050  
FXUS02 KWBC 301559  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1058 AM EST TUE JAN 30 2018  
 
VALID 12Z FRI FEB 2 2018 - 12Z TUE FEB 6 2018  
 
16Z UPDATE...  
 
THE FORECAST IS VERY SIMILAR TO THE OVERNIGHT ISSUANCE, AND THE  
MODELS CONTINUE TO SHOW GENERAL LARGE SCALE AGREEMENT TO JUSTIFY A  
BLEND OF DETERMINISTIC GUIDANCE THROUGH THE UPCOMING WEEKEND.  
SOME OF THE CMC AND UKMET WAS USED THROUGH SATURDAY, AND GREATER  
WEIGHTING TO THE GFS AND ECMWF GIVEN THEIR AGREEMENT WITH THE  
ENSEMBLE MEANS. DIFFERENCES REGARDING SHORTWAVE TIMING AND  
AMPLITUDE WITHIN THE BASE OF THE LARGE SCALE SYNOPTIC TROUGH ARE  
MORE APPARENT FOR EARLY NEXT WEEK. A SHORTWAVE TROUGH CROSSING  
THE PLAINS SUNDAY NIGHT AND INTO MONDAY WILL BE A KEY PLAYER IN  
ANY EAST COAST CYCLOGENESIS TOWARDS THE END OF THE FORECAST  
PERIOD, WITH THE 00Z ECMWF INDICATING A MORE ROBUST SHORTWAVE AS  
IT CROSSES THE MIDWEST AND OHIO VALLEY AND THE 6Z GFS INDICATING A  
FLATTER WAVE. SHORTWAVES ROTATING AROUND THE HUDSON BAY POLAR  
VORTEX ALSO INDICATE SOME TIMING DIFFERENCES BY MONDAY WHICH HAVE  
SOME DEGREE OF INFLUENCE ON THE CHARACTER OF THE BROAD UPPER  
TROUGH ACROSS THE GREAT LAKES/EASTERN U.S. GIVEN THESE  
CONSIDERATIONS, SOME OF THE EC MEAN WAS ADDED TO THE FORECAST  
PROCESS FOR MONDAY AND TUESDAY. IN TERMS OF FORECAST CONFIDENCE,  
THERE IS ABOVE AVERAGE MODEL CONFIDENCE FOR THE SYNOPTIC SCALE  
FEATURES FOR THE MEDIUM RANGE PERIOD, AND AVERAGE CONFIDENCE ON  
THE MESOSCALE FEATURES.  
 
HAMRICK  
 
***PREVIOUS DISCUSSION (ISSUED AT 700 UTC)***  
   
..OVERVIEW AND GUIDANCE PREFERENCES
 
 
MEDIUM RANGE FLOW ALOFT FEATURES RIDGING OFF THE WEST COAST AND  
CYCLONIC FLOW/TROUGHING DOWNSTREAM OVER MUCH OF THE REST OF THE  
LOWER 48. MODELS AND ENSEMBLES OFFER SIMILAR LARGER SCALE FLOW  
PATTERNS...BUT VARY WITH THE TIMING AND MAGNITUDE OF COLD HIGH  
PRESSURE DIGGING INTO THE LOWER 48 FROM CANADA. DIFFERENCES ALSO  
INCLUDE POTENTIAL FOR PROGRESSIVE EMBEDDED SHORTWAVES TO INDUCE  
AND DEVELOP FRONTAL WAVES ON THE COLD DOME PERIPHERY.  
 
THE WPC MEDIUM RANGE PRODUCT SUITE WAS PRIMARILY DERIVED FROM A  
BLEND OF THE 12 UTC GFS/ECMWF AND LESS DEFINED GEFS/ECMWF/NAEFS  
ENSEMBLES. APPLIED GREATER BLEND WEIGHTING THAN USUAL ON  
DETERMINISTIC GUIDANCE TO ALLOW MORE COOLING UNDER THE HIGH AND TO  
BETTER DEPICT 12 UTC GFS/ECMWF BASED FRONTAL LOWS GIVEN THEIR  
BETTER THAN AVERAGE SHORTWAVE CLUSTERING.  
 
   
..WEATHER HIGHLIGHTS/THREATS
 
 
OVER THE NWRN US...A SERIES OF PROGRESSIVE IMPULSES RUNNING OVER  
THE EAST PACIFIC/WEST COAST RIDGE WILL SUPPORT ADDITIONAL PERIODS  
OF MODEST PRECIPITATION. HOWEVER...EXPECT LOCALLY HEAVIER TERRAIN  
ENHANCED SNOWS ACROSS THE NWRN US AND ESPECIALLY THE NRN ROCKIES  
WITH ADDED NRN STREAM IMPULSES OF LESS CERTAIN TIMINGS. EXPECT  
ADDITIONAL POST-FRONTAL UPSLOPE SNOWS UP TO THE DIVIDE.  
UPSTREAM...THERE REMAINS A DECENT MODEL SIGNAL TO BRING ADDITIONAL  
MODEST ERN PACIFIC SYSTEM ENERGY AND PCPN TO THE PACIFIC NW EARLY  
NEXT WEEK.  
 
DOWNSTREAM...A WAVY LEAD COLD FRONT WORKS OFFSHORE INTO THE WRN  
ATLANTIC AND GULF OF MEXICO FRIDAY AS HIGH PRESSURE SETTLES OVER  
THE CENTRAL AND ERN US. HEAVY SNOW WILL TAPER OFF OVER NEW  
ENGLAND...BUT WILL PERSIST TO THE LEE OF THE GREAT LAKES IN THE  
ARCTIC AIR.  
 
NEXT WEEKEND...A CLIPPER SYSTEM WILL SPREAD SNOWS ACROSS THE  
N-CENTRAL US...GREAT LAKES REGION...THEN NORTHEAST.  
UNDERNEATH...RAINS WILL MEANWHILE INCREASE OVER THE GULF COAST BY  
SUNDAY WITH RETURN FLOW TO INCLUDE SOME LOCAL HEAVIER DOWNPOURS.  
WIDESPREAD MODERATE PCPN SHOULD EXPAND NEWD INTO EARLY NEXT WEEK  
INTO THE MID-SOUTH AND ACROSS THE ERN US.  
 
A COLD HIGH SURGE IN THE WAKE OF THE CLIPPER HAS SOME  
TIMING/STRENGTH VARIANCE IN GUIDANCE BUT BEST CLUSTERING SUGGESTS  
A PATH FROM W-CENTRAL CANADA INTO THE N-CENTRAL US SATURDAY TO  
THEN SPREAD OVER THE REST OF THE CENTRAL AND ERN US SUNDAY INTO  
MONDAY TO SHUNT ALL BUT A WAVE OVERRUNNING SNOW THREAT OFFSHORE  
DEPENDENT UPON THE POTENTIAL DEVELOPMENT OF FRONTAL WAVES.  
 
SCHICHTEL  
 

 
 
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