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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Jan 17, 2018)
 
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EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST WED JAN 17 2018

VALID 12Z SAT JAN 20 2018 - 12Z WED JAN 24 2018

...OVERVIEW...

MULTI-DAY MEANS CONTINUE TO SHOW THE ESTABLISHMENT OF BROAD
CYCLONIC FLOW AS THE PREVAILING LARGE SCALE PATTERN OVER THE LOWER
48, THOUGH WITH SOME MODEST DIFFERENCES AND RUN TO RUN VARIABILITY
FOR THE PRECISE LOCATION OF THE MEAN TROUGH'S AXIS.  WITHIN THIS
PATTERN THERE SHOULD BE TWO PROMINENT PACIFIC SYSTEMS THAT BRING
PERIODS OF HEAVY PRECIP TO THE CENTRAL/NORTHERN WEST COAST AND
SOME MOISTURE FARTHER INLAND, WHILE A LEADING UPPER TROUGH
REACHING THE WEST AS OF EARLY SAT WILL GENERATE A POTENTIALLY
SIGNIFICANT PLAINS THROUGH GREAT LAKES STORM FROM SUN INTO EARLY
NEXT WEEK.


...GUIDANCE ASSESSMENT AND PREFERENCES...

AS THE UPPER TROUGH INITIALLY OVER THE WEST PROGRESSES
EAST/NORTHEAST, GUIDANCE IN PRINCIPLE IS SHOWING AN INCREASINGLY
AGREEABLE SIGNAL THAT ASSOCIATED LOW PRESSURE WILL DEEPEN INTO A
FAIRLY STRONG STORM SYSTEM AS IT TRACKS INTO THE MS VALLEY/GREAT
LAKES.  HOWEVER THERE IS STILL A MODERATE DEGREE OF UNCERTAINTY
REGARDING THE EXACT SPEED AND TRACK OF THIS SYSTEM.  OVER THE PAST
DAY THE MOST PROMINENT TREND AMONG MOST MODELS AND ENSEMBLE
MEMBERS HAS BEEN TOWARD SLOWER TIMING.  ALSO THE 00Z/06Z GFS RUNS
ARE MOST PRONOUNCED IN PHASING SOUTHERN CANADA ENERGY WITH THE
UPPER TROUGH OF INTEREST AS IT REACHES THE GREAT LAKES.  THESE
CONSIDERATIONS SHOULD INITIALLY FAVOR TILTING THE FORECAST
SOMEWHAT MORE TOWARD THE ECMWF/ECMWF MEAN VERSUS THE FASTER
GFS/GEFS MEAN.  THAT SAID, THE 00Z UKMET/CMC STILL SUGGEST THAT
TIMING COULD BE SOMEWHAT FASTER THAN THE ECMWF-BASED SOLUTIONS
PLUS THERE HAS BEEN A FASTER/LESS AMPLIFIED TREND WITH UPSTREAM
ENERGY FLOWING INTO THE WEST EARLY NEXT WEEK.  THEREFORE CURRENTLY
PREFER ADJUSTING THE FORECAST ONLY ABOUT 2/3 TOWARD THE SLOWER
ECMWF SCENARIO.  AS FOR TRACK LATITUDE, THE 00Z CMC AND A NUMBER
OF ITS ENSEMBLE MEMBERS LEAN TOWARD THE NORTHERN SIDE OF THE FULL
SPREAD.  THE LATITUDE OF GREATEST ENSEMBLE LOW DENSITY APPEARS
FAIRLY STABLE OVER THE PAST DAY.

UPSTREAM THE GUIDANCE IS FAIRLY STABLE/AGREEABLE WITH BEST DEFINED
LOW PRESSURE TRACKING TOWARD THE BC COAST BY SUN BUT THERE ARE
SMALLER SCALE/LOWER PREDICTABILITY QUESTIONS WITH RESPECT TO
WAVINESS ALONG THE TRAILING FRONT.  SUCH DETAILS WILL LIKELY TAKE
INTO THE SHORT RANGE TIME FRAME TO GET RESOLVED SATISFACTORILY AND
WILL INFLUENCE SPECIFICS OF TIMING/INTENSITY OF PRECIP.  GUIDANCE
HAS BEEN VARIABLE WITH THE DETAILS OF SUPPORTING FLOW ALOFT THAT
STREAMS INTO THE WEST BY MON-TUE BUT THE GENERAL TREND HAS BEEN
TOWARD A FASTER/FLATTER DEPICTION OF THIS ENERGY.  AS A RESULT
WHATEVER SYSTEM(S) REACH THE CENTRAL/EAST-CENTRAL U.S. BY TUE-WED
ARE LESS DEFINED THAN IN SOME PREVIOUS FORECASTS.

