Prognostic Meteorological Discussion
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FXUS10 KWNH 261637
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1237 PM EDT WED JUL 26 2017

VALID JUL 26/1200 UTC THRU JUL 30/0000 UTC

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

12Z MODEL EVALUATION...WITH MODEL PREFERENCES AND CONFIDENCE
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...LOW/MID-LEVEL VORT/LOW OVER GA/SC...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE WEAK LOW TO MID LEVEL VORT AND ASSOCIATED LOW CENTER OVER
SOUTHERN GA WILL GRADUALLY WEAKEN AND OPEN UP INTO A TROUGH BY
THURS THAT WILL EXTEND GENERALLY FROM SC SOUTHWEST DOWN ACROSS THE
NORTHEAST GULF OF MEXICO AND LINGER INTO FRI BEFORE THEN GETTING
PICKED UP AND LIFTING NORTHEASTWARD OUT TO SEA BY THE LARGER SCALE
AMPLIFYING TROUGH OVER THE EASTERN U.S....ALTHOUGH THE GUIDANCE
DOES TEND TO MAINTAIN A WEAKNESS DOWN THAT WILL EXPAND WELL
SOUTHWEST OVER THE NORTHEAST AND CENTRAL GULF OF MEXICO. THE
MODELS ARE OVERALL IN GOOD AGREEMENT WITH THEIR MASS FIELDS...AND
SO A GENERAL MODEL BLEND WILL BE PREFERRED TO RESOLVE ANY SMALLER
SCALE DIFFERENCES AT THIS POINT.


...UPPER LOW TRACKING ACROSS ONTARIO/QUEBEC AND NORTHEAST U.S...
...RENEWED CLOSED MID-LEVEL LOW NORTH OF MAINE BY SAT MORNING...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND...THROUGH 60 HOURS
            NON-NAM CONSENSUS...AFTER 60 HOURS
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

THE MODELS AGAIN COMPARE RATHER WELL WITH THEIR MASS FIELDS WITH
THE UPPER LOW AS IT MOVES EAST AND WEAKENS ACROSS SOUTHERN
CANADA...AND AGREE IN ALLOWING ADDITIONAL ENERGY TO DIG SOUTH AND
DEVELOP ANOTHER CLOSED LOW CENTER BY SAT MORNING TO THE NORTH OF
MAINE. TOWARD THE END OF THE PERIOD THE 12Z NAM IS A LITTLE SLOWER
AND FARTHER SOUTH WITH ITS RENEWED CLOSED LOW EVOLUTION NORTH OF
MAINE. THE GLOBAL MODELS ARE GENERALLY WELL CLUSTERED WITH THEIR
TIMING AND DEPTH OF THE NEW CLOSED LOW...SO WILL PREFER A GENERAL
MODEL BLEND THROUGH 60 HOURS AND THEN FAVOR A NON-NAM CONSENSUS
THEREAFTER.


...SHORTWAVE/SURFACE LOW PRESSURE EXITING THE CENTRAL PLAINS...
...DEEPENING TROUGH OVER THE EASTERN U.S. THURS TO SAT...
...MULTIPLE LOWS CROSSING MID MS/OH VALLEYS THURS...
...AMPLIFYING LOW OVER THE MID-ATLANTIC/NEW ENGLAND FRI/SAT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: BLEND OF THE 12Z GFS AND 00Z ECENS MEAN
CONFIDENCE: SLIGHTLY BELOW AVERAGE

REGARDING THE CONVECTIVELY ENHANCED MID LEVEL SHORTWAVE NOW
BEGINNING TO EXIT THE CENTRAL PLAINS...THE ENERGY  ALONG WITH AN
ATTENDANT SURFACE LOW SHOULD BEGIN TO SHEAR OFF TO THE EAST ACROSS
THE MID MS AND OH VALLEYS TONIGHT THROUGH THURS...WITH THE
GUIDANCE AGREEING ON MULTIPLE SURFACE LOWS STRETCHING OUT ALONG
THE FRONT. MEANWHILE...A SEPARATE NORTHERN STREAM SHORTWAVE OVER
SOUTHEAST ALBERTA THIS MORNING IS EXPECTED TO DIG EAST-SOUTHEAST
ACROSS THE UPPER MIDWEST ON THURS AND THEN AMPLIFY SOUTHEAST OVER
THE GREAT LAKES/UPPER OH VALLEY AND MID-ATLANTIC REGION FRI
THROUGH FRI NIGHT...WITH EVOLUTION INTO A LARGER SCALE TROUGH
ALONG THE EASTERN SEABOARD BY SAT. THIS ENERGY WILL INTERACT WITH
THE AFOREMENTIONED FRONTAL ZONE AND CAPTURE THE SURFACE WAVE
ACTIVITY STILL TRAVERSING THE OH VALLEY ON FRI...LEADING TO A
CONSOLIDATING AND AMPLIFYING SURFACE LOW IN VICINITY OF NORTHERN
VA/D.C. AND ADJACENT AREAS OF CENTRAL MD THE NORTHERN CHESAPEAKE
BAY BY FRI EVE. THIS LOW WILL CONTINUE TO DEEPEN AND LIFT
NORTHEAST AND OFFSHORE OF SOUTHERN NEW ENGLAND BY MIDDAY SAT WITH
A TRACK LIKELY CLOSE TO THE BENCHMARK.

