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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1559Z Dec 13, 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 WED DEC 13 2017

VALID 12Z SAT DEC 16 2017 - 12Z WED DEC 20 2017

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

MOST GUIDANCE IS ADHERING TO SIMILAR IDEAS FOR THE LARGE SCALE
PATTERN EVOLUTION DURING THE PERIOD, WITH INCREASED INFLUENCE FROM
NORTH PACIFIC FLOW BUT A TENDENCY TOWARD EASTERN U.S. MEAN
TROUGHING BY NEXT TUE-WED IN RESPONSE TO A BROAD RIDGE NEAR THE
WEST COAST.  NORTHEAST PACIFIC TROUGHING SHOULD MAKE GRADUAL
PROGRESS TOWARD THE NORTHWEST TOWARD MIDWEEK.  IN CONTRAST TO THE
EXPECTED MEAN FLOW THERE ARE STILL SIGNIFICANT DETAIL AND TIMING
UNCERTAINTIES THAT REDUCE CONFIDENCE IN ANY SPECIFIC DETERMINISTIC
FORECAST.

ONE KEY UNCERTAINTY CONTINUES TO INVOLVE THE SHORTWAVE OVER THE
WEST AS OF THE START OF THE PERIOD DAY 3 SAT.  EVEN AT THIS TIME
THERE ARE SIGNIFICANT DIFFERENCES IN WHAT HAPPENS TO ENERGY IN THE
SOUTHWEST PART OF THE OVERALL FEATURE, SUCH AS THE OFFSHORE CLOSED
LOW IN RECENT ECMWF RUNS (WITH EVENTUAL MIGRATION INLAND) OR VERY
SHEARED DEPICTION IN GFS RUNS.  THE UKMET/CMC ARE BETWEEN THE TWO
EXTREMES BUT STILL WITH SUCH A SMALL FEATURE THAT CONFIDENCE IS
MINIMAL.  EVEN WITH THESE DIFFERENCES, THERE ARE POSITIVE TRENDS
TOWARD BETTER CLUSTERING BY DAY 4 SUN AS ENSEMBLE MEANS ARE
FURTHERING THEIR TRENDS TOWARD MORE ENERGY COLLECTING OVER THE
FOUR CORNERS REGION AT THAT TIME WHILE THERE IS DECENT AGREEMENT
FROM THE OPERATIONAL MODELS ALBEIT WITH TYPICAL DEPTH/AMPLITUDE
DIFFERENCES.  THEN MODELS/ENSEMBLES RAPIDLY DIVERGE FOR EJECTION
OF THIS SHORTWAVE WITH THE 00Z ECMWF/UKMET/CMC ALL BECOMING FASTER
THAN 00Z/06Z GFS RUNS.  THIS TIMING DIFFERENCE APPEARS TO BE
CONNECTED TO TIMING/AMPLITUDE OF APPROACHING NORTH PACIFIC FLOW
THAT ULTIMATELY CARVES OUT THE ANTICIPATED EASTERN U.S. TROUGH BY
MIDWEEK, AS THE GFS IS BY FAR THE SLOWEST WITH THE
ARRIVAL/AMPLIFICATION OF THAT UPSTREAM FLOW.  SO WHILE ENSEMBLE
TRENDS/GFS CONTINUITY DO SUPPORT A SLOWER SCENARIO INTO SUN-MON,
IT BECOMES MORE QUESTIONABLE TO WHAT EXTENT THE SLOW SIDE OF THE
SPREAD WILL PREVAIL THEREAFTER.  REGARDING FLOW AHEAD OF THE
INITIAL WESTERN SHORTWAVE, THERE IS NOW MUCH BETTER AGREEMENT ON
AN UPPER TROUGH/LOW EJECTING OUT OF NORTHERN MEXICO.

MEANWHILE GUIDANCE HAS BEEN SHOWING A LOT OF DIFFICULTY IN
RESOLVING THE EXTENT/TIMING OF HEIGHT FALLS REACHING THE NORTHWEST
U.S. LATE IN THE PERIOD.  JUST OVER THE PAST DAY GFS RUNS HAVE
VARIED DRAMATICALLY BETWEEN DEEP TROUGHING AND FLAT RIDGING AS OF
12Z NEXT WED, AND WHILE ECMWF RUNS HAVE TENDED TO ADVOCATE FOR THE
TROUGHING SIDE OF THE SPECTRUM ITS PAST COUPLE RUNS ARE NOT AS
AMPLIFIED AS YESTERDAY'S 12Z RUN.  00Z ENSEMBLE MEMBERS SHOW
CONSIDERABLE SPREAD AS WELL, RANGING BETWEEN RIDGE AND TROUGH
SOLUTIONS BY NEXT WED.  WITH D+8 MEANS OVER THE PAST COUPLE DAYS
FAIRLY CONSISTENT IN SHOWING A CORE OF POSITIVE HEIGHT ANOMALIES
JUST SOUTH OF MAINLAND ALASKA, TELECONNECTIONS SUGGEST THAT
POSITIVELY TILTED MEAN TROUGHING SHOULD DEVELOP OVER THE NORTHWEST
BUT EXACTLY HOW MUCH TROUGHING OCCURS IS FAIRLY SENSITIVE TO THE
EXACT STRENGTH/LOCATION OF THE POSITIVE ANOMALY CENTER.  THUS IT
WILL LIKELY TAKE ADDITIONAL TIME TO ACHIEVE BETTER
AGREEMENT/STABILITY FOR THE PATTERN OVER THE EASTERN
PACIFIC/WESTERN U.S..  RECENT GEFS/ECMWF MEAN TRENDS TO DELAY
HEIGHT FALLS SOMEWHAT LEAVE THE 00Z CMC MEAN AS HAVING THE DEEPEST
TROUGHING AMONG THE MEANS BY MIDWEEK.

