Preliminary Forecasts
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FXUS02 KWNH 220640
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
239 AM EDT MON SEP 22 2014

VALID 12Z THU SEP 25 2014 - 12Z MON SEP 29 2014

...OVERVIEW...
IN THE WEST...UNSETTLED WEATHER WILL PREVAIL THIS MEDIUM RANGE
PERIOD WITH AN AMPLIFIED TROUGH MIGRATING ACROSS THE WEST
COAST...GREAT BASIN AND NORTHERN ROCKIES. EAST OF THE
ROCKIES...THE PATTERN BECOMES A LITTLE MORE COMPLEX WITH AN
UPPER-LEVEL RIDGE SLOWLY MIGRATING EAST-NORTHEASTWARD FROM THE
WEST CENTRAL PLAINS STATES TO THE GREAT LAKES AND UPPER OHIO
VALLEY. THE COMPLEXITY LIES EMBEDDED WITHIN A CUTOFF CIRCULATION
THAT BECOMES QUASI-STATIONARY OVER THE CENTRAL PLAINS STATES OF
KANSAS...MISSOURI...NEBRASKA AND IOWA EARLY IN THE
PERIOD---MEANDERING ABOUT---FOR DAYS 4-7---BENEATH THE BUILDING
RIDGE TO ITS NORTH. A SECOND CUTOFF CIRCULATION AND ITS EVOLUTION
OVER THE SOUTHEAST STATES PRESENTS AN EQUALLY-CHALLENGING FORECAST
FOR DAYS 3-4.

...MODEL PREFERENCES...
THE 21/12Z EC AND CANADIAN ENSEMBLES GENERALLY COVER THE AMPLIFIED
PATTERN IN THE LOWER 48 AND A FAST-ZONAL FLOW ACROSS CENTRAL
CANADA FROM COAST TO COAST---WITH THE 21/18Z GEFS A SLIGHTLY
SLOWER AND BETTER OPTION FOR THE LOWER 48 THAN ITS 21/12Z
COUNTERPART.

THE MEANS PROVIDE GOOD CONTINUITY WITHIN THE HIGHER-LATITUDE
NORTHERN-STREAM FLOW BUT WERE TROUBLESOME INVOF THE SUBTROPICAL
LATITUDES---BETWEEN 20N-35N.

IN THE NORTHERN STREAM...THE MEANS OFFERED GOOD CONTINUITY WITH
`DISCERNIBLE` SHORTWAVES AND MIGRATORY WEATHER SYSTEMS MIGRATING
FROM THE GULF OF ALASKA TO HUDSON BAY. I HAD NO TROUBLE
MAINTAINING THE PROGRESSION OF FRONTS AND SURFACE WAVES FROM THE
PREVIOUS WPC SURFACE GRAPHICS. HOWEVER...THE CHALLENGE WITH USING
A HEAVILY-WEIGHTED `MEANS` SOLUTION THROUGH THIS MEDIUM RANGE
PERIOD LACKED IMPORTANT DETAIL OVER THE LOWER 48---BETWEEN 35N-45N
LATITUDE. AT THE VERY LEAST...THE 21/18Z GFS AND 21/12Z ECMWF
DETERMINISTIC RUNS HAVE DECENT CONTINUITY UNTIL 27/12Z ACROSS MUCH
OF THE LOWER 48...BEFORE THEIR SOLUTIONS UNRAVELED IN THE COLORADO
RIVER BASIN...THE NORTHEAST GULF OF MEXICO AND UPPER MISSOURI
VALLEY.

