Preliminary Forecasts
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000
FXUS02 KWNH 110643
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
142 AM EST MON DEC 11 2017

VALID 12Z THU DEC 14 2017 - 12Z MON DEC 18 2017

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

THERE WILL BE A SIGNIFICANT SHIFT IN THE SYNOPTIC-SCALE PATTERN
DURING THE PERIOD WITH A MOVEMENT FROM HIGHLY AMPLIFIED TO MORE
PROGRESSIVE WITH LOWER AMPLITUDE TRANSIENT FEATURES.
INITIALLY...THERE WILL STILL BE ROBUST 500-MB HEIGHT
ANOMALIES...GENERALLY IN THE 2 TO 2.5 STANDARD DEVIATION RANGE.
THIS CONSISTS OF A BROAD LONGWAVE TROUGH EXTENDING OVER MUCH OF
NORTHEASTERN NORTH AMERICA ALTHOUGH THE LOWEST HEIGHTS WILL BE
CENTERED WITHIN A POWERFUL UPPER LOW OVER NORTHERN QUEBEC ON
14/1200Z. ONE PRIMARY SHORTWAVE REMAINS IN THE WAVE TRAIN WITH
SOME GAIN OF AMPLITUDE ON APPROACH TOWARD THE EAST COAST.
HOWEVER...MANY SOLUTIONS DO NOT SHOW A WHOLE LOT OF SURFACE
DEEPENING OFFSHORE AS THE WAVE EXITS THE EASTERN SEABOARD. THERE
IS SOME HINT THAT A LOBE OF SHORTWAVE ENERGY WILL SPLIT OFF AND
PUSH THROUGH THE SOUTHERN U.S. ON FRIDAY/SATURDAY ALTHOUGH THIS
SCENARIO SEEMS TO BE A LOW PROBABILITY OUTLIER. LOOKING BACK TO
THE WEST COAST...A STAGNANT UPPER RIDGE IS FORECAST TO FINALLY
BUCKLE BY FRIDAY AS UPSTREAM HEIGHT FALLS SWING INTO THE PACIFIC
NORTHWEST BY EARLY FRIDAY. THIS SYSTEM IN PARTICULAR APPEARS TO BE
THE ONE WHICH FINALLY SHIFTS THE PATTERN TOWARD MORE PROGRESSIVE
AND OF SOMEWHAT LOWER AMPLITUDE WAVES. THE SCENARIO OF SOME OF
THIS ENERGY CUTTING OFF OVER THE DESERT SOUTHWEST APPEARS TO BE
DISAPPEARING RELATIVE TO THE GUIDANCE YESTERDAY. GENERALLY
SPEAKING...THIS SYSTEM SHOULD QUICKLY TRANSLATE EASTWARD WITH SOME
POTENTIAL SURFACE DEEPENING OVER THE NORTHERN TIER STATES IF ANY
NORTHERN INTERACTION TAKES PLACE. WHILE THIS PROGRESSIVE SHORTWAVE
MOVES TOWARD THE EASTERN SEABOARD BY DAY 7/DECEMBER
18...ADDITIONAL AMPLIFICATION IS LIKELY OVER THE NORTH-CENTRAL
U.S. WHILE QUASI-ZONAL FLOW IS ESTABLISHED BACK TOWARD THE EASTERN
PACIFIC.

REGARDING THE SHORTWAVE MOVING THROUGH THE OH VALLEY LATE
THURSDAY/EARLY FRIDAY...MODELS ARE STILL AT ODDS WITH ONE ANOTHER
REGARDING SURFACE EVOLUTION. ENSEMBLE SCATTER LOW PLOTS SHOW A
VAST MAJORITY OF THE 12Z ECMWF ENSEMBLES FAVORING A MORE SOUTHERN
WAVE ACROSS THE OH VALLEY ON 14/1200Z. MEANWHILE...THE LAST
SEVERAL RUNS OF THE GFS ARE MORE CONSISTENT WITH A NORTHERN LOW
PUSHING THROUGH THE LOWER GREAT LAKES AT THE SAME TIME. THEN THERE
ARE MODELS LIKE THE 00Z CMC WHICH FAVOR THE EXISTENCE OF BOTH OF
THESE SOLUTIONS. MEANWHILE...SOME SPREAD EXISTS ON THE SOUTHERN
EXTENT OF THE EJECTING LONGWAVE TROUGH AS THE 12Z ECMWF/UKMET
ATTEMPT TO SEPARATE SOME OF THIS ENERGY. THIS PARTICULAR SCENARIO
WOULD ALLOW FOR A GULF COAST WAVE TO FORM SOUTH OF THE MS/AL COAST
ON FRIDAY MORNING ALTHOUGH ITS SOLUTION IS A DISTINCT MINORITY
AMONG THE 90 AVAILABLE ENSEMBLE MEMBERS. FURTHER...THE 00Z UKMET
BACKED OFF ON THIS AS WELL WHICH WAS EXPECTED GIVEN THE DIVERGENCE
FROM THE ENSEMBLE SPAGHETTI CLUSTERING. SHIFTING THE FOCUS
WEST...GENERAL AGREEMENT EXISTS IN THE EVENTUAL EROSION OF THE
MEAN UPPER RIDGE ACROSS THE WESTERN U.S. BY SOMETIME FRIDAY.
MULTI-DAY ENSEMBLE SPAGHETTI PLOT COMPARISONS SHOW A SHIFT AWAY
FROM PINCHING OFF A CLOSED LOW OVER THE SOUTHWESTERN STATES BY
EARLY SATURDAY. THE 12Z CMC STILL SUGGESTED ITS EXISTENCE BUT HAS
SINCE BACKED OFF VIA THE 00Z RUN. THE BIGGEST ISSUE NOW IS TIMING
WITH THE 12Z ECMWF AS WELL AS PREVIOUS RUNS BEING ON THE QUICKER
SIDE OF THE ENVELOPE. THERE IS THE POTENTIAL FOR A FAIRLY
IMPRESSIVE SURFACE CYCLONE ACROSS THE GREAT LAKES BY SUNDAY
MORNING BUT MODELS ARE INCONSISTENT WITH INTENSITY AND PLACEMENT.
FOR INSTANCE...THE 12Z GFS TOOK THE SYSTEM DOWN TO A 983-MB
PRESSURE ON 17/1200Z WHILE THE RUNS AFTERWARD WERE 1003/1002-MB.
AS THIS SYSTEM ACCELERATES EASTWARD...ANOTHER DEEP CYCLONE MAY
EMERGE OVER THE NORTH-CENTRAL BY DAY 7/DECEMBER 18 BUT DETAILS ARE
AGAIN FUZZY AT BEST.

