273  
FXUS02 KWBC 211559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1059 AM EST SUN JAN 21 2018  
 
VALID 12Z WED JAN 24 2018 - 12Z SUN JAN 28 2018  
   
..OVERVIEW
 
 
EXPECT FLOW ALOFT TO MAINTAIN MODERATE PROGRESSION BUT BECOME  
AMPLIFIED WITH TIME. A VIGOROUS TROUGH OVER THE EASTERN PACIFIC  
AS OF EARLY WED WILL PROCEED INTO THE CENTRAL U.S. BY NEXT WEEKEND  
WHILE THE DOWNSTREAM RIDGE WILL REACH THE WESTERN ATLANTIC AND  
JOIN WITH RIDGING ALREADY PRESENT EAST OF THE BAHAMAS. BEHIND THE  
UPPER TROUGH, A STRONG RIDGE WILL BUILD INTO THE WEST WITH SOME  
AID FROM A DEEP TROUGH/CLOSED LOW BECOMING ANCHORED OVER THE  
NORTHEAST PACIFIC/GULF OF ALASKA BY THE LATTER HALF OF THE PERIOD.  
THE TROUGH CROSSING THE LOWER 48 WILL SPREAD A BROAD AREA OF  
ACTIVE WEATHER ACROSS THE LOWER 48 AND SUPPORT A WAVY FRONTAL  
SYSTEM SEPARATING VERY CONTRASTING AIR MASSES, WHILE EVOLUTION OF  
EASTERN PACIFIC FLOW MAY BRING INCREASING MOISTURE INTO PARTS OF  
THE PACIFIC NORTHWEST NEXT WEEKEND.  
 
   
..GUIDANCE/UNCERTAINTY ASSESSMENT AND PREFERENCES
 
 
BROADLY SPEAKING GUIDANCE CONTINUES TO SHOW BETTER THAN AVERAGE  
AGREEMENT WITH FAIRLY GOOD CONTINUITY IN MOST RESPECTS, WITH SOME  
EMBEDDED DETAIL/TIMING QUESTIONS EARLY-MID PERIOD AND LINGERING  
UNCERTAINTIES ON A SOMEWHAT LARGER SCALE BY NEXT WEEKEND. OF NOTE  
OFF THE PACIFIC NORTHWEST ON DAY 3 WED, THE 00Z ECMWF IS ON THE  
SOUTHERN FRINGE OF GUIDANCE INCLUDING NEARLY ALL 00Z  
GEFS/ECMWF/CMC ENSEMBLE MEMBERS FOR SURFACE LOW PRESSURE NEARING  
THE COAST. FARTHER EAST DIFFERENCES/ADJUSTMENTS ARE GENERALLY DUE  
TO SMALLER SCALE SHORTWAVE DETAILS THAT ARE TYPICALLY NOT WELL  
RESOLVED BEYOND THE SHORT RANGE TIME FRAME. THERE IS GOOD  
CONTINUITY WITH THE CONCEPT OF LOW PRESSURE TRACKING ACROSS THE  
EXTREME NORTHERN TIER BUT SOME ENSEMBLES AND THE UKMET ARE HINTING  
AT THE POTENTIAL FOR SLIGHTLY FASTER TIMING. THE FULL CONSENSUS  
OF MODELS/MEANS IS STILL FAIRLY CLOSE TO CONTINUITY BUT THE  
TRAILING FRONT HAS ADJUSTED A BIT FASTER DUE TO THE LOW BEING  
SOMEWHAT STRONGER THAN FORECAST YESTERDAY. AS THE WAVY FRONT  
HEADS INTO THE EAST DURING THE WEEKEND THERE IS DECENT AGREEMENT  
THAT EVOLUTION OF UPPER TROUGH ENERGY SHOULD SUPPORT CONSOLIDATION  
OF LOW PRESSURE BETWEEN THE EASTERN GREAT LAKES AND CENTRAL  
APPALACHIANS BY THE END OF DAY 7 SUN.  
 
BY NEXT WEEKEND SOME UNCERTAINTY ARISES WITH RESPECT TO THE  
PRECISE AMPLITUDE OF EASTERN PACIFIC/WESTERN U.S. FLOW ALOFT.  
GEFS/ECMWF MEANS HAVE BEEN FAIRLY STABLE OVER THE PAST DAY BUT  
INDIVIDUAL ENSEMBLE MEMBERS AND GFS/ECMWF RUNS DISPLAY SUFFICIENT  
DIFFERENCES TO HAVE AN IMPACT ON DETAILS OF THE STRONG FLOW OF  
MOISTURE (WITH SOME CONNECTION TO LOWER LATITUDES) EXPECTED TO  
REACH THE PACIFIC NORTHWEST/VANCOUVER ISLAND AREA. DETAILS HERE  
WILL ULTIMATELY AFFECT THE CHARACTER OF THE UPPER TROUGH REACHING  
THE PLAINS/MS VALLEY LATE IN THE PERIOD. MANY CANADIAN ENSEMBLES  
ARE STILL FLATTER THAN MOST OTHER GUIDANCE BUT NOT TO THE DEGREE  
SEEN YESTERDAY, WHILE THE OPERATIONAL 00Z CMC HAS ADJUSTED TOWARD  
ITS MEAN VERSUS YESTERDAY'S 00Z RUN THAT WAS MORE SIMILAR TO THE  
GFS/ECMWF AND THEIR MEANS.  
 
