962  
FXUS10 KWNH 210636  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
235 AM EDT SAT OCT 21 2017  
 
VALID OCT 21/0000 UTC THRU OCT 24/1200 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
00Z MODEL EVALUATION ALONG WITH LATEST MODEL PREFERENCES/CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
   
..LONGWAVE TROUGH EVOLUTION THROUGH WESTERN U.S
 
   
..EJECTING OUT INTO THE PLAINS ON SAT ALONG WITH COLD FRONT
 
   
..SURFACE LOW DEEPENING OVER SOUTHERN CANADA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL CONSENSUS  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS HAVE COME INTO GOOD AGREEMENT WITH THE TIMING AND DEPTH  
OF THIS UPPER TROUGH AS IT EJECTS ACROSS THE WESTERN U.S.  
OVERNIGHT AND THEN OUT ACROSS THE CENTRAL/NORTHERN PLAINS ON  
SATURDAY. A COLD FRONT ASSOCIATED WITH THIS TROUGH WILL ALSO EJECT  
OUT ACROSS THE PLAINS AS LOW PRESSURE DEEPENS ACROSS SOUTHERN  
CANADA. A GENERAL MODEL CONSENSUS WILL BE PREFERRED AT THIS TIME  
GIVEN THE GOOD MASS FIELD AGREEMENT.  
 
   
..SOUTHERN STREAM LOWER MS VALLEY CLOSED LOW DEVELOPMENT BY MON
 
   
..SURFACE COLD FRONT SWEEPING ACROSS THE MS/TN VALLEYS
 
   
..SURFACE LOW DEVELOPMENT OVER THE TN VALLEY
 
   
..DEEP PHASED CYCLONE OVER THE GREAT LAKES REGION BY TUES
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: BLEND OF THE 00Z UKMET/CMC AND 12Z ECMWF  
CONFIDENCE: SLIGHTLY BELOW AVERAGE  
 
THE MODELS BREAK AWAY THE SOUTHERN PORTION OF THE LONGER WAVE  
TROUGH EJECTING OUT OF THE WEST AND INTO THE PLAINS STATES ON  
SUNDAY...WITH THIS ENERGY DIGGING SOUTHEAST INTO THE LOWER MS  
VALLEY SUNDAY NIGHT AND RESULTING IN A CLOSED LOW FORMATION BY  
MONDAY. THIS WILL STILL BE INTERACTING WITH THE SOUTHERN PORTION  
OF A N/S ORIENTED FRONTAL ZONE AND THE MODELS SUGGEST A WAVE OF  
LOW PRESSURE WILL DEVELOP ALONG THE BOUNDARY ACROSS THE TN VALLEY  
ON MONDAY.  
 
BY MONDAY NIGHT...THE LOW CENTER WILL LIKELY BEGIN TO DEEPEN MORE  
AGGRESSIVELY AND BEGIN LIFTING NORTH TOWARD THE OH VALLEY IN  
RESPONSE TO A STRONG INTRUSION OF NORTHERN STREAM SHORTWAVE ENERGY  
WHICH WILL BE AMPLIFYING SOUTHEAST ACROSS THE MIDWEST. THE  
NORTHERN STREAM TROUGH WILL AMPLIFY INTO THE OH VALLEY AND GREAT  
LAKES REGION AND SHOULD GRADUALLY PHASE WITH THE SOUTHERN STREAM  
CLOSED LOW AS IT AND BEGINS TO INTERACT AND GRADUALLY PHASE WITH  
THE CLOSED LOW ADVANCING EAST ACROSS THE GULF COAST STATES AND THE  
SOUTHERN APPALACHIANS. THE RESULT IS EXPECTED TO BE A DEEP  
LONGWAVE TROUGH MOVING INTO THE EASTERN HALF OF THE NATION ON  
TUESDAY WITH A STRONG COLD FRONT MOVING TOWARD THE APPALACHIANS  
AND A DEEPENING CYCLONE OVER THE GREAT LAKES REGION. THE 00Z NAM  
BECOMES A SLOW OUTLIER WITH THE SOUTHERN STREAM EVOLUTION OF THE  
CLOSED LOW AND WILL BE DISMISSED. THE 00Z CMC AND 12Z ECMWF  
SOLUTIONS ARE FASTER TO LIFT THE SOUTHERN STREAM ENERGY NORTHEAST  
ACROSS THE TN/OH VALLEYS ALONG WITH THEIR RESPECTIVE SURFACE LOWS.  
THE 00Z GFS AND 12Z UKMET ARE SLOWER TO DO THIS AND KEEP MORE  
SOUTHERN ENERGY SUPPRESSED ACROSS THE SOUTH. CONSEQUENTLY...THE  
CMC AND ECMWF ARE FAVORING FASTER PHASING OF STREAMS...WITH THE  
GFS SLOWER.  
 
