Prognostic Meteorological Discussion
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FXUS02 KWBC 211600
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1059 AM EST SUN DEC 21 2014

VALID 12Z WED DEC 24 2014 - 12Z SUN DEC 28 2014

...ACTIVE CHRISTMAS HOLIDAY WEATHER PATTERN...

...OVERVIEW...
A LARGE-SCALE AND RATHER INTENSE CYCLONE CARVES OUT ACROSS
SOUTHEAST CANADA DURING THE 2-DAY CHRISTMAS PERIOD---AFFECTING A
BROAD AREA OF EASTERN NORTH AMERICA---WITH A WET AND WINDY WARM
SIDE...AND A SHORTER-DURATION POST-FRONTAL COOL SIDE---INCLUDING A
WINTRY MIX ACROSS THE GREAT LAKES.

ACROSS THE PACIFIC NORTHWEST AND INTERMOUNTAIN WEST...A TROUGH
MIGRATES MOISTURE AND COOLER TEMPERATURES DURING THE DAY 3-4
PERIOD...AND EJECTS ACROSS THE PLAINS AND CENTRAL MIDWEST/LOWER
MISSOURI VALLEY ON DAYS 4-5. THIS SYSTEM SHOULD GENERATE A SWATH
OF WINTER WEATHER ALONG ITS NORTHERN PERIPHERY AND USHER IN A
COLDER AIRMASS BEHIND ITS FRONTAL PASSAGE---EXTENDING FROM THE
SOUTHERN HIGH PLAINS TO THE CENTRAL OHIO VALLEY BY DAY 7.

...MODEL UNCERTAINTY...
THE LAKE HURON/WEST CENTRAL ONTARIO CYCLONE DETAILS EARLY IN THE
PERIOD REMAIN AN ENERGY-TRANSFER FORECAST CHALLENGE...INVOF THE
WESTERN OHIO VALLEY NORTHWARD INTO SOUTHWEST ONTARIO---AND WILL
EXTEND TO THE MESOSCALE---IN THE COMING DAYS. THE
LOUISVILLE-DETROIT CORRIDOR CONTINUES TO BE THE REGION WHERE THERE
IS A HIGH DEGREE OF VOLATILITY. WITH NEGATIVE-TILT TROUGH
SCENARIOS...THE EXACT LOCATION OF THE TRIPLE-POINT SEEMS TO BE THE
ROOT CAUSE OF LOCALLY-ACTIVE WEATHER CHANGES AND
CHARACTERISTICALLY VARIES FROM RUN-TO-RUN. OVERALL...THE
SYNOPTIC-SCALE FEATURES AND MASS FIELDS REMAIN ON A RATHER
CONSISTENT COURSE---SWEEPING SOUTHWEST TO NORTHEAST ACROSS THE
EASTERN THIRD OF THE NATION---DAY 3-4.

A GREAT DEAL OF HIGH-LATITUDE NORTH AMERICA `RESHUFFLING` TAKES
PLACE THIS MEDIUM RANGE PERIOD---WITH THE DEEP CYCLONE IN ONTARIO
WRAPPING AND FILLING THROUGH TIME. ENSEMBLE SPREAD BEGINS TO
INCREASE DRAMATICALLY---AT THE SURFACE AND ALOFT--- OVER
LABRADOR/THE DAVIS STRAIT AND IN THE NORTHERN ROCKIES...IN
RESPONSE TO THE DEGREE OF AMPLIFICATION AND TRAJECTORY OF THE
DAY3-4 CYCLONE MIGRATING INTO NORTHEAST CANADA. IN THE WAKE OF THE
DEPARTING SYSTEM...THE FLOW ACROSS THE LOWER 48 AND SPREADS OVER
THE ROCKIES PERTAINS TO THE MID-LEVEL TROUGH AND HEIGHT FALLS
DIGGING AND MIGRATING THROUGH THE GREAT BASIN.

FOR THE PACIFIC MID-LEVEL TROUGH MIGRATION INTO THE PLAINS---
THE 21/00Z GEFS/GFS SOLUTION WOULD SERVE AS THE NORTHERN ENVELOPE
SOLUTION ACROSS WYOMING/SRN MONTANA WITH THE INITIAL HEIGHT FALLS
EJECTING EAST OF THE CONTINENTAL DIVIDE AND THE UPPER HALF OF THE
MISSOURI VALLEY. THE ENERGY ALOFT APPARENTLY PHASING WITH ENERGY
OVER SOUTH CENTRAL CANADA.

