923  
FXUS10 KWNH 161634  
PMDHMD  
 
MODEL DIAGNOSTIC DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1233 PM EDT MON OCT 16 2017  
 
VALID OCT 16/1200 UTC THRU OCT 20/0000 UTC  
 
...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR  
INGEST...  
 
12Z PRELIMINARY MODEL EVALUATION WITH PREFERENCES/CONFIDENCE  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
 
...SHORTWAVE MOVING QUICKLY EAST IN ZONAL FLOW ALONG THE  
U.S.-CANADA BORDER TUESDAY THROUGH THURSDAY...   
..ASSOCIATED DEEPENING SURFACE LOW AND COLD FRONT
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 00Z ECMWF / CMC / UKMET  
CONFIDENCE: ABOVE AVERAGE  
 
A SHORTWAVE EMBEDDED IN ZONAL FLOW WILL MOVE RAPIDLY EAST ALONG OR  
JUST NORTH OF THE CANADIAN BORDER IN THE NEXT 48 HOURS...MOVING  
ABOUT 50 DEGREES LONGITUDE (150W TO 100W) BY WEDNESDAY MORNING.  
MODELS ALSO SHOW RELATIVELY STRONG LEE CYCLOGENESIS TO THE EAST OF  
THE CANADIAN ROCKIES (DEEPENING RATE OF SURFACE LOW NOT QUITE 1  
BERGERON BUT CLOSE) AND OCCLUDE THE CYCLONE IN THE VICINITY OF  
NORTHERN MANITOBA AND HUDSON BAY BY WEDNESDAY NIGHT. THE 12Z GFS  
SHOWS A SLIGHTLY STRONGER WAVE APPROACHING THE COAST...AND HAS A  
MID-LEVEL VORT MAX DISPLACED ABOUT 2 DEGREES LATITUDE FURTHER  
SOUTH THAN THE OTHER MODELS. THIS EVENTUALLY LEADS TO A SURFACE  
LOW ON THE STRONGER SIDE OF MODEL SPREAD...AND DISPLACED FURTHER  
TO THE WEST (SLOWER PROGRESSION). GIVEN THE ZONAL FLOW...PREFER  
THE CONSENSUS WITH A SLIGHTLY LOWER AMPLITUDE AND FASTER WAVE. THE  
12Z NAM HAS A SIMILAR AMPLITUDE TO THE OTHER MODELS...BUT IS  
SLOWER BY A FEW HOURS. A STRONGER VORT MAX EMBEDDED NORTH OF THE  
PRIMARY WAVE ALSO LEADS TO SURFACE LOW DEVELOPMENT FURTHER NORTH  
THAN THE OTHER MODELS. OVERALL THE 12Z NAM AND GFS DIFFERENCES ARE  
NOT LARGE...BUT ENOUGH AT SUCH SHORT TIME RANGES THAT A  
ECMWF...CMC...UKMET BLEND IS RECOMMENDED.  
 
...TROUGH DIGGING INTO THE PACIFIC NORTHWEST LATE WEDNESDAY INTO  
THURSDAY ASSOCIATED WITH AN ATMOSPHERIC RIVER EVENT...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: NON-CMC BLEND  
CONFIDENCE: SLIGHTLY ABOVE AVERAGE  
 
A DEEPER TROUGH (COMPARED TO THE TUESDAY SYSTEM) WILL DIG INTO THE  
PACIFIC NORTHWEST LATE WEDNESDAY INTO THURSDAY...WITH A STRONG  
SURFACE CYCLONE APPROACHING BRITISH COLUMBIA BY THE END OF THE  
WORK WEEK. AN ATMOSPHERIC RIVER WILL BE PUSHING INTO THE PACIFIC  
NORTHWEST JUST AHEAD OF THE TROUGH. MODEL DIFFERENCES WITH THIS  
SYSTEM ARE FAIRLY SMALL. NORMALIZED STANDARD DEVIATION OF MSLP AND  
500MB HEIGHTS ON THE 00Z ECMWF ENSEMBLES SHOWED GENERALLY NEAR OR  
SLIGHTLY BELOW NORMAL AMOUNT OF SPREAD OVER THE NORTHWEST FROM  
WEDNESDAY INTO THURSDAY. ONE EXCEPTION WAS THE 00Z CMC WHICH  
SHOWED A MUCH FLATTER WAVE ACROSS THE EASTERN PACIFIC WITH HIGHER  
HEIGHTS THROUGH MOST OF THE MID-LATITUDES (30-60N)...AS MUCH AS  
60M HIGHER THAN ENSEMBLE MEANS AT 500MB IN SOME LOCATIONS.  
THEREFORE...A GENERAL MODEL BLEND THAT EXCLUDES THE 00Z CMC IS  
PREFERRED.  
 
