Prognostic Meteorological Discussion
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659
FXUS02 KWBC 141600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST THU DEC 14 2017

VALID 12Z SUN DEC 17 2017 - 12Z THU DEC 21 2017

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

THE FIRST HALF OF THE FORECAST PERIOD WILL FEATURE A GENERAL
FLATTENING OF CONUS FLOW WITH THE EJECTION OF INITIAL TROUGHING
BETWEEN STRONG EASTERN PACIFIC AND BAHAMAS/CARIBBEAN RIDGES.  THEN
A NORTHEAST PACIFIC TROUGH WILL AMPLIFY INTO THE WESTERN U.S. IN
RESPONSE TO A STRENGTHENING RIDGE UPSTREAM WHILE SOUTHWESTERLY
FLOW OR FLAT RIDGING SHOULD BECOME ESTABLISHED OVER THE EASTERN
STATES.  GUIDANCE HAS BEEN SOMEWHAT AGREEABLE WITH THE OVERALL
EVOLUTION OF MEAN FLOW BUT WITH A LOT OF SPREAD FOR DETAILS OF THE
EJECTING WEST-CENTRAL U.S. TROUGH AND MID-LATE PERIOD ARRIVAL OF
NORTHEAST PACIFIC TROUGHING.

THE MODELS AND ENSEMBLES ARE STILL IN THE PROCESS OF RESOLVING THE
DETAILS WITHIN THE OVERALL NORTHERN PLAINS INTO NORTHWEST MEXICO
TROUGH AS OF DAY 3 SUN, WITH MEANINGFUL RUN TO RUN DIFFERENCES IN
SOME GUIDANCE.  IN CONTRAST TO 24 HOURS AGO, THE 00Z UKMET/CMC NOW
CLUSTER BETTER WITH THE SLOWER 00Z/06Z GFS VERSUS THE ECMWF WHICH
TO VARYING DEGREES HAS TENDED TO BE ON THE FASTER SIDE OF THE
ENVELOPE FOR ENERGY THAT EJECTS ACROSS THE CENTRAL LATITUDES OF
THE LOWER 48.  CURRENT ENSEMBLE MEANS ALSO SIGNAL GREATER
POTENTIAL FOR THE SLOWER GFS TYPE SCENARIO.  HOW MUCH LINGERING
TROUGHING IS COMPOSED OF FOUR CORNERS/ROCKIES ENERGY VERSUS
NORTHWESTERN MEXICO ENERGY IS STILL VERY UNCERTAIN THOUGH, AND THE
EXTENT TO WHICH INCOMING NORTHEAST PACIFIC FLOW INFLUENCES THE
EJECTING SHORTWAVE REMAINS AN UNRESOLVED UNCERTAINTY.

MEANWHILE IN RECENT DAYS MODELS/ENSEMBLE MEMBERS HAVE BEEN QUITE
VARIED FOR THE TIMING/AMPLITUDE OF TROUGHING THAT HAS BEEN
FORECAST TO MOVE INTO THE NORTHWEST, WITH DIFFERENCES IN HOW MUCH
ENERGY GETS PULLED OFF FROM THE SOUTHWEST PART OF THE TROUGH AND
EVOLUTION OF THE UPSTREAM RIDGE PLAYING A ROLE IN THE BROAD
GUIDANCE SPREAD.  MOST 00Z/06Z GUIDANCE HAS SIGNIFICANTLY IMPROVED
CLUSTERING RELATIVE TO PRIOR RUNS, WITH MORE TROUGHING THAN
YESTERDAY`S SOLUTIONS ON AVERAGE BUT PERHAPS CLOSER TO WHAT HAD
BEEN ADVERTISED 36-48 HOURS AGO.  00Z GEFS ENSEMBLES WERE HOLDING
OUT FOR STRONGER WESTERN RIDGING/SLOWER TROUGH ARRIVAL BUT THE 06Z
GEFS HAS TRENDED TOWARD THE REMAINING CONSENSUS.  NOTE THAT 00Z
ECMWF SHORTWAVE DETAILS WITHIN AND UPSTREAM FROM THE TROUGH BECOME
OUT OF SYNC WITH THE MAJORITY CLUSTER BY DAY 7 THU.
TELECONNECTIONS RELATIVE TO THE RIDGE OVER THE NORTHEAST PACIFIC
AND TROUGH NEAR HAWAI`I  SUPPORT THE IDEA OF A POSITIVELY TILTED
MEAN TROUGH EXTENDING SOUTHWESTWARD FROM NORTH OF HUDSON BAY BUT
SUGGEST THE WESTERN U.S. PART OF THE TROUGH COULD END UP BEING A
LITTLE WEAKER AND/OR EASTWARD OF WHERE INDICATED BY SOME GUIDANCE.