THE EC/CMC AND UKMET SOLUTIONS MAINTAINING SEPARATION---WITH THE
UKMET THE SOUTHERN---MOST INTENSE SOLUTION WITH ITS ENERGY HANGING
BACK OVER THE FOUR CORNERS BETWEEN 26/00Z AND 27/00Z. THIS CAMP
WOULD SUGGEST THE POTENTIAL FOR SOME ADDITIONAL OVER-RUNNING
PRECIPITATION EVENTS ALONG THE I-40 CORRIDOR BEYOND DAY5---IN THE
POST-FRONTAL `WAKE`   E EC/CMC SOLUTION IS PREFERRED /U

...MODEL PREFERENCES...
MAINTAIN THE ECENS/ECMWF AND ITS 21/00Z CYCLE SOLUTION THROUGH
FRIDAY (27/00Z) FROM THE PLAINS EASTWARD INTO ATLANTIC CANADA.
THIS WILL ALLOW MID-LEVEL ENERGY TO STREAK ACROSS THE
COLORADO/WYOMING BORDER AND CONSOLIDATE A SURFACE WAVE ALONG THE
MID-MS VALLEY/OZARKS ON FRIDAY. THE SYSTEM ORIGINATES AS A LEE
SIDE LOW RATHER THAN A CLOSED VERTICALLY-STACKED SYSTEM---SO THE
FRIDAY/SATURDAY FORECAST HAS THIS CONSOLIDATED ENERGY STREAKING
NORTHEASTWARD ALONG/NORTH OF THE OHIO VALLEY AND ST LAWRENCE
VALLEY...RATHER THAN CENTRAL UPPER GREAT LAKES---WHICH IS THE
21/00Z GEFS/GFS SOLUTION. I DO NOT WANT TO MINIMIZE THE SENSIBLE
WEATHER IMPACTS---BUT RATHER WANT TO IMPLY---THAT THERE IS `ROOM`
FOR ADDITIONAL IMPULSES TO MIGRATE ATOP THE COLD FRONT---AFTER THE
INITIAL SYSTEM EXITS THE ROCKIES. TO ACCOUNT FOR THIS---

THE CURRENT MEDIUM RANGE PACKAGE UTILIZED A 21/00Z ECENS/NAEFS
WEIGHTED BLEND BEYOND THE 27/00Z PERIOD. THIS HAS THE EFFECT
OF---INCREASING THE POPS ALONG THE PACIFIC NORTHWEST AND
PIEDMONT/OUTER BANKS FOR DAYS 6-7. THIS IS A PURELY A FUNCTION OF
THE `WET` MEMBERS---A MINORITY AT THIS TIME SCALE. THE BROAD-BRUSH
APPROACH AFTER 27/00Z ACCOUNTS FOR THE PROJECTED WEST-NORTHWEST
FLOW ALOFT FROM THE CASCADES TO THE OZARKS...AND THE FILLING OF
THE DEEP SYSTEM IN EASTERN CANADA.

...SENSIBLE WEATHER HIGHLIGHTS...
THE DEEP SURFACE LOW OVER CENTRAL ONTARIO/INVOF EASTERN SHORELINE
OF LAKE HURON AND INTENSE `WRAP-AROUND` SYNOPTIC-SCALE FLOW ACROSS
NEW ENGLAND ON CHRISTMAS EVE ALLOWS FOR MILD WARM-SECTOR AIR
TEMPERATURES TO SPREAD NORTHWARD ACROSS THE MID-ATLANTIC
STATES---WHERE HIGHS/LOWS ARE FORECAST TO BE WELL ABOVE NORMAL.
THERE SHOULD BE A WELL-DEFINED `TROWAL` FEATURE ACCOMPANYING THE
BROAD CYCLONE...BUT GENERALLY THIS PERTAINS TO THE UPPER GREAT
LAKES WITH RESPECT TO SENSIBLE WEATHER.

THE EXITING PACIFIC TROUGH AND HEIGHT FALLS WILL LIKELY `STREAK
OUT` AN ORGANIZED PRECIPITATION SHIELD ACROSS THE PLAINS AND
MID-MISSOURI VALLEY---WHICH MIGRATES ACROSS THE REGION---IN THE
DAY 4-5 TIME FRAME.

VOJTESAK

$$





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