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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0659Z Oct 24, 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
259 AM EDT FRI OCT 24 2014

VALID 12Z MON OCT 27 2014 - 12Z FRI OCT 31 2014

OVERVIEW...
AN ACTIVE PERIOD OF WEATHER IS ANTICIPATED ACROSS THE LOWER 48
THIS MEDIUM RANGE PERIOD. ALONG THE PACIFIC NORTHWEST
COAST...SOUTHERN BRITISH COLUMBIA...ADJACENT COASTAL RANGES AND
THE ROCKIES---SUBTROPICAL MOISTURE PLUME AND DEEP...ENERGETIC
CYCLONE SETTLES ALONG THE COAST OF BRITISH COLUMBIA. EAST OF THE
ROCKIES...A PROGRESSIVE SERIES OF CYCLONES MIGRATE FROM THE
NORTHERN PLAINS TO THE NORTHEAST.

UNCERTAINTY ASSESSMENT...
FOR DAYS 3-4...THE GLOBAL GUIDANCE CAPTURES THE RAPID PROGRESSION
OF REMNANT VORTICITY AND MOISTURE ASSOCIATED WITH 'ANA' CURRENTLY
INVOF 28N 170W. BY 28/12Z...THE BULK OF ITS MID-TO UPPER-LEVEL
ENERGY SETTLES OFF THE COAST OF VICTORIA ISLAND BC AND THE OLYMPIC
PENINSULA---HAVING BEEN DIRECTED EAST-NORTHEASTWARD BY A
MID-LATITUDE JET AXIS AND BENEATH THE BASE OF A SLOW-MOVING AND
SOUTHEASTWARD-MOVING CYCLONE ORIGINATING IN THE CENTRAL GULF OF
ALASKA.

THE SYNOPTIC-SCALE SETUP FOR INTENSE PERIODS OF JET-DRIVEN
RAINFALL WILL GET AN ASSIST FROM SURFACE CYCLONE DEVELOPMENT ALONG
130W ROTATING NORTHWARD ALONG THE SYSTEM'S SLOW-MOVING COLD FRONT.
ON DAY 5-6...THE BROAD CYCLONIC FLOW IN THE GULF OF ALASKA GETS AN
INFUSION OF MODIFIED-ARCTIC AIR FROM THE BERING SEA AND WESTERN
ALASKA...WITH A GREAT DEAL OF UNCERTAINTY REMAINING WITH RESPECT
TO THE NEW ORIENTATION OF THE UPPER-LEVEL TROUGH AXIS ALONG 130W
FROM HAIDI GWAII SOUTHWARD TO THE MOUTH OF THE COLUMBIA RIVER. THE
24/12Z DETERMINISTIC GFS/ECMWF SUGGEST A SHORTWAVE RIDGE BUILDS
INTO SOUTHERN BC AND THE OLYMPIC PENINSULA---AHEAD OF THE
RELOADING UPPER-LEVEL CYCLONE OVER THE NORTHWEST GULF OF ALASKA.
IN TURN...THE UPPER-LEVEL TROUGH DIGS/AMPLIFIES ALONG 140W AND
REMAINS ESSENTIALLY OFFSHORE FOR THE LATTER PORTION OF THE MEDIUM
RANGE PERIOD. THE 23/12Z DETERMINISTIC CANADIAN MAINTAINS A MORE
NEGATIVE-TILT TROUGH AXIS WITH A MARITIME POLAR AIRMASS CONTINUING
A SOUTHEASTWARD SURGE INTO NORTHERN CALIFORNIA AND THE NORTHERN
GREAT BASIN. FOR NOW...A BLEND OF THE 23/12Z NAEFS/ECENS ALLOWS
FOR A TREND TOWARDS THE NEGATIVE-TILT ORIENTATION---ALBEIT A WEAK
REFLECTION OF THIS FEATURE ALONG 130W FROM 40N-50N LATITUDE.