FOR THE UPDATED FORECAST, A BLEND OF 00Z/06Z OPERATIONAL GUIDANCE
(00Z-06Z GFS AND 00Z ECMWF/UKMET/CMC) REPRESENTED THE MOST COMMON
ASPECTS OF LATEST GUIDANCE EARLY IN THE PERIOD.  IN PARTICULAR
THIS BLEND DOWNPLAYED FASTER ECMWF SHORTWAVE PROGRESSION OVER THE
ROCKIES/PLAINS AROUND MON.  TYPICALLY DECREASING
CONFIDENCE/INCREASING SPREAD ADVOCATED FOR INCORPORATING 06Z
GEFS/00Z ECMWF MEANS INTO THE FORECAST ALONG WITH SOME OPERATIONAL
COMPONENT DAY 5 TUE ONWARD, LEADING TO MOSTLY ENSEMBLE MEAN
WEIGHTING/20 PCT GFS INPUT BY DAY 7 THU.


...WEATHER HIGHLIGHTS/THREATS...

THE GULF COAST/SOUTHERN TIER STATES WILL BE AN AREA OF INTEREST
FOR HEAVY RAINFALL POTENTIAL WITH EJECTING WEST-CENTRAL U.S.
SHORTWAVE ENERGY/ASSOCIATED COLD FRONT AS WELL AS A POSSIBLE
LEADING STATIONARY BOUNDARY PROVIDING FOCUS FOR THIS ACTIVITY.
SPECIFICS OF TIMING/COVERAGE ARE STILL UNCERTAIN GIVEN THE
DIFFICULTY THAT MODELS HAVE EXHIBITED SO FAR WITH THE DYNAMICS
ALOFT.  BEST POTENTIAL FOR HIGHEST RAINFALL TOTALS APPEARS TO BE
IN THE SUN-TUE PERIOD.  MEANWHILE THE APPROACH/ARRIVAL OF THE
NORTHEAST PACIFIC UPPER TROUGH WILL LIKELY BRING A PERIOD OF
MEANINGFUL RAIN/HIGHER ELEVATION SHOW TO FAVORED TERRAIN OF THE
PACIFIC NORTHWEST AND NORTHERN ROCKIES.  PRECIP SHOULD BE CONFINED
TO FAR NORTHERN AREAS DURING THE FIRST HALF OF THE PERIOD AND THEN
PUSH SOUTHWARD GOING INTO MIDWEEK WITH SOME MOISTURE POSSIBLY
EXTENDING INTO NORTHERN CA.  TOWARD THE END OF THE PERIOD THE
POTENTIAL FOR THE COMBINATION OF SNOW AND WIND WILL HAVE TO BE
MONITORED OVER PARTS OF THE ROCKIES/PLAINS AS LOW PRESSURE EMERGES
INTO THE CENTRAL U.S..  OVER THE EAST, SNOW SHOULD BE CONFINED TO
FAIRLY NORTHWARD LOCATIONS WITH A COMBINATION OF SYNOPTIC AND LAKE
EFFECT ACTIVITY.

FOR MOST OF THE PERIOD WARMER THAN NORMAL TEMPS WILL PREVAIL OVER
THE MAJORITY OF THE LOWER 48.  THE NORTHERN PLAINS/ROCKIES AND
SOUTHEAST SHOULD BE PARTICULARLY WARM FOR MINS DURING MON-WED AND
MON-TUE RESPECTIVELY.  THE CENTRAL PLAINS WILL LIKELY SEE THE
HIGHEST MAX TEMP ANOMALIES AFTER SUN.  EXCEPTIONS TO THE WARMTH
WILL BE BELOW NORMAL READINGS OVER NEW ENGLAND AND INTERIOR
WEST/ROCKIES ON DAY 3 SUN AND A SURGE OF BELOW NORMAL TEMPS INTO
THE WEST AND NORTHERN ROCKIES/PLAINS DAY 7 THU.

RAUSCH

$$





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