Prognostic Meteorological Discussion
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FXUS10 KWNH 181841
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
141 PM EST THU DEC 18 2014

VALID DEC 18/1200 UTC THRU DEC 22/0000 UTC

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

...AN EVALUATION OF THE 12Z MODEL GUIDANCE ALONG WITH FINAL
PREFERENCES/FORECAST CONFIDENCE...

THERE DID NOT APPEAR TO BE ANY INITIALIZATION ERRORS WITH THE 12Z
NAM/GFS DEEMED SIGNIFICANT TO IMPACT THEIR FORECASTS.


...UPPER LOW DEPARTING NEW ENGLAND...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
FORECAST CONFIDENCE: ABOVE AVERAGE

WATER VAPOR IMAGERY SHOWS A WELL-DEFINED UPPER CYCLONE GRADUALLY
EXITING UPPER NEW ENGLAND. MUCH OF NORTHERN ME CONTINUES TO SEE A
LIGHT/MODERATE AXIS OF SNOW WITH MUCH OF THE ACTIVITY WINDING DOWN
LATER IN THE DAY. MODELS SHOW SOME VARIABILITY IN HOW QUICKLY THEY
PUSH THE UPPER LOW OFFSHORE. HOWEVER...THESE DETAILS ARE SMALL SO
A GENERAL MODEL COMPROMISE SHOULD SUFFICE.


...ELONGATED SHORTWAVE MOVING FROM MO TOWARD THE MID-ATLANTIC THIS
EVENING...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
FORECAST CONFIDENCE: ABOVE AVERAGE

AN ELONGATED AXIS OF VORTICITY CURRENTLY STRETCHING FROM EASTERN
KS INTO SOUTHERN IL WILL SHIFT ITS FOCUS EASTWARD IN TIME. THIS
BATCH OF SHORTWAVE ENERGY IS FORECAST TO EVENTUALLY REACH THE
MID-ATLANTIC BY 19/0600Z. MODELS AGREE WITH THE EVOLUTION OF THIS
FEATURE AS IT UNDERCUTS A SEPARATE VORTICITY MAXIMA DROPPING DOWN
FROM THE LOWER GREAT LAKES. THE GUIDANCE DO VARY WITH THE POSITION
OF THIS SECONDARY FEATURE BUT IT DOES NOT APPEAR TO BE OF MUCH
CONSEQUENCE TO THE SENSIBLE WEATHER. THEREFORE...WILL RECOMMEND A
GENERAL MODEL COMPROMISE.


...SPLIT FLOW CARRYING ENERGY THROUGH THE NORTHERN TIER AND
SOUTHERN PLAINS ON FRIDAY...
...SURFACE LOW TRACKING ALONG THE NORTHERN GULF OF MX...

PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/00Z ECMWF ENSEMBLE
MEAN
FORECAST CONFIDENCE: AVERAGE

A COMPLEX/SPLIT FLOW REGIME IS IN PLACE THROUGHOUT MUCH OF THE
PERIOD. ACROSS THE NORTHERN STREAM...THE LEAD SHORTWAVE CROSSING
THE DAKOTAS IS FORECAST TO SLOWLY MIGRATE EASTWARD BEFORE LIFTING
NORTHWARD INTO MN AND EVENTUALLY FAR SOUTHWESTERN ONTARIO.
SUFFICIENT AGREEMENT IS NOTED WITH THIS SYSTEM. LOOKING AHEAD...A
PAIR OF SYSTEMS IN THE NORTHERN/SOUTHERN
STREAM...RESPECTIVELY...WILL MARCH THROUGH THE CENTER OF THE
COUNTRY ON FRIDAY. DIFFERENCES ALOFT ARE NOT TERRIBLY SIGNIFICANT
WITH MORE OF THE DETAIL DIFFERENCES CONCENTRATING AT THE SURFACE
OVER THE WESTERN GULF COAST. NOTABLE OUTLIERS ARE THE SLOWER 12Z
NAM AND TO A LESSER EXTENT THE 12Z CMC WHICH CARRIES THE SURFACE
LOW SLIGHTLY NORTH OF THE CONSENSUS. COMPARING THE PAST 4 MODEL
CYCLES OF ENSEMBLE LOW PLOTS SHOW A DECREASING NUMBER OF MEMBERS
IN EACH FUTURE RUN. THE 00Z ECMWF ENSEMBLE MEMBERS CLUSTER NICELY
ALONG THE TX/LA BORDER WHILE THE 00Z CMC ENSEMBLE MEMBERS ARE
SLIGHTLY TO THE NORTH. CHOOSE TO KEEP SOME EMPHASIS ON A LOW
CENTER IN THE FORECAST SO WILL LEAN IN THE DIRECTION OF THE 12Z
ECMWF. THE ENSEMBLE MEAN WILL BE THROWN IN TO ACCOUNT FOR THE
FORECAST UNCERTAINTY.


