Prognostic Meteorological Discussion
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000 FXUS02 KWBC 230617 PMDEPD 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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