588  
FXUS02 KWBC 160558  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1257 AM EST THU NOV 16 2017  
 
VALID 12Z SUN NOV 19 2017 - 12Z THU NOV 23 2017  
   
..PATTERN OVERVIEW
 
 
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 SEEMS TO INTRODUCE SIGNIFICANT UNCERTAINTY  
INTO THE FORECAST BY LATER IN THE PERIOD, PERHAPS DUE TO  
DIFFICULTIES ASSIMILATING RELATIVELY SPARSE DATA AT THESE HIGH  
LATITUDES INTO THE MODEL GUIDANCE. 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 LARGE-SCALE PATTERN  
THROUGHOUT THE MEDIUM RANGE.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES...
 
 
MODEL CONSENSUS HAS FINALLY IMPROVED SURROUNDING THE LOW PRESSURE  
SYSTEM AFFECTING THE EASTERN U.S. THIS WEEKEND, WITH THE GFS NOW  
ONLY SLIGHTLY FASTER THAN THE ECMWF WITH THE SURFACE FRONTAL  
TIMING. ADDITIONALLY, SOLUTIONS HAVE CONVERGED IN A DAY 3 (12Z  
SUN) SFC LOW POSITION ACROSS SOUTHERN QUEBEC, WITH A TREND TOWARD  
A SOMEWHAT DEEPER LOW RELATIVE TO THE PAST COUPLE DAYS. GIVEN  
THESE TRENDS, A MAJORITY DETERMINISTIC BLEND OF THE 12Z ECMWF/18Z  
GFS (WITH A BIT MORE WEIGHT TOWARD THE ECMWF GIVEN ITS RECENT  
PERFORMANCE WITH THIS SYSTEM) SERVED AS A GOOD STARTING POINT  
DURING DAYS 3-4.  
 
SIGNIFICANT UNCERTAINTY CONTINUES TO PLAGUE THE FORECAST FROM DAY  
5 (TUE) ONWARD, WITH THE GFS AND ECMWF PERSISTING WITH  
SUBSTANTIALLY DIFFERENT SOLUTIONS FOR SHORTWAVE ENERGY OF ARCTIC  
ORIGIN REACHING THE PAC NORTHWEST ON DAY 4. THE ECMWF HAS BEEN  
PERSISTENT IN FRAGMENTING THE WAVE INTO TWO COMPONENTS - ONE THAT  
TRAVERSES THE U.S. NORTHERN TIER AS IT AMPLIFIES, AND ANOTHER THAT  
DIVES SOUTH THROUGH THE FOUR CORNERS REGION EVENTUALLY CLOSING OFF  
INTO A CUTOFF UPPER LOW NEAR THE RIO GRANDE BY WED. THE ECMWF GETS  
A SUBSTANTIAL DEGREE OF SUPPORT FROM ITS ENSEMBLE, WHICH HAS  
HINTED AT THE POTENTIAL FOR THIS SCENARIO FOR SEVERAL RUNS NOW.  
THE GFS, ON THE OTHER HAND, KEEPS THE ENTIRETY OF THE WAVE  
CONFINED TO THE NORTHERN TIER RESULTING IN A HIGHLY AMPLIFIED FLOW  
PATTERN BY DAYS 6-7 ACROSS EASTERN CANADA/U.S. AND THUS THE  
TRANSPORT OF MUCH COLDER AIR SOUTHWARD INTO THE NORTHERN  
PLAINS/UPPER MIDWEST. ENSEMBLE SPAGHETTI PLOTS SHOW THE NUMBER OF  
ECENS MEMBERS SUPPORTING SOMETHING SIMILAR TO THE DETERMINISTIC EC  
INCREASING OVER THE PAST COUPLE CYCLES, AND A MINORITY OF GEFS AND  
CMCE MEMBERS HAVE SHIFTED IN THAT DIRECTION AS WELL. RECENT  
ANECDOTAL EXPERIENCE SUGGESTS THAT THE ECMWF HAS HANDLED SHORTWAVE  
ENERGY OF ARCTIC ORIGIN BETTER THAN THE GFS. BASED ON THIS ALONG  
WITH THE NOTED ENSEMBLE TRENDS, THE WPC FORECAST WAS WEIGHTED  
HEAVILY TOWARD THE 12Z ECMWF AND ECENS MEAN AFTER DAY 5.  
 
   
..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.  
 
RYAN  
 

 
 
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