Preliminary Forecasts
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000
FXUS02 KWNH 130654
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
153 AM EST WED DEC 13 2017

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

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

THE SYNOPTIC-SCALE PATTERN WILL FEATURE A SPLIT FLOW REGIME WITH A
NUMBER OF MODEL DIFFERENCES WHICH MAKE THIS A CHALLENGING AND LOW
CONFIDENCE FORECAST AHEAD. TO START WITH...A PRONOUNCED UPPER
TROUGH WILL BE IN THE PROCESS OF EXITING NEW ENGLAND ON 16/1200Z
WHILE RENEWED HEIGHT FALLS LURK UPSTREAM IN THE VICINITY OF THE
NORTHERN/CENTRAL ROCKIES. DETACHED FROM THE PREVAILING WESTERLIES
IS AN ELONGATED UPPER LOW INITIALLY POSITIONED OVER CENTRAL MX ON
SATURDAY MORNING. THIS LATTER FEATURE IS EXPECTED TO QUICKLY
BECOME ABSORBED BY THE MID-LATITUDE TROUGH PUSHING THROUGH THE
CENTRAL U.S. BY EARLY SUNDAY. AS WILL BE MENTIONED IN THE NEXT
SECTION...MODEL UNCERTAINTY PLAGUES THE EASTWARD PROGRESSION OF
THIS LONGWAVE TROUGH WITH THE POTENTIAL FOR SOME SHORTWAVE ENERGY
TO PEEL OFF AND SEPARATE OVER THE FOUR CORNERS REGION. THE SYSTEM
SHOULD EVENTUALLY REACH THE EAST COAST BY ANYWHERE BETWEEN DAY
5-7...DECEMBER 18-20...DEPENDING ON THE SELECTED MODEL. A MYRIAD
OF SURFACE WAVES ARE NOTED ON OR IN THE VICINITY OF THE
PROPAGATING BOUNDARY WITH PERHAPS CYCLONES OVER THE GREAT
LAKES/LOWER MS VALLEY/RED RIVER ON SUNDAY MORNING.
WEAKENING/SHEARING OF THE MID-LEVEL THROUGH WILL WEAKEN THE
SURFACE REFLECTION LATER INTO THE PERIOD. ALL THE
WHILE...AMPLIFIED FLOW WITHIN THE NORTHERN STREAM WILL TAKE SHAPE
OVER THE NORTHERN PLAINS/UPPER MIDWEST BY MONDAY ACCOMPANIED BY A
DEEP SURFACE LOW OVER CENTRAL CANADA. BACK WEST...IN THE WAKE OF
THE SYSTEM TRACKING TOWARD THE SOUTHERN HIGH PLAINS...A RETURN OF
UPPER RIDGING IS EXPECTED THROUGH DAY 6/TUESDAY ALTHOUGH THERE ARE
SOME HINTS THAT A CUT-OFF LOW MAY AFFECT SOUTHERN CA. EVENTUALLY A
PATTERN CHANGE WILL BE UNDERWAY AS A POWERFUL POSITIVELY-TILTED
TROUGH SWINGS SOUTHWARD FROM BRITISH COLUMBIA TOWARD THE PACIFIC
NORTHWEST ON DAY 7/DECEMBER 20.

