Prognostic Meteorological Discussion
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000
FXUS10 KWNH 230435
PMDHMD
MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1234 AM EDT SAT SEP 23 2017

VALID SEP 23/0000 UTC THRU SEP 26/1200 UTC

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

00Z MODEL EVALUATION INCLUDING PREFERENCES AND FORECAST CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...SLOW EASTWARD DRIFT OF DEEP WESTERN TROF WITH EMBEDDED INTERNAL
WAVES LIFTING NORTH THROUGH GREAT PLAINS THROUGH TUES WITH
ASSOCIATED SURFACE TROF ACROSS GREAT PLAINS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z GFS/NAM AND 12Z ECMWF THROUGH 25/06Z
            00Z GFS/NAM NORTH OF WY/NEB 25/12Z-26/12Z
            00Z GFS SOUTH OF WY/NEB 25/12Z-26/12Z
CONFIDENCE: ABOVE AVERAGE  THROUGH 25/06Z
            AVERAGE THEREAFTER

THE LARGE UPPER LOW ACROSS THE WESTERN US WILL START TO BREAK INTO
TWO MAIN AREAS OF FOCUS OVER THE NEXT DAY OR SO: THE SOUTHERN BASE
WHICH CLOSES OFF THROUGH UT/CENTRAL ROCKIES BY EARLY SUNDAY AS
WELL AS THE NORTHERN LOBE BECOMING A STRETCHED MID-LEVEL
DEFORMATION/TROF ACROSS S CANADA THAT HELPS CORRAL S/W ENERGY FROM
THE SSWLY EASTWARD ACROSS SOUTHERN CANADA ABOVE THE REX BLOCK IN
THE GREAT LAKES (SEE SECTION BELOW)  THIS EVOLUTION CONTINUES TO
BE FAIRLY WELL AGREED UPON UNTIL EARLY MONDAY AS THE TROF AS A
WHOLE BREAKS DOWN AND BECOMES FOCUSED ON SMALLER SCALE INTERNAL
WAVES INCREASING OVERALL SPREAD IN THE SLOW DEVOLUTION OF THE TROF
AS A WHOLE.  FOR EXAMPLE...THE 12Z ECMWF VERY UNCHARACTERISTICALLY
WEAKENS THE BASE OF THE TROF AND SHIFTS EMPHASIS/STRENGTH OF THE
TROF TO THE NORTHERN STREAM WAVE.  THIS ALLOWS FOR THE SHORTWAVE
IN THE NORTHERN STREAM TO ADVECT EASTWARD INTO S CANADA LATE MON
INTO TUES MUCH QUICKER THAN ANY OTHER GUIDANCE MEMBER...INCLUDING
THE BULK OF THE 12Z ECENS MEMBERS WHICH LEAD TO A SLOWER ECENS
MEAN SOLUTION MORE IN LINE WITH OVERALL RUN TO RUN
TREND/CONTINUITY.  THE 12Z CMC/UKMET ON THE OTHER SIDE OF THE
SPECTRUM ARE MUCH SLOWER OVERALL AND PREFERS TO HOLD ONTO A
STRONGER AND DEEPER MERIDIONAL TROF ACROSS THE HIGH PLAINS MON
INTO TUES...WHICH ALSO LEADS TO A NORTH AND WEST SHIFT OF THE
SURFACE TROF/WARM CONVEYOR ACROSS THE GREAT PLAINS.

