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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0654Z Dec 09, 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
154 AM EST SAT DEC 09 2017

VALID 12Z TUE DEC 12 2017 - 12Z SAT DEC 16 2017

...A MIDWEST/GREAT LAKES THROUGH NORTHEASTERN U.S. HEAVY SNOW
THREAT...


...PATTERN OVERVIEW/PREDICTABILITY ASSESSMENT/PREFERENCES...

WHAT APPEARS TO BE A SEMI-PERMANENT FEATURE TO START THE MONTH OF
DECEMBER...A BROAD LONGWAVE TROUGH CONTINUES TO ENCOMPASS REGIONS
EAST OF THE GREAT PLAINS. A SERIES OF EMBEDDED PERTURBATIONS IN
THE FLOW WILL DRIVE WAVES OF LOW PRESSURE WHICH SHOULD BE CONDUITS
TO WINTRY PRECIPITATION ANYWHERE FROM THE UPPER MIDWEST THROUGH
THE GREAT LAKES INTO THE NORTHEASTERN U.S. ONE SUCH CYCLONE IS
FORECAST TO BE IN THE PROCESS OF LIFTING AWAY FROM THE UPPER
MID-ATLANTIC TOWARD NEW ENGLAND ON TUESDAY. THE NEXT REINFORCING
SHOT OF ARCTIC AIR WILL BRING ADDITIONAL LOW PRESSURE THROUGH THE
GREAT LAKES/OH VALLEY ON THURSDAY MORNING. TOWARD THE END OF THE
PERIOD...DECEMBER 15/16...WHILE CYCLONIC FLOW REMAINS OVER THE
EASTERN HALF OF THE NATION...THERE ARE HINTS AT THE FLOW RELAXING
A BIT BASED ON SOME OF THE ENSEMBLE SPAGHETTI PLOTS. LOOKING FROM
THE ROCKIES WESTWARD...AN UPPER RIDGE WILL REMAIN A FIXTURE IN THE
FORECAST FOR MUCH OF THE PERIOD. 00Z GFS 500-MB HEIGHT ANOMALIES
ARE AROUND 2 TO 2.5 STANDARD DEVIATIONS ABOVE CLIMATOLOGY ALTHOUGH
A PATTERN CHANGE IS SUGGESTED BY SOME OF THE GUIDANCE BY NEXT
WEEKEND. BEFORE THIS OCCURS...A SHORTWAVE TRACKING ATOP THIS RIDGE
WILL DIVE SOUTHEASTWARD THROUGH THE CENTRAL HIGH PLAINS WHILE
EVENTUALLY JOINING THE PARENT DOWNSTREAM TROUGH. MID/UPPER-LEVEL
HEIGHTS REBOUND QUICKLY ACROSS THE WESTERN STATES WITH THE
EVENTUAL POTENTIAL FOR FEATURES OVER THE EASTERN PACIFIC TO BRING
MORE SUBSTANTIAL HEIGHT FALLS INTO THE PACIFIC NORTHWEST BY DAY
6/7...DECEMBER 15/16. LOOKING DOWN INTO THE SUBTROPICAL
PACIFIC...A PERSISTENT UPPER LOW IS FORECAST TO DRIFT FROM
NORTHWESTERN MX BACK INTO THE EASTERN PACIFIC AS IT BECOMES
FURTHER CUT OFF FROM THE MID-LATITUDE FLOW.

REGARDING THE STRONG SURFACE CYCLONE MOVING FROM THE LOWER GREAT
LAKES TOWARD NOVA SCOTIA ON TUESDAY/WEDNESDAY...ENSEMBLE LOW PLOTS
HAVE SHOWN MUCH STRONGER CLUSTERING IN THE 12Z MODEL CYCLE. 00Z
MODELS HAVE REMAINED FAIRLY CLOSE TO THIS CONSENSUS SOLUTION WITH
SURFACE PRESSURES IN THE 990-995 MB RANGE ON 12/1200Z. SOME
GRADUAL INTENSIFICATION IS EXPECTED DURING THE FOLLOWING 24 HOURS
WITH ADDITIONAL WAVE DEVELOPMENT OFF THE NEW ENGLAND COAST. STRONG
COLD ADVECTION IN THE WAKE WILL BRING A SIGNIFICANT SHOT OF COLD
AIR TO MUCH OF THE NORTHEASTERN QUARTER OF THE COUNTRY. BY
MID-WEEK...1000-500 MB THICKNESSES DROP TO AROUND 500 DM ACROSS
INTERIOR NY. LOOKING FARTHER UPSTREAM...FURTHER HEIGHT FALLS WILL
BRING A PAIR OF SURFACE WAVES THROUGH THE GREAT LAKES/OH
VALLEY...RESPECTIVELY. THERE IS SOME VARIABILITY IN WHETHER THERE
WILL BE A SINGLE OR PAIR OF WAVES. WHILE THE PAST FOUR RUNS OF THE
GFS AGREE ON ONLY A NORTHERN WAVE...THE 12Z/00Z ECMWF PREFER ONLY
A MORE SOUTHERN WAVE. RECENT RUNS OF THE CMC/UKMET SEEM TO FLICKER
OVER THE NUMBER OF CYCLONES PRESENT. NOT A TRIVIAL FORECAST...BUT
AT LEAST AGREE ON THE PATTERN AT HAND. THIS CONGLOMERATE OF WAVES
WILL EVENTUALLY REACH THE NORTHEASTERN U.S. BY DAY 6/FRIDAY.
TOWARD THE END OF THE PERIOD...THE 00Z GFS/12Z ECMWF ARE AT ODDS
WITH ONE ANOTHER WITH LONGWAVE TROUGH PLACEMENT AS THE LATTER IS
MUCH QUICKER AS REFLECTED BY ITS ENSEMBLE MEAN. BACK OVER THE
WESTERN U.S...ALL SOLUTIONS AGREE ON MAINTAINING A STRONG POSITIVE
HEIGHT ANOMALY FOR A MAJORITY OF THE FORECAST PERIOD. WHERE THEY
DO DIFFER IS WITH THE HANDLING OF THE MENTIONED SHORTWAVE MOVING
TOWARD THE CENTRAL HIGH PLAINS ON WEDNESDAY MORNING. AS HAS BEEN
THE CASE IN RECENT MODEL CYCLES...THE ECMWF CONTINUES TO FAVOR A
SOUTHWARD TRAJECTORY INTO THE SOUTHWESTERN U.S. WHICH IS SHOWN IN
NO OTHER MODEL SUITE. WILL SUGGEST THIS IS OUTLYING UNTIL OTHERS
JOIN THE ECMWF SOLUTION. BY THE DECEMBER 15-16 TIMEFRAME...THE
GUIDANCE SHOW SOME POTENTIAL FOR AMPLIFIED FLOW TO ERODE THE
NORTHERN EXTENT OF THE RIDGE. HOWEVER...SUCH A SCENARIO IS NOT
AGREED UPON YET BUT DOES REMAIN PLAUSIBLE GIVEN THE NUMBER OF
ENSEMBLE MEMBERS WHICH SHOW IT.