BASED ON ABOVE CONSIDERATIONS, THE UPDATED FORECAST DURING THE
FIRST HALF OF THE PERIOD INCORPORATES SOMEWHAT MORE 06Z GFS/GEFS
MEAN GUIDANCE IN TOTAL VERSUS OTHER SOLUTIONS THOUGH IT INCLUDES A
SMALL WEIGHT OF THE 00Z ECMWF/CMC/UKMET TO ACCOUNT FOR THEIR
COMMON THOUGH PERHAPS SLIGHTLY LESS LIKELY IDEAS.  THEN THE BLEND
TRENDS TOWARD A 06Z GEFS/00Z ECMWF MEAN SOLUTION GIVEN OPERATIONAL
SPREAD, WHILE MAINTAINING A SLIGHT TILT TO THE GEFS MEAN.


...WEATHER HIGHLIGHTS/THREATS...

OVER THE PAST DAY MODEL/ENSEMBLE GUIDANCE HAS COME INTO BETTER
RELATIVE AGREEMENT REGARDING THE POTENTIAL FOR AREAS OF LOCALLY
HEAVY RAINFALL FROM THE ARKLATEX REGION EASTWARD ACROSS THE
SOUTHEAST/GULF COAST, IN RESPONSE TO THE SHORTWAVE/UPPER LOW
EJECTING OUT OF MEXICO AND THE SHORTWAVE EXPECTED TO EMERGE FROM
THE FOUR CORNERS REGION AFTER SUN.  TIMING OF SUPPORTING DYNAMICS
IS STILL A QUESTION MARK BUT THERE IS IMPROVING CONFIDENCE FOR THE
OVERALL HEAVY RAIN THREAT.  FARTHER NORTH THERE WILL BE PERIODS OF
PRECIP WITH SNOW IN NORTHERNMOST AREAS.  SOME LAKE EFFECT ACTIVITY
AND/OR LAKE ENHANCEMENT WITH SYNOPTIC SYSTEMS IS POSSIBLE. 
FAVORED TERRAIN FROM THE PACIFIC NORTHWEST SOUTHEAST ACROSS THE
ROCKIES WILL BE ANOTHER AREA OF PRECIP FOCUS.  EXACT TOTALS OF
RAIN/HIGHER ELEVATION SNOW OVER THE NORTH WILL BE HIGHLY DEPENDENT
ON EXTENT OF LATE PERIOD HEIGHT FALLS ALOFT-- A FAIRLY LOW
CONFIDENCE ASPECT OF THE FORECAST AT THIS TIME.  MOST ACTIVITY
OVER THE SOUTHERN HALF OF THE ROCKIES SHOULD BE WITH THE LATE
WEEKEND/EARLY WEEK SHORTWV CROSSING THE REGION.

EXPECT THE MOST PERSISTENT WARMTH DURING SAT-WED TO BE OVER THE
NORTHERN HALF OF THE PLAINS WITH SOME LOCATIONS IN THE NORTHERN
TIER POSSIBLY SEEING MINS AVERAGING 15-20F ABOVE NORMAL FOR THE
FULL 5-DAY PERIOD.  HIGHEST MAX TEMP ANOMALIES MAY END UP BEING
CLOSER TO THE CENTRAL PLAINS.  CALIFORNIA WILL TEND TO SEE ABOVE
NORMAL TEMPS WITH BROADER COVERAGE OF POSITIVE ANOMALIES THERE AND
OVER THE GREAT BASIN NEXT WEEK, AFTER THE GREAT BASIN AND FOUR
CORNERS SEE A BRIEFLY COOLER PERIOD WITH PASSAGE OF THE SHORTWAVE
ALOFT.  THE EASTERN STATES WILL SEE SOME BELOW NORMAL READINGS
DURING THE WEEKEND BEFORE A TREND TO NEAR OR ABOVE NORMAL LEVELS.

RAUSCH