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Extended Forecast Discussion
 
(Latest Discussion - Issued 0645Z Nov 25, 2017)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
144 AM EST SAT NOV 25 2017

VALID 12Z TUE NOV 28 2017 - 12Z SAT DEC 02 2017

...PATTERN ASSESSMENT AND MODEL CHOICES...

INDIVIDUAL MODEL SOLUTIONS CONTINUE TO DIFFER BETWEEN EACH OTHER
AND FROM RUN TO RUN FOR IMPORTANT DETAILS, BUT ULTIMATELY THE
PATTERN EVOLUTION AND BEST APPROACH FOR REPRESENTING THE MOST
LIKELY SCENARIO FOR EACH DAY DURING THE EXTENDED PERIOD HAVE
CHANGED LITTLE OVER THE PAST 12-24 HOURS.

GUIDANCE AGREEMENT MAINTAINS RELATIVELY HIGH CONFIDENCE FOR THE
LARGEST SCALE ASPECTS OF THE FORECAST.  EXPECT A PROGRESSIVE WAVE
TRAIN TO BECOME ESTABLISHED ALL THE WAY FROM THE WESTERN PACIFIC
ACROSS NORTH AMERICA, THEN SLOWING AND SPLITTING AROUND A
PERSISTENT BLOCK OVER THE MIDDLE ATLANTIC OCEAN.  TIME OF YEAR AND
STRENGTH OF THE OVERALL POLE TO EQUATOR GRADIENTS OFFER POTENTIAL
FOR EMBEDDED NOTEWORTHY OR EVEN STRONG LOWS TO SPIN UP WITHIN THE
PROGRESSIVE FLOW, THOUGH FOR NOW THE DETERMINISTIC AND ENSEMBLE
GUIDANCE SUGGESTS CYCLOGENESIS WILL FOR THE MOST PART REMAIN NORTH
OF THE LOWER 48 STATES THROUGH NEXT WEEK.  LATE IN THE PERIOD THE
MAJORITY OF GUIDANCE STILL SHOWS AN AMPLIFYING UPPER TROUGH
SETTLING NEAR THE WEST COAST, DOWNSTREAM FROM AN EAST-CENTRAL
PACIFIC RIDGE.  TELECONNECTIONS RELATIVE TO THE PACIFIC/WEST COAST
PATTERN AS WELL AS ATLANTIC RIDGE SUGGEST THERE SHOULD BE A
TENDENCY TOWARD ABOVE NORMAL HEIGHTS ALOFT OVER THE EASTERN U.S..

EARLY IN THE PERIOD, GUIDANCE SHOWS SOMEWHAT IMPROVED CLUSTERING
FOR NORTHERN STREAM FLOW ACROSS THE NORTHERN TIER U.S. AND
SOUTHERN CANADA BUT MODEL/ENSEMBLE SPREAD IS STILL MUCH BROADER
THAN DESIRED FOR THE SOUTHERN STREAM SHORTWAVE/POSSIBLE CLOSED LOW
LIKELY TO BE NEAR THE FOUR CORNERS REGION AS OF EARLY TUE.  THE
18Z GFS/GEFS JUMPED WELL AHEAD OF THEIR CORRESPONDING 12Z RUNS.  A
STEADILY FASTER TREND IN THE ECMWF OVER THE 24 HOURS THROUGH ITS
12Z/24 RUN LED TO SOME HESITATION ABOUT THE 12Z UKMET THAT BECAME
THE SLOWEST OPERATIONAL SOLUTION BY WED.  A COMPROMISE AMONG THE
12Z GFS/ECMWF/CMC PROVIDED THE BEST INTERMEDIATE SOLUTION GIVEN
ONGOING UNCERTAINTY, YIELDING AT MOST A MODEST NUDGE FROM
CONTINUITY.  NEW 00Z RUNS THUS FAR SEEM TO FAVOR STAYING IN THE
MIDDLE OF THE SPREAD AS THE GFS/CMC HAVE SLOWED FROM THEIR 18Z/12Z
RUNS RESPECTIVELY WHILE THE UKMET HAS ACCELERATED.

THE UPSTREAM TROUGH APPROACHING THE WEST COAST ON TUE ALREADY HAS
SOME DETAIL DIFFERENCES AT THAT TIME.  THEN MODELS/ENSEMBLE
MEMBERS CONTINUE TO VARY WIDELY REGARDING HOW MUCH ENERGY
SEPARATES FROM THE SOUTHERN PART OF THE TROUGH OFF CALIFORNIA
VERSUS HOW MUCH CONTINUES INLAND, AND THEN WHAT HAPPENS WITH THE
OFFSHORE CALIFORNIA ENERGY AS UPSTREAM TROUGHING DEVELOPS AND
SPECIFICS OF ENERGY THAT CONTINUES ACROSS THE LOWER 48.  THUS FAR
OPERATIONAL RUNS HAVE BEEN SUFFICIENTLY INCONSISTENT TO KEEP
CONFIDENCE LOW IN ANY PARTICULAR SOLUTION.  00Z GFS TREND HAS
ALIGNED IT WITH THE 12Z ECMWF OVER THE LOWER 48 BUT BOTH BECOME
SOMEWHAT FASTER WITH THE TROUGH THAN THE MORE STABLE ENSEMBLE
MEANS OVER THE EAST.

