947  
FXUS02 KWBC 191600  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1100 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 OVERALL PATTERN IS STILL EXPECTED TO TRANSITION FROM A FAIRLY  
ZONAL/LOW AMPLITUDE PATTERN THROUGH THE WEEKEND TO A DEEPENING  
TROUGH BY THE END OF THE MEDIUM RANGE PERIOD. THROUGH THE  
BEGINNING OF THE FORECAST PERIOD, THE LARGE DIGGING TROUGH WILL  
EXIT INTO THE NORTHEAST/ATLANTIC OCEAN WHILE A SHORT WAVE SKIRTS  
ACROSS THE NORTHERN TIER STATES. ANOTHER SHORT WAVE WILL MOVE  
ACROSS THE PACIFIC NORTHWEST AND INTO THE INTERMOUNTAIN WEST.  
THESE FEATURES WILL MOVE QUICKLY EASTWARD WITH MORE INCOMING SHORT  
WAVES APPROACHING THE PACIFIC NORTHWEST. BY MONDAY, GUIDANCE STILL  
HAVE NOT CONVERGED ON THE ENERGY APPROACHING THE PACIFIC NORTHWEST  
AND CONTINUES TO HAVE SIGNIFICANT SPREAD UPSTREAM. THE ENERGY  
UPSTREAM WILL IMPACT THE TROUGH ACROSS THE WESTERN U.S. AS IT  
LIKELY WILL EJECT INTO THE PLAINS.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
ON DAYS 3 AND 4 (SATURDAY/SUNDAY), OPERATIONAL MODELS ARE OVERALL  
IN GOOD AGREEMENT BUT THE SMALL SCALE DETAILS OF THE SHORT WAVES.  
TO ACCOUNT FOR THE UNCERTAINTY, FOLLOWED SUIT WITH THE PREVIOUS  
SHIFT AND TOOK A BLEND OF THE 06Z GFS, 00Z ECMWF AND 00Z UKMET. BY  
SUNDAY, THE SHORT WAVE FORECAST TO REACH THE PACIFIC NORTHWEST  
STILL HAS SPREAD WITH RESPECT TO THE AMPLITUDE AND TIMING. ONCE  
AGAIN, THE 00Z ECMWF IS STILL FARTHER SOUTH WITH THE SURFACE LOW  
COMPARED TO OTHER GUIDANCE. BY MONDAY, THE NEXT SYSTEM APPROACHING  
THE WEST COAST WILL STRONGLY DIG ACROSS THE WESTERN U.S. AND  
NORTHERN MEXICO. THE GFS AND ECMWF HAVE BEEN CONSISTENT WITHIN  
THEMSELVES IN TERMS OF HOW THIS TROUGH EVOLVES BUT NOT WITH EACH  
OTHER. THE GFS CONTINUES TO BE MUCH FASTER--AND CONSEQUENTLY DOES  
NOT DIG THE TROUGH AS FAR SOUTH AS THE ECMWF. IT IS ALSO FASTER  
WITH UPSTREAM ENERGY IN THE PACIFIC. THE 00Z ECMWF ENSEMBLE MEAN,  
06Z GEFS MEAN, AND 00Z NAEFS LEAN TOWARD THE 00Z ECMWF WITH THE  
TROUGH BEING SLOWER AND STRETCHING TOWARD MEXICO. THUS, THE BLEND  
USED INCREASED THE ENSEMBLE MEAN USAGE BY DAY 5/MONDAY (CHRISTMAS  
EVE) WITH SOME 00Z ECMWF AND 06Z GFS AND DISCOUNTED THE 06Z GFS ON  
DAY 7/WEDNESDAY. IT IS IMPORTANT TO NOTE THAT OTHER SCENARIOS MAY  
STILL BE PLAUSIBLE WHEN VIEWED FROM A MULTI-DAY MEAN PERSPECTIVE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
THE FIRST ROUND OF HEAVY PRECIPITATION (RAIN IN LOWER  
ELEVATIONS/HIGHER ELEVATION SNOW) WILL START LATE SATURDAY NIGHT  
AND INTO SUNDAY MORNING FOR THE PACIFIC NORTHWEST. PRECIPITATION  
WILL ALSO SPREAD INTO THE INTERMOUNTAIN WEST AND NORTHERN  
CALIFORNIA ON SUNDAY. BY MONDAY, AS THIS ACTIVITY BEGINS TO TAPER  
OFF FOR NORTHERN CALIFORNIA THROUGH THE NORTHERN ROCKIES, THE NEXT  
SYSTEM WILL APPROACH THE PACIFIC NORTHWEST ONCE AGAIN. RAIN AND  
HIGHER ELEVATION SNOW WILL IMPACT THE PACIFIC NORTHWEST,  
INTERMOUNTAIN WEST, AND SOUTHWARD INTO SOUTHERN CALIFORNIA AND THE  
SOUTHWEST THROUGH THE FIRST HALF OF THE WEEK. THE 5-DAY TOTALS FOR  
THE PERIOD WILL LIKELY REACH AT LEAST SEVERAL INCHES LIQUID AT  
SOME LOCATIONS OVER WESTERN OREGON AND EXTREME NORTHWEST  
CALIFORNIA. ADDITIONALLY, 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.  
 
REINHART/  
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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