707  
FXUS02 KWBC 121559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST SUN NOV 12 2017  
 
VALID 12Z WED NOV 15 2017 - 12Z SUN NOV 19 2017  
   
..PATTERN OVERVIEW
 
 
DOWNSTREAM FROM A HIGHLY AMPLIFIED PATTERN ACROSS THE  
NORTH/CENTRAL PACIFIC, REASONABLY PROGRESSIVE FLOW SHOULD REMAIN  
IN PLACE ACROSS THE CONUS FOR MOST OF THE MEDIUM RANGE PERIOD. BY  
NEXT WEEKEND THE PATTERN SHOULD SETTLE INTO A BROAD/AMPLIFIED  
WESTERN RIDGE-EASTERN TROUGH CONFIGURATION WITH AN ELONGATED MEAN  
TROUGH OVER THE EASTERN PACIFIC. THE REX BLOCK WHICH HAS BEEN  
PERSISTENT IN THE VICINITY OF THE DATE LINE SHOWS LITTLE SIGN OF  
BREAKING DOWN OVER THE NEXT WEEK, ALTHOUGH A SLIGHT WESTWARD  
PROPAGATION IS POSSIBLE. TELECONNECTIONS ASSOCIATED WITH THE  
PROMINENT PACIFIC HEIGHT ANOMALY CENTERS SUPPORT THE EXPECTED  
NORTH AMERICAN PATTERN, WET CONDITIONS ALONG THE WEST COAST, AND  
PERIODIC INTRUSIONS OF COLD CANADIAN AIR FROM THE NORTHERN TIER  
SOUTHEASTWARD.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT
 
 
THE HANDLING OF AND DOWNSTREAM IMPACTS FROM NORTHERN  
PACIFIC/BERING SEA ENERGY DROPPING INTO THE EAST PACIFIC EARLY IN  
THE PERIOD CONTINUE TO GENERATE THE PRIMARY DIFFICULTIES IN THE  
FORECAST. MODEL/ENSEMBLE SPAGHETTI PLOTS OVER THIS PART OF THE  
PACIFIC BECOME CHAOTIC ALREADY IN THE DAYS 3-4 WED-THU TIME FRAME,  
LEADING TO THE TIMING/DETAIL DIFFERENCES THAT DEVELOP FOR A  
SIGNIFICANT EASTERN PACIFIC UPPER TROUGH EJECTING THROUGH THE  
WESTERN STATES DURING THE LATTER HALF OF THE WEEK AND A  
POTENTIALLY STRONG/WINDY STORM AFFECTING THE EASTERN HALF OF THE  
COUNTRY LATE THIS WEEK/WEEKEND.  
 
AFTER GUIDANCE HAD SHOWN A TENDENCY TOWARD AN INTERMEDIATE TIMING  
(RELATIVE TO FASTER GFS AND SLOWER ECMWF) OVER THE PAST DAY OR SO,  
NEARLY ALL MODELS/MEANS FROM THE 00Z/06Z CYCLES SUPPORT A SLOWER  
ADJUSTMENT TO THE FORECAST FOR THE UPPER TROUGH OF INTEREST FROM  
ABOUT DAY 4 THU ONWARD. WITHIN THIS SLOWER TREND THE GFS/GEFS ARE  
STILL FASTEST WHILE REMAINING MODELS/MEANS ARE SLOWER TO VARYING  
DEGREES. AS A RESULT OF RECENT TRENDS AND LATEST CLUSTERING, THE  
PREFERRED SCENARIO NOW LEANS ABOUT 2/3 TO THE ECMWF/ECMWF MEAN  
CLUSTER AFTER AN EVEN GUIDANCE BLEND DAY 3 WED INTO EARLY DAY 4  
THU. THE FORECAST INCLUDES A SMALL 00Z CMC COMPONENT INTO DAY 6  
SAT. THIS BLEND FURTHERS THE RECENT TREND TOWARD SLOWER  
PROGRESSION OF EASTERN PACIFIC/WESTERN U.S. LOW PRESSURE AND  
FRONTAL SYSTEM INTO THE PLAINS/MS VALLEY AND EASTERN STATES.  
GUIDANCE IS STILL SIGNALING POTENTIAL FOR A FAIRLY COMPLEX SURFACE  
EVOLUTION OVER THE PLAINS WITH A CENTRAL PLAINS WAVE MOST LIKELY  
TO BECOME THE DOMINANT SURFACE LOW REACHING THE GREAT LAKES.  
DETAILS OF SURFACE LOW CONSOLIDATION ARE STILL UNCERTAIN,  
TEMPERING CONFIDENCE IN LOW DEPTH AT ANY SPECIFIC TIME. HOWEVER  
OPERATIONAL MODEL RUNS AS A WHOLE DO SUGGEST A FAIRLY HIGH  
PROBABILITY THAT THE STORM WILL EVENTUALLY REACH AT LEAST AS LOW  
AS THE 980'S MB. THUS THE FORECAST BLEND MAINTAINS SOME  
OPERATIONAL MODEL COMPONENT TO BRING THE DEPTH BELOW ENSEMBLE MEAN  
FORECASTS.  
 