THE 12Z NAM OVERALL APPEARS A TAD TOO DEEP ON THE LARGER SCALE
WITH ITS HEIGHT FALLS OVER THE MID-ATLANTIC FRI NIGHT AND EARLY
SAT. MEANWHILE...THE 12Z GFS/00Z UKMET AND 00Z ECMWF SHOW FAIRLY
GOOD CLUSTERING WITH THEIR HEIGHT FALL TIMING/DEPTH. HOWEVER THERE
ARE SOME CONCERNS OVER CONVECTIVE FEEDBACK NOTED IN THE ECMWF AND
UKMET WHICH ASIDE FROM THEIR HEAVY QPF OUTPUT...APPEAR TO BE
OVER-DEEPENING THEIR MID LEVEL CENTERS AND CORRESPONDING SURFACE
LOWS JUST A BIT OVER THE MID-ATLANTIC AND OFFSHORE NEW ENGLAND
LATE FRI THROUGH SAT. THE 00Z CMC IS GENERALLY A PROGRESSIVE
OUTLIER WITH ITS HEIGHT FALLS AND ITS SURFACE LOW/FRONTAL
EVOLUTION. GENERALLY A SLIGHTLY WEAKER VERSION OF A 12Z GFS/00Z
UKMET AND 00Z ECMWF CONSENSUS WILL BE PREFERRED...WHICH OVERALL
MAY BE CLOSEST TO A 12Z GFS AND 00Z ECENS MEAN COMPROMISE. THE
LATTER WILL BE THE PREFERENCE FOR NOW PENDING THE NEW NON-NCEP
GUIDANCE.


...UPPER LOW/TROUGH OVER CA TODAY...
...SHEARING DOWNSTREAM OVER THE NORTHERN INTERMOUNTAIN REGION...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE UPPER LOW/TROUGH AXIS OVER CENTRAL/NORTHERN CA THIS MORNING
WILL BEGIN TO SLOWLY WEAKEN TODAY AND THEN ADVANCE NORTHEAST AND
SHEARING DOWNSTREAM ON THURS ACROSS THE NORTHERN INTERMOUNTAIN
REGION. THIS WILL OCCUR AS THE SUBTROPICAL RIDGE OVER THE SOUTHERN
U.S. SHIFTS FARTHER WEST IN TANDEM WITH WEAK TROUGHING ARRIVING
ACROSS THE PACIFIC NORTHWEST. SOME REMNANT ENERGY OF THIS FEATURE
WILL LIKELY EJECT OVER TOP THE RIDGE AND ACROSS THE NORTHERN HIGH
PLAINS BY FRI. THERE IS GOOD MODEL MASS FIELD AGREEMENT WITH THIS
SYSTEM AND ITS EVOLUTION AND SO A GENERAL MODEL BLEND WILL BE
PREFERRED.


...SHORTWAVE ARRIVING ACROSS THE PACIFIC NORTHWEST ON THURS...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

A STRONG SURGE OF HEIGHT FALLS DROPPING DOWN ACROSS WESTERN AK AND
INTO THE GULF OF AK WILL ALLOW A PACIFIC SHORTWAVE WELL TO ITS
SOUTHEAST TO EJECT EASTWARD AND INTO THE PACIFIC NORTHWEST BY
THURS AND SOUTHWEST CANADA ON FRI. THERE IS GOOD MODEL MASS FIELD
AGREEMENT AT THIS TIME...AND SO A GENERAL MODEL BLEND WILL BE
PREFERRED.


...SHORTWAVE REACHING NORTHERN CA ON FRI...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

THE MODELS SHOW A WEAK SOUTHERN STREAM SHORTWAVE APPROACHING
NORTHERN CA ON FRI AND THEN WEAKENING ON SAT. HOWEVER...A PORTION
OF THE ENERGY IS EXPECTED TO CLOSE OFF AT AT 700 MB AND DROP SOUTH
OFFSHORE CENTRAL CA BY LATE SAT. MODEL MASS FIELD DIFFERENCES ARE
QUITE MODEST...SO A GENERAL MODEL BLEND WILL BE PREFERRED.


...DEEP CLOSED LOW OVER THE GULF OF AK THURS/FRI/SAT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND
CONFIDENCE: ABOVE AVERAGE

MINUS SOME VERY MODEST TIMING/HEIGHT FALLS DIFFERENCES...THE
MODELS SHOW GOOD AGREEMENT WITH THE EVOLUTION OF THE STRONG CLOSED
LOW OVER THE GULF OF AK THROUGH THE PERIOD. WILL PREFER A GENERAL
MODEL BLEND WITH THIS.


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

ORRISON

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