THE NEXT SYSTEM TO AFFECT THE WEST COAST SHOULD COME INTO THE
PICTURE AROUND DAYS 6-7 TUE-WED.  THERE IS MORE UNCERTAINTY WITH
THIS SYSTEM AS IT INVOLVES A POTENTIAL INTERACTION OF MID-LATITUDE
PACIFIC FLOW AND NORTHEAST PACIFIC UPPER LOW/TROUGHING.  MINIMAL
INTERACTION WOULD ALLOW THE MID-LATITUDE ENERGY TO BRING A WAVE
INTO THE CENTRAL WEST COAST AS SUGGESTED BY THE 00Z CMC, SOME 00Z
CMC ENSEMBLES, AND A COUPLE 00Z GEFS MEMBERS.  ON THE OTHER HAND
RECENT GFS RUNS, INCLUDING THE 06Z VERSION RELATIVE TO THE 00Z
RUN, HAVE BEEN TRENDING TOWARD GREATER INTERACTION AND A FARTHER
NORTHWEST SURFACE LOW TRACK WHICH THE ECMWF AND ENSEMBLE MEANS
HAVE BEEN SHOWING.  CURRENT PREFERENCE IS TO LEAN TOWARD THIS
MAJORITY SCENARIO WITH A COMBINATION OF THE 06Z GFS/00Z ECMWF AND
THEIR MEANS PROVIDING A GOOD BALANCE OF SYNOPTIC AGREEMENT VERSUS
DETAIL.

BASED ON THE ABOVE CONSIDERATIONS, THE UPDATED FORECAST STARTED
WITH A 50/30/20 WEIGHT OF THE 00Z ECMWF/06Z GFS/00Z ECMWF MEAN
RESPECTIVELY FOR DAYS 3-5 SAT-MON FOLLOWED BY GREATER INCLUSION OF
ENSEMBLE MEAN GUIDANCE SUCH THAT THE TOTAL GEFS/ECMWF MEAN WEIGHT
REACHED 60 PERCENT BY DAY 7 WED. 


...SENSIBLE WEATHER HIGHLIGHTS/THREATS...

SEPARATE STORM SYSTEMS WILL CONTRIBUTE TO AN ACTIVE WEATHER
PATTERN OVER PORTIONS OF THE LOWER 48.  LEADING STORM EMERGING
FROM THE WEST SHOULD STRENGTHEN AS IT TRACKS FROM THE PLAINS
THROUGH THE GREAT LAKES, BRINGING A BAND OF MODERATE/HEAVY SNOW
THAT WILL MOST LIKELY BE ALIGNED FROM THE CENTRAL PLAINS THROUGH
UPPER GREAT LAKES.  WINDS MAY BE STRONG ENOUGH TO GENERATE
BLIZZARD CONDITIONS AT TIMES.  MEANWHILE IN THE WARM SECTOR EXPECT
SCATTERED TO NUMEROUS SHOWERS, WITH THUNDERSTORMS POSSIBLE FROM
EASTERN TEXAS TO THE OHIO VALLEY, AS DEEP GULF OF MEXICO MOISTURE
SURGES NORTHWARD AHEAD OF THE COLD FRONT.  SOME RAINFALL MAY BE
LOCALLY MODERATE TO HEAVY.  AS THE FRONT REACHES THE EAST COAST,
ATLANTIC INFLOW COULD ENHANCE TOTALS OVER NEW ENGLAND AS WELL. 
THE TWO PRIMARY SYSTEMS AFFECTING THE WEST COAST WILL BRING
EPISODES OF HEAVY RAIN/HIGHER ELEVATION SNOW TO NORTHERN
CALIFORNIA AND PACIFIC NORTHWEST.  THERE MAY BE BRIEF PERIODS OF
STRONG WINDS AS WELL DEPENDING ON EXACT EVOLUTION OF INDIVIDUAL
SYSTEMS.  SOME OF THE MOISTURE FROM THESE SYSTEMS WILL EXTEND INTO
THE ROCKIES.

CONTINUE TO EXPECT THE MOST EXTREME ANOMALIES FOR TEMPS TO BE IN
THE WARM SECTOR AHEAD OF THE INITIAL WESTERN/CENTRAL U.S. STORM
AND COLD FRONT, WITH THE WARMTH PROGRESSING FROM THE PLAINS SAT TO
THE EASTERN STATES IN LESS EXTREME FASHION BY MON-TUE.  SOME MIN
TEMPS MAY BE AS HIGH AS 20-30F ABOVE NORMAL SAT-MON WHILE HIGHS
MAY REACH 10-20F ABOVE NORMAL.  BEHIND THIS SYSTEM, ANTICIPATE
HIGHS OF 5-15F BELOW NORMAL OVER MUCH OF THE SOUTHERN 2/3 OF THE
WEST DURING THE WEEKEND.

RAUSCH