925  
FXUS02 KWBC 091603  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1102 AM EST WED JAN 09 2019  
 
VALID 12Z SAT JAN 12 2019 - 12Z WED JAN 16 2019  
   
..OVERVIEW
 
 
PROGRESSIVE AND AMPLIFIED FLOW ACROSS THE NORTH PACIFIC WILL  
CONTINUE TO BRING SIGNIFICANT STORM SYSTEMS TO THE WEST COAST  
EVERY COUPLE OF DAYS DURING THE MEDIUM RANGE PERIOD. A  
STRENGTHENING UPPER RIDGE ACROSS THE NORTHERN ROCKIES/GREAT BASIN  
WILL RESULT IN SPLIT FLOW AS THESE PACIFIC SYSTEMS MOVE EASTWARD,  
WITH ENERGY SEPARATING INTO NORTHERN AND SOUTHERN STREAM  
COMPONENTS. NORTHERN STREAM ENERGY WILL AMPLIFY AS IT ENTERS BROAD  
CYCLONIC UPPER-LEVEL FLOW EAST OF THE CLOSED UPPER HIGH THIS  
WEEKEND ACROSS THE MIDWEST/GREAT LAKES/NORTHEAST. THIS SETUP MAY  
RESULT IN SOME PHASING OF NORTHERN/SOUTHERN STREAM WAVES  
ALONG/NEAR THE EAST COAST AS LOW PRESSURE EXITS OFF THE SOUTHERN  
MID-ATLANTIC SUNDAY. WHILE THIS PATTERN DOES NOT FAVOR A  
WIDESPREAD OUTBREAK OF COLD AIR, PERSISTENT TROUGHING ACROSS THE  
EAST ALONG WITH THE AMPLIFIED WESTERN U.S. RIDGE WILL SUPPORT THE  
TRANSPORT OF COLD POLAR AIR SOUTHWARD INTO THE EASTERN U.S.,  
LIKELY SETTING THE STAGE FOR WINTER WEATHER POTENTIAL ON THE  
NORTHERN SIDE OF THE LOW PRESSURE SYSTEM THIS WEEKEND. BY NEXT  
WEEK, ANOTHER SYSTEM IN THE PACIFIC MAY PUSH INTO CENTRAL/SOUTHERN  
CALIFORNIA AND THE DESERT SOUTHWEST.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
MODEL SOLUTIONS SEEM TO HAVE SETTLED ON AN AMPLIFIED TROUGH  
REACHING CALIFORNIA BY THE WEEKEND, WITH AN INTENSIFYING RIDGE  
ACROSS THE NORTHERN GREAT BASIN/ROCKIES CAUSING THE TROUGH TO  
SHEAR AND SEPARATE INTO THE NORTHERN/SOUTHERN STREAM COMPONENTS BY  
SUN. MEANWHILE, PHASING OF SOUTHERN/NORTHERN STREAM SHORTWAVE  
ENERGY WILL BE UNDERWAY ACROSS THE PLAINS AND MISSISSIPPI VALLEY.  
MODELS CONTINUE TO STRUGGLE A BIT WITH THE PHASING PROCESS, WITH  
SOME RUN-TO-RUN VARIABILITY STILL DURING THE SAT-SUN TIME FRAME. A  
SURFACE LOW IS EXPECTED TO TRACK ACROSS THE GULF COAST STATES  
SAT-SUN WITH A SECOND COASTAL LOW DEVELOPING ALONG THE NORTH/SOUTH  
CAROLINA COASTLINE SAT NIGHT INTO SUN MORNING. BY DAY 5 (MON)  
THERE IS SOME DISAGREEMENT AS TO HOW QUICKLY THE LOW MOVES OUT TO  
SEA, ALTHOUGH CONSENSUS HAS IMPROVED TOWARD A TRACK MORE  
EAST-NORTHEASTERLY OUT TO SEA RATHER THAN CLOSER TO THE NORTHEAST  
U.S. COASTLINE. ALSO BY DAY 4-5 ANOTHER AMPLIFIED TROUGH AND  
ASSOCIATED FRONTAL SYSTEM WILL APPROACH THE WEST COAST. MODELS  
CONTINUE TO SHOW SOME VARIABILITY ON THE EXACT STRUCTURE OF THIS  
FEATURE (COMPACT CLOSED LOW OR MORE ELONGATED UPPER TROUGH). GIVEN  
THE VARIABILITY AT SMALLER SCALES AND GENERAL LARGE SCALE  
AGREEMENT, OPTED TO TAKE A MULTI-MODEL BLEND APPROACH DURING DAYS  
3-5, INCLUDING THE 00Z ECMWF/00Z GFS DETERMINISTIC MODELS, WITH  
SOME ENSEMBLE MEANS AND CONTINUITY FROM WPC AS WELL.  
 
