465  
FXUS02 KWBC 200700  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
159 AM EST THU DEC 20 2018  
 
VALID 12Z SUN DEC 23 2018 - 12Z THU DEC 27 2018  
 
...ACTIVE WEATHER OVER THE WEST INTO EARLY NEXT WEEK FOLLOWED BY  
SIGNIFICANT CENTRAL U.S. STORM POTENTIAL WED-THU...  
 
   
..OVERVIEW
 
 
GUIDANCE IS CONSISTENT FOR THE LARGE SCALE FORECAST, SHOWING THAT  
PROGRESSIVE/LOW AMPLITUDE FLOW ALOFT SUN-MON WILL RAPIDLY  
TRANSITION TOWARD THE ESTABLISHMENT OF AN AMPLIFIED WESTERN U.S.  
MEAN TROUGH FROM ABOUT CHRISTMAS DAY ONWARD. WITHIN THIS  
AGREEABLE EVOLUTION THERE ARE SIGNIFICANT EMBEDDED UNCERTAINTIES  
THOUGH, BOTH FOR INDIVIDUAL SHORTWAVES EMBEDDED IN THE PROGRESSIVE  
PATTERN EARLY IN THE PERIOD AND SPECIFICS OF TROUGH ENERGY THAT  
DEEPENS OVER THE WEST AND THEN LIKELY EJECTS INTO THE PLAINS.  
WHILE THE DETAILS AWAIT RESOLUTION, THERE ARE FAIRLY GOOD SIGNALS  
FROM THE GUIDANCE IN PRINCIPLE TOWARD PARTS OF THE WEST RECEIVING  
SIGNIFICANT PRECIPITATION EARLY-MID PERIOD WHILE EJECTING WESTERN  
ENERGY THEN SUPPORTS WHAT COULD BE A STRONG STORM SYSTEM ACROSS  
THE CENTRAL U.S.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
THE OVERALL FORECAST PHILOSOPHY CONTINUES TO FAVOR A  
MULTI-RUN/MULTI-MODEL APPROACH EARLY IN THE PERIOD (WITH MINORITY  
ENSEMBLE INPUT) TO DEAL WITH MEANINGFUL DIFFERENCES THAT ARISE  
FROM DAY 3 SUN ONWARD. BY THE LATTER HALF OF THE FORECAST THE  
BLEND STILL EXCLUDES GFS RUNS DUE TO THEIR STRAYING ON THE FAST  
SIDE OF GUIDANCE WITH EJECTING WESTERN ENERGY/SURFACE LOW PRESSURE  
INTO THE PLAINS, FAVORING COMPONENTS OF RECENT ECMWF/CMC RUNS AND  
SLIGHTLY MORE 12Z ECMWF MEAN INPUT RELATIVE TO THE 18Z GEFS MEAN.  
 