THE 00Z NAM AND GFS BOTH ARE A BIT SLOWER THAN THE ECMWF BUT FAVOR
THAT EVOLUTION OVER THOSE PRESENTED BY THE CMC/UKMET PARTICULARLY
IN HANDLING THE QPF ACROSS THE GREAT PLAINS.  THE 00Z NAM IS A BIT
QUICKER THAN THE 00Z GFS IN THE NORTHERN STREAM AND IN DOING SO
LEADS TO LINGERING S/W ENERGY ACROSS THE CENTRAL ROCKIES LATE MON
INTO TUES WHICH TIGHTENS AND AMPLIFIES IN A MANNER OF THE NAM`S
KNOWN DAY 3 NEGATIVE BIAS...MAKING IT LESS FAVORABLE IN CO/CENTRAL
HIGH PLAINS BY TUESDAY (BUT STILL QUITE USABLE IN THE NORTHERN
PLAINS/WESTERN GREAT LAKES.   THE 00Z GFS ALSO HAS SOME
AMPLIFICATION ISSUES PERHAPS A BIT TOO SLOW ON TUESDAY OVER THE
NORTHERN HIGH PLAINS BUT THE REMAINING TROF EVOLUTION (ROCKIES
INTO SOUTHWEST) APPEARS MORE REASONABLE IN STEP WITH 12Z ECENS
MEAN AND 18Z GEFS AND OTHER REMAINING ENSEMBLE SOLUTIONS.   ALL
CONSIDERED PREFER A 00Z NAM/GFS AND 12Z ECMWF BLEND THROUGH 25/06Z
THEN MORE 00Z GFS/NAM IN THE NORTHERN STREAM (NORTH OF WY/NEB) AND
00Z GFS SOUTH OF WY/NEB.  CONFIDENCE IS ABOVE AVERAGE THROUGH
25/06Z BUT REDUCES CONSIDERABLY WITH TIME AFTERWARD THOUGH AVERAGE
(GIVEN LARGE SCALE PATTERN REMAINS GOOD).


NORTHERN RED RIVER SFC WAVE CROSSING SOUTHERN ONTARIO AND CLIPPING
NORTHERN ME SUNDAY
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z NAM AND 12Z ECMWF/UKMET BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

SHORTWAVE ENERGY EXITING OUT OF THE SWLY FLOW OVER THE GREAT
PLAINS LIFTS NORTH THIS MORNING OUT OF THE RED RIVER VICINITY AND
STARTS SHIFTING EASTWARD THROUGH CONFLUENT FLOW (ABOVE THE REX
BLOCK) IN THE GREAT LAKES.  THE 00Z GFS AND 12Z CMC ARE SLIGHTLY
SLOWER AND THEREFORE REFLECT A SLIGHT INCREASED AMPLIFICATION OF
THE RIDGE IN E ONTARIO ALLOWING FOR LESS INTERACTION/PHASING FROM
SHORTWAVE FROM THE ARCTIC STREAM TO MAINTAIN THE SURFACE LOW
THROUGH S QUEBEC...THE 00Z NAM AND 12Z ECMWF/UKMET BEING A BIT
QUICKER PRESS THE LOW SOUTHWARD TO CLIP NORTHERN ME SUNDAY INTO
EARLY MONDAY.  ENSEMBLE SUITE TRENDS WOULD SUPPORT THIS EVOLUTION
IN STRENGTH/TIMING AS SUCH WILL FAVOR A 00Z NAM AND 12Z
ECMWF/UKMET BLEND AT SLIGHTLY ABOVE AVERAGE CONFIDENCE.


SHORTWAVE ENTERING SOUTHERN BC CLIPPING THE PACIFIC NORTHWEST WITH
WEAK FRONTAL PASSAGE LATE MON INTO TUES
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THERE REMAINS A SMALL DIFFERENCE IN TIMING OF SHORTWAVE AND
ASSOCIATED SURFACE WAVE ENTERING SOUTHERN BC MONDAY INTO TUES.
THE 00Z NAM/GFS SLIGHTLY OUTPACE THE ECMWF WITH THE UKMET FOLLOWED
LAGGING BY THE CMC.  STILL THE OVERALL SHAPE AND EVOLUTION IS NOT
TREMENDOUS AND ALL GLOBAL SOLUTIONS SHEAR THE REMAINING WAVE
BETWEEN THE NORTHERN STREAM AND NW PORTION OF THE AMPLIFIED
WESTERN US TROF BY TUESDAY.  AS SUCH A GENERAL MODEL BLEND SEEMS
APPROPRIATE AT ABOVE AVERAGE CONFIDENCE.


