731  
FXUS02 KWBC 070647  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
147 AM EST MON JAN 07 2019  
 
VALID 12Z THU JAN 10 2019 - 12Z MON JAN 14 2019  
   
..OVERVIEW
 
 
A RELATIVELY AMPLIFIED BUT STILL PROGRESSIVE FLOW REGIME WILL  
PERSIST ACROSS THE CONUS DURING THE EXTENDED FORECAST PERIOD. 500  
HPA D+8 CHARTS SHOW A PERSISTENT MEAN TROUGH JUST OFF THE U.S.  
WEST COAST, WITH RIDGING ACROSS THE U.S. AND CANADIAN PLAINS, AND  
MEAN A MEAN TROUGH ACROSS NEW ENGLAND. AMPLIFIED SYSTEMS SHOULD  
REACH THE WEST COAST EVERY COUPLE DAYS, WITH SOME SPLITTING OF  
ENERGY INTO NORTHERN/SOUTHERN STREAM COMPONENTS AS THE WAVES MOVE  
INTO THE ROCKIES/CENTRAL U.S. MODELS CONTINUE TO SUGGEST THE  
POTENTIAL FOR SOME DEGREE OF NORTHERN/SOUTHERN STREAM PHASING  
ACROSS THE EASTERN U.S. BY THE WEEKEND, WITH POTENTIAL INITIAL  
SURFACE LOW DEVELOPMENT IN THE NORTHERN GULF OF MEXICO, AND ENERGY  
TRANSFERRING TO A NEW LOW OFF THE SOUTHEAST COAST BY SAT NIGHT/SUN.  
 
   
..GUIDANCE EVALUATION/PREFERENCES
 
 
A DETERMINISTIC MULTI-MODEL BLEND (ECMWF/GFS/UKMET) WAS USED AS A  
FORECAST STARTING POINT ON DAYS 3-4 (THU-FRI). RELATIVELY LOW  
MODEL SPREAD AND GOOD RUN-TO-RUN CONTINUITY DURING THIS TIME FRAME  
SUPPORTED USE OF THIS BLENDED CONSENSUS SOLUTION. THE MOST  
SIGNIFICANT FEATURES DURING THIS TIME FRAME ARE AN AMPLIFIED UPPER  
TROUGH EXITING THE EASTERN U.S., AND ANOTHER AMPLIFIED TROUGH  
APPROACHING THE WEST COAST. MODELS DID SHOW SOME SPREAD AS TO THE  
STRUCTURE AND INTENSITY OF THE LOW PRESSURE SYSTEM OFF THE WEST  
COAST BY FRI, BUT GIVEN THAT NONE OF THESE SOLUTIONS WERE DEEMED  
OUTLIERS, A BLENDED APPROACH WAS BEST.  
 
BY DAYS 5-7 (SAT-MON), THE AFOREMENTIONED SPLIT FLOW BEGINS TO  
PLAY A LARGER ROLE AS THE PACIFIC ENERGY SEPARATES INTO NORTHERN  
AND SOUTHERN STREAM COMPONENTS, WHILE YET ANOTHER AMPLIFIED WAVE  
APPROACHES THE WEST COAST. SOUTHERN STREAM SHORTWAVE ENERGY SHOULD  
CROSS THE FOUR CORNERS REGION FRI AND REACH THE SOUTHERN PLAINS  
SAT, WHILE THE NORTHERN STREAM COMPONENT INITIALLY WEAKENS AS IT  
ENCOUNTERS ANTICYCLONIC FLOW BEFORE AMPLIFYING ONCE AGAIN AS IT  
REACHES THE NORTHERN PLAINS/MIDWEST FRI-SAT. THE SAT-SUN TIME  
PERIOD IS WHEN THE COMPLEXITIES BEGIN AS TO HOW THESE TWO FEATURES  
WILL (OR WILL NOT) INTERACT, POTENTIALLY RESULTED IN A COASTAL LOW  
PRESSURE SYSTEM OFF THE EASTERN SEABOARD BY SUN-MON. THE GFS HAS  
FOR A COUPLE RUNS SHOWN A HIGHLY PHASED SYSTEM WITH A RELATIVELY  
INTENSE LOW PRESSURE SYSTEM OFF THE EAST COAST, BUT HAS ALSO BEEN  
FAIRLY INCONSISTENT. THE ECMWF, ON THE OTHER HAND, HAS SHOWN A  
LESS PHASED/MORE SUPPRESSED SYSTEM THAT MOVES MORE QUICKLY OUT TO  
SEA, AND HAS ALSO BEEN QUITE CONSISTENT IN DEPICTING THIS TYPE OF  
SOLUTION (WITH A FAIR DEGREE OF SUPPORT FROM ECENS/GEFS MEANS AS  
WELL). GIVEN THE LOW PREDICTABILITY OF SUCH AN EVOLUTION ON DAYS  
6-7, OPTED TO GO HEAVILY TOWARD A SOLUTION MORE LIKE THE ENSEMBLE  
MEANS AND ECMWF (AND GFS FROM 12Z SUN). THE NEXT SYSTEM  
APPROACHING THE WEST COAST BY DAY 6-7 IS REFLECTED BY A RANGE OF  
POSSIBLE MODEL SOLUTIONS VARYING IN TIMING AS WELL AS AMPLITUDE.  
THUS, AN ENSEMBLE MEAN APPROACH IS PREFERRED HERE AS WELL. GIVEN  
THESE CONSIDERATIONS, THE FORECAST DURING DAYS 5-7 WAS GRADUALLY  
TRENDED TO WEIGHT ECENS AND GEFS ENSEMBLE MEANS MORE HEAVILY THAN  
DETERMINISTIC GUIDANCE.  
 
   
..WEATHER HIGHLIGHTS/HAZARDS
 
 
EXPECT THE POTENTIAL FOR MULTIPLE ROUNDS OF HEAVY PRECIPITATION  
(LOW ELEVATION RAIN AND MOUNTAIN SNOWS) ACROSS THE WEST COAST  
DURING THE FORECAST PERIOD AS A RAPID SUCCESSION OF PACIFIC  
SYSTEMS IMPACTS THE REGION. AS SHORTWAVE ENERGY REACHES THE  
SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY BY FRI-SAT, AREAS OF  
HEAVY RAIN WILL BE POSSIBLE, WITH PRECIPITATION SPREADING EAST  
INTO THE SOUTHEAST/SOUTHERN MID-ATLANTIC BY SAT NIGHT/SUN. COLD  
AIR TO THE NORTH OF THE SYSTEM COULD RESULT IN AREAS IF WINTRY  
PRECIPITATION. THE DEGREE OF WINTER WEATHER POTENTIAL FROM THE  
SOUTHERN APPALACHIANS TO THE MID-ATLANTIC SAT-SUN IS HIGHLY  
UNCERTAIN FOR THE REASONS DESCRIBED ABOVE.  
 
ABOVE AVERAGE TEMPERATURES WILL PERSIST THROUGH THE FORECAST  
PERIOD ACROSS THE NORTH CENTRAL U.S., WITH MAX TEMP ANOMALIES IN  
THE +10 TO +15 DEG F RANGE (EVEN WARMER FOR SOME AREAS) THROUGH  
MUCH OF THE PERIOD. THE COLDEST TEMPERATURES WILL BE ACROSS THE  
NORTHEAST, WHERE TEMPERATURES AS COLD AS 10 TO 20 DEG BELOW  
AVERAGE WILL BE POSSIBLE FROM FRI THROUGH THE WEEKEND INTO EARLY  
NEXT WEEK.  
 
RYAN  
 
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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