Prognostic Meteorological Discussion
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000
FXUS10 KWNH 270445
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1144 PM EST FRI DEC 26 2014

VALID DEC 27/0000 UTC THRU DEC 30/1200 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR
INGEST...

...AN EVALUATION OF THE 00Z NAM/GFS ALONG WITH PRELIMINARY
PREFERENCES/FORECAST CONFIDENCE...

THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 00Z
NAM/GFS DEEMED SIGNIFICANT ENOUGH TO IMPACT THEIR FORECASTS.


...FULL-LATITUDE TROF MOVING ACROSS THE MIDDLE OF THE COUNTRY ON
SATURDAY...
...ADVANCING COLD FRONT/LOW PRESSURE OVER THE GREAT LAKES...

PRELIMINARY PREFERENCE: MODEL COMPROMISE BETWEEN THE 00Z GFS AND
12Z ECMWF/UKMET
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

WATER VAPOR IMAGERY SHOWS A WELL DEFINED UPPER TROF ANCHORING THE
FOUR CORNERS REGION WITH A LEAD IMPULSE CURRENTLY LIFTING THROUGH
THE CENTRAL HIGH PLAINS. LIGHT TO MODERATE SNOW WAS NOTED FALLING
ALONG THE ASSOCIATED SURFACE BOUNDARY STRETCHING FROM THE CENTRAL
PLAINS UP INTO THE MIDDLE MS VALLEY/UPPER MIDWEST. THE INITIAL
SHORTWAVE IS FORECAST TO PUSH INTO THE UPPER GREAT LAKES ON
SATURDAY CARRYING THE DEVELOPING WAVE OF LOW PRESSURE ACROSS THE
LAKES AND EVENTUALLY INTO SOUTHERN ONTARIO. MODELS SEEMS TO HAVE A
DECENT GRASP ON THIS PATTERN WITH ONLY MINOR DIFFERENCES NOTED. A
GENERAL MODEL COMPROMISE IS SUFFICIENT WITH THIS ASPECT OF THE
SYSTEM.

WHILE THE INITIAL SHORTWAVE ENERGY MOVES INTO THE GREAT
LAKES...THE REMAINDER OF THE UPPER TROF WILL SLOWLY EJECT OUT OF
THE FOUR CORNERS REGION. AN IMPRESSIVE SHORTWAVE WILL CROSS THE
RIO GRANDE BY 28/1200Z WITH THE 00Z GFS TRENDING SLOWER CARRYING
IT CLOSER TO THE MODEL CONSENSUS. THIS LEAVES THE 18Z GEFS MEAN AS
A FASTER OUTLIER. IN TIME...THE SHORTWAVE IS EXPECTED TO GRADUALLY
SHEAR OUT AS IT RACES TOWARD THE MID-ATLANTIC BY LATE IN THE
WEEKEND. STRONG SOUTHWESTERLY FLOW ALOFT WILL ALLOW THE FRONTAL
ZONE TO HANG UP SOUTH OF THE MID-ATLANTIC TOWARD THE END OF THE
PERIOD WITH MODEL DIFFERENCES NOTED. MOST NOTABLY...THE 00Z NAM IS
MUCH SLOWER WHILE DEPICTING A SURFACE WAVE FARTHER NORTH THAN ANY
MODEL. THE SOLUTIONS DEPICTED BY THE 00Z GFS AND 12Z ECMWF/UKMET
LOOK REASONABLE AND ARE WITHIN THE REALM OF POSSIBILITIES IN THE
ENSEMBLE LOW PLOTS. WILL FAVOR A COMPROMISE BETWEEN THESE
SOLUTIONS.


...PRONOUNCED SHORTWAVE CROSSING THE PACIFIC NORTHWEST LATE
SATURDAY...
...VORTICITY STRETCHING OUT OVER THE CENTRAL U.S...

PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS OR 00Z GFS-PARALLEL
WITH THE 12Z ECMWF
FORECAST CONFIDENCE: AVERAGE

WITH A SPRAWLING UPPER RIDGE SET UP ALONG THE EASTERN PACIFIC...A
RATHER POTENT SHORTWAVE WILL BE ABLE TO DIVE FROM THE GULF OF AK
DOWN TOWARD THE PACIFIC NORTHWEST EARLY IN THE FORECAST. MODELS
AGREE THIS FEATURE WILL ENTER THE STATE OF WA BY 28/0000Z. AS THE
HEIGHT FALLS SLIDE EASTWARD...THE VORTICITY SHOULD GRADUALLY
BECOME MORE ELONGATED IN NATURE ACROSS THE GREAT PLAINS BY LATE
SUNDAY/EARLY MONDAY. THE BIGGEST OUTLIER IS THE 00Z NAM WHICH IS
SLOWER AND MORE AMPLIFIED OVER THE CENTRAL ROCKIES ON 29/1200Z.
THEREAFTER...THE ENSEMBLE MEANS WHICH SEEM TO BE PLACED MORE IN
THE MIDDLE GROUND SUGGEST THE 12Z CMC/UKMET ARE TOO QUICK TO EJECT
THE ENERGY TOWARD THE MS RIVER VALLEY. THE RECOMMENDATION IS TO
STAY CLOSER TO THE CENTER OF THE SPREAD WHICH MAKES A COMBINATION
OF EITHER THE 00Z GFS/GFS-PARALLEL AND 12Z ECMWF REASONABLE HERE.


...ROBUST UPPER TROF DIVING INTO THE WESTERN U.S. EARLY NEXT
WEEK...
...ASSOCIATED PLUNGE OF ARCTIC AIR...

PRELIMINARY PREFERENCE: A NON-12Z UKMET MODEL COMPROMISE
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

ON THE WINGS OF THE INITIAL SYSTEM CROSSING THE GREAT BASIN WILL
BE A SIGNIFICANT SHOT OF COLD AIR TO THE WEST AS A LARGE UPPER
TROF DROPS DOWN FROM FAR WESTERN CANADA. BY EARLY TUESDAY...A
BROAD POSITIVELY-TILTED TROF WILL ANCHOR THE WEST WITH PRETTY
REASONABLE AGREEMENT SUGGESTED ON THE 552-DM SPAGHETTI PLOT. THE
EXCEPTION IS THE 12Z UKMET WHICH IS MUCH MORE AGGRESSIVE ALOFT
WHICH LEADS TO A QUICKER SURFACE SOLUTION IN THE DESERT SOUTHWEST.
WPC RECOMMENDS A NON-12Z UKMET MODEL COMPROMISE GIVEN THIS
ASSESSMENT.


...MODEL TREND GRAPHICS AT
www.wpc.ncep.noaa.gov/html/model2.shtml...
...500 MB FORECASTS AT
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml...

RUBIN-OSTER

$$





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