Prognostic Meteorological Discussion
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FXUS02 KWBC 260555
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
154 AM EDT SUN OCT 26 2014

VALID 12Z WED OCT 29 2014 - 12Z SUN NOV 02 2014

...OVERVIEW...
AN `ATMOSPHERIC RIVER` SET-UP IN THE PACIFIC NORTHWEST COAST AND
NORTHERN DIVIDE---IN PART...THE RESULT OF REMNANT MOISTURE AND
ENERGY FROM `ANA`...RESHUFFLES THE FLOW PATTERN DOWNSTREAM OVER
SOUTH CENTRAL AND EASTERN CANADA DURING THE FIRST HALF OF THE
FORECAST PERIOD. WITH AN AMPLIFIED PATTERN CARVING OUT ACROSS THE
EASTERN THIRD OF THE LOWER 48 AND A BROAD UPPER RIDGE ANCHORING
ACROSS THE CENTER OF THE CONTINENT BY PERIOD`S END---THE WEST
COAST REMAINS ACTIVE DURING THE LATTER HALF OF THE PERIOD. THE
SENSIBLE WEATHER CHANGES IN THE WEST ARE LINKED TO THE GRADUAL
SOUTHEASTWARD MIGRATION OF A COLD AND UNSTABLE UPPER-LEVEL TROUGH
WITH ORIGINS IN WESTERN ALASKA AND THE BERING SEA.

...UNCERTAINTY ASSESSMENT...
THE BIGGER DIFFERENCES IN THE 25/12Z DETERMINISTIC GUIDANCE ARE
EMBEDDED IN THE DETAILS ON OR/JUST AFTER DAY 5. WHILE THE 25/12Z
ECENS/GEFS AND NAEFS ENSEMBLE MEANS MAINTAIN A STABLE WAVE PATTERN
FOR DAYS 3-7---WITH RESPECT TO AMPLITUDE AND PROGRESSION OF THE
FLOW FROM COAST TO COAST---THE DETERMINISTIC GUIDANCE TENDS TO
FLIP-FLOP THE EVENTUAL OUTCOME OF THE FLOW ACROSS NORTH CENTRAL
CANADA AND DOWNSTREAM IN THE EAST. RATHER THAN STEP THROUGH THE
FLIP FLOPS AND ATTEMPT TO TRACE THE ERRORS...THE CRUCIAL PART OF
THIS MEDIUM RANGE FORECAST CYCLE---BETWEEN 1/00Z-1/12Z---REMAINS
INTACT OVER THE CENTRAL CONUS AND REMAINS ESSENTIALLY UNCHANGED
FROM 2-3 DAYS AGO. IN PART...THE STABILITY OF THE FLOW
PATTERN---RESULTS FROM THE RELATIVELY-ACCURATE INCORPORATION OF
`ANA` INTO THE MID-LATITUDE WESTERLY FLOW---AND THE GRADUAL BUT
STEADY PROGRESSION OF TWO MIGRATORY UPPER-LEVEL TROUGH AXES---ONE
IN THE EAST NEAR 85W AND THE OTHER IN THE WEST NEAR 130W BY DAY 7.

