Prognostic Meteorological Discussion
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FXUS02 KWBC 250657
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
256 AM EDT SAT OCT 25 2014

VALID 12Z TUE OCT 28 2014 - 12Z SAT NOV 01 2014

...REMNANTS OF `ANA` THREATEN BRITISH COLUMBIA AND THE PACIFIC
NORTHWEST...

OVERVIEW...
AN `ATMOSPHERIC RIVER` LOOKS TO SET UP ALONG THE PACIFIC NORTHWEST
COAST DURING THE FIRST HALF OF THIS MEDIUM RANGE PERIOD. WITH
`ANA` IN AN EXTRATROPICAL TRANSITION PHASE OVER THE CENTRAL
PACIFIC PRIOR TO THE START OF THE MEDIUM RANGE---SEE THE CPHC
FORECAST FOR MORE DETAILS---THE EXPECTED JET-LEVEL WINDS WILL PLAY
AN IMPORTANT ROLE IN THE EVOLUTION OF THE WAVE AMPLITUDE
DOWNSTREAM ACROSS THE LOWER 48 THROUGH DAY 7.

UNCERTAINTY ASSESSMENT...
LIKE YESTERDAY...THE FORECAST CHALLENGE IS TO PROJECT---HOW
ORGANIZED WILL A DAY 2-3 CYCLONE IN THE WEST BE? WITH REMNANTS OF
A TROPICAL SYSTEM---ON DAY 4-5 AFTER CROSSING THE ROCKIES---THEN
THE MIDWEST. TO CREATE SIGNIFICANT OR NOTEWORTHY SENSIBLE
WEATHER---WITH LITTLE TO NO LOW-LEVEL INFLOW TO WORK WITH...THE
ONLY THING DISCONCERTING ABOUT THE FORECAST---AT THIS POINT---WILL
BE THE `UN-ANTICIPATED` ACCELERATION OF THE 700MB-250MB WINDS
ASSOCIATED WITH IT.

MODEL PREFERENCES...
THE 24/12Z ECENS/NAEFS/GEFS MEANS SEEMED TO CAPTURE THE AMPLITUDE
OF THE WAVE PATTERN SETTING UP ACROSS THE LOWER 48 THIS MEDIUM
RANGE PERIOD. BY DAY 7...THE ECENS MEAN IS A SLIGHTLY FASTER
SOLUTION ALOFT ALONG THE WEST COAST AND A SLIGHTLY DEEPER SOLUTION
IN THE EAST. AFTER DAY 5...THE DETERMINISTIC 24/12Z ECMWF/GFS AND
CANADIAN GENERATE TOO MUCH SPREAD TO COUNT ON ANY OF THEM TO
GENERATE `PERFECT PROGS` RESULTS...ESPECIALLY IN THE WEST.

IN THE EAST---THE 24/12Z GFS/ECMWF DETERMINISTIC RUNS SHOW DECENT
UTILITY IN THE EASTERN PORTION OF NORTH AMERICA FOR DAYS 3 THROUGH
7---ESPECIALLY WITH THE CYCLONE TRACK AND PROGRESSION OF THE
SYSTEM`S COLD FRONT FROM THE MISSISSIPPI VALLEY TO THE WESTERN
ATLANTIC SHORELINE BETWEEN DAYS 3-5. THEIR OBVIOUS DIFFERENCES
CONCERN THE EVOLUTION/INTENSIFICATION OF THE SURFACE LOW BETWEEN
WEDNESDAY NIGHT AND FRIDAY MORNING...AND THE DEPTH OF A SHORTWAVE
(AN ALBERTA CLIPPER) TRANSITING THE BASE OF THE UPPER-LEVEL TROUGH
WEDNESDAY NIGHT TO FRIDAY MORNING. THIS SHORTWAVE GENERALLY
FOLLOWS THE ENTIRE COURSE OF BOTH---THE MISSOURI AND OHIO RIVERS.
THE ONE CONCERN WITH THIS CLIPPER WILL BE THAT ITS CONTENTS
INCLUDE SOME RESIDUAL ENERGY/VORTICITY FROM POST-TROPICAL
`ANA`---SCHEDULED TO ARRIVE ALONG THE WEST COAST ON DAY 3.

IN THE WEST...THE 24/12Z ECMWF LOOKS TO BE THE SLOW OUTLIER---WITH
THE OTHER DETERMINISTIC RUNS AND ITS OWN 12Z ECENS MEAN SOLUTION
ON/AFTER 30/00Z. USING THE ECMWF `WHOLESALE` WOULD DELAY THE
ARRIVAL OF THE MODIFIED POLAR AIRMASS INTO NORTHERN CALIFORNIA BY
A GOOD DAY---AND IN THE DAY 6-7 PERIOD...RECOMMEND SOMETHING A TAD
FASTER GIVEN THE SIGNIFICANT HEIGHT FALLS EXPECTED ALONG THE
OREGON/WASHINGTON COASTS WITH THE ANTICIPATED/IMMINENT ARRIVAL OF
`ANA` AND HER REMNANTS.

SENSIBLE WEATHER HIGHLIGHTS...
SURFACE CYCLONE MIGRATING FROM THE UPPER GREAT LAKES ON DAY 3 TO
NORTH CENTRAL QUEBEC ON DAY 5...PRODUCES A BROAD AREA OF WARM
ADVECTION AND ABOVE NORMAL TEMPERATURES FOR A LARGE PORTION OF THE
EAST...A NARROW BAND OF MODERATE RAINFALL INTENSITY ALONG ITS COLD
FRONT...AND A SOUTHEASTWARD SURGE OF COOLER CANADIAN AIR BEHIND
ITS COLD FRONT. THE COLDER AIRMASS SPREADS ACROSS THE GREAT LAKES
AND THE NORTHEAST FOR DAYS 6-7...AND PROVIDE A NOTABLE DIFFERENCE
ACROSS THE SOUTHEAST AND GULF COAST---PRIMARILY A SIGNIFICANT DEW
POINT CHANGE.

IN THE WEST...CONSIDERABLE PERIODS OF STRONG SOUTH AND SOUTHEAST
WIND BURSTS AT THE COAST AND WINDWARD COASTAL RANGES---WITH
PERIODS OF WARM ADVECTION STRATI-FORM PRECIPITATION SPREADING LEE
OF THE CASCADES. THE JET-DRIVEN...HEAVY RAINS GENERALLY
CONCENTRATING FROM THE US/CANADA BORDER ON DAY 3...FOLLOWED BY
HEIGHT FALLS DOWN THE COAST INTO SOUTHWEST OREGON BY DAY
5...SIGNAL THE ARRIVAL OF THE POLAR JET AND THE MODIFIED MARITIME
POLAR AIRMASS ASSOCIATED WITH THE GULF OF ALASKA.

NORTHERN ROCKIES...ADJACENT HIGH PLAINS SHOULD SEE WIDESPREAD
DOWNSLOPE CONDITIONS...INCLUDING NOTABLY HIGHER WIND SPEEDS AND
RAPIDLY-FLUCTUATING AMBIENT AIR TEMPERATURES---BUT LITTLE IN THE
WAY OF RAINFALL. ASIDE FROM THE WESTERN SLOPES OF THE NORTHERN
DIVIDE WHERE THE MID-LEVEL WARM ADVECTION RAINS ARE LIKELY...THE
ONLY ADDITIONAL PRECIPITATION SOURCE WILL BE GENERATED BY THE
ALBERTA CLIPPER RACING THROUGH THE MISSOURI AND OHIO RIVER VALLEYS
ON WEDNESDAY AND THURSDAY.

VOJTESAK







$$





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