030  
FXUS02 KWNH 180652  
PREEPD  
 
PRELIMINARY EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
151 AM EST WED JAN 18 2017  
 
VALID 12Z SAT JAN 21 2017 - 12Z WED JAN 25 2017  
   
..OVERVIEW
 
 
LITTLE HAS CHANGED WITH THE EXPECTATION THAT STRONGLY POSITIVE HGT  
ANOMALIES/RIDGING ALOFT OVER ERN CANADA WILL ENCOURAGE VIGOROUS  
ERN PAC SYSTEMS TO PROGRESS INTO/ACROSS THE LOWER 48. BEHIND A  
COMPACT SYSTEM LIFTING NWD OVER THE NRN PLAINS DURING THE  
WEEKEND... THE EXTENDED FCST WILL FOCUS ON TWO DOMINANT FEATURES.  
THE FIRST SHOULD EMERGE FROM THE WEST EARLY IN THE PERIOD LEADING  
TO A PSBL SEVERE WEATHER EVENT ALONG THE GULF COAST REGION DURING  
THE WEEKEND AND THEN BRING STRONG ONSHORE FLOW AND AREAS OF HVY  
RNFL TO THE EAST COAST STATES. THE SECOND WILL LIKELY AFFECT THE  
WEST COAST WITH HVY RNFL/STRONG WINDS BY SUN AND THEN BRING LOW  
PRESSURE INTO THE PLAINS/MS VLY BY TUE-WED WITH A BAND OF SNOW TO  
THE NW OF THE LOW TRACK. THE WEST SHOULD SEE A PERIOD OF DRIER  
WEATHER BEHIND THIS SYSTEM.  
 
   
..GUIDANCE EVALUATION AND WEATHER HIGHLIGHTS/THREATS
 
 
EXISTING SPREAD IN THE GUIDANCE THROUGH THE 12Z/18Z CYCLES FAVORED  
ONLY MODEST ADJUSTMENTS IN THE FCST. INDIVIDUAL SOLNS COMPARED  
BETTER OR WORSE TO THE CONSENSUS OR AVG DEPENDING ON THE AREA...  
REQUIRING VARIOUS WEIGHTINGS OF RECENT GFS/ECMWF RUNS AND THEIR  
ENSEMBLE MEANS WITH A LITTLE 12Z CMC INCLUDED EARLY AS WELL.  
 
GUIDANCE HAS REMAINED FAIRLY WELL CLUSTERED AS THE EARLY SAT FOUR  
CORNERS SYSTEM REACHES THE PLAINS BY EARLY SUN. THEN GUIDANCE  
DIVERGES OVER THE EAST WITH LITTLE REDUCTION IN SPREAD OVER 24 HRS  
AGO. GFS/GEFS MEAN RUNS HAVE BEEN WAFFLING BTWN THE AVG OF  
ECMWF/ECMWF MEAN RUNS AND A FARTHER SWD TRACK THAT SOME CMC RUNS  
HAVE BEEN SHOWING AS WELL. INTERESTINGLY THE CORRESPONDING CMC  
MEAN IS MORE SIMILAR TO RECENT ECMWF RUNS. BEYOND THE GENERAL  
LATITUDE ISSUE ARE THE DETAILS OF HOW ENERGY/SFC LOW PRES MAY  
SPLIT. THE SMALL SCALE OF THIS ASPECT OF EVOLUTION SUGGESTS LOW  
PREDICTABILITY 5-7 DAYS OUT IN TIME BUT SPECIFICS WILL BE VERY  
IMPORTANT FOR SENSIBLE WEATHER IMPACTS OVER THE NRN MID ATLC/NEW  
ENGLAND. CURRENTLY THE BALANCE OF GUIDANCE APPEARS TO LEAN  
SLIGHTLY IN THE DIRECTION OF THE PAST COUPLE OF ECMWF RUNS THROUGH  
THE 12Z CYCLE WITH A SOMEWHAT MORE OFFSHORE WAVE AS IN THE ECMWF  
MEANS ALSO A POSSIBILITY.  
 