THE ONE QUESTION THAT WILL BE A FORECAST CHALLENGE AS THE PATTERN
RESHUFFLES---AND WILL LIKELY CHANGE OVER TIME IS---HOW AMPLIFIED
DOES THE MID-CONTINENTAL RIDGE BECOME ALONG 110W THROUGH TIME AS
THE UPPER-LEVEL TROUGH IN ALASKA REACHES THE NORTHEAST PACIFIC? IN
 EARLIER FORECAST CYCLES...THERE APPEARED TO BE VERY LITTLE
SOUTHEASTWARD MOMENTUM AT HIGH LATITUDE---AND THE UPPER RIDGE WAS
TO AMPLIFY (OR NOT) ALONG 110W-120W THEN PERHAPS PINCH OFF A
CUTOFF 500MB FEATURE ALONG 40-45N 135W (OR NOT). THE 25/12Z MEANS
AND THE DETERMINISTIC GUIDANCE HAVE `LET GO` OF THIS
SOLUTION---AND THE MEANS AGREE THAT THE TROUGH ADVANCES FROM THE
GULF OF ALASKA...TO THE WEST CENTRAL COAST OF NORTH AMERICA AND
THE LEADING EDGE REACHING THE CONTINENTAL DIVIDE AROUND DAY 7.
OBVIOUSLY...THE EXACT DETAILS OF SECONDARY SURFACE LOW DEVELOPMENT
INVOF THE AK PANHANDLE AND ACROSS THE ENTIRE ARCHIPELAGO OF
SOUTHWEST BRITISH COLUMBIA VARY ALONG THE LEADING EDGE OF THIS
UPPER-LEVEL TROUGH...BUT THE IDEA THAT A SYSTEM CAN MOVE
NORTHEASTWARD INTO THE RIDGE...SUGGESTS THE BROAD UPPER-LEVEL
RIDGE DOWNSTREAM HAS ROOM TO AMPLIFY AND MIGRATE EASTWARD RATHER
THAN BECOME A `BLOCKY` FEATURE OF THE PATTERN OVER NORTH CENTRAL
CANADA. BECAUSE THE FLOW IS RESHUFFLING OVER NORTH CENTRAL
CANADA...DIFFICULT TO BUY OFF ON ANY `TREND` IN THE EAST.

THE NATURE OF THE UPPER-LEVEL RIDGE...MIGRATORY OR
QUASI-STATIONARY...EVENTUALLY WILL DICTATE THE OUTCOME OF THE
`RESHUFFLE`...BUT THE 25/12Z DETERMINISTIC GUIDANCE TRIES TO
GENERATE A SOUTHEASTWARD-MOVING SHORTWAVE OVER WEST CENTRAL
ONTARIO-NORTHERN MANITOBA AROUND DAY 4 THAT DESCENDS THE EASTERN
FLANKS OF THE RIDGE. THE MAIN REASON I THINK THE 25/12Z
DETERMINISTIC GUIDANCE STRUGGLES IN THE EAST---THERE WILL BE A
CLIPPER SYSTEM TRANSITING THE BASE OF THE DEEP TROUGH IN NORTHERN
QUEBEC AHEAD OF THE AMPLIFYING UPPER-LEVEL RIDGE. AND UNTIL THAT
MID-LEVEL CIRCULATION MOVES THROUGH THE BASE OF THIS TROUGH...VERY
LITTLE NORTHWEST FLOW FROM ARCTIC CANADA CAN ADVANCE INTO THE
LOWER 48. BY THEN...THE TROUGH IN THE GULF OF ALASKA HAS MOVED
INLAND ALONG THE WEST COAST. FOR THE EAST TO GET PROGRESSIVELY
COLDER...THE RIDGE HAS TO REMAIN QUASI-STATIONARY...AND DOUBT THIS
PART OF THE FORECAST...IS STABLE ENOUGH...TO OCCUR AT THE DAY 4-5
TIME FRAME.

...MODEL PREFERENCES...
A BLEND OF THE 25/12Z ECENS/GEFS AND NAEFS OFFERED A VERY
REASONABLE STARTING POINT FOR DAYS 3-7 AND A BLEND OF THE
DETERMINISTIC 25/12Z ECMWF/GFS INTO THE MID-POINT OF DAY
5...SHOULD WASH OUT THE FLIP-FLOP FORECASTS ALONG THE WEST
COAST---AS HEIGHT FALLS AND THE PACIFIC MARITIME AIRMASS MOVES
INLAND.

