Prognostic Meteorological Discussion
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000
FXUS10 KWNH 201903
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
303 PM EDT MON APR 20 2015

VALID APR 20/1200 UTC THRU APR 24/0000 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 ENOUGH TO IMPACT THEIR FORECASTS.


...ANOMALOUS CLOSED LOW AFFECTING THE GREAT LAKES...
...ASSOCIATED SURFACE LOW/ADVANCING COLD FRONT...

FINAL PREFERENCE: MODEL COMPROMISE OF THE 12Z GFS/ECMWF/UKMET
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A DEEP UPPER LOW WILL CONTINUE TO REMAIN A DOMINANT FORCE ACROSS
THE GREAT LAKES REGION AS A SERIES OF VORTICITY MAXIMA ROTATE
AROUND THE CYCLONE. MODELS VARY GREATLY REGARDING EACH INDIVIDUAL
IMPULSE WHICH IS TO BE EXPECTED GIVEN THE COMPLEX NATURE OF THESE
BROAD CIRCULATIONS. LOOKING AT THE SURFACE BOUNDARIES...THE
GUIDANCE HAS A GOOD HANDLE ON THE INITIAL FRONTAL SYSTEM EXITING
THE EASTERN U.S. WITH SLIGHTLY MORE UNCERTAINTY WITH THE SECONDARY
COLD FRONT. THE 12Z NAM APPEARS OUTLYING AS IT DEVELOPS A STRONG
WAVE OF LOW PRESSURE ACROSS THE OH VALLEY ON WEDNESDAY MORNING
WHICH IS NOT SEEN IN ANY OTHER MODEL. ADDITIONALLY...THE 00Z CMC
APPEARED MUCH TOO QUICK RELATIVE TO THE MORE STABLE LOOKING MEANS.
THE 12Z GFS/ECMWF/UKMET SEEM TO BEST SUPPORT THESE MEANS AND A
MODEL COMPROMISE OF THESE SOLUTIONS IS RECOMMENDED.


...INITIAL CUT-OFF LOW OFFSHORE OF BAJA CA...
...MORE ELONGATED UPPER LOW SETTING UP ACROSS THE SOUTHWESTERN
U.S. BY LATE WEDNESDAY...

FINAL PREFERENCE: BLEND OF THE 12Z UKMET WITH THE 12Z GEFS/00Z
ECMWF ENSEMBLE MEANS
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

THE INITIAL WEAK CIRCULATION OFF OF SOUTHERN CA SHOULD SLOWLY
STRENGTHEN IN TIME AS IT MOVES TOWARD INLAND LOCALES ON TUESDAY
EVENING. MODELS SEEM TO AGREE WELL WITH THIS EVOLUTION BUT THINGS
BECOME MORE COMPLICATED AS ADDITIONAL UPSTREAM ENERGY APPROACHES
THE REGION. BY MID-WEEK...THE SPREAD GROWS CONSIDERABLY WITH THE
12Z NAM BEING ON THE FAR WESTERN SIDE OF THE SOLUTION ENVELOPE.
MEANWHILE...THE 00Z CMC IS QUICKER TO SLIDE HEIGHT FALLS TOWARD
THE DESERT SOUTHWEST. AT THE CENTER OF THE BUILDING SPREAD ARE THE
12Z UKMET/00Z ECMWF AND 12Z GEFS/00Z ECMWF ENSEMBLE MEANS. IT
APPEARS THE 12Z ECMWF TRENDED SIGNIFICANTLY SLOWER AND POTENTIALLY
AWAY FROM THE STRONGER ENSEMBLE SUPPORT. PLAN ON INCORPORATING THE
12Z UKMET IN PLACE OF THE 00Z ECMWF FROM THE PREVIOUS PREFERENCE.


...STRONG HEIGHT FALLS REACHING THE PACIFIC NORTHWEST BY TUESDAY
NIGHT...

FINAL PREFERENCE: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS
FORECAST CONFIDENCE: BELOW AVERAGE

A PRONOUNCED AREA OF HEIGHT FALLS WILL WORK THEIR WAY FROM THE
NORTHEAST PACIFIC TOWARD THE PACIFIC NORTHWEST ON TUESDAY. THE
MODELS BEGIN TO DIVERGE FROM ONE ANOTHER PRETTY QUICKLY ONCE THE
SYSTEM PUSHES INLAND WITH MANY SOLUTIONS SUGGESTING THE SYSTEM
WILL BECOME CUT-OFF IN NATURE. THE TWO SOLUTIONS PRESENTING
OPPOSING IDEAS ARE THE 12Z GFS WHICH IS MUCH SLOWER AND AT TIMES
ALMOST STATIONARY WHILE THE 00Z UKMET ACCELERATES THE SYSTEM OFF
TOWARD THE NORTH AND EAST. UNFORTUNATELY THE DETERMINISTIC MODELS
ARE NOT REALLY SUPPORTING THEIR OWN ENSEMBLE MEANS AT THIS TIME
WHICH SIGNIFICANTLY LOWERS THE CONFIDENCE IN THIS FORECAST. WHILE
A VAST MAJORITY OF THE DETERMINISTIC MODELS SUGGEST THE SYSTEM
WILL BE CLOSED OFF...IT IS DIFFICULT TO PINPOINT HOW QUICKLY THIS
SYSTEM WILL SLIDE EASTWARD. ALTHOUGH ENSEMBLE MEANS SHOW MORE OF
AN OPEN WAVE IN THEIR MASS FIELDS...THIS IS THE BEST WAY TO HANDLE
THE SITUATION.


...TRAILING CLOSED LOW NEARING THE WA COAST BY 24/0000Z...

FINAL PREFERENCE: BLEND OF THE 12Z GEFS/00Z ECMWF ENSEMBLE MEANS
FORECAST CONFIDENCE: BELOW AVERAGE

IN THE WAKE OF THE INITIAL CLOSED LOW MOVING TOWARD THE NORTHERN
ROCKIES WILL BE A MORE ROBUST CIRCULATION APPROACHING VANCOUVER
ISLAND BY 24/0000Z. THERE APPEARS TO BE A LARGE ARRAY OF SOLUTIONS
AT HAND WHICH IS SIGNIFIED IN THE MESSY LOOKING 540/552-DM
SPAGHETTI PLOTS. THE 00Z ECMWF ENSEMBLE MEMBERS SEEM TO SHOW MORE
AMPLIFICATION...PARTICULARLY IN THE PAST TWO MODEL CYCLES. IT
LOOKS LIKE THE 12Z ECMWF HAS MOVED AWAY FROM ITS ENSEMBLE MEAN
SHIFTING WELL TO THE NORTH. THERE DOES NOT SEEM TO BE GREAT
RUN-TO-RUN CONTINUITY IN THE MODEL SOLUTIONS WHICH MAKES IT MORE
REASONABLE TO STICK WITH AN ENSEMBLE APPROACH AS STATED IN THE
PREVIOUS PREFERENCE.


...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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