...SERIES OF PACIFIC SHORTWAVES MOVING TOWARD THE NORTHERN
ROCKIES...
...EVENTUAL SURFACE CYCLONE CROSSING THE DAKOTAS BY SUNDAY
EVENING...

PRELIMINARY PREFERENCE: BLEND OF THE 12Z GFS-PARALLEL/UKMET WITH
THE 00Z ECMWF ENSEMBLE MEAN
FORECAST CONFIDENCE: AVERAGE

AN ACTIVE PACIFIC JET WILL ALLOW A SERIES OF SYSTEMS TO TRACK
ACROSS WESTERN CANADA/PACIFIC NORTHWEST THROUGHOUT THE PERIOD. THE
INITIAL CLOSED LOW ACROSS THE NORTHEAST PACIFIC WILL ALLOW AN AXIS
OF HEIGHT FALLS TO PUSH INTO THE WA/OR COASTS BY 19/1200Z. THIS
SHORTWAVE ENERGY IS EXPECTED TO REACH THE NORTHERN ROCKIES BY
SATURDAY MORNING WITH THE 12Z NAM ON THE SLIGHTLY FASTER SIDE OF
THE SPREAD. MODELS SHOW SOME SORT OF SPLIT ACROSS THE NORTHERN
HIGH PLAINS WITH THE GUIDANCE VARYING IN WHICH WAVE THEY FORECAST
TO DOMINATE. ON ITS WINGS...A MORE IMPRESSIVE BATCH OF VORTICITY
WILL MOVE ALONG THE U.S./CANADIAN BORDER TOWARD THE HIGH PLAINS
REGION LATE IN THE PERIOD. THIS LEADS TO A RATHER STRONG AREA OF
SURFACE LOW PRESSURE ACROSS THE NORTHERN PLAINS BY SUNDAY. THE
ENSEMBLE LOW PLOTS PLACE MUCH OF THE CLUSTERING UP ALONG THE MT/ND
BORDER ON 22/0000Z WITH PREVIOUS MODEL CYCLES ALSO FAVORING THIS
LOCATION. THIS MAKES THE 12Z GFS APPEAR TOO FAR SOUTH WHILE THE
12Z CMC SHOWS NOTHING GIVEN IT SUGGESTS A MORE ZONAL FLOW WILL
DOMINATE RELATIVE TO THE EXPECTED AMPLIFICATION IN THE MID-LEVEL
FLOW. OVERALL...IT APPEARS THERE IS A GROWING DETERMINISTIC MODEL
CONSENSUS TOWARD THE CLUSTERING VISIBLE ALONG THE MT/ND BORDER.
THE 12Z UKMET/GFS-PARALLEL FAVOR THIS ENSEMBLE SOLUTION SO THESE
WILL BE INCORPORATED.


...NEXT SOUTHERN STREAM FEATURE CROSSING NORTHERN MX LATE IN THE
WEEKEND...

PRELIMINARY PREFERENCE: BLEND OF THE 12Z ECMWF/00Z ECMWF ENSEMBLE
MEAN
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

BY SUNDAY...THE ACTIVE SOUTHERN BRANCH OF THE JET WILL ALLOW
ANOTHER SYSTEM TO MOVE ACROSS MX AND EVENTUALLY NEARING THE
WESTERN GULF COAST BY SUNDAY EVENING. THE TIMING DIFFERENCES ARE
STILL EVIDENT AFTER REVIEWING MUCH OF THE 12Z GUIDANCE. THE 12Z
NAM CONTINUES TO BE QUICKER WHILE THE 12Z CMC SHOWS SOMETHING
COMPLETELY DIFFERENT. THE ENSEMBLE MEANS APPEAR RATHER STABLE WITH
THE 12Z ECMWF TRENDING SLOWER WHICH BRINGS IT CLOSER TO THIS
ENSEMBLE MEAN CLUSTER. WPC WILL FAVOR A COMBINATION OF THE 12Z
ECMWF/00Z ECMWF ENSEMBLE MEAN.


...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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