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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Dec 15, 2017)
 
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST FRI DEC 15 2017

VALID 12Z MON DEC 18 2017 - 12Z FRI DEC 22 2017

...OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT...

DURING THE FIRST HALF OF NEXT WEEK NORTHERN STREAM FLOW ACROSS
CANADA AND NORTHERN TIER U.S. WILL PUSH ALONG FAIRLY STRONG
CANADIAN LOW PRESSURE AND TRAILING FRONT CROSSING THE
CENTRAL/EASTERN STATES, WHILE A SOUTHERN ROCKIES/EXTREME NORTHWEST
MEXICO SHORTWAVE WILL ACCELERATE EASTWARD.  A NORTHEAST PACIFIC
TROUGH WILL APPROACH THE NORTHWESTERN U.S. TUE AND THEN
PROGRESS/AMPLIFY TO THE SOUTH AND EAST THEREAFTER IN RESPONSE TO A
BUILDING/SHARPENING UPSTREAM RIDGE.  THIS EVOLUTION WILL YIELD A
BROAD POSITIVELY TILTED TROUGH EXTENDING FROM NEAR HUDSON BAY INTO
NORTHWEST MEXICO BY THE END OF NEXT WEEK.  BAHAMAS/CARIBBEAN
RIDGING WILL PERSIST THROUGH THE PERIOD WITH SOME FLUCTUATIONS IN
STRENGTH.

TYPICAL MODEL/ENSEMBLE BEHAVIOR SUGGESTS FURTHER ADJUSTMENTS MAY
BE IN STORE BUT AT LEAST FOR THE TIME BEING THE CONSIDERABLE
GUIDANCE SPREAD AND RUN TO RUN VARIABILITY FOR SIGNIFICANT
FEATURES OVER RECENT DAYS HAS SETTLED DOWN SOMEWHAT SO CONFIDENCE
IN THE OVERALL FORECAST IS GRADUALLY IMPROVING.

THE RECENT SLOWER TREND IN THE ECMWF HAS NARROWED THE SOLUTION
ENVELOPE A BIT FOR THE EJECTING SOUTHERN ROCKIES SHORTWAVE.  THE
00Z ECMWF AND SOME GEFS/ECMWF ENSEMBLE MEMBERS SHOW POTENTIAL FOR
THE FEATURE TO EJECT IN TWO PIECES, ONE CLOSE TO THE MAJORITY
GFS/UKMET/CMC CLUSTER AND A SECOND THAT TRAILS BY ABOUT 12-18
HOURS.  NO MODEL SYSTEM HAS BEEN IDEAL BUT THE GFS HAS GENERALLY
HELD ONTO THE MOST STABLE EVOLUTION OF THIS FEATURE IN RECENT DAYS.

INTO DAY 6 THU THE LATEST MODEL/ENSEMBLE MEAN RUNS AGREE WELL IN
PRINCIPLE FOR THE UPPER TROUGH AMPLIFYING INTO THE WEST.  OVER THE
PAST COUPLE DAYS SOME GEFS MEMBERS HAVE BEEN MORE HESITANT TO
BRING IN LEADING HEIGHT FALLS COMPARED TO MOST OTHER GUIDANCE SO
PREFERENCE WOULD BE TO UNDERWEIGHT THE GEFS MEAN TO SOME EXTENT. 
ENSEMBLE MEANS END UP CONVERGING SOMEWHAT TOWARD THE END OF THE
PERIOD THOUGH.  BY DAY 7 FRI ALL THREE OF THE MAJOR ENSEMBLE
SYSTEMS SHOW THE GREATEST SPREAD ALOFT OVER THE WEST
COAST/SOUTHWEST/GREAT BASIN AND OVER/NEAR THE GREAT LAKES.  NOT
SURPRISINGLY THE OPERATIONAL GUIDANCE SHOWS MORE DIVERGENCE WITH
DETAILS ALOFT BY NEXT FRI AS WELL.  THE WESTERN AREA OF SPREAD
MAKES SENSE AS TELECONNECTION-FAVORED FLOW DIFFERS DEPENDING ON
THE RELATIVE DOMINANCE OF THE CORE OF POSITIVE HEIGHT ANOMALIES
OFF THE ALASKA PANHANDLE AND NEGATIVE HEIGHT ANOMALIES JUST NORTH
OF HAWAI'I, WITH THE FORMER SUPPORTING MORE SOUTHWEST U.S.
TROUGHING THAN THE LATTER.  AT THE SAME TIME THE
STRENGTH/EVOLUTION OF WHAT APPEARS TO BE A FAIRLY LIKELY UPPER MS
VALLEY/GREAT LAKES WAVE BY THU-FRI IS FAIRLY UNCERTAIN GIVEN THE
SOLUTION SPREAD FOR DETAILS ALOFT, WITH OPERATIONAL RUNS DIFFERING
BY 10-20MB.  RECENT ECMWF RUNS HAVE TENDED TO BE ON THE STRONGER
SIDE OF THE ENVELOPE WHILE THE GFS HAS VARIED BETWEEN THE STRONGER
(00Z RUN) AND WEAKER (06Z RUN) AND CMC RUNS HAVE BEEN INCONSISTENT
AS WELL.  FOR BOTH OF THESE AREAS OF UNCERTAINTY THE DETERMINISTIC
MANUAL FORECAST REPRESENTS A FAIRLY CONSERVATIVE MODEL/ENSEMBLE
BLENDED APPROACH WHILE AWAITING IMPROVED CLUSTERING/STABILITY.