UPDATE: THE 00Z UKMET HAS TRENDED VERY SHARPLY TOWARD THE 00Z  
CMC/12Z ECMWF CAMP AND AWAY FROM THE SLOWER GFS. THE 00Z ECMWF  
THOUGH HAS TRENDED MORE PROGRESSIVE IN LIFTING ENERGY NORTH FROM  
THE TN VALLEY AND ACROSS THE OH VALLEY AND GREAT LAKES REGION AND  
HAS A FASTER SURFACE LOW AS A RESULT. THIS IS A BIT MORE OUT OF  
TOLERANCE RELATIVE TO THE MODEL CONSENSUS AND THE SUITE OF EURO  
AND CANADIAN ENSEMBLE MEMBERS. GENERALLY THERE HAS BEEN A  
MULTI-DAY TREND TOWARD QUICKER PHASING AND WITH A STRONGER  
INTRUSION OF NORTHERN STREAM ENERGY TO FACILITATE A MORE AMPLIFIED  
SOLUTION...BUT THE 00Z ECMWF IS PROBABLY A BIT TOO PROGRESSIVE.  
GIVEN THE MANY EURO/CANADIAN ENSEMBLE MEMBERS AND SOME GEFS  
MEMBERS THAT WERE MORE STRONGLY FAVORING THE PRIOR 12Z ECMWF  
RUN...THE PREFERENCE WILL BE TO INCORPORATE THE LATEST UKMET/CMC  
AND PRIOR ECMWF RUN AS A CONSENSUS.  
 
   
..SHORTWAVE REACHING B.C./PAC NORTHWEST ON SAT WITH WARM FRONT
 
   
..ATMOSPHERIC RIVER ARRIVING OVER THE PACIFIC NORTHWEST SAT
 
   
..UPPER TROUGH SWEEPING ACROSS THE NORTHERN PLAINS/MIDWEST MON
 
   
..WEST COAST RIDGE/SANTA ANA WIND DEVELOPMENT MON/TUES
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL CONSENSUS  
CONFIDENCE: ABOVE AVERAGE  
 
THE MODELS ARE IN GOOD AGREEMENT THROUGH EARLY SATURDAY WITH THE  
ARRIVAL OF AN UPPER SPEED MAX/SHORTWAVE AND ATTENDANT WARM FRONT  
ACROSS THE PACIFIC NORTHWEST AND SOUTHWEST BRITISH COLUMBIA. THIS  
ENERGY WILL ADVANCE RAPIDLY NORTHEAST INTO SOUTHWEST CANADA AND  
LEAD THE WAY FOR A LONG-FETCH ATMOSPHERIC RIVER TRAVERSING THE  
PACIFIC BASIN TO COME CRASHING INTO THE COASTAL RANGES AND  
CASCADES OF THE PACIFIC NORTHWEST THROUGH SATURDAY NIGHT AND EARLY  
SUNDAY. THE ONSHORE FLOW SHOULD THEN BEGIN TO SLACKEN BY LATER  
SUNDAY AS LARGER SCALE RIDGING BEGINS TO BUILD ALONG THE WEST  
COAST. MEANWHILE...THE INITIAL ARRIVAL OF SHORTWAVE ENERGY WILL  
THEN AMPLIFY DOWNSTREAM ACROSS THE MIDWEST AND TOWARD THE EASTERN  
U.S. LEADING THE WAY FOR A RATHER HIGH AMPLIFIED LONGWAVE PATTERN  
ACROSS THE U.S. BY TUESDAY. UNFORTUNATELY...THE DEVELOPING UPPER  
LEVEL RIDGE OVER THE WEST BY MONDAY AND TUESDAY WILL ALLOW  
STRONGER SURFACE HIGH PRESSURE TO SETTLE OVER THE GREAT BASIN AND  
LEAD TO THE DEVELOPMENT OF RATHER STRONG SANTA ANA WINDS OVER  
CALIFORNIA. WILL PREFER A GENERAL MODEL CONSENSUS THROUGH THE  
PERIOD GIVEN GOOD LARGE SCALE MASS FIELD AGREEMENT.  
 
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ORRISON  
 

 
 
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