FOR DAYS 3-5 WED-FRI A BLEND OF THE 00Z/06Z GFS AND 00Z ECMWF  
ALONG WITH LESSER WEIGHT OF THE 00Z CMC AND UKMET REPRESENTED  
CONSENSUS WELL FOR THAT PART OF THE FORECAST, WITH INPUT FROM  
REMAINING MODELS HELPING TO DOWNPLAY THE LESS CONFIDENT 00Z ECMWF  
DETAILS OFF THE PACIFIC NORTHWEST ON WED. DAYS 6-7 SAT-SUN BEGAN  
TO INCLUDE 06Z GEFS/00Z ECMWF ENSEMBLE MEANS ALONG WITH THE  
GFS/ECMWF RUNS (WHILE PHASING OUT THE CMC) TO RESOLVE UNCERTAINTY  
OF SPECIFICS WITHIN THE FAIRLY AGREEABLE LARGE SCALE PATTERN.  
 
   
..SENSIBLE WEATHER HIGHLIGHTS/THREATS
 
 
THE STRONG SYSTEM HEADING INTO THE WEST WILL INITIALLY FOCUS  
HEAVIEST RAIN/HIGHER ELEVATION SNOW OVER/NEAR NORTHERN CALIFORNIA  
ON WED WITH LESSER AMOUNTS TO THE NORTH/NORTHEAST. PRECIP SHOULD  
LESSEN IN INTENSITY SOMEWHAT, BUT STILL WITH POTENTIAL FOR  
LOCALIZED ENHANCEMENT, AS MOISTURE EXTENDING SOUTH AND EAST  
EVENTUALLY CROSSES MUCH OF THE NORTHERN/CENTRAL WESTERN STATES  
THROUGH LATE WEEK. EMERGENCE OF THE LEADING COLD FRONT OVER THE  
PLAINS BY FRI AND FLOW AROUND EASTERN U.S. INTO WESTERN ATLANTIC  
HIGH PRESSURE WILL BRING GULF OF MEXICO MOISTURE NORTHWARD,  
EVENTUALLY GENERATING A LARGE AREA OF PRECIP OVER THE EASTERN HALF  
OF THE COUNTRY FROM FRI NIGHT THROUGH THE WEEKEND. THE FULL ARRAY  
OF MODEL/ENSEMBLE GUIDANCE CURRENTLY SIGNALS TWO AREAS THAT SHOULD  
BE MOST FAVORED FOR HEAVY RAINFALL POTENTIAL, ONE FROM THE  
SOUTH-CENTRAL MS VALLEY INTO THE OH VALLEY/LOWER GREAT LAKES AND A  
SECOND FROM THE CENTRAL/EASTERN GULF COAST INTO SOUTHERN  
APPALACHIANS/MID ATLANTIC. EXTREME NORTHERN TIER LOCATIONS MAY  
SEE SNOW IN THE VICINITY OF ONE OR MORE SURFACE WAVES.  
 
A MAJORITY OF THE WEST WILL SEE A DRIER TREND BY NEXT WEEKEND AS  
THE ABOVE SYSTEM DEPARTS. THE ONE EXCEPTION WILL BE OVER PORTIONS  
OF THE PACIFIC NORTHWEST INTO VANCOUVER ISLAND AND POSSIBLY TO  
SOME DEGREE INTO THE EXTREME NORTHERN ROCKIES AS EVOLUTION OF FLOW  
ALOFT SUPPORTS A POTENTIAL ATMOSPHERIC RIVER EVENT. CURRENTLY THE  
FAVORED TERRAIN OVER WA AND VANCOUVER ISLAND HAVE THE HIGHEST  
PROBABILITY OF SEEING HEAVY PRECIP NEXT WEEKEND BUT THERE IS  
ENOUGH SPREAD IN GUIDANCE TO ALLOW FOR THE POTENTIAL THAT SOME OF  
THIS MOISTURE COULD EXTEND SOMEWHAT FARTHER SOUTH.  
 
AROUND MIDWEEK EXPECT MOSTLY LIGHT LAKE EFFECT SNOW IN THE WAKE OF  
THE STRONG/WINDY SHORT RANGE SYSTEM TRACKING AWAY FROM NEW  
ENGLAND, WHILE A FRONTAL PASSAGE MAY BRING A PERIOD OF RAIN TO  
SOUTHERN FLORIDA.  
 
WARM FLOW AHEAD OF THE SYSTEM PROGRESSING INLAND FROM THE WEST  
COAST WILL INITIALLY BRING MODERATELY ABOVE NORMAL READINGS TO THE  
WESTERN STATES AND THEN MUCH ABOVE NORMAL TEMPS TO THE  
CENTRAL-EASTERN STATES THU ONWARD, WITH SOME PLUS 15-30F  
ANOMALIES. TRAILING COLDER AIR CROSSING THE WEST LATE WEEK AND  
REACHING THE CENTRAL U.S. NEXT WEEKEND SHOULD BE LESS EXTREME,  
ONLY 5-15F BELOW NORMAL AT MOST LOCATIONS.  
 
RAUSCH  
 

 
 
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