Preliminary Forecasts
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FXUS02 KWNH 230617
PREEPD

PRELIMINARY EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
216 AM EDT SAT MAY 23 2015

VALID 12Z TUE MAY 26 2015 - 12Z SAT MAY 30 2015

...OVERVIEW...
THE LOWER 48`S WEATHER PATTERN WILL LOOK A BIT MORE LIKE SUMMER
THIS MEDIUM RANGE PERIOD --- WITH A MUCH SLOWER WEST-TO-EAST
PROGRESSION OF ORGANIZED WEATHER SYSTEMS --- A RATHER EXPANSIVE
RIDGE IN THE SOUTHEAST PRODUCING A RATHER MUGGY WEATHER PATTERN
BENEATH IT --- AND A SCATTERED BUT DISORGANIZED
SHOWER/THUNDERSTORM REGIME FOR MUCH OF THE CENTRAL PORTION OF THE
COUNTRY AND CONTINENTAL DIVIDE.

...MODEL PREFERENCES/UNCERTAINTY ASSESSMENT...
DID NOT NOTICE MUCH DIFFERENCE IN THE 22/12Z GUIDANCE --- TO MAKE
ALTERATIONS TO THE CURRENT MEDIUM RANGE FORECAST. THE AMPLIFYING
RIDGE IN THE EAST BETWEEN DAYS 3-6 --- SHOULD CHALLENGE THE
EASTWARD ADVANCE OF A MID-CONTINENTAL FRONTAL PROGRESSION. AND THE
UPPER-LEVEL RIDGE OVER WESTERN CANADA CONTINUES TO BE `UNDERCUT`
BY PACIFIC FLOW ORIGINATING AT HIGH AND LOW LATITUDES.

EAST OF THE CONTINENTAL DIVIDE --- THE ECMWF/GFS WERE IN FAIRLY
GOOD AGREEMENT HEADING INTO DAY 4 --- EJECTING A SHORTWAVE ACROSS
THE MID-MISSOURI VALLEY AND MIGRATING THIS SYSTEM ACROSS THE UPPER
GREAT LAKES. ITS ASSOCIATED FRONT MAKES SOME PROGRESS ACROSS THE
GREAT LAKES---THE MID OHIO VALLEY AND ST LAWRENCE VALLEY ---
BEFORE WEAKENING AS IT INTERACTS WITH A RATHER SUBSTANTIAL SURFACE
RIDGE ALONG/EAST OF THE CENTRAL APPALACHIANS AND CAROLINAS. TO ME
--- THE GFS CONTINUED TO BE THE MORE AGGRESSIVE SOLUTION ---
ALLOWING THE FRONT TO MIGRATE BRIEFLY INTO THE NORTHEAST WITH A
SLIGHTLY DEEPER SURFACE CYCLONE DEVELOPING OVER EASTERN QUEBEC
VERSUS THE DETERMINISTIC ECMWF. THESE DIFFERENCE TRANSLATE TO
SLIGHTLY COOLER TEMPERATURES READINGS ACROSS THE EAST (WHICH HAVE
BEEN GENERALLY DOWNPLAYED BY WPC) GIVEN THE RATHER STRONG
DEEP-LAYERED RIDGE DEVELOPING ACROSS THE SOUTHEAST AND
MID-ATLANTIC REGION BETWEEN DAY 3-6. THEN AGAIN --- THE COOLER
READINGS MAY BE TEMPERED BY CONVECTIVE CLOUDS AND SCATTERED
SHOWERS THAT WILL ACCOMPANY THE FRONT. THE 22/12Z UKMET --- BEING
THE STRONGER `CONVECTIVE` SOLUTION OVER THE MIDWEST ON THURSDAY
MORNING --- LED ME TO THE CONCLUSION THAT TEMPERATURES `MAY` BE
LOCALLY COOLER INVOF THE SOUTHEAST RIDGE AND ITS MUGGY AIRMASS.

WEST OF THE CONTINENTAL DIVIDE --- A SPLIT-FLOW PATTERN EARLY IN
THE PERIOD CONTAINS TWO MIGRATORY WAVES. THE STRONGER OF THE TWO
WAVES ORIGINATES IN THE ALASKA PANHANDLE AND WEST CENTRAL BC
MOUNTAINS AND MIGRATES ACROSS THE NORTHERN ROCKIES DAYS 3-4. A
WEAKER WAVE MOVES INTO THE CALIFORNIA COAST AND MIGRATES ACROSS
THE SOUTHERN GREAT BASIN. THE ECMWF/GFS SOLUTIONS LOOKED
REASONABLE WITH THEIR CLOSED 564DM TO 570DM 500MB LOW MIGRATING
INTO WESTERN MONTANA BY 28/00Z. GENERALLY ACCEPTED THE ECMWF
VERSION AND SEQUENCE OF EVENTS --- GIVEN ITS BETTER RUN-TO-RUN
CONTINUITY --- BUT THERE ARE ENOUGH DIFFERENCES BETWEEN THE GFS
AND ECMWF WITH THE TRACK OF THE WEAKER MIGRATORY IN THE SOUTHERN
PORTION OF THE SPLIT-FLOW PATTERN --- TO RECOMMEND SOMETHING
CLOSER TO THE ECENS/NAEFS MEANS BEYOND DAY 5 AS THIS WAVE MOVES
INTO THE FOUR CORNERS AND EJECTS ACROSS THE SOUTHERN ROCKIES
AROUND DAY 6.

SENSIBLE WEATHER HIGHLIGHTS...
SULTRY WEATHER FOR THE EAST ---WITH POTENTIAL FOR WIDESPREAD UPPER
80S TO MID 90S AND HIT-N-MISS AFTERNOON AND EVENING AIRMASS
SHOWERS/THUNDERSTORMS.

A SLOW-MOVING FRONTAL BOUNDARY WILL BE THE FOCUS FOR
MORE-ORGANIZED PRECIPITATION AND PERIODS OF LOCALLY HEAVY RAINFALL
ACROSS THE SOUTHERN PLAINS...MISSISSIPPI VALLEY...MIDWEST AND
GREAT LAKES.

VOJTESAK

$$




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