Preliminary Forecasts
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000
FXUS02 KWNH 120650
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
149 AM EST TUE DEC 12 2017

VALID 12Z FRI DEC 15 2017 - 12Z TUE DEC 19 2017

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

AN EVOLVING PATTERN IS EXPECTED DURING THE PERIOD WITH A SHIFT
FROM ARCTIC DOMINATION EAST OF THE GREAT PLAINS TO BEING MORE
STRONGLY INFLUENCED BY PACIFIC AIR MASSES. ON 15/1200Z...AMPLIFIED
FLOW ACROSS THE EASTERN THIRD OF THE COUNTRY WILL BE IN PLACE WITH
A POTENT SHORTWAVE ACCELERATING FROM THE MID-SOUTH TOWARD THE
SOUTHEASTERN U.S. ON FRIDAY. THIS SHOULD INDUCE AN AREA OF LOW
PRESSURE OFF THE CAROLINA COAST WITH A TRACK EAST OF THE NC/VA
BORDER BY 16/0000Z. QUICK PROGRESSION OF THIS WAVE IS EXPECTED
WITH A POSITION EAST OF NOVA SCOTIA 12 HOURS LATER BEFORE PUSHING
INTO THE CANADIAN MARITIMES. LOOKING BACK TO THE WEST COAST...A
STAGNANT UPPER RIDGE WILL BE IN THE PROCESS OF ERODING AS A
LOW-AMPLITUDE SHORTWAVE OVER THE EASTERN PACIFIC MOVES INTO THE
PACIFIC NORTHWEST SOME TIME FRIDAY. PROGRESSIVE HEIGHT FALLS WILL
MARCH EASTWARD TOWARD THE GREAT PLAINS WITH POTENTIAL NORTHERN
STREAM INTERACTION WHICH LEADS TO SOME FORECAST UNCERTAINTY WITH
TIMING AND DEPTH OF RELEVANT SURFACE CYCLONES. GUIDANCE IS
SCATTERED ALL ABOUT BUT THIS SYSTEM SHOULD REACH THE EASTERN
SEABOARD DURING THE DAY 6/7...DECEMBER 18/19. IN ITS WAKE...THE
SYNOPTIC-SCALE PATTERN OVER THE WESTERN/CENTRAL U.S. SHOULD REMAIN
DE-AMPLIFIED WITH THE NEXT SHORTWAVE AFFECTING THE NORTH-CENTRAL
STATES EARLY NEXT WEEK. AT THE SAME TIME...LOW-AMPLITUDE RIDGING
IS LIKELY OVER THE WEST COAST WHILE LOWER HEIGHTS LURK WELL
OFFSHORE.

MODEL DIFFERENCES ARE COMMONPLACE THROUGHOUT THE FORECAST WHICH
LED TO SOME BELOW AVERAGE CONFIDENCE FOR THE DAY 6/7...DECEMBER
18/19 PERIOD. AT THE START...THERE ARE DETAIL DIFFERENCES WITH THE
EVOLUTION OF THE EASTERN U.S. LONGWAVE TROUGH. THE LARGEST OUTLIER
REMAINS THE PAST SEVERAL RUNS OF THE UKMET WHICH HAS HAD A
TENDENCY TO BE SLOWER/DEEPER WITH THE SOUTHERN EXTENT OF HEIGHT
FALLS. RECENT CYCLES OF THE GFS/ECMWF HAVE BEEN IN CLOSER
AGREEMENT AND HAVE MADE A WESTWARD SHIFT BASED ON MULTI-DAY
ENSEMBLE LOW PLOTS. THE 00Z GFS HAS MOVED A TOUCH BIT WEST OF ITS
PRECEDING RUN WHICH WOULD MARK A SNOWFALL THREAT FOR SECTIONS OF
THE MID-ATLANTIC THROUGH FRIDAY NIGHT. EXCEPT FOR A FEW RANDOM
ENSEMBLE MEMBERS...A VAST MAJORITY OF THE 90 CMC/GEFS/ECMWF
ENSEMBLE MEMBERS WOULD BE WELL EAST OF NEW ENGLAND BY 16/1200Z.
SHIFTING FOCUS TO THE WESTERN/CENTRAL U.S...MODEL UNCERTAINTY IS
NOTED QUITE EARLY WITH THE 00Z/12Z CMC BEING DECIDEDLY SLOWER
EJECTING SHORTWAVE ENERGY INTO THE PACIFIC NORTHWEST ON FRIDAY.
EVEN WITH THE 00Z GUIDANCE ARRIVING...THEY ADD MORE QUESTIONS THAN
ANSWERS TO THE PATTERN OVER THE WEEKEND. THE 00Z CMC/UKMET HAVE
MOVED TOWARD A SOLUTION WHICH FAVORS A BROAD UPPER LOW PEELING OFF
FROM THE PREVAILING WESTERLIES WITH AN EVENTUAL POSITION OVER THE
DESERT SOUTHWEST BY SUNDAY. THIS IS A GENERAL SHIFT FROM THEIR
PRECEDING RUNS AS WELL AS THE MORE PROGRESSIVE GFS/ECMWF SUITES.
WHILE THE 564-DM ENSEMBLE SPAGHETTI PLOT SHOWS SOME 12Z GEFS/ECMWF
ENSEMBLE MEMBERS WITH THIS SCENARIO...IT STILL APPEARS TO BE A
LOWER PROBABILITY SOLUTION AT THIS TIME. CONSIDERING THE 00Z/18Z
GFS VERSUS THE 12Z ECMWF...THE LATTER CONTINUES TO BE MUCH QUICKER
WHILE EVEN BECOMING QUICKER THAN ITS PARENT ENSEMBLE MEAN BY AS
EARLY AS SATURDAY. AT THE SURFACE...THERE IS THE POTENTIAL FOR TWO
DOMINANT WAVES...ONE ACROSS THE GREAT LAKES WHILE THE OTHER WILL
EMERGE OUT THE TX/OK PANHANDLES. ENSEMBLE LOW SPREAD IS LESS WITH
THE SOUTHERN WAVE WHILE IMMENSE UNCERTAINTY DOMINATES THE FORMER
TO THE NORTH. RUN-TO-RUN CONTINUITY IS POOR WITH INTENSITY AND
POSITION WHICH SIGNIFICANTLY DECREASES THE FORECAST CONFIDENCE
HERE. WHILE THIS SYSTEM SLIDES TOWARD THE EAST COAST...RENEWED
HEIGHT FALLS WILL LIKELY REACH THE NORTH-CENTRAL U.S. BY EARLY
NEXT WEEK.

