260  
FXUS02 KWBC 190700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
200 AM EST WED DEC 19 2018  
 
VALID 12Z SAT DEC 22 2018 - 12Z WED DEC 26 2018  
 
...ACTIVE WEATHER OVER THE WEST, POSSIBLY REACHING THE CENTRAL  
U.S. NEXT WED...  
 
...WIND AND SOME LINGERING PRECIPITATION AS STRONG SYSTEM DEPARTS  
FROM THE EAST....  
 
   
..OVERVIEW
 
 
THE ONE ASPECT OF THE FORECAST THAT HAS REASONABLE CONFIDENCE IS  
THE EXPECTED PATTERN TRANSITION FROM ONE OF FAIRLY LOW AMPLITUDE  
AT THE START OF THE WEEKEND TOWARD A DEEPENING MEAN TROUGH OVER  
THE WEST. GUIDANCE HAS BEEN CONSISTENT IN SHOWING THE DEVELOPMENT  
OF MEAN RIDGING/POSITIVE HEIGHT ANOMALIES OVER THE PACIFIC AROUND  
140W OR SO WITH TELECONNECTIONS SUPPORTING THE DOWNSTREAM TROUGH.  
HOWEVER BEYOND THAT THE CONFIDENCE IN SPECIFICS IS MUCH LOWER.  
FROM THE START OF THE PERIOD GUIDANCE DIFFERS FOR DETAILS OF  
INDIVIDUAL SHORTWAVES PROGRESSING FROM THE EASTERN PACIFIC ACROSS  
THE LOWER 48. BY DAY 5 MON EVEN LATEST 00Z GUIDANCE BECOMES 180  
DEGREES OUT OF PHASE WITH SOME OF THESE SHORTWAVES. THERE IS SOME  
SEMBLANCE OF CONSENSUS THAT A SHORTWAVE FORECAST TO APPROACH THE  
WEST COAST BY MON WILL BE THE ONE THAT ESTABLISHES THE WESTERN  
U.S. MEAN TROUGH. HOWEVER THERE ARE DIFFERENCES IN TIMING OF THIS  
ENERGY'S AMPLIFICATION, AND BEYOND THAT, SIGNIFICANT SPREAD IN  
UPSTREAM NORTH PACIFIC ENERGY THAT MAY INFLUENCE HOW THE EARLY  
WEEK WESTERN TROUGH MAY ULTIMATELY EJECT INTO THE PLAINS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
IN ORDER TO EMPHASIZE OPERATIONAL GUIDANCE DURING THE FIRST HALF  
OF THE PERIOD WHILE DOWNPLAYING LOWER-CONFIDENCE SHORTWAVE  
DETAILS, THE FORECAST BLEND WAS MOSTLY WEIGHTED TOWARD THE LAST  
TWO AVAILABLE GFS (12Z-18Z/18) AND ECMWF (00Z-12Z/18) RUNS ALONG  
WITH A LITTLE 12Z UKMET FOR DAY 3 SAT INTO DAY 5 MON. AFTER THAT  
TIME THE FORECAST TRENDED TOWARD MORE 12Z NAEFS/ECMWF MEAN INPUT  
WHILE MAINTAINING SOME OPERATIONAL ECMWF WEIGHT AS THE AVERAGE OF  
THE PAST TWO RUNS ALIGNED MORE CLOSELY TO THE MEANS FOR THE  
WESTERN TROUGH VERSUS THE FASTER-EJECTING GFS.  
 
BEHIND THE RAPIDLY DEPARTING DEEP EAST COAST UPPER TROUGH,  
GUIDANCE VALID THIS SAT GENERALLY SHOWS ONE AREA OF SHORTWAVE  
ENERGY CONCENTRATED OVER SOUTHERN CANADA/NORTHERN PLAINS/UPPER  
MISSISSIPPI VALLEY AND MORE DIFFUSE ENERGY FARTHER SOUTH FOLLOWED  
CLOSELY BY WESTERN U.S. ENERGY REACHING THE CENTRAL STATES BY  
EARLY SUN. THERE HAS BEEN SOME DEGREE OF CLUSTERING TOWARD THIS  
TRAILING SHORTWAVE NEARING THE EAST COAST BY MON. HOWEVER THE 18Z  
GEFS MEAN BECOMES CURIOUSLY SLOW WITH THIS FEATURE WHILE THE NEW  
00Z CMC BECOMES SLOW/AMPLIFIED AND THE 00Z UKMET TRENDS SLOWER AS  
WELL.  
 
THE NEXT DEFINED SHORTWAVE IS FORECAST TO REACH NEAR THE WEST  
COAST BY EARLY SUN. THERE IS STILL SIGNIFICANT SPREAD FOR  
SHORTWAVE AMPLITUDE AND RESULTING TRACK/DEPTH OF SURFACE LOW  
PRESSURE. ECMWF RUNS ARE FAIRLY FAR SOUTHWARD WITH THE SURFACE  
LOW RELATIVE TO THE MAJORITY OF ENSEMBLE MEMBERS, FAVORING AT  
LEAST SOME DEGREE OF COMPROMISE WITH OTHER GUIDANCE. DIFFERENCES  
FOR THIS SHORTWAVE ENERGY PERSIST/WIDEN AS IT CONTINUES EASTWARD,  
COMPLICATING THE SENSIBLE WEATHER FORECAST.  
 
