Prognostic Meteorological Discussion
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FXUS02 KWBC 040601
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
200 AM EDT FRI SEP 04 2015

VALID 12Z MON SEP 07 2015 - 12Z FRI SEP 11 2015

...OVERVIEW...
AN ACTIVE HIGH-LATITUDE JET STREAM WILL PREVAIL AND DOMINATE THE
FLOW PATTERN FOR A LARGE PORTION OF THE WESTERN HEMISPHERE. THIS
PROMINENT FLOW WILL GRADUALLY `SUPPLANT` THE PREVAILING
UPPER-LEVEL RIDGE OVER THE SOUTHEAST PORTION OF THE CONTINENT.
VERY UNUSUAL FLOW PATTERN IN THE SUBTROPICAL PACIFIC---WITH 4
TROPICAL SYSTEMS  MEANDERING---IN ONE WAY...SHAPE OR FORM---
BETWEEN 20N AND 40N LATITUDE THIS MEDIUM RANGE PERIOD. SEVERAL OF
THESE SYSTEMS WILL ENTRAIN VARYING PORTIONS OF THEIR MOISTURE
PLUMES INTO THE MID-LATITUDE FLOW AND PROVIDE SOME SENSIBLE
WEATHER CHANGES ACROSS NORTH AMERICA. OVERALL...A 3/12Z ECENS/GEFS
AND DETERMINISTIC BLEND OF THE GFS/ECMWF WORKS THROUGH THE
MID-POINT (9/12Z) OF THE `NEW DAY 5`---THEN A 70/30 BLEND OF THE
TWO MEANS THEREAFTER WILL KEEP GOOD CONTINUITY AND NOT PRODUCE
MUCH `CHANGE` FOR THE MEDIUM RANGE FORECAST.

....UNCERTAINTY ASSESSMENT...
ASIDE FROM THE 3/12Z CANADIAN ENSEMBLE MEAN...THE MODEL ENSEMBLE
CONSENSUS IS DEPICTING A BROAD HEMISPHERIC TROUGH AND COOLER
UPPER-LEVEL FLOW PATTERN AT/ABOVE 55N LATITUDE FROM THE BERING SEA
TO HUDSON BAY AND ATLANTIC CANADA. THIS PATTERN WILL KEEP
`SEPARATION` FROM THE SUBTROPICAL RIDGE PATTERN PREVAILING ACROSS
THE SOUTHERN HALF OF THE LOWER 48---WITH THE EXCEPTION OF A SERIES
OF MID-LEVEL LONG WAVES PROJECTED TO TRAVEL ALONG THE NORTHERN
STREAM AND MIGRATE EASTWARD---STARTING IN THE SHORT RANGE AND
CONTINUING THROUGHOUT THIS MEDIUM RANGE PERIOD. THE UNCERTAINTY
LIES IN THE ABILITY OF THESE SHORTWAVES TO ENTRAIN SUBTROPICAL
MOISTURE AND GRADUALLY WEAKEN THE NORTHERN PERIPHERY OF THE
UPPER-LEVEL RIDGE. AND PRIMARILY ACROSS EAST AND SOUTHEAST NORTH
AMERICA...THE CHALLENGE WILL BE TO TIME THE EVACUATION OF THE
WARM/HUMID AND DRY WEATHER NOW IN PLACE---AND THE LATITUDE WHERE A
COOLER...WETTER WEATHER PATTERN CAN `DISPLACE` THE STAGNANT TROUGH
INVOF THE GULF OF MEXICO AND `MOVE IT` DURING THE MEDIUM RANGE
PERIOD.

...MODEL PREFERENCES...
ACROSS THE WEST COAST OF NORTH AMERICA...THE NORTHERN
STREAM---WITH ORIGINS IN NORTHEAST SIBERIA WILL HAVE ENTRAINED
SOME OF `IGNACIO` (BETWEEN 5/12-7/12Z) PER CPHC/OPC
COORDINATION---WITH THE MOISTURE STREAM FOCUSING BETWEEN 55N-60N
LATITUDE THROUGH THE END OF THE MEDIUM RANGE PERIOD. THE 3/12Z
GEFS/ECENS AND THEIR DETERMINISTIC RUNS REASONABLY HANDLE THE
SYSTEM MIGRATION AND SENSIBLE WEATHER---WITH A BROAD WARM
ADVECTION SCENARIO FOR THE AK PANHANDLE/BRITISH COLUMBIA AND
BRUSHING ACROSS PUGET SOUND (DAY3-4). THIS WARM FRONT AND MODIFIED
NORTHEAST PACIFIC MOISTURE PLUME APPEARS TO HAVE SOME `ROOM` TO
EXPAND DOWNSTREAM ALONG AND EAST OF THE DIVIDE TO THE GREAT
LAKES...BUT THE DETERMINISTIC RUNS (ALL 4 OF THEM...UK/EC/GFS/CMC)
ALL HAVE DIFFERENT `LOOKS` WITH THE FORWARD SPEED AND AREAL EXTENT
OF THE WARM ADVECTION ALONG THE CANADIAN BORDER ASSOCIATED WITH
THE MOISTURE PLUME.

