Prognostic Meteorological Discussion
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000
FXUS02 KWBC 221605
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1104 AM EST MON DEC 22 2014

VALID 12Z THU DEC 25 2014 - 12Z MON DEC 29 2014

...FORECAST PATTERN...

TO BEGIN THE PERIOD ON CHRISTMAS MORNING...A PAIR OF SIGNIFICANT
SYSTEMS WILL BE ON THE MAP DOMAIN WITH A FULL-LATITUDE TROF
ANCHORING THE EASTERN SEABOARD WHILE A WELL DEVELOPED SHORTWAVE
WILL BE SEEN DIGGING THROUGH THE DESERT SOUTHWEST. THE FORMER
FEATURE IS FORECAST TO EVENTUALLY EVOLVE INTO A CLOSED LOW WITH
RAPID CYCLOGENESIS TAKING PLACE OVER QUEBEC. AS A BROAD SURFACE
HIGH BUILDS ACROSS THE EAST ON FRIDAY...THE AFOREMENTIONED WAVE
ACROSS THE SOUTHWESTERN U.S. WILL GATHER STRENGTH ACROSS THE
CENTRAL PLAINS WITH A PRONOUNCED SURFACE LOW TRACKING THROUGH THE
MIDDLE MS VALLEY AND INTO THE UPPER GREAT LAKES BY LATE FRIDAY.
THIS FEATURE IS NOT COMPLETELY RESOLVED YET AS SOME OF THE MODELS
SHOW A SECONDARY BATCH OF SHORTWAVE ENERGY TRAILING BEHIND IT WITH
EVENTUAL AMPLIFICATION BY SATURDAY OVER THE SOUTHERN
PLAINS/ARKLATEX REGION. TOWARD THE END OF THE PERIOD SPANNING LATE
IN THE WEEKEND THROUGH EARLY NEXT WEEK...A STRONG UPPER RIDGE
SHOULD BUILD ACROSS THE EASTERN PACIFIC WITH HEIGHTS BEGINNING TO
FALL DOWNSTREAM ACROSS THE WESTERN U.S. THIS IS EXPECTED TO LEAD
TO GENERAL SOUTHWESTERLY ALOFT FROM THE GREAT PLAINS EASTWARD
DURING THE LATTER COUPLE DAYS OF THE MEDIUM RANGE.


...MODEL UNCERTAINTY/WPC PREFERENCES...

