Prognostic Meteorological Discussion
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FXUS10 KWNH 210435
PMDHMD
MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1234 AM EDT THU SEP 21 2017

VALID SEP 21/0000 UTC THRU SEP 24/1200 UTC

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

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

...OCCLUDED LOW OVER SOUTH-CENTRAL CANADA TODAY...
...ASSOCIATED COLD FRONT OVER THE UPPER MIDWEST DIMINISHING
TODAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

ACROSS THE UPPER MIDWEST/WESTERN GREAT LAKES THERE IS STRONG MODEL
AGREEMENT TO SUPPORT A GENERAL MODEL BLEND AT ABOVE AVERAGE
CONFIDENCE EVEN THOUGH THE DIFFERENCES IN TIMING OF THE WAVE IN
CANADA (NAM/GFS WELL OUTPACING THE CMC FOLLOWED BY THE ECMWF AND
UKMET).


...WESTERN CONUS TROUGH AMPLIFICATION AND MOVEMENT THROUGH
SATURDAY...
...SURFACE LEE CYCLOGENESIS BEGINNING LATE THURSDAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: 00Z GFS AND 12Z UKMET BLEND
CONFIDENCE: ABOVE AVERAGE THROUGH FRI...AVERAGE THEREAFTER

GOES-WV MOSAIC SHOWS STRONG WESTERN LOBE OF THE LARGE GLOBAL TROF
THAT DOMINATES THE WESTERN/NORTH CENTRAL PORTION OF NORTH AMERICA.
 MODEL GUIDANCE IS QUITE STRONGLY AGREED UPON THE TIMING AND
EVOLUTION OF THE LOBE AS IT DESCENDS ORIENTING THE OVERALL TROF
MORE N-S THOUGH FRIDAY WHILE CONCURRENTLY CLOSING OFF ACROSS THE
NORTHERN GREAT BASIN/SNAKE RIVER VALLEY BY LATE FRIDAY.  IT IS
AFTER 18Z ON FRIDAY THAT MODEL SPREAD BEGINS TO INCREASE AND
TRADITIONAL MODEL BIASES SEEM TO TAKE ROOT.  AS THE LEAD HEIGHT
FALLS ROTATE NORTH ALONG THE CLOSED LOW...THE WAVE BEGINS TO
UNFOLD AGAIN ON SAT...WITH THE 00Z GFS SHEDDING MORE ENERGY
NORTHWARD A BIT QUICKER WITH SUPPORT OF THE 00Z NAM AND 12Z UKMET
WHILE THE CMC/ECMWF LAG A BIT.  THE 00Z NAM HOWEVER SHOWS SOME
INTERNAL OVER STRENGTHENING OF A VORT CENTER THAT APPEARS TO
DESTABILIZE THE OVERALL TROF EVOLUTION AND SUPPORTING A VERY
STRONG/NEARING NEGATIVE TILT TO THE BASE BY 12Z SUNDAY...WHILE
PLAUSIBLE ITS FALLS IN LINE WITH TYPICAL DAY 3 NAM BIAS.   THE 12Z
ECMWF BEING A BIT SLOW WITH THE JET STREAK AROUND THE BASE OF THE
TROF BY MIDDAY SAT ALLOWS FOR A DEEPER/SOUTHWARD ORGANIZATION OF
THE SHORTWAVE FEATURES WHICH EVENTUALLY LEADS TO A CLOSED LOW WELL
SOUTH AND SLOW BY 12Z SUN EVEN COMPARED TO A MAJORITY OF ITS
ENSEMBLE MEMBERS.   THIS SLOWER/WEAKER INITIAL HEIGHT FALLS
APPEARS TO TO NEGATIVELY AFFECT (TOO WEAK) THE LEE CYCLOGENSIS FRI
THROUGH SAT WITH ESPECIALLY WITH THE LEAD WAVE LIFTING INTO N MN
BY EARLY SAT.   ALL CONSIDERED A 00Z GFS AND 12Z UKMET BLEND SEEMS
PRUDENT AND REPRESENTS THE OVERALL ENSEMBLE SOLUTIONS.  CONFIDENCE
IS FAIRLY HIGH THROUGH FRIDAY BUT DEGRADES TO AVERAGE AS THE
SPREAD INCREASES THROUGH THE WEEKEND.


