Prognostic Meteorological Discussion
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000
FXUS10 KWNH 280452
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1252 AM EDT THU AUG 28 2014

VALID AUG 28/0000 UTC THRU AUG 31/1200 UTC

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

==================================================
00Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES
==================================================
NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO
SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT.
==================================================

...CENTRAL AND NORTHERN U.S. FROM THE PLAINS TO GREAT LAKES...

PREFERENCE: BLEND OF 00Z GFS/12Z CANADIAN

A CLOSED LOW IN COLORADO WILL MOVE EAST AND THEN GRADUALLY
DE-AMPLIFY AS IT LIFTS TOWARD THE GREAT LAKES. AT THE SAME TIME A
NORTHERN STREAM TROUGH WILL RIDE ALONG THROUGH SOUTHERN
CANADA...PUSHING A COLD FRONT INTO MONTANA LATE THURSDAY AND ON
TOWARD THE UPPER MIDWEST BY FRIDAY EVENING. THE 12Z UKMET
REPRESENTED THE DEEP END OF THE MODEL SOLUTIONS...AS IT MAINTAINED
A CLOSED UPPER CIRCULATION AND ROBUST LOW LEVEL CYCLONE...REACHING
THE GREAT LAKES ON DAY 3. ALTHOUGH THIS WAS AT ONE END OF THE
SPECTRUM REGARDING STRENGTH...THE ENSEMBLE MEANS DO SUPPORT TIMING
THAT IS MORE LIKE THE UKMET RATHER THAN THE FASTER 00Z NAM AND 12Z
ECMWF. THE PREVIOUS 00Z ECMWF HAD...IN FACT...BEEN MUCH SLOWER.
MORE RECENTLY...THE TREND IN THE 00Z GFS AND NAM WAS FOR SLIGHTLY
LESS AMPLIFIED AND SLIGHTLY FASTER SOLUTIONS THAN THEIR 12Z
COUNTERPARTS. LOOKING FOR A MIDDLE POINT IN ALL THIS EVIDENCE
POINTS TOWARD THE 00Z GFS AND 12Z CANADIAN.


...TROUGH EXITING THE NORTHEAST BY FRIDAY...

PREFERENCE: NONE

THE MODELS ARE IN GOOD AGREEMENT WITH RESPECT TO THE
TIMING/AMPLITUDE OF THE 500 MB TROUGH AND ASSOCIATED SURFACE FRONT.


...SLOW MOVING EASTERLY WAVE ALONG THE WESTERN GULF COAST...

PREFERENCE: BLEND OF 12Z ECMWF/UKMET

THIS PORTION OF THE MAP FEATURES SOME FORECAST CHALLENGES...AS A
WEAK POINT IN THE FLOW...OR COL POINT EXISTS OVER TEXAS. THE
MODELS VARY IN THEIR DEPICTION OF VORTICITY MAXIMA EAST OF THE COL
AND THEIR SUBSEQUENT MOTION. ONE FEATURE WAS NEARING THE SOUTH
TEXAS COAST AS OF EARLY THURSDAY MORNING...AND WAS WELL DEFINED IN
SATELLITE IMAGERY...PARTICULARLY THE VISIBLE CHANNEL DATING BACK
TO WEDNESDAY AFTERNOON. ANOTHER DISTURBANCE...ARRIVING AS AN
EASTERLY WAVE FROM THE YUCATAN...IS FORECAST BY SEVERAL MODELS TO
DEEPEN AS IT MOVES INTO THE WESTERN GULF OF MEXICO AND THEN LIFTS
THROUGH THE LOWER MISSISSIPPI VALLEY AHEAD OF THE GREAT PLAINS
TROUGH. BOTH OF THESE ARE SMALL FEATURES IN WEAK STEERING
FLOW...AND ONE WAS NOT YET VERY EVIDENT IN OBSERVATIONAL DATA...SO
CONFIDENCE IS LOW. WE DISCOUNT...HOWEVER...THE 00Z GFS WHICH WAS
ILL DEFINED WITH THE CIRCULATION NEAR TEXAS EARLY IN THE FORECAST.
THE 12Z CANADIAN WAS GENERALLY FASTER THAN THE OTHER MODELS...
LEAVING THE 00Z NAM AND 12Z ECMWF/UKMET AS A REASONABLY SIMILAR
CLUSTER OF SOLUTIONS. THE NAM...HOWEVER...ALLOWS THE COMBINED
SHEAR AXIS DEFINED BY THESE DISTURBANCES TO DRIFT FARTHER NORTH
AND WEST...PERHAPS ALLOWED DUE TO THE WARMER/FLATTER SOLUTION THAT
MODEL HAS IN THE NORTHERN STREAM. THIS YIELDS SOME SIGNIFICANT QPF
DIFFERENCES ON DAY 3. FOR QPF...WPC IS RECOMMENDING LESS WEIGHT ON
THE NAM IN THIS REGION ON DAY 3.


...DEEP TROUGH DIGGING INTO THE NORTHWEST BY FRI AND SAT...

PREFERENCE: BLEND OF 00Z GFS WITH 12Z ECMWF/CANADIAN

MODELS GENERALLY AGREE TO THE TIMING OF A MEDIUM WAVELENGTH TROUGH
THAT IS FORECAST TO SLIDE THROUGH THE WESTERN U.S. WHILE
UNDERGOING SOME BROAD AMPLIFICATION ON FRIDAY/SATURDAY. ALTHOUGH
THE NAM IS INCLUDED IN THIS STATEMENT...IT FEATURES NOTICEABLY
WARMER HEIGHTS AND ALSO MOVES OUT OF PHASE WITH THE OPERATIONAL
MODEL CONSENSUS IN THE UPSTREAM FLOW ON DAY 3. OTHERWISE...THE
GLOBAL MODELS COULD BE BLENDED TO REACH AN OVERALL CONSENSUS...BUT
WE CHOSE TO OMIT THE 12Z UKMET...WHICH WAS DISTINCTLY SHARPER WITH
SHORTWAVE ENERGY IN THE BASE OF THE TROUGH...AFFECTING THE DAY 3
FORECAST OVER THE GREAT BASIN AND FOUR CORNERS AREA.

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

BURKE
$$





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