...REX BLOCK OVER EASTERN THIRD OF US...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE GUIDANCE IS FAIRLY WELL AGREED UPON ON THE OVERALL EVOLUTION
OF THE WEAK BROAD SHORTWAVE ACROSS THE SOUTHERN STATES THROUGH
MONDAY...ALL GENERALLY PLACE THE CENTER OF THE WAVE NEAR THE S
MS/E LA BOARDER BY 12Z MONDAY THOUGH THE 00Z NAM AND 12Z UKMET
CONTINUE TO BE DEEPER/MORE CONSOLIDATED.  THE DEEPER SOLUTIONS
REMAIN SLIGHTLY EAST OF THE WEAKER 00Z GFS/12Z ECMWF AS THERE IS A
SLIGHT EASTWARD DRIFT OF THE LOW INTO TUESDAY.  AS SUCH A GENERAL
MODEL BLEND SEEMS SUFFICE AT ABOVE AVERAGE CONFIDENCE.


...JOSE REMNANTS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-GFS BLEND
CONFIDENCE: ABOVE AVERAGE

VERY SLOW SOUTHWARD MEANDERING IS EXPECTED OVER THE NEXT DAY OR SO
BEFORE BECOMING GENERALLY TOO WEAK TO HAVE CLOSED MAIN ISOBAR
AROUND 00Z MONDAY THOUGH A WEAK LOW/REMNANT SWIRL SHOULD STILL
EXIST.  THE ONLY SIGNIFICANT DEPARTURE FROM AN OTHERWISE VERY
TIGHT CLUSTER CONTINUES TO BE THE 00Z GFS WHICH REMAINS A BIT EAST
AND NORTH OF THE MAIN CLUSTERING THOUGH THE 12Z ECMWF IS CLOSEST
REMAINING GLOBAL GUIDANCE MEMBER NEAR IT.  STILL THE OVERALL
PATTERN OF THE HEIGHT FIELDS AND SURFACE PRESSURE SUGGEST A
NON-GFS PREFERENCE.  WITH SMALL SPREAD CONFIDENCE CAN BE
CONSIDERED ABOVE AVERAGE IN THIS PREFERENCE.


...HURRICANE MARIA...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z GFS AND 12Z ECMWF BLEND
CONFIDENCE: REFERENCE NHC PRODUCTS

THROUGH 48HRS THE GLOBAL GUIDANCE IS QUITE TIGHT TO THE OFFICIAL
NHC FORECAST WITH ONE EXCEPTION BEING A VERY FAST AND WEST LEANING
TRACK BY THE OPERATIONAL 12Z CMC.  AFTER 48HRS...THE 00Z NAM/GFS
AND ECMWF SIGNIFICANTLY SLOW WITH THE NAM THE SLOWEST OVERALL.
THE 18Z GEFS MEMBERS ALSO INDICATE A LARGE VARIATION BECOMING
QUICKER AND SHIFTING MORE WESTERLY UNDER A STRONGER SUB-TROPICAL
RIDGE INFLUENCE LEAVING THE OPERATIONAL 18Z GFS THE SLOW NEAR
LONER WITHIN THE SUITE. THE 00Z GFS HOWEVER SHIFTED CLOSER TO THE
18Z GEFS MEAN SHOWING A SLIGHTLY FASTER AND WESTWARD JOG TOWARD
DAY 3. THE 12Z ECMWF WAS SLOWER AND SLIGHTLY EAST OF THE FORECAST
TRACK...AS SUCH A COMBINATION OF THE 12Z ECMWF AND 00Z GFS WOULD
LIKELY SERVE AS A MODERATE PROXY FOR THE OFFICIAL NHC FORECAST.

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

GALLINA

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