756  
FXUS02 KWBC 161600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 AM EST SUN DEC 16 2018  
 
VALID 12Z WED DEC 19 2018 - 12Z SUN DEC 23 2018  
 
...PERIODS OF HEAVY PRECIPITATION EXPECTED TO CONTINUE OVER THE  
NORTHWEST THROUGH NEXT WEEKEND...  
 
...LATE WEEK STORM LIKELY BRINGS WIDESPREAD WIND AND RAIN FOR MUCH  
OF THE EASTERN U.S....  
 
   
..OVERVIEW
 
 
SOME DETAILS REMAIN IN QUESTION BUT THERE IS GOOD  
AGREEMENT/CONTINUITY WITH THE GENERAL IDEA THAT A RAPIDLY  
BUILDING/SHARPENING RIDGE OVER THE WEST WED-THU WILL SUPPORT  
DOWNSTREAM DIGGING OF SHORTWAVE ENERGY INTO AN AMPLIFIED TROUGH  
OVER THE MISSISSIPPI VALLEY BY THU. THE SHARPENING TROUGH SHOULD  
THEN CLOSE OFF NEAR THE SOUTHERN APPALACHIANS AS A SIGNIFICANT  
SURFACE CYCLONE WILL LIKELY MOVE UP THE APPALACHIANS ON FRIDAY.  
MEANWHILE, INCOMING PACIFIC SHORTWAVE ENERGY SHOULD DAMPEN THE  
WESTERN RIDGE, LEADING TO A NOTICEABLY LESS AMPLIFIED OVERALL MEAN  
FLOW WITH THE HINT OF A SPLIT FLOW PATTERN JUST INLAND FROM THE  
WEST COAST BY NEXT SUNDAY. THE PROGRESSIVE/LOW AMPLITUDE NATURE  
OF FLOW IN THIS TIME FRAME WILL LIKELY CREATE DIFFICULTIES IN  
RESOLVING SHORTWAVE SPECIFICS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
GLOBAL MODELS AND ENSEMBLES ARE ALL COMING INTO AGREEMENT FOR A  
SIGNIFICANT CYCLONE FORECAST TO IMPACT MUCH OF THE EASTERN U.S.  
FRIDAY TO SATURDAY, WITH THE STORM TRACK SLOWLY TRENDING FARTHER  
AND FARTHER INLAND. APT TO USE MORE OF THE 00Z ECMWF SOLUTION FOR  
THE SEA-LEVEL PRESSURE FIELDS FOR THIS TIME-FRAME, WHICH DEPICTS A  
DOUBLE-BARREL LOW PRESSURE SYSTEM MOVING UP THE APPALACHIANS IN  
FAVOR OF A LARGER SEPARATION BETWEEN THE UPPER-LEVEL CUT-OFF LOW  
TO THE SOUTHWEST AND THE TRIPLE-POINT LOW TO THE NORTHEAST. FOR  
DAYS 3 AND 4, AND FURTHER OUT FOR DAYS 6 AND 7, MAINLY A CONSENSUS  
OF THE 00Z ECMWF/EC MEAN AND THE 06Z GFS/GEFS WAS USED, WHICH IS  
KEEPING GOOD CONTINUITY WITH THE PREVIOUS WPC FORECASTS.  
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
ONE WAVE OF HEAVY RAIN/HIGHER ELEVATION SNOW OVER THE NORTHWEST  
DURING THE SHORT RANGE PERIOD SHOULD BE TAPERING OFF BY NEXT WED.  
THEN LOW PRESSURE LIKELY TO TRACK JUST OFFSHORE THE PACIFIC  
NORTHWEST INTO WESTERN CANADA THU-THU NIGHT SHOULD BRING ANOTHER  
EPISODE OF SIGNIFICANT PRECIPITATION. THIS EVENT MAY HAVE  
SOMEWHAT MORE LIMITED EXTENT OF HEAVIEST ACTIVITY, MOST LIKELY  
FOCUSED OVER THE OLYMPICS AND NORTHERN CASCADES, BUT STILL EXPECT  
SOME MOISTURE TO REACH INTO EXTREME NORTHERN CALIFORNIA. THERE  
MAY BE A PERIOD OF FAIRLY STRONG WINDS IF THE SURFACE LOW IS ON  
THE DEEPER SIDE OF THE FULL GUIDANCE SPREAD. EXPECT LIGHTER AND  
MORE SCATTERED PRECIPITATION FRI FOLLOWED BY PERIODS OF MOSTLY  
LIGHT-MODERATE RAIN/SNOW DURING THE WEEKEND.  
 
THE STORM SYSTEM EXPECTED TO AFFECT THE EAST SHOULD SPREAD A BROAD  
AREA OF MOISTURE OVER MOST OF THE EASTERN HALF OF THE LOWER 48  
DURING THE LATTER HALF OF THE WEEK, WITH CHILLY CYCLONIC FLOW  
SUPPORTING LAKE EFFECT/TERRAIN-ENHANCED PRECIPITATION DURING THE  
WEEKEND. WHILE SOME DETAILS OF SYSTEM EVOLUTION HAVE CHANGED OVER  
THE PAST DAY, BEST GUIDANCE SIGNALS FOR HIGHEST RAINFALL TOTALS  
FOR THIS EVENT CONTINUE TO EXIST OVER THE FLORIDA PENINSULA AND  
NORTH CAROLINA COAST. OTHER RELATIVE MAXIMA MAY EXIST WITH  
ATLANTIC INFLOW ACROSS THE NORTHERN HALF OF THE EAST COAST AND TO  
THE NORTHWEST OF THE STORM TRACK. THIS SHOULD BE A FAIRLY WARM  
SYSTEM FOR THE TIME OF YEAR, WITH MOST SNOW CONFINED TO THE  
WESTERN PERIPHERY AND THEN OVER THE GREAT LAKES AND VICINITY AFTER  
SYSTEM PASSAGE.  
 
EXPECT THE GREATEST TEMPERATURE ANOMALIES TO BE OF THE WARM  
VARIETY OVER THE CENTRAL U.S. DURING WED-FRI, WITH SOME AREAS IN  
THE NORTHERN PLAINS SEEING MIN TEMPERATURES IN PARTICULAR  
EXCEEDING 20F ABOVE NORMAL. SUCH ANOMALIES MAY REACH DAILY  
RECORDS FOR WARM MINS. MEANWHILE THE EASTERN U.S. STORM MAY PULL  
IN A BRIEF SURGE OF WARMTH TO THE MID-ATLANTIC/NORTHEAST LATE IN  
THE WEEK. THE PACIFIC ORIGIN OF SUPPORTING ENERGY FOR THIS STORM  
SHOULD KEEP COLD SECTOR TEMPERATURES FROM REACHING MUCH LOWER THAN  
NORMAL, THOUGH SOME COOLER READINGS MAY BE POSSIBLE OVER THE  
SOUTHEAST AROUND FRI-SAT. THE OVERALL FLATTER TREND FOR FLOW  
ALOFT SHOULD PROMOTE MODERATION OF TEMPERATURES TOWARD NORMAL OVER  
MANY AREAS OF THE COUNTRY BY NEXT SUN.  
 
KONG/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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