THE CLIPPER`S TRACK AND DEPTH SEEMS TO HAVE BETTER CONTINUITY AND
DETAIL VERSUS 24 HOURS AGO AND IN PART WILL BE TRACED BACK TO
RESIDUAL VORTICITY THAT HAS MODIFIED SUBTROPICAL ORIGINS (`ANA`)
AND TRACKING ACROSS THE EASTERN PACIFIC AT 35 KTS. THE CANADIAN
SURFACE HIGH BUILDING IN BEHIND THE CLIPPER LOOKS MORE THAN
REASONABLE FROM THE 25/12Z GFS/ECMWF SOLUTIONS. IT`LL BE WORTH
WATCHING...BUT AT THIS POINT IN TIME...PREFER TO HAVE THE CLIPPER
MAINTAIN A MORE EAST-NORTHEASTWARD PROGRESSION...AND REINFORCE THE
COLD FRONT`S DEPARTURE INTO THE WESTERN ATLANTIC. AS MENTIONED BY
THE PREVIOUS SHIFT...THERE REALLY WAS NOT MUCH ENSEMBLE SUPPORT
FOR THE 25/12Z ECMWF SOLUTION IN THE CAROLINAS.

SENSIBLE WEATHER HIGHLIGHTS...
SURFACE CYCLONE MIGRATING FROM THE UPPER GREAT LAKES TO NORTH
CENTRAL QUEBEC WILL PRODUCE A BROAD AREA OF WARM ADVECTION AND
ABOVE AVERAGE TEMPERATURES IN ITS WARM SECTOR... FOLLOWED BY A
NARROW BAND OF MODERATE-INTENSITY RAINFALL ALONG THE COLD FRONT.
BEHIND THE FRONT...A SURGE OF COOLER/DRIER CANADIAN AIR REACHES
THE ATLANTIC COAST ON DAY 4-5. AN ALBERTA CLIPPER...REINFORCING
THE COOLER/DRIER AIRMASS...SHOULD SPREAD THE DRIER AIRMASS ACROSS
THE GREAT LAKES...THE NORTHEAST AND ULTIMATELY THE EASTERN HALF OF
THE CONUS BY DAY 6. THE CANADIAN SURFACE HIGH BUILDING INTO THE
MIDWEST ON DAY 6 SHOULD PROVIDE A NOTABLE DROP IN DEW POINTS FOR
THE SOUTHEAST AND MOST OF FLORIDA---POSSIBLY DROPPING INTO THE 40F
TO 45F RANGE ALONG THE I-20 AND I-10 CORRIDOR---THURSDAY AND
FRIDAY.

IN THE WEST...WHAT STARTS OUT AS A WINDY...WET BUT MILD PACIFIC
FLOW WITH EMBEDDED STRONG SOUTH AND SOUTHEAST WIND BURSTS AT THE
COAST AND WINDWARD COASTAL RANGES---IS FOLLOWED BY SIGNIFICANT
HEIGHT FALLS...AND THE ARRIVAL OF THE POLAR JET AND MODIFIED
MARITIME POLAR AIRMASS---ORIGINATING IN THE GULF OF ALASKA.

FOR THE NORTHERN ROCKIES AND ADJACENT HIGH PLAINS...WIDESPREAD
DOWNSLOPE CONDITIONS ARE EXPECTED...WITH NOTABLY HIGHER WIND
SPEEDS...PERIODS OF HIGH CLOUDS AND RAPIDLY-FLUCTUATING AMBIENT
AIR TEMPERATURES. LITTLE IN THE WAY OF RAINFALL IS
EXPECTED...ASIDE FROM THE WESTERN SLOPES OF THE NORTHERN DIVIDE
WHERE THE MID-LEVEL WARM ADVECTION IN ASSOCIATION WITH REMNANT
`ANA` MID-LEVEL MOISTURE MIGRATES EASTWARD.

THE ONLY ADDITIONAL PRECIPITATION SOURCE FOR THE MIDWEST WILL BE
GENERATED BY THE ALBERTA CLIPPER RACING THROUGH THE MISSOURI AND
OHIO RIVER VALLEYS ON WEDNESDAY NIGHT...THURSDAY AND THURSDAY
NIGHT.

VOJTESAK

$$





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