792  
FXUS10 KWNH 271828  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
227 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  
CONFIDENCE: 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. THE  
MODELS ARE IN SUFFICIENT AGREEMENT THROUGH THE PERIOD FOR A  
GENERAL MODEL BLEND TO BE PREFERRED.  
 
   
..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 12Z 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 STRONG  
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 GIVEN  
PROXIMITY OF WHAT WILL BE AN ANOMALOUSLY STRONG...3.5-4.0 SIGMA  
BELOW NORMAL...CLOSED 500 MB LOW OVER THE MID-ATLANTIC STATES.  
 
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 12Z  
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 12Z UKMET IS VERY CLOSE TO THE GFS/ECMWF  
CAMP...BUT THE 12Z CMC IS OVERALL THE WEAKEST WITH THE TROUGH  
AMPLIFICATION AND IS THE 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 EJECTING ACROSS THE NORTHERN GREAT BASIN TODAY
 
   
..ENERGY SHEARING ACROSS THE NORTHERN PLAINS ON FRI
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS TAKE A RATHER WELL-DEFINED SHORTWAVE TROUGH OVER NV/ID  
OFF TO THE EAST-NORTHEAST THROUGH TONIGHT AND ACROSS NORTHERN WY  
AND SOUTHERN MT. ON FRI...THE ENERGY SHOULD EJECT OUT ACROSS THE  
NORTHERN PLAINS AND GRADUALLY DISSIPATE OVER THE UPPER MS VALLEY  
FRI NIGHT. THE MODELS SHOW MINIMAL MASS FIELD SPREAD...SO A  
GENERAL MODEL BLEND WILL BE PREFERRED.  
 
   
..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: 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 700 MB AND DROP SOUTH  
OFFSHORE CENTRAL AND SOUTHERN CA BY LATE SAT THROUGH SUN. THE  
GUIDANCE IS GENERALLY WELL-CLUSTERED WITH THEIR SOLUTIONS...SO A  
GENERAL MODEL BLEND 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.  
 
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ORRISON  
 

 
 
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