INITIALLY WENT WITH AROUND 60 TO 80 PERCENT OF 12Z GFS/ECMWF/CMC
FOR DAYS 3/4...THURSDAY/FRIDAY TO ACCOUNT FOR THE SYSTEM MOVING
TOWARD THE EAST COAST. GENERALLY STAYED AWAY FROM ENSEMBLE MEANS
AS THEY AVERAGED THE TWO LOWS MOVING THROUGH THE OH VALLEY/GREAT
LAKES ON THURSDAY. DID QUICKLY REDUCE CONTRIBUTIONS OF THE 12Z
ECMWF ON DAYS 4/5...FRIDAY/SATURDAY TO AROUND 10 TO 15 PERCENT
GIVEN THE OUTLYING SOLUTION OVER THE SOUTHEASTERN U.S. BEYOND THIS
PERIOD...PUSHED TOWARD A FULL ENSEMBLE MEAN APPROACH GIVEN
UNCERTAINTIES WITH THE TIMING OF THE PROGRESSIVE TROUGH AS WELL AS
THE POTENTIAL FOR DEEP SURFACE LOWS ALONG THE NORTHERN TIER. GAVE
A SLIGHT EDGE TO THE 12Z GEFS MEAN RELATIVE TO THE 12Z ECMWF
ENSEMBLE MEAN DURING THE END OF THE PERIOD.


...WEATHER HIGHLIGHTS/THREATS...

THERE WILL ONLY BE A BRIEF PERIOD OF BELOW AVERAGE TEMPERATURES
AND THIS APPEARS TO BE CONFINED TO LOCATIONS EAST OF THE MS RIVER
ON THURSDAY/FRIDAY. THE FINAL ARCTIC SURGE SHOULD BRING A
REINFORCEMENT OF COLD AIR TO THE REGION WITH THE MOST ANOMALOUS
TEMPERATURES BEING OVER NEW ENGLAND WITH ANOMALIES IN THE 10 TO 20
DEGREE BELOW CLIMATOLOGY RANGE. THIS SUGGESTS HIGHS ON THURSDAY
SHOULD STRUGGLE TO GET OUT OF THE UPPER SINGLE DIGITS AND TEENS
OVER INTERIOR NEW ENGLAND. OTHERWISE...MUCH OF THE COUNTRY SHOULD
SEE ABOVE AVERAGE TEMPERATURES AS THE FLOW FLATTENS AND BECOMES
MORE PACIFIC DOMINANT. DOWNSLOPING FLOW IN THE LEE OF THE ROCKIES
WILL RAISE TEMPERATURES INTO THE 50S AS FAR NORTH AS EASTERN MT ON
FRIDAY. THIS IS EASILY 15 TO 20 DEGREES ABOVE AVERAGE.

LOOKING AT PRECIPITATION DURING THE PERIOD...SOME LAKE EFFECT
ACTIVITY IS POSSIBLE ALTHOUGH IT SHOULD BE MORE TAME GIVEN A
CHANGING SYNOPTIC-SCALE PATTERN. WHILE PRECIPITATION CHANCES WILL
LOOM THROUGHOUT THE PERIOD...IT SHOULD BE WARM ADVECTION DRIVEN
VERSUS MESOSCALE BANDING RELATED. OUT WEST...CONDITIONS WILL BE
MUCH MORE ACTIVE THAN THEY HAVE BEEN THIS MONTH GIVEN THE
FLATTENING OF THE RIDGE. WIDESPREAD RAIN/SNOW SHOULD BLANKET THE
PACIFIC NORTHWEST AND LOCATIONS DOWNSTREAM INTO THE CENTRAL GREAT
BASIN/UPPER INTERMOUNTAIN WEST/NORTHERN ROCKIES. GIVEN A LACK OF
COLD AIR...SNOWFALL SHOULD BE RELEGATED TO THE HIGHER ELEVATIONS.
THIS THREAT COULD CONTINUE THROUGH DAY 7/DECEMBER 18 GIVEN THE
SEMI-ZONAL FLOW IN PLACE. ELSEWHERE...WARM ADVECTION SHOWERS ARE
POSSIBLE FROM THE WESTERN GULF COAST TOWARD THE TN
VALLEY/SOUTHEASTERN U.S. THIS WEEKEND ALTHOUGH MODEL SPREAD IS
HIGH WITH THE THE 00Z GFS BEING WET WHILE THE 12Z ECMWF IS
COMPLETELY DRY.


RUBIN-OSTER

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