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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1553Z Dec 20, 2014)
 
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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
1053 AM EST SAT DEC 20 2014

VALID 12Z TUE DEC 23 2014 - 12Z SAT DEC 27 2014

---WET AND WINDY FOR THE HOLIDAYS---
EAST OF THE MISSISSIPPI...PACIFIC NORTHWEST AND GREAT BASIN.

...OVERVIEW...
THE EMERGENCE OF A NEGATIVE-TILT TROUGH OVER THE NORTHEAST POTION
OF THE CONTINENT IS EXPECTED TO PRODUCE AN INTENSE SURFACE CYCLONE
OVER SOUTHEAST CANADA AND WINDY CONDITIONS ACROSS THE NORTHEAST
QUARTER OF THE CONUS. THE LARGE EXPANSE OF TROUGHING IN EASTERN
CANADA WILL GENERATE A NORTHWEST FLOW PATTERN IN THE WEST...AND A
PACIFIC STORM TRACK THAT SLOWLY CARVES OUT ACROSS THE GREAT BASIN
AND SOUTH CENTRAL ROCKIES DURING THE LATTER HALF OF THE MEDIUM
RANGE PERIOD.

...UNCERTAINTY ASSESSMENT...
HONESTLY...THE UNCERTAINTY IS BEYOND DAY 7---AND HOW THE DEEP
SURFACE CYCLONE 'RESHUFFLES' THE PATTERN OVER THE NORTHEAST
QUARTER OF CANADA AND ACROSS THE HIGH-LATITUDE WESTERN HEMISPHERIC
CIRCULATION.

MIDWEST/GREAT LAKES--
THE 20/00Z MODEL GUIDANCE---GFS/ECMWF/CANADIAN ARE VERY REASONABLY
CLUSTERED ON CHRISTMAS EVE MORNING (24/12Z) WITH THE SURFACE
REFLECTION AND ITS DEPTH...INCLUDING THE INITIAL SEQUENCING OF
TRIPLE-POINT LOW FORMATION MOVING NORTHWARD THROUGH THE OHIO
VALLEY. GIVEN THE DYNAMICAL MODEL SCENARIOS...THE CONSISTENT
SIGNAL FOR MULTIPLE TRIPLE-POINT REFLECTION(S) DURING THE STRONG
SYSTEM'S "MORPHOLOGY" AND A RAPID DEEPENING OF THE CYCLONE WITHIN
A 24-HOUR PERIOD---BETWEEN LOUISVILLE AND DETROIT---ANTICIPATE A
SUB 976MB LOW CENTER INVOF LAKE HURON (AT 25/00Z) AND A
WELL-DEFINED 'TROWAL' INVOF LAKE SUPERIOR. THE 'TROWAL' AND AXIS
OF BEST MID-LEVEL FRONTOGENESIS IS 'ANOTHER UNCERTAINTY'---AND
PROBABLY NOT A MAJOR MEDIUM RANGE ISSUE---EXCEPT FOR CENTRAL
CANADA AND POST-FRONTAL CLOUD COVER ISSUE OVER THE UPPER LAKES
AFTER THE DAY 4-5 TIME FRAME.

SOUTHEAST/EASTERN SEABOARD--
THE FRONTAL PASSAGE 'TIMING' IS SHOWING LITTLE RUN-TO-RUN
VARIANCE. WHAT DIFFERENCES REMAIN---ARE TIED TO THE DEPTH/BREATH
OF THE INDIVIDUAL DETERMINISTIC SOLUTIONS AND THEIR 'LOCALIZED'
INFLOW SCENARIOS FROM ATLANTIC OCEAN TO EASTERN GREAT LAKES. IT IS
SOMEWHAT OF AN UNDERSTATEMENT HERE---DESCRIBING THE VOLATILITY
DEVELOPING OVER THE GULF STREAM AND ITS IMPACTS ACROSS SOUTHERN
NEW ENGLAND AND THE INTERIOR MOUNTAINS. TO STILL HAVE ABOVE
AVERAGE CONFIDENCE IN A SUB-970MB CYCLONE CENTER NEAR/INVOF OTTAWA
ONTARIO AT 25/12Z AND COMBINE THIS WITH THE UPPER-LEVEL SUPPORT TO
GENERATE THE INTENSE LOW-LEVEL GRADIENTS AND ATLANTIC MOISTURE
'INFLOW' WITHIN THE SYSTEM'S WARM SECTOR---IS...EQUALLY MIRRORED
AND SUPPORTED BY THE DRY SLOT AND DRAMATIC WIND SHIFT AND WESTERLY
FLOW COMPONENT ON THE BACKSIDE OF THE FRONT. THE CONSISTENT SIGNAL
IS ILLUSTRATED BY THE 925MB-850MB WIND FORECASTS---PROJECTED NOW
FOR SEVERAL DAYS WORTH OF FORECAST CYCLES. THE POST-FRONTAL SHEAR
IS JUST AS IMPRESSIVE ALONG THE ENTIRE STRETCH OF THE APPALACHIAN
CHAIN.

