820  
FXUS02 KWBC 171601  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1200 PM EDT SUN MAR 17 2019  
 
VALID 12Z WED MAR 20 2019 - 12Z SUN MAR 24 2019  
   
..OVERVIEW AND GUIDANCE ASSESSMENT/PREFERENCES
 
 
LATEST GUIDANCE CONTINUES TO ANTICIPATE A SPLIT FLOW REGIME OVER  
WESTERN NORTH AMERICA, WITH A PERSISTENT WESTERN CANADA MEAN RIDGE  
DEFLECTING PERIODIC BUNDLES OF SHORTWAVE ENERGY INTO THE WESTERN  
LOWER 48 AND THE REST INTO OR AROUND MAINLAND ALASKA. ASIDE FROM  
SOME SPECIFICS THERE HAS BEEN DECENT AGREEMENT THAT THE LEADING  
FEATURE COMING INTO THE WEST WILL FORM AN UPPER LOW NEAR THE  
CALIFORNIA COAST AROUND MIDWEEK AND THEN GRADUALLY WEAKEN AS IT  
REACHES THE CENTRAL U.S. ANOTHER SHORTWAVE SHOULD REACH THE WEST  
BY EARLY NEXT WEEKEND BUT THERE IS GREATER UNCERTAINTY FOR  
SPECIFICS OF THIS FEATURE AND UPSTREAM FLOW. MEAN TROUGHING WILL  
PREVAIL OVER EASTERN NORTH AMERICA MID-LATE WEEK AND THEN  
GRADUALLY SHIFT INTO THE WESTERN ATLANTIC. WITHIN THIS TROUGH  
THERE IS STILL A LOT OF GUIDANCE SPREAD/RUN-TO-RUN VARIABILITY  
WITH INDIVIDUAL SHORTWAVES AND THUS LOW CONFIDENCE IN FORECAST  
SPECIFICS.  
 
COMPARED TO THE PAST COUPLE DAYS GUIDANCE SEEMS TO BE BETTER  
CLUSTERED FOR THE LEADING SYSTEM MOVING INTO/THROUGH THE WEST AND  
EVENTUALLY REACHING THE PLAINS. A CONSENSUS APPROACH SEEMS  
REASONABLE GIVEN THE LOW PREDICTABILITY OF EMBEDDED SMALL SCALE  
IMPULSES THAT WILL PLAY A ROLE IN DETERMINING EXACTLY WHERE THE  
UPPER LOW CENTER WILL BE AT A PARTICULAR POINT IN TIME. UPSTREAM  
DETAILS RAPIDLY DIVERGE DURING THE LATTER HALF OF THE PERIOD.  
THERE ARE ESSENTIALLY TWO CLUSTERS OF GUIDANCE, THE GFS/GEFS WHICH  
STRAY FASTER WITH NORTH PACIFIC FLOW FROM EARLIER IN THE PERIOD  
VERSUS MOST OTHER SOLUTIONS (ECMWF/ECMWF MEAN/CMC/UKMET/NAVGEM AND  
TO SOME DEGREE THE CMC MEAN) THAT ARE SLOWER. AS A RESULT BY LATE  
IN THE PERIOD THE GFS CLUSTER ALREADY HAS ANOTHER STRONG TROUGH  
NEARING THE WEST COAST BY DAY 7 SUN AND RAPIDLY WEAKENS THE  
SHORTWAVE REACHING THE COAST THE PREVIOUS DAY. ON THE OTHER HAND  
OTHER SOLUTIONS ARE SLOW ENOUGH UPSTREAM TO ALLOW MORE OF THE DAY  
6 SAT ENERGY TO MOVE INTO THE SOUTHERN HALF OF THE WEST ON SUN.  
PREFER TO LEAN 3/4 TOWARD THE ECMWF CLUSTER GIVEN THE MORE  
PRONOUNCED GUIDANCE SIGNAL TOWARD THAT SCENARIO AND THE FACT THAT  
IT WOULD REPRESENT SOMEWHAT BETTER PERSISTENCE OF THE MEAN PATTERN  
THAT HAS BECOME ESTABLISHED.  
 
CONFIDENCE REMAINS WELL BELOW AVERAGE FOR DETAILS OVER THE  
CENTRAL/EASTERN U.S., INCLUDING POSSIBLE SURFACE LOW DEVELOPMENT  
OFF THE EAST COAST. THE CLOSED UPPER HIGH FORECAST NEAR OR NORTH  
OF THE NORTHERN MONTANA BORDER AS OF EARLY WED IS CONTINUING TO  
CAUSE HAVOC WITH THE FORECAST OF SHORTWAVE ENERGY OVER THE  
PLAINS/MISSISSIPPI VALLEY AT THAT TIME. THE 00Z ECMWF/NAVGEM ARE  
IN THE MINORITY TODAY, HOLDING MORE ENERGY OVER THE NORTH-CENTRAL  
PLAINS ON WED VERSUS ESTABLISHED CONSENSUS. FURTHER COMPLICATING  
THE FORECAST, THERE ARE VARIOUS POSSIBILITIES DEPICTED FOR  
AMPLIFICATION/TIMING OF CANADIAN ENERGY THAT MAY DROP INTO THE  
EASTERN U.S. WITH SOME POTENTIAL FOR INTERACTION/PHASING TO OCCUR.  
THE 06Z FV3 GFS IS ON THE EXTREME SIDE OF GUIDANCE REGARDING  
ASSOCIATED SURFACE DEVELOPMENT AND EFFECTS ALONG PARTS OF THE EAST  
COAST BUT THERE IS IN GENERAL A STRONGER SIGNAL FOR SURFACE  
DEVELOPMENT JUST CLOSE ENOUGH TO THE COAST TO MERIT CLOSE  
MONITORING. NOTE THAT THE 12Z/16 ECMWF CONTRIBUTES TO THIS  
SIGNAL, WITH AN INITIAL PLAINS/MS VALLEY SHORTWAVE MORE SIMILAR TO  
THE CURRENT MAJORITY. ADDITIONAL SHORTWAVES MAY FEED INTO THE  
MEAN TROUGH ALOFT BEHIND THE POSSIBLE LATE WEEK ATLANTIC  
DEVELOPMENT BUT SHOULD REFLECT MERELY AS COLD FRONTS AT THE  
SURFACE.  
 