ADJUSTMENTS WITH THE ABOVE SYSTEM HAVE LED TO A STRONGER TREND FOR  
THE UPPER TROUGH REACHING THE NORTHEAST BY LATE DAY 4 THU. AS A  
RESULT THERE IS INCREASED POTENTIAL FOR SURFACE DEVELOPMENT JUST  
OFF THE NEW ENGLAND COAST INTO THE CANADIAN MARITIMES AROUND LATE  
THU-FRI. THE 00Z UKMET IS SLOWEST/MOST ENTHUSIASTIC WITH THIS  
DEVELOPMENT BUT THE MODEL HAS RECENTLY HAD SUCH A BIAS SO WOULD  
CONSIDER IT A LOWER PROBABILITY SOLUTION.  
 
ALSO WORTH NOTING, THE 00Z CMC COMPONENT WAS SUFFICIENTLY SMALL  
NOT TO HAVE AN ADVERSE IMPACT ON THE FORECAST OVER THE EASTERN  
PACIFIC, WHERE THE MODEL BROUGHT EASTERN PACIFIC UPPER TROUGHING  
FARTHER EAST THAN MOST OTHER GUIDANCE. REMAINING DIFFERENCES AS  
THE MEAN TROUGH INCREASES IN SCALE BECOME MODEST RELATIVE TO  
TYPICAL DAYS 5-7 ERRORS.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
PERSISTENT ONSHORE FLOW AND THE ARRIVAL OF A COUPLE LOW PRESSURE  
SYSTEMS (ONE ON WED AND ANOTHER SAT-SUN) WILL KEEP PRECIPITATION  
IN THE PICTURE ALONG THE WEST COAST THROUGH MUCH OF THE PERIOD.  
HEAVY RAIN AND MOUNTAIN SNOW ARE LIKELY ALONG WITH GUSTY WINDS,  
ESPECIALLY WED-THU WITH THE ARRIVAL OF THE STRONGER OF THE TWO  
SYSTEMS. A BRIEF LULL IS POSSIBLE LATE IN THE WEEK BEFORE THE  
SECOND SYSTEM APPROACHES, BRINGING ANOTHER ROUND OF PRECIP OVER  
THE WEEKEND. EXPECT HIGHEST PRECIP TOTALS FROM FAVORED TERRAIN  
OVER THE PACIFIC NORTHWEST INTO NORTHERN CA AND THE SIERRA NEVADA  
RANGE. SOME MOISTURE WILL EXTEND INTO THE ROCKIES. FARTHER EAST,  
A SURFACE FRONT WILL BRING AREAS OF SHOWERS TO THE MS/OH VALLEYS  
AND GREAT LAKES-NORTHEAST WED-THU. IN THE WAKE OF THIS FRONT AND  
POSSIBLE OFFSHORE-CANADIAN MARITIMES LOW PRESSURE, NEW ENGLAND MAY  
SEE A BRIEF PERIOD OF BRISK WINDS AROUND FRI. MORE SIGNIFICANT  
PRECIP IS POSSIBLE WITH THE DEEP/WINDY GREAT LAKES CYCLONE FRI-SAT  
FROM THE OH VALLEY/GREAT LAKES TO THE NORTHEAST. SUFFICIENT COLD  
AIR MAY BE IN PLACE NORTH OF THE SYSTEM TO PRODUCE SOME SNOWFALL  
POTENTIAL LATE IN THE WEEK FROM THE UPPER MIDWEST TO THE GREAT  
LAKES AND PERHAPS EVEN NORTHERN NEW ENGLAND.  
 
AHEAD OF THE FRONT CROSSING THE WEST DURING THE FIRST HALF OF THE  
PERIOD, HIGH TEMPS REACHING 10-20F ABOVE NORMAL ARE LIKELY FROM  
THE FOUR CORNERS STATES INTO THE CENTRAL/SOUTHERN PLAINS. WITH  
LESSER ANOMALIES THE SOUTHERN ROCKIES TO INTERIOR SOUTHWEST SHOULD  
REMAIN SOMEWHAT ABOVE NORMAL INTO NEXT WEEKEND AS BROAD UPPER  
RIDGING BUILDS INTO THE WEST. EXPECT THE STRONG STORM AFFECTING  
THE EASTERN HALF OF THE COUNTRY LATE WEEK INTO THE WEEKEND TO PULL  
DOWN SOME COLD TEMPS (10-15F BELOW NORMAL FOR HIGHS) FROM THE  
NORTHERN TIER SOUTHEAST TO AT LEAST AS FAR AS THE APPALACHIANS  
OVER THE COURSE OF FRI-SUN.  
 
RAUSCH/RYAN  
 

 
 
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