Prognostic Meteorological Discussion
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091
FXUS10 KWNH 231653
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1252 PM EDT SAT SEP 23 2017

VALID SEP 23/1200 UTC THRU SEP 27/0000 UTC

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

...AN EVALUATION OF THE 12Z NAM/GFS ALONG WITH PRELIMINARY
PREFERENCES/FORECAST CONFIDENCE...


...HURRICANE MARIA...

PRELIMINARY PREFERENCE: THROUGH 26/0000Z: 09Z SREF
MEAN...THEREAFTER: BLEND OF THE 06Z GFS/00Z ECMWF
FORECAST CONFIDENCE: ---

AS OF THE 1500Z NHC ADVISORY...MARIA IS CURRENTLY A CATEGORY 3
HURRICANE WITH A CENTRAL PRESSURE OF 952-MB. IT IS MOVING AT
AROUND 7 KT TOWARD THE NORTH-NORTHWEST. THE EXPECTATION IS FOR
GRADUAL WEAKENING TO TAKE PLACE WITH SOME SLOWING TO TAKE PLACE
THROUGH MID-WEEK. UNFORTUNATELY...THE AVAILABLE GUIDANCE IS
SCATTERED ABOUT SUGGESTING A LARGER THAN NORMAL SPATIAL SPREAD TO
CONTEND WITH. THERE ARE SOME MORE EXTREME WESTERN OUTLIERS
RELATIVE TO THE OFFICIAL TRACK WHICH INCLUDES THE 00Z CMC/UKMET.
RUN-TO-RUN CONTINUITY IS RATHER POOR AFTER NOTING THE 00Z ECMWF
JUMPED 150 TO 200 MI WEST OF ITS PRECEDING FORECAST. AT THIS POINT
IN TIME...THE 09Z SREF MEAN OFFERS A TRACK CLOSE TO THAT OF NHC
BEFORE IT DIVERGES TOWARD THE EAST. THEREAFTER...AN AVERAGE OF THE
06Z GFS/00Z ECMWF WOULD BE THE BEST WAY TO PROCEED INTO DAY 3.

...REMNANT CIRCULATION FROM JOSE...

PRELIMINARY PREFERENCE: THROUGH 26/0000Z: 09Z SREF
MEAN...THEREAFTER: BLEND OF THE 06Z GFS/00Z ECMWF
FORECAST CONFIDENCE: AVERAGE

THE CIRCULATION WHICH REMAINS FROM JOSE CONTINUES TO SPIN OFF THE
NEW ENGLAND COAST. ITS MUCH WEAKENED STATE SHOULD LIMIT THE
IMPACTS ACROSS THE REGION. EASTWARD EXPANSION OF AN UPSTREAM RIDGE
AXIS SHOULD BRIEFLY FORCE THIS SYSTEM FARTHER OUT TO SEA. AN
EVENTUAL WESTWARD RETURN IS NOTED...BUT IN A MUCH MORE SHEARED
STATE AS SUGGESTED BY A VAST MAJORITY OF THE GUIDANCE. SOME
REMNANT MID-LEVEL VORTICITY COULD STREAM TOWARD NEW ENGLAND ON
MONDAY BUT WITH LITTLE TO NO IMPACTS. PLAN ON STAYING CLOSER TO
THE PREFERENCE FOR MARIA GIVEN ANY INTERACTION BETWEEN THE TWO
ENTITIES AND THE RESULTANT WESTWARD EXPANSION OF THE UPPER RIDGE
STEERING MARIA.


...INITIAL SURFACE WAVE EJECTING FROM SOUTHWESTERN ONTARIO TOWARD
EXTREME NORTHERN ME...

PRELIMINARY PREFERENCE: GENERAL MODEL COMPROMISE
FORECAST CONFIDENCE: SLIGHTLY ABOVE AVERAGE

A SURFACE WAVE CURRENTLY ACCELERATING THROUGH SOUTHWESTERN ONTARIO
SHOULD GRADUALLY WEAKEN ON ITS PATH TOWARD SOUTHERN QUEBEC. THE
ATTENDANT COLD FRONT SHOULD SEE ITS FORWARD PROGRESSION HALTED
WHILE ENCOUNTERING A DOWNSTREAM RIDGE. EVENTUALLY A NORTHWARD
RETURN AS A WARM FRONT IS EXPECTED IN ADVANCE OF THE LONGWAVE
TROUGH ANCHORING THE WESTERN U.S. GIVEN ONLY SUBTLE DIFFERENCES IN
THE NOTED PATTERN...A GENERAL MODEL COMPROMISE APPEARS WARRANTED
HERE.