WITHIN THE PROGRESSIVE FLOW IN PLACE EARLY IN THE PERIOD,  
MODELS/ENSEMBLES ARE STILL HAVING SOME DIFFICULTY WITH A LEADING  
CENTRAL-EASTERN U.S. SHORTWAVE DURING SUN-MON. FORTUNATELY ANY  
ASSOCIATED PRECIPITATION WITH THIS FEATURE SHOULD BE FAIRLY LIGHT  
AND A GENERAL BLEND ACCOUNTS WELL FOR THE DETAIL UNCERTAINTY OF  
THIS FEATURE. THEN OFF THE NORTHERN PACIFIC NORTHWEST COAST THERE  
IS STILL SIGNIFICANT SPREAD FOR APPROACHING LOW PRESSURE AS OF  
EARLY SUN. TRACK PREFERENCE BASED ON GUIDANCE THROUGH 18Z DATA  
WAS FOR AN AVERAGE AMONG THE 18Z GEFS/12Z ECMWF MEANS AS A GOOD  
INTERMEDIATE STARTING POINT, WITH ECMWF RUNS IN THE SOUTHERN 1/3  
OF THE ENSEMBLE SPREAD AND TRENDING NORTHWARD OVER RECENT RUNS  
WHILE GFS RUNS HAVE BEEN WELL NORTHWARD. THE NEW 00Z GFS HAS  
ADJUSTED CLOSE TO THIS SCENARIO. CONSULT THE MODEL DIAGNOSTIC  
DISCUSSION PMDHMD FOR UPDATED PREFERENCES BASED ON THE FULL SUITE  
OF 00Z GUIDANCE. AS ASSOCIATED SHORTWAVE ENERGY MOVES INLAND  
THERE IS STILL TREMENDOUS TIMING/AMPLITUDE SPREAD. THUS FAR ECMWF  
RUNS HAVE BEEN REMARKABLY CONSISTENT IN BEING STRONGER THAN MOST  
OTHER SOLUTIONS ALL THE WAY FROM THE WEST ACROSS THE LOWER 48.  
THE NEXT CLOSEST IDEAS COME FROM THE PAST COUPLE UKMET RUNS AND  
00Z CMC THAT BECOME BETTER DEFINED ONCE THE ENERGY REACHES THE  
PLAINS. THE ECMWF'S CONTINUITY AND HINT OF THE FEATURE IN RECENT  
ECMWF MEANS REQUIRED SOME ACCOUNTING FOR THIS FEATURE, BUT IN MORE  
SUBDUED FASHION GIVEN TYPICALLY LOWER PREDICTABILITY OF SHORTWAVES  
EMBEDDED IN FAST FLOW. ONCE THIS ENERGY REACHES THE EASTERN U.S.  
THE ECMWF/CMC HAVE BEEN SUGGESTING POSSIBLE INTERACTION WITH  
EASTERN CANADA ENERGY. AGAIN PREFER TO TONE DOWN THIS EVOLUTION  
SOMEWHAT BY WAY OF THE MEANS AS THE DEVELOPING WESTERN TROUGH  
SHOULD START TO PROMOTE RISING EAST COAST HEIGHTS.  
 
BY DAY 4 MON EXPECT ANOTHER SHORTWAVE TO APPROACH THE WEST COAST,  
ULTIMATELY AMPLIFYING INTO THE WELL-ADVERTISED DEEP WESTERN  
TROUGH. AS OF MON THE 12Z/18Z GFS RUNS WERE THE LEAST FAVORED AS  
THEY BROUGHT SHORTWAVE ENERGY AND SURFACE LOW PRESSURE FARTHER  
EAST THAN MOST OTHER GUIDANCE INCLUDING THE GEFS MEAN. THE NEW  
00Z GFS MAY BE SOMEWHAT CLOSER TO CONSENSUS ALOFT BUT STILL HAS A  
QUESTIONABLY FAST LEADING SURFACE WAVE. THE SPECIFICS OF HOW MUCH  
THE WESTERN TROUGH AMPLIFIES AND THEN THE TIMING OF EJECTION OF  
ENERGY INTO THE PLAINS CONTINUES TO BE COMPLICATED BY AN UPSTREAM  
NORTH PACIFIC FEATURE WHOSE ENERGY MAY FEED DOWN TOWARD THE  
OVERALL MEAN TROUGH. AS HAS BEEN THE CASE OVER THE PAST DAY,  
LATEST GFS RUNS ARE ON THE FASTER SIDE OF THE SPREAD WITH THE  
WESTERN TROUGH AND EJECTION OF ITS ENERGY INTO THE PLAINS IN  
RESPONSE TO DIFFERENCES OVER THE NORTH PACIFIC. CONTINUE TO FAVOR  
LEANING CLOSER TO THE SLOWER CLUSTER OF GUIDANCE INCLUDING THE  
ECMWF/ECMWF MEAN AND CMC/CMC MEAN GIVEN THE STRONGLY AMPLIFYING  
NATURE OF THE OVERALL PATTERN. HOWEVER IT IS STILL WORTH NOTING  
THAT ANY OF THESE TIMING OPTIONS WOULD STILL FIT WITHIN THE  
EXPECTED MEAN PATTERN FROM A MULTI-DAY MEAN PERSPECTIVE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT THE TWO SYSTEMS AFFECTING THE NORTHWEST DURING THE FIRST  
HALF OF THE PERIOD TO BRING HEAVIEST RAIN/HIGHER ELEVATION SNOW TO  
FAVORED TERRAIN OF WESTERN OREGON AND NORTHWESTERN CALIFORNIA.  
SEVERAL INCHES LIQUID REMAIN POSSIBLE OVER SOME LOCATIONS WITHIN  
THIS REGION. WESTERN WASHINGTON WILL LIKELY SEE SIGNIFICANT  
PRECIPITATION AS WELL BUT WITH SOMEWHAT LESS CONFIDENCE THAT  
TOTALS WILL REACH LEVELS THAT MAY BE SEEN IMMEDIATELY TO THE  
SOUTH. SOME OF THIS MOISTURE WILL SPREAD INLAND TO BRING  
MEANINGFUL PRECIPITATION INTO THE NORTHERN/CENTRAL INTERMOUNTAIN  
WEST AND ROCKIES. AS UPPER TROUGHING AMPLIFIES THE SIERRA NEVADA  
RANGE SHOULD SEE A PERIOD OF ENHANCED RAIN/SNOW, AND SOMEWHAT  
LIGHTER ACTIVITY MAY EXTEND AS FAR SOUTH AS SOUTHERN CALIFORNIA  
AND THE SOUTHWEST.  
 