THE GUIDANCE SEEMS TO HAVE A REASONABLE HANDLE ON THE EASTERN
SYSTEM ON CHRISTMAS DAY WHILE THERE ARE DIVERGING OPINIONS OF THE
SHORTWAVE ACROSS THE WEST COAST. THE 00Z ECMWF/UKMET AND 06Z
GFS-PARALLEL WERE IN THE SLIGHTLY FASTER CAMP RELATIVE TO THE 06Z
GFS/00Z CMC. THE TREND APPEARED TO BE IN FAVOR OF THE
ECMWF/UKMET/GFS-PARALLEL CAMP BASED ON THE PAST FEW SPAGHETTI
PLOTS. THIS PARTICULAR SHORTWAVE BECOMES THE INITIAL TROUBLE SPOT
ACROSS THE CONUS WITH MODELS VARYING IN TIMING ALONG WITH WHETHER
OR NOT THE SYSTEM EJECTS OUT IN TWO DISCRETE PARTS. THE PAST
SEVERAL RUNS OF THE GFS REMAIN QUICKER WITH THE PATTERN ACROSS THE
COUNTRY WHICH SEEMS TO BE AN OUTLIER AT THIS TIME. ON THE MUCH
SLOWER SIDE IS THE 00Z UKMET WHICH ACTUALLY CLOSES OFF AN UPPER
LOW OVER THE SOUTHWESTERN STATES ON FRIDAY. THE 00Z ECMWF/06Z
GFS-PARALLEL OFFER A NICE MIDDLE GROUND SOLUTION WHICH SHOW AN
INITIAL SHORTWAVE LIFTING THROUGH THE CENTRAL PLAINS/MIDDLE MS
VALLEY ON FRIDAY WITH ADDITIONAL HEIGHT FALLS APPROACHING THE
SOUTHERN HIGH PLAINS BY THE FOLLOWING DAY. DETERMINISTIC GUIDANCE
BECOMES MUCH MORE DIFFICULT TO USE BY THE WEEKEND ONWARD GIVEN A
LACK OF RUN-TO-RUN CONTINUITY AND DIFFERENCES FROM ONE ANOTHER.
THE ISSUE AT HAND APPEARS TO BE FROM A PRONOUNCED SHORTWAVE
DROPPING DOWN FROM THE NORTHEAST PACIFIC TOWARD THE PACIFIC
NORTHWEST COAST. EXACTLY WHERE THIS SYSTEM DIGS INTO THE WESTERN
U.S. IS A BIG QUESTION MARK WITH THE GFS SUITE AND 00Z CMC BEING
WELL EAST OF THE SLOWER ECMWF SOLUTION SUITE. EVEN THE FLOW
DOWNSTREAM IS RATHER UNCERTAIN AS THE PAST SEVERAL ECMWF RUNS
DEPICT A STRONG SHORTWAVE EMBEDDED IN THE SOUTHWESTERLY FLOW
ALOFT. FELT INCLINED TO UTILIZE ENSEMBLE MEANS SLIGHTLY EARLIER
THAN NORMAL GIVEN ALL OF THE ABOVE. FOR DAYS 3 AND 4...THE
SIMILARITY OF THE 00Z ECMWF/06Z GFS-PARALLEL ALLOWED FOR ITS USE
BEFORE SHIFTING TO A COMBINATION OF THE 00Z ECMWF ENSEMBLE/NAEFS
MEANS THEREAFTER. SLIGHTLY WEIGHTED IT TOWARD THE 00Z ECMWF
ENSEMBLE MEAN AS IT CAPTURES THE MULTI-DAY SLOWING TREND IN THE
SPAGHETTI PLOTS.


...SENSIBLE WEATHER HIGHLIGHTS...

ON CHRISTMAS DAY...EXPECT CONVECTION TO BE EXITING THE
SOUTHEASTERN U.S. IN RESPONSE TO THE STRONG COLD FRONT MOVING
THROUGH WHILE COMMA-HEAD PRECIPITATION MAY BE ONGOING ACROSS UPPER
NEW ENGLAND. OUT WEST...WET CONDITIONS SHOULD PREVAIL OVER MUCH OF
THE WESTERN U.S. GIVEN THE TROF DIGGING THROUGH THE REGION.
CONDITIONS BECOME MORE QUIET ACROSS THE EAST WITH A SURFACE HIGH
ANCHORING THE AREA. IT IS EXPECTED TO BECOME A BIT MORE ACTIVE
ACROSS THE CENTER OF THE COUNTRY AS THE HEIGHT FALLS SPREAD
EASTWARD INTO THE GREAT PLAINS. OF COURSE IF THE SLOWER SOLUTIONS
LIKE THE 00Z ECMWF/UKMET VERIFY...IT COULD BECOME QUITE WET ACROSS
THE WESTERN/CENTRAL GULF COAST AND ALONG THE ADVANCING COLD FRONT.
THE PATTERN BECOMES A BIT MORE NEBULOUS THEREAFTER DEPENDING ON
THE POSITION OF THE WESTERN U.S. TROF. HOWEVER...THIS SHOULD BRING
ANOTHER ROUND OF PRECIPITATION TO SECTIONS OF THE WEST AT THE VERY
LEAST.

IN TERMS OF TEMPERATURES...READINGS AT 10 TO 15 DEGREES ABOVE
NORMAL WILL BE COMMONPLACE OVER THE CENTRAL U.S. ON THE CHRISTMAS
DAY. THESE POSITIVE ANOMALIES WILL GRADUALLY SHIFT EASTWARD AHEAD
OF THE ADVANCING COLD FRONT. MEANWHILE...BELOW NORMAL TEMPERATURES
SHOULD SPILL INTO MUCH OF THE U.S. TOWARD THE LATTER HALF OF THE
PERIOD AS A CONTINENTAL POLAR AIR MASS FILLS THE REGION.


RUBIN-OSTER

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