028  
FXUS02 KWBC 180700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST TUE DEC 18 2018  
 
VALID 12Z FRI DEC 21 2018 - 12Z TUE DEC 25 2018  
   
..ACTIVE WEATHER OVER THE NORTHWEST/CENTRAL WEST COAST
 
 
...WIDESPREAD PRECIPITATION AND WIND WITH LATE WEEK EASTERN  
SYSTEM....  
 
   
..OVERVIEW
 
 
DURING FRI-SAT DEEP TROUGHING ALOFT WITH A LIKELY EMBEDDED UPPER  
LOW WILL PROGRESS RAPIDLY NORTHWARD OVER THE EASTERN U.S., PUSHING  
ALONG A STRONG SURFACE SYSTEM THAT WILL REACH EASTERN CANADA BY  
SAT. INCOMING PACIFIC SHORTWAVE ENERGY HEADING INTO A  
WEST-CENTRAL U.S. RIDGE AS OF EARLY FRI WILL USHER IN A SHORT  
PERIOD OF LOW AMPLITUDE FLOW CONTAINING MULTIPLE FEATURES  
AFFECTING THE WEST COAST AND THEN CONTINUING RAPIDLY EASTWARD--A  
PATTERN THAT TYPICALLY HAS LOW PREDICTABILITY FOR SPECIFICS. WITH  
TYPICAL UNCERTAINTY IN THE DETAILS, GUIDANCE IS CONSISTENT AND  
FAIRLY AGREEABLE THAT THE PATTERN WILL EVOLVE TOWARD AMPLIFIED  
WESTERN U.S. MEAN TROUGHING BY AROUND CHRISTMAS DAY NEXT TUE.  
MULTI-DAY MEANS BY D+8 CONTINUE TO SHOW AN AREA OF POSITIVE HEIGHT  
ANOMALIES AROUND 140-150W LONGITUDE WITH TELECONNECTIONS  
SUPPORTING AN AMPLIFIED WESTERN TROUGH. THIS ENHANCES CONFIDENCE  
IN THE GENERAL THEME OF MOST GUIDANCE AT THAT TIME.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
FOR THE LATE WEEK EASTERN SYSTEM RECENT TRENDS IN THE THU-THU  
NIGHT TIME FRAME (NOW IN THE SHORT RANGE) HAVE BEEN TOWARD FARTHER  
SOUTH FORMATION/TRACK OF THE UPPER LOW, LEADING TO A SLIGHTLY  
FARTHER SOUTH POSITION OF THE SURFACE LOW AT THE START OF THE  
EXTENDED PERIOD EARLY FRI. HOWEVER WITH UPSTREAM FLOW RAPIDLY  
FLATTENING THE SYSTEM'S NORTHWARD PROGRESS SEEMS TO BE FAIRLY  
CLOSE TO CONTINUITY BY EARLY SAT. UNCERTAINTY PERSISTS WITH  
SMALLER SCALE DETAILS DUE IN PART TO INTERACTION WITH NORTHERN  
STREAM ENERGY AND EFFECTS OF FLOW FARTHER WESTWARD.  
 
GUIDANCE IS STILL HAVING DIFFICULTY IN RESOLVING DETAILS OF  
SHORTWAVE ENERGY REACHING WESTERN NORTH AMERICA BY FRI. A  
COMPROMISE AMONG GFS/ECMWF RUNS AND THEIR ENSEMBLE MEANS HAS  
PROVIDED THE BEST STABILITY OVER RECENT CYCLES. AFTER MANY RUNS  
DIFFERING WILDLY FROM CONSENSUS, THE PAST COUPLE OF UKMET RUNS  
HAVE ADJUSTED CLOSER TO THE MOST LIKELY SCENARIO. ON THE OTHER  
HAND THE PAST TWO CMC RUNS HAVE TRENDED MUCH SLOWER THAN MOST  
OTHER SOLUTIONS. AFTER FRI CONFIDENCE IN ANY PIECE OF GUIDANCE  
FURTHER DECREASES AS AMPLITUDE/TIMING DIVERGE DUE IN PART TO WEAK  
LOW-PREDICTABILITY SHORTWAVES STREAMING INTO THE WEST THROUGH SAT.  
REGARDING THIS TRAILING SHORTWAVE ENERGY, ECMWF RUNS HAVE TONED  
IT DOWN A BIT FROM 24 HOURS AGO BUT STILL DEPICT A SHORTWAVE  
REACHING THE PLAINS SUN AND EAST COAST MON WITH SOMEWHAT MORE  
SURFACE REFLECTION/PRECIPITATION COVERAGE THAN SOME OTHER  
GUIDANCE. OTHER RECENT MODELS HINT AT THIS FEATURE ALOFT SUN-MON  
BUT WITH A VARIETY OF IDEAS FOR TIMING/AMPLITUDE.  
 
