028  
FXUS02 KWBC 291512  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1012 AM EST MON JAN 29 2018  
 
VALID 12Z THU FEB 01 2018 - 12Z MON FEB 05 2018  
 
15Z UPDATE...  
 
FORECAST WAS NOT CHANGED SIGNIFICANTLY FROM OVERNIGHT ISSUANCE.  
MODELS CONTINUED TO SHOW GENERAL LARGE SCALE AGREEMENT TO JUSTIFY  
HEAVY USE OF DETERMINISTIC GUIDANCE THROUGH DAY 5. THUS, THE WPC  
FORECAST FOR DAYS 3-5 WAS BASED ON A BLEND OF THE 00Z  
ECMWF/CMC/06Z GFS. MODELS BEGIN TO SHOW SOME SMALL SCALE  
DIFFERENCES DURING DAYS 6-7. ONE OF THE MORE SIGNIFICANT  
DIFFERENCES NOTED DURING THIS TIME FRAME IS THE AMPLITUDE OF  
PACIFIC SHORTWAVE ENERGY CROSSING THE CREST OF THE RIDGE AND  
APPROACHING THE PAC NW. A TREND WAS NOTED AMONG RECENT  
DETERMINISTIC RUNS TOWARD SOMEWHAT GREATER AMPLITUDE WITH THIS  
FEATURE, BUT WOULD LIKE TO SEE MORE RUN-TO-RUN CONSISTENCY HERE.  
ADDITIONALLY, SHORTWAVES ROTATING AROUND THE HUDSON BAY UPPER LOW  
SHOW SOME TIMING DIFFERENCES BY DAYS 6-7 WHICH HAVE SOME DEGREE OF  
INFLUENCE ON THE CHARACTER OF THE BROAD UPPER TROUGH ACROSS THE  
GREAT LAKES/EASTERN U.S. GIVEN THESE CONSIDERATIONS, ENSEMBLE MEAN  
(00Z ECENS/NAEFS) WEIGHTING WAS INCREASED SIGNIFICANTLY DURING THE  
DAY 6-7 PERIOD. OVERALL, THIS RESULTED IN A FORECAST FAIRLY  
CONSISTENT WITH CONTINUITY. THE MOST SIGNIFICANT ADJUSTMENT WAS AN  
INCREASE IN THE FORWARD SPEED OF THE COLD FRONT CROSSING THE MS  
VALLEY/EASTERN U.S. EARLY NEXT WEEK, WHICH HAD SOME BROAD SUPPORT  
AMONG THE GUIDANCE.  
 
RYAN  
 
PREVIOUS DISCUSSION (ISSUED AT 0657 UTC)...  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
THE MEDIUM RANGE PERIOD FLOW ALOFT WILL FEATURE RIDGING CENTERED  
NEAR THE WEST COAST AND TROUGHING SPREAD DOWNSTREAM OVER MOST OF  
THE REST OF THE LOWER 48. ENSEMBLES MEANS ARE IN GENERAL AGREEMENT  
OVERALL AND A COMPOSITE BLEND OFFERS A GOOD STARTING POINT FOR THE  
MEDIUM RANGE FORECAST IN LIEU OF CHASING TIMING/AMPLITUDE  
DETERMINISTIC MODEL DIFFERENCES.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
IN THE WEST, A SERIES OF PROGRESSIVE IMPULSES WILL SUPPORT  
ADDITIONAL PERIODS OF MODEST PRECIPITATION INCLUDING HEAVIER  
TERRAIN ENHANCED SNOWS ACROSS THE NWRN US AND ESPECIALLY THE NRN  
ROCKIES WITH ADDED NRN STREAM IMPULSES OF UNCERTAIN TIMINGS.  
EXPECT ANOTHER ROUND OF PRECIPITATION (MOSTLY SNOW) ALONG THE  
DIVIDE VIA UPSLOPE FLOW NEXT WEEKEND IN THE WAKE OF A CLIPPER  
SYSTEM. UPSTREAM...THERE IS ALSO SOME MODEL SIGNAL TO BRING  
ADDITIONAL MODEST ERN PACIFIC SYSTEM ENERGY AND PCPN TO THE  
PACIFIC NW AGAIN BY DAYS 6/7.  
 
DOWNSTREAM...A LEAD FRONT REACHES THE EAST-CENTRAL US BY THU. THE  
MAIN SFC LOW CENTER WILL TRACK THROUGH SRN CANADA WITH MOST OF THE  
PRECIPITATION TO START...BUT INCREASED GULF MOISTURE WILL SUPPORT  
AN EXPANDED PRECIPITATION AREA ALONG/AHEAD OF THE COLD FRONT  
ACROSS THE E-CENTRAL THEN ERN US. ENOUGH COLD AIR SHOULD BE IN  
PLACE TO THE NORTH OF THE FRONT TO SUPPORT A SWATH OF SNOW  
PARALLEL TO ANY SFC LOW TRACK OR AT LEAST PARALLEL TO THE UPPER  
TROUGH AXIS PROPAGATION. HOWEVER...THE MODELS AND ENSEMBLES ARE  
NOW TRENDING AWAY FROM LOW DEVELOPMENT. EARLIER RELEASED WPC PROGS  
PLUGGED IN A FRI MID-ATLANTIC WAVE FOR CONTINUITY AS THERE WAS A  
SMALL CLUSTER OF SUPPORTING ENSEMBLES AND THE 12 UTC  
CANADIAN...BUT THE 00 UTC CANADIAN US NOW MUCH MORE PROGRESSIVE  
WITH THE FRONT. NEXT WEEKEND, A CLIPPER SYSTEM WILL SPREAD SOME  
GENERALLY LIGHT SNOW TO THE CENTRAL STATES INTO THE GREAT LAKES  
WITH RAIN INCREASING AND EXPANDING INLAND FROM THE GULF COAST AS  
RETURN FLOW RESUMES...WITH WIDESPREAD MODERATE PCPN EARLY NEXT  
WEEK INTO THE ERN STATES.  
 
SCHICHTEL  
 

 
 
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