379  
FXUS02 KWBC 161540  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1039 AM EST THU NOV 16 2017  
 
VALID 12Z SUN NOV 19 2017 - 12Z THU NOV 23 2017  
   
..PATTERN OVERVIEW AND GUIDANCE/UNCERTAINTY PREFERENCES...
 
 
A HIGHLY AMPLIFIED PATTERN WILL REMAIN IN PLACE ACROSS THE  
CENTRAL/NORTHERN PACIFIC THROUGH THE MEDIUM RANGE. AN ANOMALOUSLY  
STRONG RIDGE, THE NORTHERN PORTION OF A VERY PERSISTENT REX BLOCK  
EXTENDING SOUTH GENERALLY ALONG THE DATE LINE, WILL CAUSE THE  
PRIMARY SOURCE OF SHORTWAVE ENERGY AFFECTING THE CONUS DURING THE  
MEDIUM RANGE TO BE ACROSS THE HIGH LATITUDES OF SIBERIA AND THE  
ARCTIC REGIONS. THIS CONTINUES TO INTRODUCE UNCERTAINTY INTO THE  
FORECAST THROUGH THE FORECAST PERIOD, ESPECIALLY WITH THE GFS.  
LOOKING TO TELECONNECTIONS FOR SOME GUIDANCE, BOTH ANOMALY CENTERS  
ASSOCIATED WITH THE REX BLOCK SUPPORT SOME DEGREE OF PERSISTENT  
(ALBEIT NOT PARTICULARLY DEEP) EASTERN U.S. TROUGHING, AND FLAT TO  
MODESTLY ANTICYCLONIC UPPER-LEVEL FLOW ACROSS THE WESTERN U.S,  
WITH AN ELONGATED AREA OF TROUGHING ACROSS THE EAST PACIFIC  
DOWNSTREAM OF THE BLOCK. THIS IS GENERALLY CONSISTENT WITH THE  
FORECAST FROM THE 00Z ECMWF/CANADIAN/UKMET. THE 00Z/06Z GFS WERE  
CLUSTERED RELATIVELY CLOSELY TO THE OTHERS ONLY TO START THE  
PERIOD (SUN) BEFORE DIFFERENCES IN HANDLING THE INCOMING WESTERN  
VORTICITY RENDER ITS FORECAST OUT OF SYNC. BY NEXT TUE-THU, UPPER  
HIGH TO THE WEST OF BAJA CALIFORNIA SUPPORTS EASTERN CONUS  
TROUGHING AND A POSSIBLE EMBEDDED SOUTHERN CLOSED LOW IN THE GULF  
(NOT EXPLICITLY SHOWN IN THE 500MB GRAPHICS). 06Z GEFS MEAN AND  
00Z ECMWF ENSEMBLE MEAN WERE FAIRLY CLOSE SO THEIR CONSENSUS  
(ALONG WITH OVERNIGHT CONTINUITY) FORMED THE BASE OF THE DAY 6-7  
FORECAST.  
 
FRACASSO/RYAN  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
LOW PRESSURE SYSTEM/FRONTAL BOUNDARY MOVING OFF THE EASTERN  
SEABOARD ON SUN WILL BRING AND END TO RAIN FOR MUCH OF THE EAST  
COAST, ALTHOUGH RAIN AND PERHAPS SOME SNOW MAY LINGER ACROSS NEW  
ENGLAND INTO THE DAY BEHIND THE SYSTEM. EXPECT BELOW AVERAGE  
TEMPERATURES (5 TO 15 DEG F FOR HIGHS) IN THE WAKE OF THIS COLD  
FRONT. A SECOND CLIPPER-LIKE SYSTEM WILL QUICKLY CROSS THE GREAT  
LAKES TUE-WED BRINGING SOME POTENTIAL FOR SCATTERED AREAS OF SNOW  
AND A RENEWED BLAST OF COLD TEMPERATURES. UNCERTAINTY REMAINS  
SURROUNDING HOW SUBSTANTIAL THIS SECOND AREA OF COLD AIR WILL BE,  
WITH THE GFS SOLUTIONS LOOKING MUCH COLDER THAN THE PREFERRED  
ECMWF. FINALLY, WHETHER OR NOT SOME VARIATION OF THE LOWER  
LATITUDE CUTOFF LOW MENTIONED ABOVE ACTUALLY FORMS BY DAYS 6-7  
WILL PLAY A SIGNIFICANT ROLE IN THE WEATHER ACROSS THE GULF COAST  
BY THAT PERIOD, WITH UPPER LOW DEVELOPMENT SUPPORTING MORE  
MOISTURE TRANSPORT NORTHWARD ACROSS A LINGERING SURFACE FRONT, AND  
PERHAPS EVEN DEVELOPMENT OF A FRONTAL WAVE WHICH WOULD FURTHER  
ENHANCE PRECIPITATION.  
 
A WEAKENING FRONT WILL BRING HEAVY RAIN AND MOUNTAIN SNOW TO AREAS  
FROM NORTHERN CA NORTH INTO THE PAC NW SUN-MON. NEXT SYSTEM  
APPROACHES BRINGING A RETURN OF PRECIP TO THE PAC NW BY TUE-WED.  
BY THE MIDDLE OF NEXT WEEK, RISING HEIGHTS/SNOW LEVELS ACROSS THE  
WEST WILL KEEP SNOW CONFINED TO THE HIGHER ELEVATIONS OF THE  
NORTHERN CASCADES. ALSO AS A RESULT OF THE RIDGING, TEMPERATURES  
WILL BE INCREASINGLY ABOVE AVERAGE ACROSS MUCH OF THE WEST FROM  
MON ONWARD, WITH HIGHS REACHING 10 TO 20 DEG F ABOVE AVERAGE OVER  
MUCH OF THE INTERIOR WEST. RECORD HIGHS MAY BE POSSIBLE MIDWEEK IN  
THE DESERT SOUTHWEST.  
 
RYAN  
 

 
 
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