Preliminary Forecasts
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FXUS02 KWNH 210636
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
235 AM EDT THU AUG 21 2014

VALID 12Z SUN AUG 24 2014 - 12Z THU AUG 28 2014

...OVERVIEW...
PROGRESSIVE PATTERN IN THE NORTHEAST PACIFIC TRANSLATES TO AN
ABOVE NORMAL RAINFALL PRECIPITATION PATTERN IN THE NORTHERN
ROCKIES AND UPPER MIDWEST-GREAT LAKES THIS FORECAST PERIOD. ACROSS
THE SOUTH...A BROAD SUBTROPICAL RIDGE IS EXPECTED TO ANCHOR HOT
AND HUMID CONDITIONS IN THE LOWER MS VALLEY...OZARKS AND SOUTHERN
PLAINS.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
THE 20/12Z GEFS...NAEFS AND ECENS AND THEIR DETERMINISTIC RUNS
CONTINUE TO PROJECT BELOW NORMAL HEIGHTS...AN ACTIVE JET-LEVEL
WIND REGIME AND NOTABLE MID-LATITUDE PACIFIC TROUGH MIGRATION
ACROSS THE NORTHWEST THIS WEEKEND AND THE NORTHERN PLAINS EARLY
NEXT WEEK. ALTHOUGH THE BULK OF THE COLD CORE ASSOCIATED WITH THE
UPPER-LEVEL TROUGH AND EMBEDDED 500MB CIRCULATION ORIGINATING IN
THE NORTHEAST PACIFIC DO NOT RELEASE INTO THE NORTHERN HIGH PLAINS
UNTIL LATE SATURDAY/SUNDAY MORNING...THE DETERMINISTIC GUIDANCE
CONTINUES TO SUGGEST THAT ENERGETIC SOUTHWEST FLOW ALOFT WITH
SUBTROPICAL ORIGINS WILL PRECEDE THE UPPER-LEVEL TROUGH MIGRATION
ACROSS THE CENTRAL PORTION OF THE CONTINENT---TRACKING SEVERAL
ORGANIZED MID-LEVEL SYSTEMS NORTHEASTWARD FROM THE COLORADO
ROCKIES INTO THE UPPER MISSISSIPPI VALLEY-CENTRAL LAKES
REGION---AHEAD OF THE LARGER HEIGHT FALLS FOR DAYS 3-5. A
COMBINATION OF THE 19/12Z ECENS/NAEFS MEANS APPEARED TO MAINTAIN A
REASONABLE PROGRESSION OF THE LOWER-AMPLITUDE PACIFIC WAVE TRAIN
MIGRATION ACROSS SOUTH-CENTRAL CANADA FOR DAYS 5-7.

A QUICK LOOK AT THE 21/00Z GUIDANCE MAINTAINS GOOD CONTINUITY WITH
THE CURRENT WPC FORECASTS...WITH THE GFS-ECMWF SOLUTIONS SLIGHTLY
MORE AMPLIFIED THAN THE 19/12Z GEFS-ECENS MEANS. THIS SHOULD ALLOW
THE HEAT AND HUMIDITY BENEATH THE SUBTROPICAL RIDGE TO HOLD FIRM
OVER THE OZARKS AND SOUTHERN PLAINS THROUGH THE WEEKEND AND
GENERALLY SPREAD NORTHEASTWARD INTO THE MID-MS VALLEY-LOWER OHIO
VALLEY AHEAD OF THE UPPER-LEVEL TROUGH MIGRATION ACROSS THE
NORTHERN PLAINS AND UPPER MIDWEST. THOSE SOLUTIONS WERE NOT NEARLY
AS PROGRESSIVE AS THE 21/00Z CANADIAN FOR DAYS 6-7.

THE PACIFIC COLD FRONT MOVING THROUGH THE NORTH CENTRAL STATES ON
DAY 3-4 SHOULD BE AN ACTIVE ONE...AND REMAIN ACTIVE FOR DAYS
4-5--PERIODICALLY INTERACTING WITH SUBTLE SUBTROPICAL WAVE ENERGY
ORIGINATING IN THE NORTHERN GULF OF MEXICO.

...SENSIBLE WEATHER HIGHLIGHTS...
ENERGETIC MID-LEVEL PACIFIC CYCLONE MIGRATES ACROSS THE
DAKOTAS---EARLY IN THE PERIOD---AND SPREADS A RATHER CHILLY
AIRMASS FOR LATE AUGUST INTO THE NORTHERN ROCKIES AND UPPER HALF
OF THE MISSOURI VALLEY THROUGH MONDAY NIGHT AND TUESDAY MORNING.
HIGH TEMPERATURES--ESPECIALLY IN MONTANA AND NORTHERN
WYOMING---WILL BE A GOOD 15F-20F BELOW NORMAL. THIS COOLER AIRMASS
EVENTUALLY SETTLES INTO THE NORTH CENTRAL ROCKIES OF SOUTHERN
WYOMING AND CENTRAL COLORADO ON TUESDAY AND WEDNESDAY. TEMPERATURE
PROFILES SUGGEST THE HIGHER ELEVATIONS OF TETONS WILL RECEIVE
LIGHT SNOWFALL FROM THIS SYSTEM.

THIS MID-LEVEL CYCLONE WILL HAVE A WET SIDE TO IT---WITH DAY 4-7
PRECIPITATION AMOUNTS GENEROUSLY DISTRIBUTED ACROSS THE UPPER HALF
OF THE MISSOURI BASIN AND PORTIONS OF THE UPPER MIDWEST. A BLEND
OF THE ECMWF/GFS DETERMINISTIC GUIDANCE ANTICIPATES WIDESPREAD 1+
INCH AMOUNTS DURING THE 96-HOUR FORECAST PERIOD. MUCH LESS
CONFIDENCE WITH THE DAY 4-7 DETERMINISTIC DETAILS CONCERNING
CONVECTIVE PRECIPITATION CONTRIBUTIONS AND DISTRIBUTIONS  FROM THE
CENTRAL ROCKIES--- EASTWARD ACROSS KANSAS, MISSOURI, IOWA,
ILLINOIS AND MICHIGAN ALONG THE PROGRESSIVE COLD FRONT.

A SUBTLE FEATURE OF THE MEDIUM RANGE FORECAST WILL BE THE COOLER
CONDITIONS ALONG THE EAST COAST FROM MAINE INTO NORTHEAST FLORIDA.
HIGH PRESSURE TO THE LEE OF THE APPALACHIANS WILL BE VERY SLOW TO
WEAKEN AND DISPERSE--LEADING TO BELOW NORMAL TEMPERATURES FOR MUCH
OF THE PERIOD.

VOJTESAK

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