AS DISCUSSED PREVIOUSLY...A LOW CONFIDENCE FORECAST AT BEST WITH
ENSEMBLE SPAGHETTI PLOTS FROM DAY 4 LOOKING CLOSER TO WHAT ONE
WOULD EXPECT ON DAY 6/7. WHILE THE UPPER TROUGH EXITING NEW
ENGLAND APPEARS TO BE WELL AGREED UPON...THE OPPOSITE CAN BE SAID
FOR THE EVOLVING SYSTEM PUSHING TOWARD THE HIGH PLAINS ON
SATURDAY. INITIALLY...AN UPPER LOW OVER CENTRAL MX IS A NEW
FEATURE ON THE MAP AS PREVIOUS DAYS HAD IT WELL WEST BACK TOWARD
BAJA CA. WHILE EARLY PLACEMENT IS VARIABLE...ALL OF THE GUIDANCE
AGREES ON LIFTING THIS CLOSED LOW NORTH-NORTHEASTWARD WITH GRADUAL
ABSORPTION INTO THE APPROACHING LONGWAVE TROUGH. REGARDING THIS
LATTER FEATURE...THE MOST STABLE SOLUTION ON A RUN-TO-RUN BASIS
HAS BEEN THE GFS. IT HAS STUCK WITH ITS SLOWER FORECAST ALL ALONG
AS IT ALLOWS FOR A LOBE OF VORTICITY TO PEEL OFF AND DRIFT
EASTWARD FROM THE FOUR CORNERS REGION TO THE OZARKS FROM SUNDAY
THROUGH TUESDAY. SUCH A SCENARIO HAS SOME NEIGHBORS...MOST NOTABLY
THE 12Z CMC/UKMET. MEANWHILE...THE ECMWF SYSTEM HAS BEEN THE
FASTEST GLOBAL MODEL ALL ALONG ALTHOUGH IT HAS MADE A DRAMATIC
WESTWARD SHIFT DURING THE 12Z/00Z RUNS. UNFORTUNATELY...IT IS
STILL THE QUICKEST PIECE OF GUIDANCE WHILE ALSO BEING THE MOST
VIGOROUS IN TERMS OF POSITIVE VORTICITY ADVECTION OVER THE
SOUTHEASTERN U.S. SOMEWHAT UNNERVING...THE 00Z CMC/UKMET HAVE
TRENDED QUICKER BUT STILL REMAIN WEST OF THE MENTIONED ECMWF
SOLUTIONS. FROM WHAT HAS BEEN SEEN OF THE 00Z ECMWF...IT HAS MOVED
WESTWARD WHICH IS A GOOD SIGN AS THE EARLIER RUNS WERE FOR THE
MOST PART REJECTED AS OUTLIERS. BUT IT STILL IS THE QUICKEST 00Z
OPERATIONAL MODEL AT THIS TIME.

CONSIDERING THE REMAINDER OF THE MAP...AMPLIFIED FLOW WITHIN THE
NORTHERN STREAM WILL BEGIN TO AFFECT THE NORTHERN PLAINS/UPPER
MIDWEST BY AS EARLY AS MONDAY. BASED ON THE ENSEMBLE SPAGHETTI
PLOTS...TEMPORAL DIFFERENCES ARE ALSO EVIDENT HERE WITH THE 12Z
ECMWF ENSEMBLE SYSTEM WELL AHEAD OF THE OTHER GUIDANCE. OVER THE
WEST COAST...THE PAST THREE RUNS OF THE ECMWF ARE THE ONLY MODELS
TO SHOW A CUT-OFF UPPER LOW MOVING TOWARD SOUTHERN CA ON DAY
6/7...DECEMBER 19/20. BACKTRACKING SUGGESTS THIS SYSTEM ACTUALLY
PEELS OFF FROM THE INITIAL HEIGHT FALLS MOVING THROUGH THE WESTERN
U.S. ON SATURDAY. GIVEN NO OTHER MODEL SHOWS THIS EVOLUTION...WILL
CONTINUE TO DISMISS IT AS AN OUTLIER. LOOKING NORTHWARD...A
POSITIVELY-TILTED TROUGH IS FORECAST TO DIP DOWN FROM BRITISH
COLUMBIA INTO THE PACIFIC NORTHWEST LATE IN THE PERIOD. RELATIVE
TO THE REST OF THE PERIOD...IT CONSISTS OF THE LOWEST MODEL SPREAD
DESPITE BEING A DAY 7 FORECAST. RUN-TO-RUN CONTINUITY IS A LITTLE
SKETCHY THOUGH AS THE 00Z GFS SLOWED BY LEAST 12 HOURS WHICH WOULD
KEEP HEIGHT FALLS OUT OF WESTERN WA ON DECEMBER 20.