REGARDING THE MODEL PREFERENCE...INITIALLY FAVORED A 3-WAY
COMBINATION OF THE 18Z GFS AND 12Z ECMWF/UKMET WITH MORE EMPHASIS
ON THE FORMER TWO MODELS. THIS HELPED MAINTAIN A DEEPER SOLUTION
DURING THE DAY 3/4...DECEMBER 12/13 PERIOD. AS UNCERTAINTY GREW
WITH DETAILS OF THE UPSTREAM SYSTEM MOVING THROUGH THE GREAT
LAKES...GRADUALLY INCREASED ENSEMBLE MEAN INFLUENCES FROM THE 18Z
GEFS MEAN AND 12Z NAEFS/ECMWF ENSEMBLE MEANS. DID KEEP 30 PERCENT
OF THE OPERATIONAL MODELS...18Z GFS/12Z ECMWF...INTO DAY 7 GIVEN
DECENT SYNOPTIC-SCALE AGREEMENT. THIS ALLOWS FOR GENERAL
NORTHWESTERLY FLOW ALOFT OVER THE PACIFIC NORTHWEST AS THE RIDGE
ERODES A BIT.


...WEATHER HIGHLIGHTS/THREATS...

GIVEN THE STAGNANT HIGH-AMPLITUDE PATTERN IN PLACE...PRONOUNCED
TEMPERATURE ANOMALIES WILL RULE THE COUNTRY. HOWEVER...EXPECTED
HIGHS/LOWS DO NOT SEEM TO BE EXTREME ENOUGH TO BREAK ANY
WIDESPREAD DAILY TEMPERATURE RECORDS. GENERALLY
SPEAKING...READINGS SHOULD BE AROUND 10 TO POSSIBLY 20 DEGREES
ABOVE CLIMATOLOGY OVER THE WESTERN HALF OF THE U.S. THE EXCEPTION
APPEARS TO BE AREAS OF THE PACIFIC NORTHWEST/CENTRAL GREAT BASIN
WHICH WILL LIKELY HAVE A PERSISTENT LOW-LEVEL TEMPERATURE
INVERSION THUS KEEPING TEMPERATURES CLOSER TO AVERAGE. FROM THE MS
RIVER VALLEY EASTWARD...THE INFLUENCE OF ARCTIC AIR IN THE
FORECAST WILL KEEP HIGHS 10 TO 20 DEGREES BELOW AVERAGE. THE
COLDEST AIR WILL BE MOVE THROUGH WEDNESDAY AS HIGHS STRUGGLE TO
GET ABOVE THE FREEZING MARK OVER THE LOWER OH VALLEY/CENTRAL
APPALACHIANS/MID-ATLANTIC.

SNOWFALL WILL BE COMMONPLACE ANYWHERE FROM THE UPPER MIDWEST
THROUGH THE GREAT LAKES INTO THE CENTRAL/NORTHERN APPALACHIANS AND
NEW ENGLAND. PERSISTENT CYCLONIC FLOW WILL KEEP THE LAKE EFFECT
TRAIN GOING ALTHOUGH SOME BREAKS ARE LIKELY GIVEN SHIFTS IN THE
PREVAILING LOW-LEVEL WIND DIRECTION. THE INITIAL SYSTEM ON
TUESDAY/WEDNESDAY COULD BRING MODERATE/HEAVY SNOW TO AREAS OF NEW
ENGLAND...PARTICULARLY FARTHER INLAND. ELSEWHERE...A RETURN OF
PRECIPITATION IS LIKELY LATE IN THE PERIOD OVER THE PACIFIC
NORTHWEST AS THE UPPER RIDGE POSSIBLY BUCKLES.


RUBIN-OSTER