ACROSS THE DESERT SOUTHWEST...THE TROPICAL MOISTURE PLUME
ASSOCIATED WITH THE SYSTEM CURRENTLY KNOWN AS `KEVIN` WILL BE
MIGRATING NORTHWARD---AND PER THE NHC 4/03Z FORECAST
ADVISORY...THE SYSTEM IS NOT EXPECTED TO LOOK LIKE THE 3/12Z
CANADIAN AND UKMET SOLUTIONS. THAT SAID...A PORTION OF ITS
MOISTURE PLUME SHOULD BE LIFTING NORTHEASTWARD AHEAD OF THE DAY
2-3 TROUGH...AND THE PREVIOUS WPC FORECAST PHILOSOPHY AND MODEL
CHOICES WERE CARRIED FORWARD BY THE 3/12Z ECENS/GEFS MEANS. NOT
SURE THE DETAILS OF THE 3/12Z GFS/ECMWF DETERMINISTIC RUNS (AS
PREFECT PROGS) CAPTURE THE INTERACTION BETWEEN THE MID-LATITUDE
AND SUBTROPICAL MOISTURE/ENERGY EXCHANGE BETWEEN 30N-40N ALONG
125W---AT/BEYOND 9/12Z.

CAUGHT IN THE `CROSSFIRE` WITH THE DEVELOPMENTS IN THE
PACIFIC...IS THE QUASI-STATIONARY UPPER-LEVEL TROUGH LODGED IN THE
SOUTHWESTERN GULF OF MEXICO AND THE DECAYING STATIONARY FRONTAL
DRAPE ALONG 30N FROM THE MS DELTA EASTWARD TO THE FIRST COAST OF
FLORIDA. THE PREVAILING UPPER-LEVEL RIDGE TO ITS NORTH---
RESPONSIBLE FOR THE WARM/HUMID EARLY-SEPTEMBER WEATHER IN THE
EAST---WILL BE SLOW TO BREAK DOWN---WITH A SLOWER-MOVING FRONTAL
PROGRESSION. THE 3/12Z ECENS/GEFS MEANS GAVE THE IMPRESSION THAT
THE MEAN UPPER-LEVEL TROUGH RETROGRADES VERY SLOWLY WESTWARD
THROUGH TIME. BUT THE PATTERN APPEARS TO BE MORE OF A
PERSISTENT-TYPE FORECAST BENEATH THE RIDGE (IN THE LOWER LAYERS)
ACROSS THE SOUTHEAST. AND ALOFT IN THE WESTERN GULF---LOOKS FAR
MORE DEPENDENT UPON WHAT IMPACT THE SUBTROPICAL JET-LET OVER
MEXICO AND MOISTURE PLUME FROM `KEVIN` HAVE ACROSS SOUTH TEXAS AND
THE MOISTURE TRANSLATION DOWNSTREAM OVER THE LOWER MISSOURI VALLEY
AHEAD OF THE FRONT. OVERALL...THE 3/12Z DETERMINISTIC ECMWF WOULD
BE THE FASTER/STRONGER SOLUTION ACROSS THE SOUTHERN PLAINS...LOWER
MS VALLEY AND THROUGH THE NORTHEAST WITH THE PROGRESSION OF THE
LOW-LEVEL FRONT VERSUS THE 3/12Z GFS. AT THIS POINT...IT SEEMS TO
BE A TOSS UP...IE  WHICH SOLUTION WILL PREVAIL AND WHETHER THE
FRONT CAN ACTUALLY MAKE A STRONG PUSH INTO THE GULF OF MEXICO IN
THE DAY6-7 PERIOD.

...SENSIBLE WEATHER HIGHLIGHTS...
THE DAY 3-6 FRONTAL PROGRESSION AND SURFACE CYCLONE TRACK APPEAR
TO BE THE PRIMARY REGIONS FOR CONVECTIVE PRECIPITATION ACROSS THE
PLAINS AND MIDWEST.

RESIDUAL MOISTURE FROM `KEVIN` WILL HAVE THE GREATEST IMPACTS ON
THE UPPER RIO GRANDE VALLEY AND FOUR CORNERS/SOUTHERN
ROCKIES---PRIOR TO DAY 5---BEFORE THE MOISTURE TRANSLATES
DOWNSTREAM INTO THE OZARKS AND MID-MS VALLEY.

VOJTESAK



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