...PROGRESSIVE TROUGH MOVING FROM THE EASTERN GREAT LAKES TODAY TO  
THE CANADIAN MARITIMES TUESDAY...  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: GENERAL MODEL BLEND  
CONFIDENCE: ABOVE AVERAGE  
 
A BROAD TROUGH OVER THE NORTHEASTERN CONUS WILL GRADUALLY  
DEAMPLIFY AND QUICKLY EXIT OVER THE ATLANTIC DURING THE NEXT 24  
HOURS...WITH THE SURFACE COLD FRONT GENERALLY ALREADY OFFSHORE.  
MODELS SHOW NO SUBSTANTIAL DISAGREEMENT IN THE MASS FIELDS WITH  
THIS SYSTEM SO A GENERAL MODEL BLEND IS RECOMMENDED WITH HIGH  
CONFIDENCE.  
 
   
..LINGERING BROAD TROUGH OVER THE SOUTHEAST STATES
 
   
..STALLING COLD FRONT IN THE VICINITY OF FLORIDA
 
 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
PREFERENCE: 12Z GFS / 00Z ECMWF  
CONFIDENCE: AVERAGE  
 
ALTHOUGH THE COMPONENT OF THE EASTERN CONUS WAVE TIED MORE CLOSELY  
TO THE NORTHERN STREAM JET SHOULD BE PROGRESSIVE...A BROAD TROUGH  
IS LIKELY TO LINGER OVER THE SOUTHEAST THROUGH THE REMAINDER OF  
THE WEEK. INITIALLY THIS WILL LIKELY BE RELATED TO THE SOUTHERN  
PORTION OF THE INITIAL TROUGH...HOWEVER SEVERAL REINFORCING  
SHORTWAVES CURRENTLY NEAR THE NORTHWESTERN U.S. ARE LIKELY TO  
ARRIVE OVER THE NEXT SEVERAL DAYS. AT THE SURFACE...A COLD FRONT  
WILL STALL OUT NEAR THE SOUTHERN PERIPHERY OF THE LOW-LEVEL MID  
ATLANTIC ANTICYCLONE.  
 
IN GENERAL...THE 00Z ECMWF AND 12Z GFS ARE CLOSEST TO THE ENSEMBLE  
MEANS IN THEIR DEPICTION OF THE MASS FIELDS. BOTH MAINTAIN  
SLIGHTLY LOWER HEIGHTS ALOFT THROUGH THE REGION...WHICH IS  
GENERALLY PREFERRED GIVEN THE SOUTHWARD EXTENSION OF THE TROUGH  
NOTED ON WATER VAPOR SATELLITE AT THE MOMENT...AS WELL AS SOME  
CONVECTION OVER THE CENTRAL GULF THAT MAY BE TIED TO A WEAK  
MID-LEVEL LOW. THE CMC...UKMET...AND NAM (TO A SLIGHTLY LESSER  
EXTENT) TEND TO DEAMPLIFY THE TROUGH AND BUILD HEIGHTS  
FASTER...AND DEPICT A STRONGER SURFACE HIGH ACROSS THE EASTERN  
CONUS. GIVEN THE SUPPORT FROM THE 00Z ENSEMBLE MEANS...PREFER A  
BLEND OF THE 12Z GFS AND 00Z ECMWF IN THIS REGION.  
 
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