Prognostic Meteorological Discussion
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000
FXUS10 KWNH 211714
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
113 PM EDT SUN SEP 21 2014

VALID SEP 21/1200 UTC THRU SEP 25/0000 UTC

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

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12Z NAM/GFS EVALUATION AND PRELIMINARY PREFERENCES
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NAM/GFS INITIALIZATION ERRORS DO NOT APPEAR TO
SIGNIFICANTLY AFFECT THEIR LARGE SCALE OUTPUT.
==================================================

...EASTERN U.S THROUGH MONDAY...

PREFERENCE: 12Z NAM/GFS

A WELL DEFINED CYCLONE WILL PUSH EAST FROM NEW ENGLAND...ABSORBING
A COASTAL LOW LIFTING UP FROM THE CAROLINAS. MODEL SPREAD IS
MINIMAL...SO WE WILL SIDE WITH THE SMALL SCALE TRENDS IN THE MORE
RECENT 12Z GUIDANCE. THE 00Z ECMWF WAS WEAKER THAN CONSENSUS WITH
THE COASTAL LOW...AND WAS SLIGHTLY LESS PRONOUNCED AT THE BASE OF
THE NORTHERN STREAM UPPER TROUGH OVER SOUTHERN NEW ENGLAND AND THE
MID ATLANTIC.


...EASTERN U.S TUE/WED...

PREFERENCE: ECMWF ENSEMBLE MEAN

LARGE MODEL SPREAD LEADS TO BELOW AVERAGE CONFIDENCE AND LEADS US
TO PREFER AN ENSEMBLE MEAN OVER THE SOUTHEAST U.S. BY TUE/WED. THE
SYNOPTIC PATTERN ALLOWS FOR DEVELOPMENT OF A BROAD UPPER LOW AS
SEEN IN RECENT ECMWF RUNS...AS NORTHERN STREAM AMPLIFIES FROM
COAST TO COAST...AND SHORTWAVE ENERGY DESCENDS TOWARD A WEAKNESS
BETWEEN TWO RIDGES OVER THE SOUTHEAST U.S. AND NORTHERN GULF OF
MEXICO. A MODEREATLY STRONG CLUSTER OF 00Z ECMWF ENSEMBLE MEMBERS
SUPPORTS THIS NOTION. THE GFS/GEFS HAVE LENDED ALMOST NO SUPPORT
TO THE IDEA OVER THE PAST FEW CYCLES...BUT THE 12Z NAM DID TREND
IN THE DIRECTION OF THE ECMWF ENSEMBLE MEAN...WHILE NOT BEING
QUITE AS SLOW OR ROBUST. IN ANY CASE...THIS WILL OR WILL NOT OCCUR
ON THE HEELS OF A LARGE SCALE TROUGH...INITIALLY ATOP STABLE
AIR...SO THE VERY AGGRESSIVE QPF SOLUTIONS FROM THE 00Z
ECMWF/UKMET MAY BE TOO OPTIMISTIC AT PULLING MOISTURE INLAND. THE
BEST COMPROMISE SEEMS TO BE THE ECMWF ENSEMBLE MEAN...INDICATIVE
OF SOME MEANINGFUL RAIN ALONG THE COASTS OF GA/SC/NC ON
WEDNESDAY...AND LIGHTER RAIN INLAND 100 OR 200 MILES.


...SHEAR AXIS TRAPPED IN THE UPPER RIDGE BUILDING OVER THE
SOUTHERN PLAINS...

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

DATING BACK OVER THE PAST SEVERAL CYCLES...AND BEYOND THE FIRST 12
HOURS OF THE FORECAST...MODELS HAVE STRUGGLED TO MAINTAIN
DEFINITION OF A 850-700 MB SHEAR AXIS TRAPPED BENEATH A
STRENGTHENING UPPER LEVEL RIDGE. CONVECTIVE FLARE-UPS WITHIN A
PSEUDO-TROPICAL AIRMASS CONTINUE TO DRIVE A HEAVY RAIN THREAT OVER
PARTS OF TX/NM. THIS ACTIVITY MAY GRADUALLY LIFT AND REFORM NORTH
AND EAST TOWARD THE TX PANHANDLE AND ADJACENT PARTS OF OK THROUGH
TUE/WED. THE MODEL SOLUTIONS ARE SIMILAR. THE NAM MAY ALSO BE OF
USE...BUT IT IS AN OUTLIER IN THE STRENGTH OF MID LEVEL WINDS IT
PREDICTS AROUND THE DEVELOPING RIDGE. CONFIDENCE REGARDING DETAILS
OF THE CONVECTIVE FORECAST IS DIMINISHED AT LONGER LEAD TIMES
OWING TO CONTINUED SMALL SCALE FEEDBACK BEING OBSERVED AND NOT
OFTEN APPEARING IN THE MODELS.


...SYSTEM TRACKING FROM THE GREAT BASIN INTO THE PLAINS...

PREFERENCE: 12Z GFS

THERE HAS BEEN A TREND FOR THE MODELS TO SLOW THIS SYSTEM AND
DEPICT A SOMEWHAT MORE PRONOUNCED CIRCULATION BY THE TIME IT PULLS
INTO THE CENTRAL PLAINS. RECENT GFS RUNS...ESPECIALLY THE 06Z/12Z
RUNS...FIT BEST WITH THE ENSEMBLE TRENDS. THE 00Z ECMWF IS PERHAPS
IN THE LEAST AMOUNT OF AGREEMENT...AS IT IS WEAKER AND FASTER BY
DAY 3. THE 12Z NAM TRENDED EVEN SLOWER...AND ALSO CLOSED THE
CIRCULATION FARTHER SOUTH INTO KS/NE. WE PREFER THE GFS SOLUTION
IN WHICH THE CLOSED LOW ENDS UP NEAR NE/IA BY 25/00Z...WHICH IS
MORE CONSISTENT WITH THE EJECTION OF THIS SYSTEM FROM NORTHEAST
WYOMING ON DAY 2.


...DEEP TROUGH WITH INCREASING IMPACT TO THE PACIFIC NORTHWEST...

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

THIS TROUGH WILL BEING TO PRODUCE MORE MEANINGFUL IMPACTS AND
WIDESPREAD RAIN BY WEDNESDAY. MODEL SPREAD IS MINIMAL...BUT THE
12Z NAM BECOMES MORE AMPLIFIED THAN CONSENSUS...AND THE 00Z
CANADIAN BECOMES SLOWER THAN CONSENSUS BY 25/00Z.

...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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