Prognostic Meteorological Discussion
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FXUS10 KWNH 270500
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1259 AM EDT THU AUG 27 2015

VALID AUG 27/0000 UTC THRU AUG 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.


...SHORTWAVE EXITING THE NORTHEASTERN U.S. THURSDAY AFTERNOON...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
FORECAST CONFIDENCE: HIGH

A BROAD SHORTWAVE WILL MIGRATE NORTHEASTWARD IN TIME WHILE EXITING
THE NORTHEASTERN U.S. BY THURSDAY AFTERNOON. THE MODEL DIFFERENCES
DO NOT SEEM TO ARISE UNTIL THE SYSTEM HAS CROSSED INTO NOVA SCOTIA
AND POINTS NORTHWARD. A GENERAL MODEL COMPROMISE APPEARS SUITABLE
HERE.


...ELONGATED UPPER TROUGH LINGERING ACROSS THE GULF COAST REGION...

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

AS THE CORE OF THE MID-LEVEL HEIGHTS REMAIN TO THE NORTH...AN
ELONGATED AXIS OF SHORTWAVE ENERGY WILL HANG BACK ALONG THE GULF
COAST REGION. THROUGH FRIDAY EVENING...MODELS AGREE ON THE
LOCATION OF THIS AXIS BUT THE 12Z CMC/UKMET FAVOR A SLIGHTLY
STRONGER 500-MB CYCLONIC CIRCULATION. MOVING AHEAD...THE 12Z UKMET
TAKES A JOG TOWARD THE EAST AWAY FROM THE STRONGER CONSENSUS
CENTERED ALONG THE CENTRAL GULF COAST BY 30/0000Z.
CONVERSELY...THE 00Z NAM SLIDES TOWARD THE WEST WHICH APPEARS TO
BE A LOWER PROBABILITY SOLUTION AT THIS TIME. THE ENSEMBLE MEANS
BEST SUPPORT A COMBINATION OF THE 00Z GFS/12Z ECMWF WHICH WILL BE
THE RECOMMENDED SOLUTION.


...POTENT SHORTWAVE TRACKING FROM THE CENTRAL/NORTHERN HIGH PLAINS
TO THE GREAT LAKES...
...ASSOCIATED WAVE OF LOW PRESSURE/HEAVY RAINFALL THREAT...

PRELIMINARY PREFERENCE: BLEND OF THE 00Z GFS/12Z ECMWF THROUGH
29/1200Z...THEREAFTER A BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE
MEANS
FORECAST CONFIDENCE: SLIGHTLY BELOW AVERAGE

SHORTWAVES EMERGING OUT OF THE ROCKIES WILL CONTINUE TO BECOME
BETTER ORGANIZED AS IT MOVES INTO THE GREAT PLAINS. THIS WILL POSE
A SIGNIFICANT HEAVY RAINFALL THREAT AS IT TAPS INTO BETTER
MOISTURE. BY MIDDAY FRIDAY...THE 12Z CMC/UKMET BEGIN TO DIVERGE
FROM THE MODEL CLUSTER WITH THE LATTER BECOMING MUCH MORE
PROGRESSIVE. ALTERNATIVELY...THE 12Z CMC SEEMS TO BE FOCUS THE
BETTER ENERGETICS TO THE SOUTH WHICH HAS NO SUPPORT FROM ANY OTHER
PIECE OF GUIDANCE. ONE THING ALL OF THE MODELS AGREE ON IS THE
GRADUAL DEGRADATION OF THE SURFACE LOW AS IT MOVES FROM THE MIDDLE
MS VALLEY TO THE GREAT LAKES REGION. OVERALL RUN-TO-RUN CONTINUITY
IS NOT GREAT WHICH LIMITS THE CONFIDENCE OF THIS FORECAST. PLAN ON
SPLITTING THE DIFFERENCES BETWEEN THE 00Z GFS/12Z ECMWF SOLUTIONS
THROUGH DAY 2 WHILE COMBINING THEIR RESPECTIVE MEANS BY THE
FOLLOWING DAY.


...EASTERN PACIFIC/WEST COAST MEAN TROUGH THROUGH THE WEEKEND...

PRELIMINARY PREFERENCE: BLEND OF THE 18Z GEFS/12Z ECMWF ENSEMBLE
MEANS
FORECAST CONFIDENCE: AVERAGE

A VERY ACTIVE PERIOD WILL UNFOLD ACROSS THE NORTHWESTERN U.S. AS A
SERIES OF SHORTWAVES PROPEL THEIR WAY FROM THE EASTERN PACIFIC
TOWARD POINTS INLAND. THE INITIAL FEATURE IS A CLOSED LOW
CURRENTLY OFF THE WA/OR COAST WHICH IS FORECAST TO CARRY AN AXIS
OF HEIGHT FALLS INLAND BY SATURDAY MORNING. THE MULTI-DAY
SPAGHETTI PLOTS SHOW A QUICKER TREND AS A WHOLE WHICH SUGGESTS THE
12Z CMC/UKMET ARE MUCH TOO SLOW. FOLLOWING IN THE WAKE OF THIS
INITIAL SYSTEM IS ANOTHER MORE ROBUST TROUGH PLOWING THROUGH THE
PACIFIC NORTHWEST BY THE WEEKEND. WHILE THE GUIDANCE ALL AGREE ON
A SYNOPTIC-SCALE TROUGH ANCHORING THE EASTERN PACIFIC AND WESTERN
U.S...THEY EXHIBIT DIFFERENCES WITH THE INDIVIDUAL SHORTWAVES
WITHIN THE TROUGH. ENSEMBLE MEANS WILL BE THE COURSE OF ACTION
UNTIL THESE MESOSCALE DIFFERENCES BECOME BETTER RESOLVED.


...TROPICAL STORM ERIKA CROSSING THE BAHAMAS BY SUNDAY MORNING...

PRELIMINARY PREFERENCE: MODEL COMPROMISE BETWEEN THE 00Z NAM/GFS
AND 12Z ECMWF
FORECAST CONFIDENCE: ---

THIS TROPICAL CYCLONE CLOSES IN ON THE MAP DOMAIN BY EARLY SUNDAY.
MANY OF THE RECENT MODELS ARE IN CLOSE ALIGNMENT WITH THE NHC
FORECAST TRACK WITH THE EXCEPTION OF THE 12Z CMC AND 12Z ECMWF
ENSEMBLE/21Z SREF MEANS. THE 12Z UKMET IS A HAIR EAST OF THE TRACK
BUT IS SIGNIFICANTLY DEEPER FOR A SYSTEM WITH A LOT OF INTENSITY
UNCERTAINTY BASED ON THE RECENT NHC FORECAST DISCUSSIONS. SOME
FORM OF COMPROMISE BETWEEN THE 00Z NAM/GFS AND 12Z ECMWF SHOULD
ALIGN CLOSELY WITH THE FORECAST.


...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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