Quantitative Precipitation Forecast
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000
FOUS11 KWBC 310810
QPFHSD

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
410 AM EDT FRI OCT 31 2014

VALID 12Z FRI OCT 31 2014 - 12Z MON NOV 03 2014

DAYS 1-3...

...CALIFORNIA/INTERMOUNTAIN WEST/NORTHERN ROCKIES...

A POWERFUL STORM SYSTEM OUT OVER THE PAC WILL GRADUALLY DRIFT
TOWARDS CA ON FRI.  IN ADVANCE OF THE UPPER TROUGH...A JET STREAK
WILL CROSS CENTRAL CA AND CENTRAL TO NORTHERN NV.  PAC MOISTURE
TRANSPORT WITH VERY FAVORABLE OROGRAPHICS WILL PRODUCE NEEDED
PRECIPITATION WITH SNOW ACROSS THE CENTRAL/SRN SIERRA.   SEVERAL
INCHES ARE EXPECTED WITH UP TO A FOOT OF SNOW IS POSSIBLE NEAR THE
SIERRA CREST.
ON SAT (DAY 2) AS THE UPPER TROUGH CROSSES INTO THE GREAT BASIN...
A SURFACE LOW WILL FORM AND UPPER LEVEL DIVERGENCE/MID LEVEL
DEFORMATION  IS MAXIMIZED OVER CENTRAL ID TO WESTERN MT.  THE
MODELS ARE CONVERGING ON SNOWS WITH OROGRAPHIC ENHANCEMENT FOR
SIGNIFICANT ACCUMULATIONS FOR THE RANGES OF THIS AREA.  ON DAY 2.
TIMING DIFFERENCES START TO DEVELOP ON DAY 3 AS THE 700 MB LOW
DEPARTS OUT OF MT INTO CANADA...WITH SUBSEQUENT DECREASE ON SHOW
SHOWER COVERAGE/INTENSITY.  THE 12Z THU ECMWF WAS SLOWER BUT THE
00Z ECMWF WAS A FEW HOURS FASTER...RESULTING IN LESS PRECIP AND
SUBSEQUENT POTENTIAL SNOW AMOUNTS. MANUAL PROGS BLENDED THE 2
ECMWF RUNS AND 12Z SREF TO ACCOUNT FOR QPF/TEMPERATURE PROFILE
DIFFERENCES.

...CENTRAL/SOUTHERN APPALACHIANS...

A ZONE OF MID LEVEL CYCLONIC VORTICITY ADVECTION CROSSES THE
WESTERN GREAT LAKES FRI BEFORE PROCEEDING INTO THE OH/TN
VALLEYS. AN AREA OF ENHANCED MOISTURE AND ASCENT IN ADVANCE OF THE
700 MB FRONT PRODUCES PRECIPITATION.
THE FIRST SIGNIFICANT SNOW OF THE SEASON APPEARS LIKELY FRI
NIGHT-SAT ACROSS PARTS OF THE CENTRAL/SRN APPALACHIANS. THE
DYNAMIC SHORT WAVE TROUGH WILL PRODUCE A BAND OF WELL DEFINED MID
LEVEL DEFORMATION THAT ROTATES ACROSS THE CENTRAL TO SOUTHERN
APPALACHIANS.
A POCKET OF ENHANCED VERTICAL LIFT AND VERY COLD ANOMALOUS AIR
ALOFT WILL SURGE ACROSS THE APPALACHIAN SPINE TO COMBINE WITH
UPSLOPE FLOW FOR SNOWFALL IN THE MTNS AND ONLY MINOR ACCUMULATIONS
AWAY FROM THE TERRAIN.  THE MODEL/FORECAST SPREAD HAS NARROWED
CONSIDERABLY AND MANUAL PROGS RELIED HON A BLEND OF CONTINUITY AND
THE 21Z SREF MEAN/00Z NAM/ECMWF FOR QPF/TEMPERATURE PROFILES USED
IN GENERATING SNOW AMTS/PROBABILITIES.  SNOW SHOULD DECREASE IN
COVERAGE/INTENSITY AS THE UPPER TROUGH DEPARTS THE MOUNTAINS OVER
LOWER ELEVATIONS AND THEN THE COAST.

...MAINE...
THE MODELS INDICATE A PAIR OF LOWS TO FORM OVER THE WESTERN
ATLANTIC.  THE FIRST LOW MOVES NORTH OVER THE OPEN WATERS WITH
UPPER DIVERGENCE DEVELOPING/MID LEVEL FRONTOGENESIS OVER MAINE AS
A JET STREAK CROSSES EASTERN MAINE AND ADJACENT COASTAL WATERS.
SNOW DEVELOPS WITHIN THE FRONTAL CIRCULATION WITH SEVERAL INCHES
POSSIBLE.  ON SUN(DAY 3) THE SECOND LOW AT THE BASE OF THE UPPER
TROUGH INTENSIFIES AND BECOMES THE PRIMARY LOW.  THE MODEL ARE
STARTING TO CONVERGE ON TRACKS...BUT LOTS OF UNCERTAINTY REMAINS
BECAUSE THE CIRCULATION MOVING NORTH MAINTAINS AN UPPER
DIVERGENCE/MID LEVEL FRONTOGENESIS SIGNATURE THAT MAY PRODUCE
BANDED MODERATE TO HEAVY SNOW DEPENDING ON THE PHASING OF THE
UPPER TROUGH AND LOW LEVEL CYCLONE TRACK.  THE 00Z ECMWF INCREASED
AMOUNTS OVER EASTERN MAINE THIS RUN AS IT NUDGED THE LOW LEVEL
CYCLONE A BIT CLOSER TO MAINE.  THE HIGHER AMOUNTS IN THE GFS
RESULTS FROM A LITTLE FURTHER WEST TRACK WHILE THE CONVERSE IS
TRUE FOR THE UKMET AND CANADIAN GLOBAL MODELS.  LOW TO MODERATE
PROBABILITIES FOR SNOW ARE SHOWN GIVEN CONTINUING QPF UNCERTAINTY
FROM THE MODEL SPREAD AND SHARP QPF GRADIENTS IN MAINE. A
CONSENSUS APPROACH WAS TAKEN WITH ORE WEIGHTING ONT HE 00Z ECMWF
AND 21Z SREF MEAN.

THE PROBABILITY OF SIGNIFICANT ICING IS LESS THAN 10 PERCENT.

PETERSEN



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