...DEEP/ANOMALOUS UPPER TROUGH INITIALLY POSITIONED OVER THE
WESTERN U.S...
...GRADUAL EASTWARD PROGRESSION OF THE SYSTEM...REACHING THE
DAKOTAS BY 26/1800Z...
...ATTENDANT SURFACE FRONT/WAVES OF LOW PRESSURE...

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

WATER VAPOR IMAGERY SHOWED A DEEP UPPER TROUGH PUSHING THROUGH THE
DESERT SOUTHWEST WITH AN ATTENDANT CLOSED LOW CENTER SPINNING OVER
SOUTHERN NV. THE 00Z GFS INDICATED THE SYSTEM FEATURES SIGNIFICANT
500-MB HEIGHT ANOMALIES ON THE ORDER OF 3 TO 4 STANDARD DEVIATIONS
BELOW AVERAGE. THE HIGHLY MERIDIONAL ORIENTATION OF THE LONGWAVE
PATTERN ALLOWS ABUNDANT POLEWARD MOISTURE TRANSPORT THROUGH THE
GREAT PLAINS FOSTERING A MULTI-DAY HEAVY RAINFALL EVENT. THE SLOW
FORWARD PROGRESSION OF THIS TROUGH IS CONSISTENT AMONG THE
GUIDANCE ALTHOUGH THE 09Z SREF MEAN QUICKLY BECOMES TOO SLOW.
DURING THE PERIOD...THE 12Z NAM EMPHASIZES A NUMBER OF EMBEDDED
SHORTWAVES WHICH MAY EXIST OR COULD BE DRIVEN BY CONVECTIVE
FEEDBACK AS SUGGESTED BY THE EXTREME 24-HR QPF AMOUNTS. WHILE
OTHER SOLUTIONS AGREE ON THE LONGWAVE PATTERN...THERE ARE A NUMBER
OF SURFACE FRONTAL DIFFERENCES VISIBLE. THE 12Z NAM IS MUCH
QUICKER RELATIVE TO THE CONSENSUS WHILE THE 00Z ECMWF/UKMET ARE
FASTER THAN THE 12Z GFS. WILL SUGGEST A SPLIT BETWEEN THE 00Z
ECMWF/UKMET AND 12Z GFS WHILE AWAITING ADDITIONAL 12Z GUIDANCE.


...WEAK HEIGHT FALLS DESCENDING FROM THE PACIFIC NORTHWEST TOWARD
THE CENTRAL GREAT BASIN...
...EVENTUAL CLOSED LOW PINCHING OFF OVER THE DESERT SOUTHWEST ON
TUESDAY...

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

ALONG THE EASTERN EXTENT OF AN UPPER RIDGE ACROSS THE PACIFIC...A
SHORTWAVE WILL SLIDE SOUTHWARD THROUGH THE CENTRAL GREAT BASIN AND
INTO THE SOUTHWESTERN STATES. THIS EVENTUALLY HELPS ESTABLISH A
CLOSED LOW ACROSS THE DESERT SOUTHWEST TOWARD THE END OF THE
PERIOD. DIVERSITY IN THE GUIDANCE CONTINUES WITH THE 00Z GFS/CMC
ENSEMBLES FAVORING A MORE AMPLIFIED/SLOWER EVOLUTION RELATIVE TO
THE 00Z ECMWF ENSEMBLE MEMBERS. GIVEN BOTH SOLUTIONS HAVE REMAINED
IN THEIR OWN CAMPS...WILL FAVOR A BLEND OF THE TWO...12Z GFS/00Z
ECMWF.



www.wpc.ncep.noaa.gov/html/model2.shtml
www.wpc.ncep.noaa.gov/h5pref/h5pref.shtml

RUBIN-OSTER

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