IN TERMS OF BOTH DEPTH AND POSITION THE ECMWF MEAN RUNS CONTINUE
TO PROVIDE THE MOST CONSISTENT EVOLUTION FOR THE UPPER TROUGH
SETTLING NEAR THE WEST COAST BY DAYS 6-7 FRI-SAT.  ECMWF/CMC RUNS
HAVE GENERALLY SHARED SOME SIMILARITY WITH THE ECMWF MEAN IN
PRINCIPLE.  GFS RUNS HAVE TENDED TO BE SOMEWHAT SHARPER/SLOWER
WHILE RECENT GEFS MEANS HAVE SEEMED TO WAFFLE BETWEEN ECMWF MEAN
AND GFS IDEAS AND WITH HIGHER HEIGHTS THAN THE ECMWF MEAN.  NOTE
THAT THE 18Z/00Z GFS RUNS WEAKEN THE NORTHERN PART OF THE PACIFIC
RIDGE MORE THAN OTHER GUIDANCE TOWARD DAY 7 SAT, LOWERING
CONFIDENCE IN THE GFS FCST NEAR THE WEST COAST BY DAY 7 AND
BEYOND.  WITHIN THE 12Z/18Z COLLECTION OF MEANS THE CMC MEAN WAS A
SLIGHTLY WEAKER VERSION OF THE ECMWF MEAN WHILE THE 12Z GEFS MEAN
WAS CLOSER TO THE ECMWF MEAN THAN THE 18Z RUN.  THIS FAVORS THE
12Z RUN FOR THE GEFS COMPONENT OF THE FORECAST.

BASED ON PREFERENCES FOR THE INITIAL FOUR CORNERS FEATURE THE
UPDATED FORECAST STARTED WITH A BLEND OF 12Z GFS/ECMWF/CMC RUNS
FOR DAYS 3-4 TUE-WED.  THIS BLEND REPRESENTED THE MOST COMMON
IDEAS OF GUIDANCE ELSEWHERE.  CONTINUED DETAIL SPREAD LED TO A
RAPID INCREASE OF ENSEMBLE WEIGHTING AROUND MID-PERIOD ON THE WAY
TO AN EXCLUSIVELY ENSEMBLE MEAN FORECAST BY DAYS 6-7 FRI-SAT.  FOR
THIS LATTER TIME FRAME, PERSISTENCE OF THE ECMWF MEAN MERITED 2/3
WEIGHTING RELATIVE TO THE 12Z GEFS MEAN.


...WEATHER HIGHLIGHTS...

EXPECT FAIRLY LOW COVERAGE OF SIGNIFICANT PRECIP ACROSS THE LOWER
48 DURING THE PERIOD.  THE FEATURE EMERGING FROM THE FOUR CORNERS
REGION MAY PRODUCE A BRIEF PERIOD OF SNOW OVER THE ROCKIES
INITIALLY, FOLLOWED BY RAINFALL ACROSS PARTS OF THE PLAINS/MS
VALLEY.  LACK OF AGREEMENT ON TIMING/TRACK OF THIS SYSTEM KEEPS
CONFIDENCE LOW FOR INTENSITY AND PRECISE LOCATION OF THIS RAIN. 
LEADING FRONT CROSSING THE NORTHEAST AROUND MIDWEEK SHOULD BE
FAIRLY DRY EXCEPT FOR EXTREME NORTHERN AREAS.  NEXT FRONT ARRIVING
INTO THE NORTHWEST ON TUE WILL BRING A PERIOD OF RAIN/SNOW WITH
BEST FOCUS OVER THE FAVORED TERRAIN OF WESTERN WA/OR AND NORTHERN
ROCKIES.  PRECIP COVERAGE SHOULD EXPAND WITH THE FRONT AS IT
CROSSES THE EAST LATER IN THE WEEK.  ANY SNOW OVER THE EAST SHOULD
BE CONFINED TO FAR NORTHERN AREAS/HIGH ELEVATIONS.  DURING THE
LATTER HALF OF THE PERIOD EXPECT THE UPPER TROUGH NEARING THE WEST
COAST TO BRING AN INCREASE OF MOISTURE TO THE REGION, BUT WITH
LOWER THAN DESIRED CONFIDENCE ON SPECIFICS DUE TO DETAIL
UNCERTAINTIES WITH THE TROUGH ITSELF AS WELL AS
STRENGTH/LOCATION/TIMING QUESTIONS WITH ENERGY THAT MAY BE
LINGERING OFFSHORE CALIFORNIA AHEAD OF THE TROUGH.

THERE WILL BE DECENT COVERAGE OF WELL ABOVE NORMAL TEMPS OVER THE
PLAINS/EAST-CENTRAL U.S. ON TUE WITH HIGHEST ANOMALIES OF PLUS
15-25F POSSIBLE FOR MINS THAT MORNING.  PROGRESSION OF THE INITIAL
GREAT LAKES-PLAINS FRONT WILL LOWER TEMPS TO MODESTLY ABOVE NORMAL
LEVELS OVER MOST OF THE EAST AND THEN THE NEXT FRONT REACHING THE
EAST BY THU-FRI SHOULD BRING READINGS DOWN TO A FEW DEGREES F
EITHER SIDE OF NORMAL.  HOWEVER SOME PLUS 10-15F ANOMALIES WILL
LIKELY PERSIST OVER/NEAR THE NORTHERN PLAINS THROUGH THE PERIOD. 
NEAR TO SLIGHTLY ABOVE NORMAL TEMPS SHOULD TREND A LITTLE BELOW
NORMAL OVER THE WEST COAST STATES BY NEXT SAT AS UPPER TROUGHING
ARRIVES.

RAUSCH/BURKE