WHILE THE EARLY PART OF THE FORECAST COULD BE SALVAGED VIA A 18Z
GFS/12Z ECMWF BLEND...ENSEMBLE MEANS HAD TO BE INCORPORATED FROM
DAY 4/SATURDAY ONWARD GIVEN EVERYTHING MENTIONED EARLIER. BY THE
CONCLUSION OF THE WEEKEND...WAS ONLY ABLE TO KEEP 20 PERCENT OF
THE OPERATIONAL MODELS IN THE PICTURE WHILE SPLITTING BETWEEN THE
18Z GEFS AND 12Z ECMWF/NAEFS ENSEMBLE MEANS. BY THE REMAINING TWO
DAYS OF THE PERIOD...WENT FULLY ENSEMBLE MEANS FAVORING 50 PERCENT
OF THE 12Z ECMWF ENSEMBLE MEAN WHILE UTILIZING 25 PERCENT EACH OF
THE 18Z GEFS/12Z NAEFS ENSEMBLE MEANS.


...WEATHER HIGHLIGHTS/THREATS...

THE LAST SHOT OF ARCTIC AIR MOVING THROUGH THE EASTERN U.S. WILL
MAINTAIN BELOW AVERAGE TEMPERATURES THROUGH SATURDAY. WHILE
FORECAST TEMPERATURE ANOMALIES ARE REDUCED A BIT FROM WHAT WILL BE
EXPERIENCED BEFORE THIS PERIOD...EXPECT HIGHS AROUND 5 TO 15
DEGREES BELOW CLIMATOLOGY. THE MOST ANOMALOUS COLD AIR WILL BE
OVER THE GREAT LAKES UP INTO NEW ENGLAND AS HIGHS STRUGGLE TO GET
OUT OF THE TEENS AND 20S ON FRIDAY. HOWEVER...THE INCREASED
DOMINANCE OF PACIFIC AIR MASSES SHOULD ALLOW FOR WELL ABOVE NORMAL
TEMPERATURES TO PREVAIL...MAXIMIZING IN THE 1O TO 15 DEGREE RANGE
ACROSS THE CENTRAL U.S. ON FRIDAY...FORECAST HIGHS WILL BE IN THE
LOW/MID 50S AS FAR NORTH AS EASTERN MT.

A FEW ACTIVE AREAS OF PRECIPITATION ARE LIKELY ON FRIDAY WITH THE
EASTERN U.S. TROUGH FAVORING A MIXTURE OF RAIN/SNOW OVER THE GREAT
LAKES WHILE SHOWERS FALL ALONG THE COLD FRONT ACROSS FL. DEPENDING
ON THE TRACK OF THE COASTAL LOW...SOME RAIN/SNOW MAY PUSH BACK
INTO THE MID-ATLANTIC. BACK WEST...THE EROSION OF THE MEAN UPPER
RIDGE WILL ALLOW FOR A RETURN OF WIDESPREAD PRECIPITATION FROM THE
PACIFIC NORTHWEST EASTWARD INTO THE UPPER INTERMOUNTAIN
WEST/NORTHERN ROCKIES. QUASI-ZONAL FLOW BEING ESTABLISHED SHOULD
KEEP CONDITIONS WET OVER THE PACIFIC NORTHWEST WITH UPSLOPE FLOW
FAVORING HIGHER AMOUNTS OVER THE WA CASCADES. AS THE LEAD SYSTEM
MOVES TOWARD THE CENTRAL U.S. THIS WEEKEND...SOME THREAT FOR
LIGHT/MODERATE RAINFALL WILL BE POSSIBLE FROM THE ARKLATEX
EASTWARD TO THE LOWER MID-ATLANTIC. HOWEVER...THIS IS HIGHLY MODEL
DEPENDENT GIVEN THE VAST SPREAD AT HAND. THERE SHOULD ALSO BE
ENOUGH COLD AIR IN PLACE TO MAINTAIN A SNOWFALL THREAT FROM THE
LOWER GREAT LAKES INTO THE NORTHEASTERN U.S. ON SUNDAY/MONDAY.


RUBIN-OSTER

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