Preliminary Forecasts
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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

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