IN THE EASTERN HALF OF THE NATION...THE GENERAL CONSENSUS AMONG
THE DETERMINISTIC 23/12Z GFS/CANADIAN/ECMWF POINTS TO A
NORTHEASTWARD-MOVING SURFACE CYCLONE TRACKING FROM THE NORTH
CENTRAL PLAINS TO GREAT LAKES...EVENTUALLY CLOSING OFF OVER
NORTHEAST ONTARIO AS A CANADIAN ARCTIC AIRMASS CONGEALS INTO THE
BROAD WARM ADVECTION SPREADING OUT ACROSS THE CENTRAL MISSISSIPPI
AND WESTERN OHIO VALLEYS. WHAT UNCERTAINTY REMAINS...WILL BE THE
DEPTH AND ASSOCIATED STORM TRACK OF A MID-LEVEL ALBERTA CLIPPER
MIGRATING SOUTHEASTWARD UNDERNEATH THE BASE OF THE CLOSED
UPPER-LEVEL CYCLONE. BY DAY 6-7...THIS CLIPPER SLIDES A SECONDARY
SHOT OF COOLER CANADIAN AIR INTO THE GREAT LAKES...GLIDING
EASTWARD ACROSS THE EASTERN GREAT LAKES...UPPER OHIO VALLEY AND
NORTHERN MID-ATLANTIC STATES. THE FORECAST CHALLENGE---IS THE
INFUSION OF SUBTROPICAL ENERGY/MOISTURE AT JET-LEVEL (ON DAY 2)
GOING TO ACCURATELY DEPICT A SYSTEM---FROM BEGINNING TO END?

MODEL PREFERENCES...
THE 23/12Z GUIDANCE CYCLE APPEARED TO HAVE A DECENT HANDLE ON THE
EMERGING DETAILS OFF THE WEST COAST FOR A PROLONGED 36-HOUR TO
48-HOUR PERIOD OF JET-DRIVEN PRECIPITATION. THE PERSISTENCE OF THE
UPPER-LEVEL JET MAINTAINS A QUASI-STATIONARY POSITION
PERPENDICULAR TO THE COASTAL RANGES OF NORTHERN OREGON AND WESTERN
WASHINGTON...AND THAT AGREEMENT TRANSLATES DOWNSTREAM INTO A
MEDIUM-AMPLITUDE UPPER TROUGH DOWNWIND OF THE ROCKIES. THE
CULMINATION OF A CLOSED UPPER-LEVEL LOW IN ONTARIO BY DAY 5...WAS
A REASONABLE CONCLUSION. THE 12Z GFS AND ECMWF SHOWED GOOD UTILITY
WITH BOTH FEATURES IN THIS PATTERN. THEIR UTILITY DWINDLES RATHER
QUICKLY AROUND THE 29/00Z TIME FRAME...WHEN A TROUGH IN WESTERN
ALASKA...BEGINS TO IMPINGE UPON THE QUASI-STATIONARY TROUGH INVOF
SOUTHWEST CANADA AND ALTERS THE JET-LEVEL WIND FIELDS. THE EXIT
REGION OF THE UPPER-JET AXIS SHIFTS DOWNSTREAM INTO THE CENTRAL
PORTION OF THE CONTINENT...AND THE 'NEW' JET BEGINS TO REFORM IN
THE NORTHEAST PACIFIC...PRODUCING AMPLITUDE TO THE FLOW BETWEEN
130W AND 140W. LEANED UPON THE ECENS/NAEFS MEANS...WHICH WILL
ALLOW THE COLDER AIRMASS FROM THE GULF OF ALASKA TO REPLENISH THE
WEST COAST AROUND DAY 7. SAME HOLDS TRUE IN THE EAST BEYOND DAY
5...USING THE ECENS/NAEFS BLENDS GIVES THE CLIPPER SYSTEM 'ROOM'
TO ENTER THE BASE OF THE UPPER TROUGH AND CLOSED LOW INVOF
85W...AND KEEP A MODERATELY-AMPLIFIED FLOW ALONG/EAST OF THE
APPALACHIANS AT THE DAY 7 TIME FRAME.

SENSIBLE WEATHER HIGHLIGHTS...
ABOVE NORMAL PRECIPITATION TOTALS FOR THE WEST COAST AND CASCADIA
FROM THE CANADIAN BORDER TO HUMBOLDT BAY...WITH THE PROSPECTS FOR
LARGE SNOWFALL ACCUMULATIONS IN THE CASCADES AND ENTIRE CHAIN OF
VOLCANIC PEAKS FROM MT BAKER TO MT SHASTA.

ABOVE NORMAL TEMPERATURES FOR THE PLAINS STATES DAYS 3-4...WITH
THESE POSITIVE TEMPERATURE ANOMALIES MIGRATING DOWNSTREAM INTO THE
OHIO VALLEY...EASTERN LAKES AND MID-ATLANTIC REGION/SOUTHERN NEW
ENGLAND FOR DAYS 4-5. MODERATE PRECIPITATION AND EMBEDDED DEEP
CONVECTIVE ELEMENTS PRECEDE THE FRONTAL MIGRATION FROM THE OZARKS
TO THE DELMARVA.

VOJTESAK