PACIFIC NORTHWEST/NORTHERN CONTINENTAL DIVIDE/GREAT BASIN--
THE MODEL SPREAD IS TIED TO THE DEPTH OF A CANADIAN AIRMASS 'PUSH'
INTO THE NORTHWEST QUARTER OF THE NATION. THE INDIVIDUAL
RUN-TO-RUN SOLUTIONS SEEM TO STRUGGLE WITH HOW/WHEN/WHERE TO EJECT
THE ENERGY ACROSS THE DIVIDE---AND WHERE TO DELINEATE THE WESTWARD
EXTENT/INFLUENCE OF BROAD-SCALE CYCLONIC FLOW ESTABLISHED OVER THE
MIDWEST---WITH RESPECT TO THE DOWNSTREAM NEGATIVE-TILTED TROUGH.
ALOFT...THERE IS FAR LESS DISAGREEMENT WITH THE
SOUTHEASTWARD-MOVING MID/UPPER-LEVEL PACIFIC TROUGH AXIS

...MODEL PREFERENCES...
THE MEDIUM RANGE FORECAST IS WEIGHTED WITH THE 20/00Z ECENS/ECMWF
SOLUTION IN MIND.

OVERALL...THE 20/00Z ECENS MEAN WILL BE SUFFICIENT TO HANDLE THE
EVOLVING NORTH AMERICA FLOW PATTERN ALOFT. AT THE SURFACE...IT
CARRIES THE DEEPEST SURFACE SOLUTION IN SOUTHEAST CANADA---AND FOR
ALL KEY FEATURES AT THE SYNOPTIC-SCALE BETWEEN DAY4-7. THE CONCERN
IS THE ECENS MEAN---OR ANY ENSEMBLE MEAN FOR THAT MATTER--- WILL
NOT PROVIDE THE GOOD DETAIL---THE WIND FORECAST DESERVES.

OF THE DETERMINISTIC GUIDANCE...
THE 20/00Z ECMWF WOULD BE MY RECOMMENDATION FOR THE NEGATIVE-TILT
TROUGH AND THE EVOLUTION OF THE SURFACE FEATURES IN THE SOUTHERN
HIGH PLAINS THROUGH THE LATTER HALF OF DAY 5(26/00). BUT THE ISSUE
WITH THE DETERMINISTIC ECMWF IS ITS BIAS--AND INCORRECTLY
DEPICTING THE SYNOPTIC PATTERN ACROSS THE CENTRAL PLAINS. IN
THAT...ITS DYNAMICAL SOLUTION IS TOO DEEP AND GENERATES A MATURING
SURFACE CYCLONE THAT MIGRATES ACROSS THE SOUTHERN HIGH PLAINS AND
MID-MS VALLEY FOR DAYS 5-7. THE DETERMINISTIC SEEMS TO 'LIKE'
CRASHING A  DEEP SURFACE SURFACE CYCLONE INTO DOWNSTREAM PATTERNS
THAT ARE OBVIOUSLY 'BLOCKED'. IN THE DAY 5-6 SCENARIO---THIS
'BLOCK' IS THE DEEP CYCLONIC FLOW OVER CANADA---NOT A STABLE
RIDGE. ITS MEAN SOLUTION AND MORE OF AN INVERTED TROUGH SOLUTION
WITH A WEAK/FLAT SURFACE WAVE IS MORE LIKELY.
 
A GFS/GEFS-LIKE SOLUTION WOULD 'HEAP' A TREMENDOUS AMOUNT OF WARM
ADVECTION AND ATLANTIC MOISTURE ACROSS SOUTH CENTRAL
ONTARIO---INTO THE DEVELOPING 'TROWAL'---AND IF ONE FELT
COMFORTABLE USING THAT SOLUTION---THEN EXPECT MUCH LESS WINTER
P-TYPE CONDITIONS IN THE POST-FRONTAL ENVIRONMENT OVER THE GREAT
LAKES. THE GEFS SOLUTION BEYOND DAY 5 IS MORE WESTERN
CANADA-PACIFIC INFLUENCED THAN THE ECENS/ECMWF SCENARIO. 

... SENSIBLE WEATHER HIGHLIGHTS/THREATS...
THIS PARTICULAR CHRISTMAS HOLIDAY FORECAST CONTINUES TO BE A WARM
SECTOR-TYPE STORM SYSTEM FOR THE EASTERN SEABOARD. AND VERY
WET...LOCALLY 'CONVECTIVE' AND WINDY AT TIMES. POST-FRONTAL COLD
ADVECTION AND 'TROWAL' INFLUENCES REMAIN POSITIONED OVER THE GREAT
LAKES...WITH SOME UPSLOPE-ASSISTED---BUT 'LOCALIZED' SNOW SHOWERS
OVER THE APPALACHIANS.

LOCAL LAKE-EFFECT CONDITIONS APPLY WITHIN THE SOUTHWEST QUADRANT
OF THE EXITING SURFACE CIRCULATION.

THE PACIFIC MID-LEVEL TROUGH AND LEADING EDGE OF THE HEIGHT FALLS
WILL BE THE FOCAL POINTS FOR PRECIPITATION IN THE WEST...AND A
CONSOLIDATION OF THE DIGGING PACIFIC TROUGH AXIS WILL BE
INFLUENTIAL IN THE DEVELOPMENT OF PRECIPITATION ALONG THE WINDWARD
SLOES OF THE GREAT BASIN AND CONTINENTAL DIVIDE THROUGH DAY 5.

EXPECT A SLOW-EVOLVING WINTER WEATHER P-TYPE SCENARIO FOR THE
CENTRAL AND SOUTHERN PLAINS STATES AS THE PACIFIC ENERGY EJECTS A
SERIES OF OVER-RUNNING TYPE PRECIPITATION EVENTS.   

VOJTESAK