THE EARLY-MID PERIOD BLEND INCORPORATED OPERATIONAL
GFS/ECMWF/UKMET/CMC GUIDANCE TO REPRESENT MAJORITY CLUSTER
CONCEPTS FOR DETAIL.  THEN THE FORECAST TRANSITIONED TO 35-60 PCT
ENSEMBLE MEAN GUIDANCE FOR DAYS 6-7 THU-FRI RESPECTIVELY AS
GUIDANCE SPREAD INCREASED.


...WEATHER HIGHLIGHTS/THREATS...

ASIDE FROM TYPICAL DETAIL ADJUSTMENTS GUIDANCE HAS BEEN FAIRLY
STABLE IN SHOWING THE POTENTIAL FOR A PERIOD OF LOCALLY HEAVY
RAINFALL OVER AND INLAND FROM THE GULF COAST REGION, SUPPORTED BY
THE EJECTING SOUTHERN ROCKIES SHORTWAVE/ASSOCIATED FRONT AS WELL
AS A LEADING STATIONARY FRONT.  CURRENTLY THE MOST LIKELY AXIS OF
HIGHEST RAINFALL EXTENDS FROM EAST/SOUTHEAST TX INTO GA.  THE
MAJORITY OF THIS EVENT SHOULD TAKE PLACE DURING THE FIRST HALF OF
THE WEEK.  OVER THE WEST, EXTREME NORTHERN AREAS WILL SEE SOME
EARLY WEEK RAIN/HIGHER ELEVATION SNOW ACCOMPANYING A STATIONARY
FRONT.  THEN THE APPROACH/ARRIVAL OF THE NORTHEAST PACIFIC
TROUGH/SURFACE SYSTEM WILL BRING A PERIOD OF ENHANCED MOISTURE
FOCUS INTO THE PACIFIC NORTHWEST/NORTHERN ROCKIES, FOLLOWED BY A
DRYING TREND BY THU-FRI.  AS ASSOCIATED LOW PRESSURE TRACKS INTO
THE PLAINS AND GREAT LAKES/NORTHEAST DURING THE LATTER HALF OF THE
WEEK, EXPECT SNOW AND BREEZY TO WINDY CONDITIONS TO BE POTENTIAL
THREATS IN THE COLD SECTOR.  SNOWFALL OVER THE UPPER MS VALLEY AND
UPPER GREAT LAKES WILL BE PARTICULARLY SENSITIVE TO THE EXACT
STRENGTH OF THE SURFACE LOW.  EARLIER IN THE WEEK A FAST MOVING
FRONT CROSSING THE NORTHERN TIER WILL BRING A PERIOD OF BREEZY
WEATHER AND SOME LAKE EFFECT SNOW IN ITS WAKE.

LOCATIONS FROM THE NORTHERN PLAINS TO SOUTHEAST SHOULD SEE THE
HIGHEST TEMPERATURE ANOMALIES IN THE EARLY-MID WEEK PERIOD.  THOSE
TWO AREAS HAVE THE BEST POTENTIAL TO SEE SOME MIN TEMPS AT LEAST
15-20F ABOVE NORMAL WHILE SUCH ANOMALIES FOR MAX TEMPS MAY FOCUS
CLOSER TO THE CENTRAL PLAINS/MS VALLEY ESPECIALLY AROUND MIDWEEK. 
SYSTEM PUSHING OUT OF THE WEST WILL BRING A MUCH COLDER TREND TO
THE INTERIOR WEST THEN EXTENDING INTO THE PLAINS/MS VALLEY
MID-LATE WEEK, WITH SOME HIGHS 10-20F BELOW NORMAL.

RAUSCH