...TROPICAL STORM JOSE...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: REFER TO OFFICIAL NHC PRODUCTS
BEST PROXY TO 03Z NHC FORECAST: NON-CMC BLEND

MUCH OF THE GUIDANCE SUITE CONTINUES TO DEPICT A STALLED JOSE WITH
STEADY FILLING OVER TIME. THE 12Z CMC SEEMS TO BE THE ONLY MAIN
GLOBAL DETERMINISTIC GUIDANCE BREAKING FROM THE STRONG NEARLY
STATIONARY CONSENSUS AS IT STARTS TO DRIFT WEST OR SW WITH TIME
STARTING LATE FRI/EARLY SAT.  AS SUCH A NON-CMC BLEND IS A SOLID
PROXY FOR THE OFFICIAL 03Z NHC ADVISORY.


...WEAK UPPER LEVEL TROUGH EXTENDING SOUTHWEST FROM
JOSE...RETROGRADES ACROSS SOUTH/GULF COAST BY SUN...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: NON-GFS BLEND
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

GOES-WV SUITE DEPICTS A MODEST WAVE OVER THE CAROLINAS WITH
ANOTHER WEAK TWIST ACROSS THE LOWER MS VALLEY.  GUIDANCE IS FAIRLY
CONSISTENT WITH THESE FEATURES` PLACEMENT AS THE RIDGE/CLOSED
ANTICYCLONE ESTABLISHES OVER THE OHIO VALLEY BACK TOWARD THE
OZARKS BY SAT MORNING.  A BINARY INTERACTION BETWEEN THE WAVES
WILL DRAW THE CAROLINA WAVE SW INTO THE SOUTH WHILE DIGGING THE
LOWER MS VALLEY WAVE UNDER THE BASE INTO THE CENTRAL GULF.  THE
00Z NAM ALREADY DEPICTS THE WAVE A BIT STRONG AND SHOWS A TIGHTER
INTERACTION/RADIUS BETWEEN THE WAVES POSING A DEEPER CLOSED SYSTEM
(582DM) LINE ON SUN AT THE MOUTH OF THE MS RIVER.   THE 00Z GFS
CONTINUES TO FAVOR THE STRONGER CAROLINAS WAVE WITH LITTLE
RESOLUTION OF THE LOWER MS VALLEY WAVE.  AS SUCH A WEAKENED MS
VALLEY WAVE ALLOWS THE CAROLINA WAVE TO RETROGRADE BUT WITH LITTLE
SOUTHWARD TRACK CONSEQUENTLY ERODING THE BASE OF THE UPPER RIDGE
IN COMPARISON TO OTHER GUIDANCE.

THE 12Z UKMET/ECMWF BOTH ARE MORE SUBDUED THAN THE 00Z NAM BUT
TRACK SIMILARLY IN PLACEMENT/TIMING WITH RESPECT TO THE RIDGE AS
WELL AS THE MEDIAN SOLUTION OF THE ENSEMBLE GUIDANCE MEMBERS.
WHILE THE 12Z CMC HAVING A SLIGHTLY DISPLACED (SW) JOSE SHOWING
WEAKER AGREEMENT OVER THE MID-ATLANTIC/CAROLINAS... THE PLACEMENT
OF THESE TWO WEAKER SOLUTIONS FALLS QUITE SIMILAR TO THE
UKMET/ECMWF.  WHILE THE 00Z NAM IS DEEPER...THIS MAY BE MORE
RELATED TO THE HIGHER FIDELITY OF THE WAVE WITHIN THE HIGHER
RESOLUTION OVERALL...AS SUCH WILL SUPPORT A NON-GFS BLEND AT
SLIGHTLY ABOVE AVERAGE CONFIDENCE...THOUGH WEIGHTING MORE TOWARD
THE 12Z ECMWF/UKMET MORE THAN THE CMC/NAM.


...HURRICANE MARIA...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE/CONFIDENCE: REFER TO OFFICIAL NHC PRODUCTS
BEST PROXY TO 03Z FORECAST: 00Z GFS AND 12Z ECMWF/UKMET

OVERALL THE GUIDANCE SUITE IS FAIRLY COMPACT WITH RESPECT TO
MARIA...THOUGH THE 00Z NAM CONTINUES TO BE SLIGHTLY EAST AND
SLOWER AND THE 12Z CMC APPEARS WEST THOUGH WELL PACED TO THE
OFFICIAL NHC FORECAST.  THE 00Z GFS HAS SHOWN A CONSIDERABLE
SLOWING OF MARIA COMPARED TO THE 18Z RUN AND NOW ALONG WITH THE
THE 12Z ECMWF AND UKMET PRESENT THE BEST PROXY TO THE OFFICIAL 03Z
NHC FORECAST (WHICH IS VERY NEAR THE HFIP MULTI-MODEL CONSENSUS).

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

GALLINA

$$




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