AS THIS SYSTEM CROSSES THE CNTRL-ERN STATES IT WILL BE ACCOMPANIED  
BY A BROAD SHIELD OF PCPN WITH HEAVIEST RNFL EXPECTED FROM THE ERN  
HALF OF THE GULF COAST NEWD ALONG THE APLCHNS/EAST COAST. CONSULT  
SPC OUTLOOKS FOR LATEST INFO REGARDING POTENTIAL SEVERE WEATHER  
ALONG THE GULF COAST DURING THE WEEKEND. RAIN OVER THE EAST COAST  
STATES WILL BE ENHANCED BY STRONG ATLC INFLOW. THUS FAR THERE HAS  
BEEN ENOUGH UNCERTAINTY OVER EXACT SYSTEM EVOLUTION TO REDUCE  
CONFIDENCE IN PINPOINTING LOCATION/TIMING/STRENGTH OF HIGHEST WIND  
SPEEDS. WITH LESSER TOTALS THERE MAY ALSO BE A RELATIVE MAXIMUM  
FROM THE S-CNTRL PLAINS INTO THE LWR OH VLY. PCPN TYPE STILL  
LOOKS TO BE NEARLY ALL RAIN WITH ANY WINTRY WEATHER CONFINED TO  
NEW ENGLAND... AND PSBLY LOCALLY WITH THE UPR LOW.  
 
FOR THE SECOND SYSTEM GUIDANCE IS STILL IN THE PROCESS OF TRYING  
TO RESOLVE SFC DETAILS OFF THE WEST COAST BY SAT NIGHT-SUN...  
FAVORING AN INTERMEDIATE SOLN WHILE AWAITING BETTER CLUSTERING.  
12Z/18Z GFS RUNS MAY BE SOMEWHAT QUICK TO BRING LEADING HGT FALLS  
INTO THE NORTHWEST BUT OTHERWISE THERE IS REASONABLE AGREEMENT  
AMONG 12Z/18Z SOLNS FOR THE OVERALL TROUGH ALOFT UNTIL THE 12Z  
ECMWF/ECMWF MEAN TREND FASTER AFTER ABOUT LATE MON. THIS REFLECTS  
A FASTER TREND FROM THE ROCKIES THROUGH MS VLY AND IS A SWITCH  
FROM YDAY WHEN GFS RUNS WERE ON THE FASTER SIDE. WITH THE TIMING  
OF GFS/ECMWF CLUSTERS THE REVERSE OF HISTORICAL BIASES A  
COMPROMISE APPEARS BEST. THE NEW 00Z CYCLE ADDS NO CLARITY AS THE  
GFS TRENDED FASTER BUT THE GEFS MEAN/CMC REMAIN A LITTLE FARTHER  
BACK.  
 
THE MOST PRONOUNCED EFFECTS OF THIS SYSTEM ALONG THE WEST COAST  
SHOULD BE IN THE SAT NIGHT-MON TIME FRAME. HEAVIEST PCPN SHOULD  
FIRST ARRIVE AT THE CNTRL WEST COAST AND THEN CONTINUE SWD WITH  
TIME. FAVORED COASTAL LOCATIONS AND SIERRA NEVADA RANGE MAY SEE  
UP TO SEVERAL INCHES LIQUID. STRONGEST WINDS WILL LIKELY AFFECT  
THE CNTRL-NRN WEST COAST AROUND SAT NIGHT-SUN. EXPECT AN AREA OF  
MOSTLY SNOW TO CROSS MUCH OF THE WEST SUN INTO EARLY NEXT WEEK  
WITH RAIN OVER THE EXTREME SOUTH. AS THIS SYSTEM TRACKS NEWD  
ACROSS THE PLAINS THERE WILL LIKELY BE A BAND OF SNOW THE NW OF  
THE LOW TRACK.  
 
HIGHS WILL BE BELOW NORMAL OVER MUCH OF THE WEST DURING THE PERIOD  
WITH SOME AREAS MORE THAN 10F COLDER THAN AVG ON ONE OR MORE DAYS.  
MOST OF THE REMAINDER OF THE LOWER 48 SHOULD BE ABOVE TO MUCH  
ABOVE NORMAL ASIDE FROM THE FL PENINSULA AFTER A WARM WEEKEND AND  
THE PLAINS WHICH WILL BEGIN TO SEE SOME OF THE COOLER AIR MOVE IN  
FROM THE WEST BY TUE-WED. THE FCST IS CONSISTENT WITH THE UPR MS  
VLY REGION SEEING MULTIPLE DAYS OF MORNING LOWS 30-40F ABOVE  
NORMAL WITH SOME RECORD WARM MINS.  
 
RAUSCH  
 

 
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab HPC Page Main Text Page