ABOUT 24-36 HOURS LATER THE NEXT SYSTEM SHOULD BE NEARING THE WEST  
COAST, WITH THE UPPER TROUGH THEN STRONGLY DIGGING OVER THE  
WESTERN STATES AND NORTHERN MEXICO. OVER THE PAST 1-2 DAYS THE  
ECMWF MEAN HAS BEEN THE MOST CONSISTENT GUIDANCE SOURCE FOR THE  
OVERALL TROUGH THROUGH DAY 7 WED. THE GEFS MEAN HAS TRENDED  
SLOWER OVER THE PAST DAY TOWARD THE ECMWF MEAN. BY THE LAST  
COUPLE DAYS OF THE FORECAST ONE BIG QUESTION WILL BE THE  
STRENGTH/SPEED OF UPSTREAM ENERGY ROUNDING THE EASTERN PACIFIC  
RIDGE. LATEST GFS/CMC RUNS ARE STRONG/FAST ENOUGH WITH THE  
UPSTREAM ENERGY FOR IT TO CAUSE A QUICKER EJECTION OF THE WESTERN  
TROUGH INTO THE PLAINS. 12Z/18Z FV3 GFS RUNS ARE SLOWER THAN THE  
OPERATIONAL GFS, PLUS THE CMC MEAN (WHICH HAS HAD A PRONOUNCED  
FLAT/PROGRESSIVE TENDENCY IN AMPLIFIED REGIMES OVER RECENT WEEKS)  
CONTINUES TO BE EVEN A LITTLE SLOWER WITH THE WESTERN TROUGH THAN  
THE ECMWF MEAN. THESE CONSIDERATIONS LEAD TO FAVORING THE  
NAEFS/ECMWF MEANS AND LAGGED-AVERAGE ECMWF APPROACH. IT IS  
IMPORTANT TO NOTE THAT OTHER SCENARIOS MAY STILL BE PLAUSIBLE WHEN  
VIEWED FROM A MULTI-DAY MEAN PERSPECTIVE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE TWO MAIN SYSTEMS LIKELY TO AFFECT THE WEST COAST SHOULD FOCUS  
THE HEAVIEST RAIN AND HIGHER ELEVATION SNOW FROM THE PACIFIC  
NORTHWEST INTO NORTHWESTERN CALIFORNIA, THEN EVENTUALLY THE SIERRA  
NEVADA RANGE AS UPPER TROUGHING BEGINS TO AMPLIFY SOUTHEASTWARD.  
5-DAY TOTALS FOR THE PERIOD WILL LIKELY REACH AT LEAST SEVERAL  
INCHES LIQUID AT SOME LOCATIONS OVER WESTERN OREGON AND EXTREME  
NORTHWEST CALIFORNIA. MOISTURE/SHORTWAVE ENERGY SPREADING INLAND  
WILL BRING PERIODS OF RAIN/HIGHER ELEVATION SNOW THROUGH AT LEAST  
THE NORTHERN 2/3 OF THE INTERIOR WEST/ROCKIES. SOME PRECIPITATION  
MAY REACH INTO SOUTHERN CALIFORNIA AND THE SOUTHWEST BUT WITH  
AMOUNTS VERY SENSITIVE TO UNCERTAIN SPECIFICS ALOFT. DEPENDING ON  
EVOLUTION OF THE SYSTEMS, THERE MAY BE ONE OR MORE PERIODS OF  
STRONG WINDS ESPECIALLY ALONG THE WEST COAST.  
 
FARTHER EAST, THE PLAINS MAY START TO SEE MORE ORGANIZED  
PRECIPITATION AROUND THE MIDDLE OF NEXT WEEK IN ADVANCE OF THE  
AMPLIFYING WESTERN TROUGH. BEFORE THEN LEADING SHORTWAVES WILL  
LIKELY PRODUCE SOME AREAS OF PRECIPITATION BUT POOR GUIDANCE  
AGREEMENT FOR DETAILS WILL KEEP CONFIDENCE LOW FOR SPECIFIC  
COVERAGE/TIMING/AMOUNTS. ANY MOISTURE REACHING THE NORTHERN HALF  
OR THIRD OF THE AREA EAST OF THE ROCKIES MAY BE IN THE FORM OF  
SNOW. OVER THE EAST EXPECT BRISK WINDS AND LAKE  
EFFECT/TERRAIN-ENHANCED SNOW TO LINGER INTO THE WEEKEND BEHIND THE  
STRONG STORM TRACKING INTO EASTERN CANADA.  
 
THE GENERAL PATTERN EVOLUTION FROM LOW AMPLITUDE FLOW ALOFT TO A  
WESTERN MEAN TROUGH WILL PROMOTE ABOVE NORMAL TEMPERATURES OVER  
MUCH OF THE CENTRAL U.S. DURING THE PERIOD, WITH HIGHEST ANOMALIES  
LIKELY TO BE FOR MORNING LOWS ON SAT AND WED. MEANWHILE THE WEST  
WILL TREND FROM HAVING TEMPERATURES A FEW DEGREES F ON EITHER SIDE  
OF NORMAL FOR HIGHS (GENERALLY MORE ABOVE NORMAL FOR MINS) OVER  
THE WEEKEND TOWARD BELOW NORMAL READINGS NEXT WEEK. THE EXTREME  
SOUTHEAST/FLORIDA WILL MODERATE AFTER A CHILLY DAY ON SAT.  
 
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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