MOST GUIDANCE STILL SHOWS TWO WELL-DEFINED SYSTEMS NEARING THE  
PACIFIC NORTHWEST MID-LATE PERIOD, THE FIRST APPROACHING ON SUN  
(GFS RUNS POTENTIALLY A BIT ON THE FAST SIDE AND CMC PLUS ENSEMBLE  
MEANS PERHAPS A LITTLE NORTH) AND THE NEXT ON MON (GFS AND ECMWF  
IN BETTER AGREEMENT FOR THIS ONE). THEN THE MAJORITY CLUSTER HAS  
THE SUPPORTING SHORTWAVE FOR THE MON SYSTEM AMPLIFYING AS IT HEADS  
INTO THE WEST. BY LATE IN THE PERIOD THE 12Z/18Z GEFS MEANS  
BECOME THE MOST QUESTIONABLE GUIDANCE AS THEY DAMPEN THE RIDGE  
EXPECTED OFF THE WEST COAST, THUS PUSHING THE WESTERN MEAN TROUGH  
FARTHER EASTWARD. GFS RUNS MAY BEGIN TO EXHIBIT SOMEWHAT FAST  
PROGRESSION OF AN UPSTREAM NORTH PACIFIC SYSTEM BUT STILL SHOW  
BETTER WESTERN TROUGH AMPLIFICATION THAN THE GEFS MEAN.  
INTERESTINGLY THE 12Z CMC MEAN ACTUALLY AMPLIFIES ITS TROUGH A  
LITTLE WEST OF THE ECMWF MEAN, AFTER CMC MEANS HAVE TENDED TO BE  
CHRONICALLY UNDER-AMPLIFIED VERSUS CONSENSUS IN MULTIPLE CASES  
OVER THE PACIFIC/NORTH AMERICA DOMAIN IN RECENT WEEKS.  
 
BASED ON FORECAST CONSIDERATIONS THIS CYCLE, THE FIRST PART OF THE  
FORECAST EMPHASIZED THE 18Z GFS/12Z ECMWF WITH LESSER INPUT FROM  
THE 12Z UKMET/ECMWF MEAN. THE LATTER HALF OF THE FORECAST FAVORED  
THE 18Z GFS AND 12Z ECMWF/ECMWF MEAN, WITH GEFS MEAN INPUT KEPT  
LOW ENOUGH NOT TO HAVE AN ADVERSE IMPACT ON THE PREFERRED PATTERN  
OVER THE WEST.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
LOW AMPLITUDE SHORTWAVE ENERGY CROSSING THE WEST DURING FRI-SAT  
MAY PRODUCE SOME AREAS OF LIGHT/SCATTERED PRECIPITATION. THEN  
EXPECT A PERIOD OF MORE SIGNIFICANT WEATHER NEAR THE WEST COAST  
FROM SUN INTO EARLY NEXT WEEK. THERE IS STILL CONSIDERABLY  
UNCERTAINTY IN THE SPECIFICS BUT THE CURRENT MOST LIKELY EVOLUTION  
WOULD HAVE ONE SYSTEM BRINGING AREAS OF ENHANCED RAIN/HIGHER  
ELEVATION SNOW TO THE CENTRAL/NORTHERN WEST COAST BY LATE SAT-SUN  
WITH ADDITIONAL MOISTURE ACCOMPANYING ANOTHER SYSTEM THAT ARRIVES  
BY MON. AFTER MON THE PRECIPITATION SHOULD EXTEND FARTHER  
SOUTHEASTWARD THAN WITH PRECEDING SYSTEMS AS UPPER TROUGHING  
AMPLIFIES SOUTHEASTWARD. ONE OR BOTH OF THESE SYSTEMS MAY PRODUCE  
STRONG WINDS OVER SOME AREAS.  
 
THE STRONG STORM AFFECTING THE EAST LATE THIS WEEK INTO THE  
WEEKEND WILL BE ACCOMPANIED BY AREAS OF HEAVY RAINFALL ON FRI,  
ESPECIALLY ALONG THE NORTHEASTERN COAST WHERE A PERIOD OF STRONG  
ATLANTIC INFLOW MAY ENHANCE AMOUNTS. A TRAILING BAND OF ENHANCED  
RAINFALL MAY EXTEND OVER OR JUST WEST OF THE APPALACHIANS. THE  
RAINFALL FROM THIS SYSTEM MAY BE PROBLEMATIC OVER SOME AREAS DUE  
TO THE SOIL BEING SATURATED FROM THE RECENT MULTI-DAY EVENT. AS  
THE SYSTEM TRACKS FARTHER NORTHWARD EXPECT SOME LAKE  
EFFECT/TERRAIN-ENHANCED SNOW DURING THE WEEKEND. SOME AREAS COULD  
SEE STRONG WINDS WITH THIS SYSTEM AS WELL.  
 
OTHERWISE THE CENTRAL/EASTERN STATES WILL LIKELY SEE MODEST  
COVERAGE OF PRECIPITATION, AND FAIRLY LIGHT INTENSITY OF WHAT  
OCCURS. SPECIFICS WILL TAKE ADDITIONAL TIME TO RESOLVE GIVEN THE  
LOW PREDICTABILITY OF SUPPORTING SHORTWAVES ALOFT. AT LEAST SOME  
OF ANY MOISTURE ACROSS THE NORTHERN THIRD TO HALF OF THE AREA EAST  
OF THE ROCKIES MAY FALL IN THE FORM OF SNOW.  
 
EARLY IN THE PERIOD THE EASTERN STORM WILL BRING A BRIEF WARM  
SURGE INTO THE MID-ATLANTIC/NORTHEAST WITH SOME READINGS AT LEAST  
10-20F ABOVE NORMAL, WHILE BRINGING LESS EXTREME BELOW NORMAL  
ANOMALIES INTO THE SOUTHEAST/FLORIDA. THE CENTRAL U.S. WILL TEND  
TO SEE ABOVE NORMAL TEMPERATURES DURING THE PERIOD BUT WITH  
SOMEWHAT LESS EXTREME ANOMALIES AFTER FRI-SAT. WESTERN SYSTEMS  
SHOULD PROMOTE NEAR TO SLIGHTLY BELOW NORMAL HIGHS OVER THE  
NORTHWEST, WITH SOME EXPANSION OF COOLER AIR INTO THE SOUTHWEST  
TOWARD THE END OF THE PERIOD.  
 
RAUSCH  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND 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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