BY WED-THU THERE IS DECENT POTENTIAL FOR A SIGNIFICANT STORM  
SYSTEM TO EMERGE OVER AND TRACK THROUGH THE PLAINS AS UPPER TROUGH  
ENERGY EMERGES FROM THE WEST. LOCATIONS OVER THE NORTHERN HALF OR  
THIRD OF THE PLAINS WOULD SEE SNOW WITH THIS SYSTEM BASED ON THE  
CURRENT MOIST LIKELY SCENARIO. STRONG WINDS MAY ADD TO THE  
HAZARDOUS CONDITIONS. MEANWHILE WARM SECTOR RAINFALL SHOULD  
EXPAND AN INTENSIFY WITHIN AN AREA FROM THE CENTRAL PLAINS THROUGH  
THE CENTRAL GULF COAST REGION.  
 
AHEAD OF THIS STORM, FORECASTING PRECIPITATION COVERAGE/TIMING  
OVER THE CENTRAL-EASTERN STATES IS STILL DIFFICULT DUE TO LACK OF  
AGREEMENT FOR PROGRESSIVE SHORTWAVES. IT IS LIKELY THAT MOST  
PRECIPITATION THAT FALLS WILL BE ON THE LIGHT SIDE, WITH THE  
RAIN/SNOW LINE TENDING TO LIE NEAR 40N LATITUDE.  
 
THE LOW AMPLITUDE PATTERN ALOFT FROM LATE WEEKEND INTO CHRISTMAS  
EVE WILL PROMOTE NEAR TO ABOVE NORMAL TEMPERATURES OVER MOST OF  
THE LOWER 48. WARMEST READINGS VERSUS NORMAL SHOULD BE OVER THE  
NORTHERN HALF OF THE PLAINS WITH SOME PLUS 10-15F ANOMALIES FOR  
MORNING LOWS IN PARTICULAR. THEN COOLER AIR WILL BEGIN TO FILTER  
INTO THE WEST AS UPPER TROUGHING STARTS TO AMPLIFY OVER THE REGION  
WHILE THE PLAINS REMAIN WARM. BY WED-THU BELOW NORMAL  
TEMPERATURES WILL SPREAD ACROSS THE WEST INTO THE HIGH PLAINS WITH  
HIGHS AT LEAST 5-10F BELOW NORMAL. AT THE SAME TIME  
CENTRAL/EAST-CENTRAL AREAS WILL TREND WARMER WITH EXPANDING  
COVERAGE OF PLUS 10-20F ANOMALIES.  
 
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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