GIVEN THE ROBUST AMOUNT OF MODEL SPREAD...IT WAS NECESSARY TO LEAN
MORE HEAVILY ON THE ENSEMBLE MEANS THAN USUAL. THROUGH DAY
4/SUNDAY...TOOK A MULTI-MODEL BLEND APPROACH WITH THE 12Z GFS/GEFS
MEAN/UKMET BEING 80 PERCENT OF THE BLEND WITH ONLY 20 PERCENT OF
THE 12Z ECMWF/ECMWF ENSEMBLE MEAN BEING UTILIZED.
THEREAFTER...QUICKLY REMOVED THE 12Z ECMWF WHILE LEANING MUCH MORE
HEAVILY ON THE 12Z GFS/GEFS MEAN/NAEFS ENSEMBLE MEAN. DID KEEP 20
PERCENT OF THE 12Z ECMWF ENSEMBLE MEAN IN THE MIX DURING THE DAY
5-7...DECEMBER 18-20 PERIOD TO ACCOUNT FOR GENERAL MODEL
UNCERTAINTY.


...WEATHER HIGHLIGHTS/THREATS...

GENERALLY SPEAKING...TEMPERATURES SHOULD BE AT OR ABOVE
CLIMATOLOGY THROUGH THE FORECAST PERIOD. EXCEPTIONS INCLUDE THE
EAST COAST ON SATURDAY IN THE WAKE OF A STRONG FRONTAL PASSAGE
EXITING INTO THE WESTERN ATLANTIC. THIS EXTENDS DOWN ALONG THE
GULF COAST AS THE BOUNDARY SWEEPS SOUTHWARD THROUGH CUBA ON
16/1200Z. DEPARTURES FROM NORMAL WILL LIKELY ONLY BE IN THE 5 TO
10 DEGREE RANGE. BY SUNDAY...COOLER WEATHER IS EXPECTED UNDERNEATH
THE EVOLVING UPPER TROUGH ACROSS THE FOUR CORNERS/SOUTHERN HIGH
PLAINS. OTHERWISE...THE ENTIRE COUNTRY CAN EXPECT MILD CONDITIONS
FOR THE THIRD WEEK OF DECEMBER GIVEN THE DOMINANCE OF PACIFIC AIR
MASSES. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES WILL ALLOW FOR
HIGHS TO BE ANYWHERE FROM 10 TO 15 DEGREES ABOVE NORMAL OVER THE
CENTRAL U.S. BY THE MIDDLE OF NEXT WEEK...HIGH TEMPERATURES MAY
REACH THE MID 50S ALL THE WAY UP TO THE NEB PANHANDLE.

WITH A LACK OF ARCTIC AIR TO WORK WITH...ANY WINTRY PRECIPITATION
WILL LIKELY BE CONFINED TO HIGHER ELEVATIONS OVER THE PACIFIC
NORTHWEST/UPPER INTERMOUNTAIN WEST/ROCKIES AS WELL AS ACROSS THE
GREAT LAKES AND INTERIOR NEW ENGLAND. CONSIDERING PRECIPITATION IN
GENERAL...A STREAK OF LIGHT/MODERATE RAIN/SNOW IS LIKELY OVER THE
CENTRAL/NORTHERN ROCKIES AS THE UPPER TROUGH TRACKS SOUTHEASTWARD
ON SATURDAY. INCREASING WARM ADVECTION AHEAD OF THIS SYSTEM WILL
LEAD TO ENHANCED HEAVY RAINFALL CHANCES FROM THE ARKLATEX EASTWARD
TO THE SOUTHEASTERN U.S. COAST. THERE ARE MANY MODELS SHOWING 2 TO
3 INCH 24-HOUR AMOUNTS DURING PART OF THE PERIOD. TIMING IS OF
COURSE AN ISSUE GIVEN THE PREPONDERANCE OF SOLUTIONS IN PLAY. BACK
TO THE PACIFIC NORTHWEST...AN ATMOSPHERIC RIVER SIGNATURE SHOWS UP
IN THE GUIDANCE BY THE EARLY/MIDDLE PART OF NEXT WEEK.
HOWEVER...THERE ARE CHANCES THIS SETS UP NORTH OF THE
INTERNATIONAL BORDER WITH CANADA AS SHOWN IN THE 00Z GFS.


RUBIN-OSTER

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