BY TUE-WED (DAYS 6-7), ADDITIONAL SOUTHERN STREAM SHORTWAVE ENERGY  
SHOULD CROSS THE SOUTHEAST (THE ENERGY WHICH BEGAN NEAR CA ON DAY  
3) WHILE THE NEXT PACIFIC TROUGH MOVES INTO THE FOUR CORNERS  
REGION AND EVENTUALLY THE PLAINS (MODELS HAVE ALMOST UNIVERSALLY  
SPED THIS FEATURE UP COMPARED TO PREVIOUS RUNS WHICH KEPT A CLOSED  
LOW LINGERING OFFSHORE FOR LONGER). MODEL/ENSEMBLE SOLUTIONS HAVE  
SHOWN A LARGE DEGREE OF INCONSISTENCY/VARIABILITY BY THIS TIME  
FRAME SO WAS LEERY OF LEANING TOO HEAVILY TOWARD DETERMINISTIC  
GUIDANCE. THUS, WEIGHT WAS SHIFTED TOWARD THE ECENS/GEFS ENSEMBLE  
MEANS BY DAYS 6-7.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT THE POTENTIAL FOR ROUNDS OF PRECIPITATION (LOW ELEVATION  
RAIN AND MOUNTAIN SNOWS) ACROSS THE WEST COAST, PARTICULARLY  
CALIFORNIA, AS MULTIPLE PACIFIC SYSTEMS IMPACT THE REGION. LIGHT  
PRECIPITATION IS EXPECTED FOR THE SOUTHWEST AS WELL. AS SHORTWAVE  
ENERGY REACHES THE LOWER MISSISSIPPI VALLEY/SOUTHEAST BY SAT-SUN,  
PRECIPITATION WILL OVERSPREAD MUCH OF THE REGION, AS FAR NORTH AS  
THE OHIO VALLEY, APPALACHIANS, AND MID-ATLANTIC. COLD AIR ON THE  
NORTHERN SIDE OF THE SYSTEM WILL RESULT IN A THREAT OF WINTRY  
PRECIPITATION FROM PORTIONS OF THE OHIO VALLEY TO THE SOUTHERN  
MID-ATLANTIC. CONFIDENCE HAS INCREASED FOR THE POTENTIAL OF  
ACCUMULATING SNOWS IN THESE AREAS, BUT THE EXACT AREAS TO RECEIVE  
ANY POTENTIALLY HEAVY SNOW REMAIN MORE UNCERTAIN AT THIS TIME AND  
WILL DEPEND ON THE EXACT TRACK OF THE SURFACE LOW PRESSURE SYSTEM  
OFF THE SOUTHEAST/SOUTHERN MID-ATLANTIC COASTLINE, AS WELL AS THE  
PLACEMENT OF THE HIGH IN SOUTHEASTERN CANADA.  
 
GENERALLY ABOVE AVERAGE TEMPERATURES WILL PERSIST THROUGH MUCH OF  
THE FORECAST PERIOD ACROSS THE NORTH CENTRAL U.S. WITH MAX  
TEMPERATURE ANOMALIES IN THE +10 TO +15 DEGREES F RANGE (EVEN  
WARMER FOR SOME AREAS), DUE TO THE UPPER-LEVEL RIDGING THERE. THE  
COLDEST TEMPERATURES DURING THE FORECAST PERIOD WILL BE ACROSS THE  
NORTHEAST DURING THE WEEKEND; HIGHS IN THE SINGLE DIGITS ARE  
POSSIBLE IN NORTHERN NEW ENGLAND, 10 TO 20 DEGREES BELOW AVERAGE.  
SOME MODERATION IS LIKELY BY NEXT WEEK AS THE TROUGH MOVES OFF THE  
COAST IN THE WAKE OF THE WEEKEND SYSTEM.  
 
RYAN/TATE  
 
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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