Prognostic Meteorological Discussion
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000
FXUS10 KWNH 271616
PMDHMD

MODEL DIAGNOSTIC DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1214 PM EDT THU JUL 27 2017

VALID JUL 27/1200 UTC THRU JUL 31/0000 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
SOUTHEAST GA WILL GRADUALLY WEAKEN AND OPEN UP INTO A TROUGH BY
LATER TONIGHT OR EARLY FRI. THE MODELS ARE IN GOOD AGREEMENT WITH
THE DETAILS OF THIS SYSTEM...SO A GENERAL MODEL BLEND WILL BE
PREFERRED.


...UPPER TROUGH TRACKING ACROSS QUEBEC AND NORTHEAST U.S...
...RENEWED CLOSED UPPER LOW NORTH OF MAINE BY SAT MORNING...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND...THROUGH 60 HOURS
            12Z GFS/00Z ECMWF BLEND...AFTER 60 HOURS
CONFIDENCE: SLIGHTLY ABOVE AVERAGE

ADDITIONAL SHORTWAVE ENERGY DIGGING INTO THE BACK SIDE OF A
WEAKENING UPPER TROUGH OVER QUEBEC AND THE NORTHEAST WILL ALLOW A
NEW UPPER LOW TO EVOLVE AND DROP A BIT FARTHER SOUTH TO A POSITION
NORTH OF MAINE BY SAT MORNING. NO SIGNIFICANT MASS FIELD
DIFFERENCES ARE NOTED THROUGH SAT WITH THIS SYSTEM...AND IT SHOULD
THEN EXIT NORTHEAST THROUGH THE CANADIAN MARITIMES ON SUN. ON
SUN...THE 00Z CMC WAS THE SLOWEST TO EXIT THE SYSTEM...WITH THE
00Z UKMET THE FASTEST. THE 12Z NAM/12Z GFS AND 00Z ECMWF SPLIT THE
DIFFERENCE...BUT THE GFS AND ECMWF TEND TO CLUSTER TOGETHER BETTER
AND ARE CLOSE TO THE MODEL CONSENSUS. 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 ECMWF
CONFIDENCE: AVERAGE

THE LATEST GOES WV IMAGERY SHOWS AN ELONGATED AXIS OF SHORTWAVE
ENERGY WITH SOME OF IT CONVECTIVELY INDUCED GRADUALLY SHEARING OFF
TO THE EAST ACROSS THE MID MS AND OH VALLEYS. A SURFACE FRONT IS
NOTED ACROSS THE REGION GRADUALLY MOVING EAST AND WITH MULTIPLE
WAVES OF LOW PRESSURE TRAVERSING IT AT THIS TIME. THE LATEST MODEL
GUIDANCE CONTINUES TO GENERALLY SHEAR THIS ENERGY EAST THROUGH THE
OH VALLEY THROUGH TONIGHT. MEANWHILE...A SEPARATE NORTHERN STREAM
SHORTWAVE NOW ENTERING THE UPPER MS VALLEY IS EXPECTED TO 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...BUT WITH A CLOSED OFF
MID/UPPER LEVEL LOW OVER THE MID-ATLANTIC REGION SAT INTO SUN.
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 THE DELMARVA
FRI NIGHT. THIS LOW WILL LIFT NORTHEAST AND OFFSHORE OF SOUTHERN
NEW ENGLAND BY MIDDAY SAT WHILE ALSO BECOMING ELONGATED AS
ADDITIONAL HEIGHT FALLS LINGER OVER THE MID-ATLANTIC REGION WHICH
MAY KEEP A SEPARATE LOW CENTER NEAR THE DELMARVA ALONG WITH AN
INVERTED TROUGH EXTENDING BACK WEST OVER THE MID-ATLANTIC COASTAL
PLAIN. ON SUN...ALL OF THE GUIDANCE ALLOW A LEAD SURFACE LOW TO
EXIT AWAY FROM NEW ENGLAND WITH A LOW CENTER BECOMING REFOCUSED
JUST EAST OF THE MID-ATLANTIC COAST.

THE 12Z NAM HAS TRENDED A BIT FURTHER NORTH FROM ITS 00Z RUN AND
IS CLOSER TO THE MODEL CONSENSUS LED BY THE 12Z GFS AND 00Z
ECMWF...ALTHOUGH IT TENDS TO DIG ENERGY OVERALL A BIT SOUTH OF
THESE MODELS LATE SAT AND SUN OVER THE MID-ATLANTIC AND PARTS OF
THE SOUTHEAST. THE 00Z UKMET IS VERY CLOSE TO THE GFS/ECMWF
CAMP...BUT THE 00Z CMC IS OVERALL THE WEAKEST WITH THE TROUGH
AMPLIFICATION AND IS THE FARTHEST SOUTH AND MOST PROGRESSIVE WITH
ITS SURFACE WAVE ACTIVITY. THE BEST MODEL CLUSTERING AT THIS POINT
FAVORS THE GFS AND ECMWF PREDOMINANTLY...SO WILL PREFER A BLEND OF
THESE MODELS AT THIS TIME.


...SHORTWAVE ARRIVING ACROSS THE PACIFIC NORTHWEST TODAY...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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 INTO THE PACIFIC NORTHWEST TODAY 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: NON-CMC CONSENSUS
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 700 MB AND DROP SOUTH
OFFSHORE CENTRAL AND SOUTHERN CA BY LATE SAT THROUGH SUN. THE 00Z
CMC IS A BIT OF A WEAKER OUTLIER COMPARED TO THE REMAINING
GUIDANCE WHICH IS WELL-CLUSTERED. SO A NON-CMC CONSENSUS WILL BE
PREFERRED.


...DEEP CLOSED LOW OVER THE GULF OF AK THROUGH FRI AND SAT...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
PREFERENCE: GENERAL MODEL BLEND...THROUGH 60 HOURS
            NON-NAM CONSENSUS...AFTER 60 HOURS
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 FRI AND SAT. ON SUN...THE UPPER
LOW BECOMES MORE ELONGATED SW/NE ACROSS THE REGION AS ADDITIONAL
ENERGY DIGS SOUTH AROUND THE WESTERN PERIPHERY OF THE UPPER LOW.
THE 12Z NAM BECOMES FOCUSED PROBABLY A BIT TOO FAR NORTHEAST WITH
THE CORE OF ITS UPPER LOW ON SUN AND IS PROBABLY TOO DEEP AT THAT
POINT. THE GLOBAL MODELS TEND TO REMAIN IN BETTER AGREEMENT BY
COMPARISON. SO WILL PREFER A GENERAL MODEL BLEND THROUGH 60
HOURS...AND A NON-NAM CONSENSUS THEREAFTER.


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

ORRISON

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