...GUIDANCE ASSESSMENT...
THE OVERALL PREDICTABILITY IN THE WESTERN US HAS IMPROVED WITH
`INBOUND` PACIFIC SYSTEMS ORIGINATING IN THE SOUTHEAST GULF OF
ALASKA AND SOUTHERN AK PANHANDLE. BUT THAT PREDICTABILITY QUICKLY
DROPS OFF BY DAY 3---EAST OF THE MISSISSIPPI---WITH THE OUTCOME OF
AN AMPLIFIED TROUGH CURRENTLY DIGGING ACROSS THE EASTERN GREAT
LAKES. MUCH OF THE UNCERTAINTY IN THE EAST IS TIED TO A CUTOFF
THAT FORMS ALONG THE BASE OF THE DIGGING SHORTWAVE INVOF THE
SOUTHEAST US AND HOW IT INTERACTS WITH THE LOW-LEVEL FRONTAL
BOUNDARY PRESSING SOUTHWARD INTO THE GULF OF MEXICO AND OFF THE
GEORGIA COAST. INITIALLY...THE FRONT IS MORE OF CANADIAN COLD
FRONT...THEN QUICKLY MODIFIES INTO A STATIONARY FRONT ONCE IT
INTERACTS WITH RELATIVELY MILD SEA SURFACE TEMPERATURES AND A
SURFACE HIGH BUILDS TO ITS NORTH. THAT SURFACE HIGH MAINTAINS A
DECENT AND PERSISTENT `WEDGE` ALONG AND EAST OF THE APPALACHIANS
FROM DAYS 3-6.

THE SECOND COMPONENT OF THE UNPREDICTABILITY IN THE EAST IS THE
EXACT PATH OF THE COMPACT CLOSED UPPER-LEVEL SYSTEM---CURRENTLY
OBSERVED ON WATER VAPOR SATELLITE IMAGERY (AS OF 22/0545Z) INVOF
THE NEVADA-UTAH-IDAHO TRI-STATE BORDER. BEYOND 84 HOURS...THE
21/12Z ECENS...GEFS AND NAEFS MEANS `WASH OUT` THE CIRCULATION AND
MUCH OF ITS RESIDUAL VORTICITY IS SWEPT NORTHEASTWARD INTO
SOUTHEAST CANADA. BUT THEIR CORRESPONDING DETERMINISTIC RUNS
MAINTAIN A QUASI-STATIONARY DISTURBANCE---A REMNANT REGION OF
VORTICITY---MEANDERING INVOF THE LOWER MISSOURI VALLEY STATES
DURING THE ENTIRE MEDIUM RANGE PERIOD. THE 21/18Z GFS BEING `MORE
PREFERRED` INVOF THE OZARK VS THE 21/12Z GFS SOLUTION (BY THE END
OF DAY 5).

AFTER DAY 5...THE INTERACTION OF THE EASTWARD-MOVING PACIFIC
TROUGH WITH THE TRAPPED UPPER-LEVEL LOW AND/OR ITS REMNANTS
CREATES A WIDE VARIETY OF QPF SOLUTIONS. OF THE 21/12Z
DETERMINISTIC SOLUTIONS...THE ECMWF MAINTAINS A MUCH SLOWER
SOLUTION VERSUS THE CANADIAN AND GFS---AND CONSEQUENTLY---LOOKED
WETTER OVER THE NORTHERN ROCKIES...DESERT SOUTHWEST AND
MID-ATLANTIC REGION...AND DRIER OVER THE MIDWEST. THE DIFFERENCES
ARE EQUALLY APPARENT IN THE NORTHERN GULF COAST REGION...WITH THE
ECMWF THE WETTEST SOLUTION ALONG THE GULF COAST`S PORTION OF THE
I-10 CORRIDOR FROM PENSACOLA TO JACKSONVILLE.

...SENSIBLE WEATHER HIGHLIGHTS...
PACIFIC TROUGH REMAINS ON TRACK FOR LATE WEDNESDAY AND
THURSDAY---RAIN AND RAIN SHOWER EVENT---WITH ONSHORE FLOW PATTERN
FROM CANADIAN BORDER TO NORTHERN CALIFORNIA.

OTHER HIGHLIGHTS FOR DAYS 3-4 INCLUDE THE +15F TO +25F HIGH
TEMPERATURE ANOMALIES ACROSS THE UPPER MISSOURI VALLEY. THESE
TEMPERATURE ANOMALIES DO MODIFY SLIGHTLY DOWNSTREAM OVER THE GREAT
LAKES AND NORTHEAST (RANGING FROM +10F TO +15F ABOVE NORMAL) FOR
DAYS 4-6 AND DAYS 5-7 RESPECTIVELY.

A MOIST EASTERLY LOW-LEVEL SHOULD YIELD A RATHER HUMID AND LOCALLY
WET...MEDIUM RANGE PERIOD FOR FLORIDA AND PORTIONS OF THE I-20
CORRIDOR FROM SOUTH CAROLINA TO MISSISSIPPI.

VOJTESAK

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