393  
FXUS02 KWBC 131632  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1131 AM EST THU DEC 13 2018  
 
VALID 12Z SUN DEC 16 2018 - 12Z THU DEC 20 2018  
   
..SIGNIFICANT PRECIPITATION FOR THE NORTHWEST
 
 
   
..OVERVIEW
 
 
PROGRESSIVE YET STILL AMPLIFIED FLOW WILL DOMINATE THE LOWER 48  
NEXT WEEK AS A LEAD UPPER LOW EXITS OFF THE EAST COAST SUNDAY. AN  
INCOMING TROUGH INTO THE WEST WILL SEPARATE INTO NORTHERN AND  
SOUTHERN COMPONENTS LATE MONDAY INTO TUESDAY. THE STRENGTH OF THE  
SOUTHERN TROUGH OR CLOSED LOW WILL MODULATE THE AMOUNT OF  
PRECIPITATION MIDWEEK OVER TEXAS. IN THE PACIFIC NORTHWEST, A  
SERIES OF SYSTEMS WILL PRODUCE SEVERAL BOUTS OF MODERATE TO  
OROGRAPHICALLY-ENHANCED LOCALLY HEAVY RAIN/SNOW.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODELS AND ENSEMBLES CONTINUE TO DIFFER ON THE TIMING/TRACK OF THE  
EMBEDDED SYSTEMS ACROSS THE LOWER 48, BUT FORECAST CLUSTERING HAS  
IMPROVED OVER THE PAST FEW RUNS. ACCORDINGLY, THE WPC MEDIUM RANGE  
PRODUCT SUITE WAS PRIMARILY DERIVED FROM A COMPOSITE BLEND OF  
DETERMINISTIC MODELS TO START (SUN-TUE) AND MOSTLY ENSEMBLES TO  
END THE FORECAST (NEXT WED/THU) AS SPREAD INCREASED OVER  
ESPECIALLY THE CENTRAL/EASTERN STATES. UNCERTAINTY LIED DOWNSTREAM  
OF A STRONG PACIFIC JET STARTING NEXT TUESDAY IN THE SOUTHWEST  
WITH THE DEGREE OF FLOW SEPARATION -- THE GFS/GEFS INDICATING A  
STRONGER SOUTHERN CLOSED LOW WHILE THE ECMWF/CANADIAN AND MOST OF  
THEIR ENSEMBLE MEMBERS INDICATING A WEAKER TROUGH -- WHICH WILL  
HAVE A DIRECT AFFECT ON THE AMOUNT OF PRECIPITATION OVER TEXAS  
NEXT WEEK. PREFER A SOLUTION A BIT ON THE SLOWER SIDE OF THE  
GUIDANCE BUT NOT AS SLOW AS THE GFS. TO THE NORTH, AMOUNT OF  
NORTHERN STREAM TROUGHING WILL BE AFFECTED BY THE SLOWNESS OF THE  
SOUTHERN STREAM, BUT FOR NOW WILL SPLIT THE DIFFERENCE BETWEEN THE  
QUICKER ECMWF ENSEMBLES AND SLOWER GEFS ENSEMBLES GIVEN  
UNCLEAR/CONFLICTING UP/DOWNSTREAM SIGNALS.  
 
   
..PATTERN OVERVIEW AND WEATHER HIGHLIGHTS/HAZARDS
 
 
A LEAD MAIN SOUTHEAST/MID-ATLANTIC SYSTEM WILL SHIFT OFFSHORE  
SUNDAY. LINGERING WRAPBACK RAINS AND NORTHERN FRINGE SNOW WILL  
SLOWLY EXIT BY EARLY NEXT WEEK AS THE COASTAL STORM/MARITIME  
THREAT LIFTS TO THE CANADIAN MARITIMES. A CANADIAN COLD FRONT WILL  
SWEEP THROUGH THE NORTHEAST MONDAY BRINGING IN COOLER TEMPERATURES  
TUE/WED (ABOUT 5-10 DEG F BELOW AVERAGE). MUCH MILDER TEMPERATURES  
(10-30 DEG F ABOVE AVERAGE) ARE FORECAST OVER THE CENTRAL STATES,  
ESPECIALLY THE HIGH PLAINS, AS UPPER RIDGING AND OCCASIONAL  
DOWNSLOPING WILL FAVOR RELATIVELY "WARM" TEMPERATURES (MAINLY IN  
THE 40S TO LOW 50S).  
 
A HEAVY PRECIPITATION AND HIGH WIND THREAT FOR THE PACIFIC  
NORTHWEST/NORTHERN CA RESURGES NEXT WEEK AS A SUCCESSION OF  
ENERGETIC PACIFIC SYSTEMS APPROACH THE REGION. SOME DEEPER LOWER  
LATITUDE MOISTURE INFUSION AND FAVORABLE FLOW SUGGESTS LOCALLY  
SIGNIFICANT RAIN AND HIGHER ELEVATION SNOW. THIS WILL PUSH  
EASTWARD TO THE NORTHERN GREAT BASIN/ROCKIES, AIDED BY A  
PRONOUNCED NORTHERN STREAM FLOW. TO THE SOUTH, A SPLIT/SEPARATED  
SOUTHERN STREAM TROUGH WILL CARVE OUT OVER THE SOUTHWEST  
U.S./NORTHERN MEXICO BY EARLY NEXT WEEK, BUT THE PRECIPITATION  
AMOUNT REMAINS UNCERTAIN. THAT SYSTEM SHOULD TRACK EAST OF 90W AND  
INCREASINGLY TAP GULF OF MEXICO MOISTURE LATER NEXT WEEK TO  
ENHANCE THE RAINFALL POTENTIAL OVER THE LOWER MISSISSIPPI VALLEY  
EASTWARD.  
 
FRACASSO/SCHICHTEL  
 
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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