DUE TO LOWER CONFIDENCE IN THE 00Z ECMWF EAST OF THE ROCKIES  
EARLY-MID PERIOD, THE PRIMARILY OPERATIONAL MODEL BLEND WEIGHTS  
THAT SOLUTION SOMEWHAT LOWER THAN WOULD NORMALLY BE THE CASE.  
THEN BY NEXT WEEKEND THE FORECAST PHASES OUT THE LESS PROBABLE GFS  
OVER THE PACIFIC/WESTERN U.S. WHILE EMPHASIZING THE 00Z  
ECMWF/ECMWF MEAN/CMC AND MODEST WEIGHT OF THE 06Z GEFS MEAN.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
THE SOUTHERN STREAM SYSTEM INITIALLY HEADING INTO THE WEST WILL  
BRING RAIN AND MOUNTAIN SNOW TO MOST LOCATIONS FROM CALIFORNIA  
EASTWARD INTO THE CENTRAL/SOUTHERN ROCKIES MID-LATE WEEK. MOST OF  
THE PRECIPITATION MAY STAY JUST NORTH OF THE DESERT  
SOUTHWEST/SOUTHERN ARIZONA THOUGH. TYPICALLY FAVORED TERRAIN  
SHOULD SEE THE HIGHEST TOTALS WITH THIS SYSTEM. EXPECT THE  
CENTRAL/SOUTHERN PLAINS TO SEE GREATER COVERAGE AND INTENSITY OF  
SHOWERS/THUNDERSTORMS DURING THE LATE WEEK INTO WEEKEND TIME FRAME  
AS MOISTURE INCREASES AHEAD OF THE WESTERN SYSTEM. SOME AREAS OF  
HEAVY RAINFALL ARE POSSIBLE. THE NEXT SYSTEM REACHING THE WEST  
COAST LATE FRI-SAT WILL LIKELY FOCUS RAIN AND HIGH ELEVATION SNOW  
OVER CENTRAL/NORTHERN AREAS WITH LIGHTER ACTIVITY EXTENDING  
FARTHER SOUTH. AREAS NEAR THE CENTRAL WEST COAST MAY SEE A BRIEF  
PERIOD OF MODERATE-HEAVY PRECIPITATION. CONFIDENCE DECREASES IN  
COVERAGE/AREAS OF GREATER FOCUS OVER THE WEST BY NEXT SUN. TO THE  
EAST OF THE ROCKIES, SOME RAINFALL MAY ACCOMPANY A WEAKENING  
WAVE/FRONTAL SYSTEM OVER THE PLAINS/EAST-CENTRAL U.S. EARLY IN THE  
PERIOD. FRONTAL PASSAGES MAY BRING ONE OR MORE EPISODES OF MOSTLY  
LIGHT RAIN/SNOW TO THE GREAT LAKES/NORTHEAST. SPECIFICS OF  
POTENTIAL SURFACE LOW DEVELOPMENT OFF THE EAST COAST CONTINUE TO  
BE A MAJOR UNCERTAINTY AROUND THU-FRI. THE MOST PROBABLE SCENARIO  
IS FOR ANY SYSTEM TO TRACK FAR ENOUGH OFFSHORE TO KEEP MOST  
PRECIPITATION AWAY FROM EAST COAST LOCATIONS. HOWEVER THERE IS  
STILL AN ONGOING LESSER THREAT FOR MORE MEANINGFUL EFFECTS THAT  
REQUIRE MONITORING.  
 
THE OVERALL PATTERN SHOULD KEEP DAYTIME HIGHS SOMEWHAT BELOW  
NORMAL OVER THE CENTRAL/SOUTHERN LATITUDES OF THE WEST, THOUGH  
CLOUDS/PRECIPITATION WILL KEEP LOWS ON THE WARMER SIDE OF NORMAL  
(BY 10-15F IN SOME CASES). ASIDE FROM PERIODIC COOL PUSHES INTO  
THE NORTHEAST, THERE SHOULD BE EXPANDING COVERAGE OF ABOVE NORMAL  
TEMPERATURES TO THE EAST OF THE ROCKIES DURING FRI-SUN, WITH SOME  
PLUS 10-20F ANOMALIES POSSIBLE OVER PARTS OF THE  
PLAINS/MISSISSIPPI VALLEY.  
 
RAUSCH  
 
ADDITIONAL 3-7 DAY HAZARDS INFORMATION CAN BE FOUND ON THE WPC  
MEDIUM RANGE HAZARDS CHART AT:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/THREATS/THREATS.PHP  
 
WPC MEDIUM RANGE 500MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDICES ARE AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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