAGE/INTENSITY. THE NAM WAS UNDER
WEIGHTED AS A RESULT OF ITS OUTLIER STRONGER SOLUTION. A LOW RISK
OF FOUR INCHES IS SHOWN AS CONSENSUS QPF IS UNDER A QUARTER INCH
AND THE 850 MB LOW OPEN UP INTO AN OPEN WAVE...RESULTING IN
WEAKENING LOW LEVEL CONVERGENCE. 
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
DAY 3...
CA...
THE MODELS ALL INDICATE A CLOSED LOW SFC/ALOFT GRADUALLY
APPROACHES AND DIFLUENT FLOW ALOFT SUPPORTS LG AREAS OF DIVERGENCE
ALOFT.  THIS RESULTS IN MDT TO HEAVY PRECIP IN CA.  A SUSTAINED
VEERING WIND PROFILE WITH A 700 MB JET NEAR 50 KT SUPPORTS ASCENT
ACROSS THE SHASTA/SISKIYOUS/NORTHERN SIERRAS.  PHASING DIFFERENCES
AND TIMING DIFFERENCES WITH THE SFC LOW RESULT IN A WIDE QPF
SPREAD ACRS THE STATE.  THE NAM WAS AN OUTLIER WITH ITS MASS AND
QPF FIELDS AND WAS GIVEN LITTLE WEIGHT.  MANUAL PROGS BLENDED THE
ECMWF/CANADIAN/GFS MODEL SOLUTIONS.  HVY QPF TOTALS INDICATED ON
THE ECMWF/GFS AND CANADIAN GLOBAL SUPPORT THE POSSIBILITY OF
SEVERAL FEET OF SNOW OVER A MULTI-DAY PERIOD WITHIN THE FAVORABLE
OROGRAPHIC LOCATIONS.
THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.
PETERSEN
$$