Prognostic Meteorological Discussion
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000 FXUS02 KWBC 271522 PMDEPD EXTENDED FORECAST DISCUSSION NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 1122 AM EDT MON MAR 27 2017 VALID 12Z THU MAR 30 2017 - 12Z MON APR 03 2017 ...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES... THE PATTERN ACROSS NORTH AMERICA THROUGH THE MEDIUM RANGE WILL CONTINUE TO FEATURE AN ACTIVE FLOW REGIME FROM THE NORTH PACIFIC INTO BRITISH COLUMBIA AND THE U.S. NORTHWEST. THE FLOW THROUGHOUT THE PERIOD WILL SPLIT AS IT MOVES EAST, WITH A NUMBER OF RELATIVELY LOW AMPLITUDE WAVES PROPAGATING THROUGH THE NORTHERN STREAM ACROSS CANADA, AND A HIGHLY AMPLIFIED, LOW WAVENUMBER SOUTHERN STREAM FLOW ACROSS THE CONUS. PARTIAL PHASING OF THE TWO STREAMS WILL OCCUR PERIODICALLY ACROSS SOUTHEASTERN CANADA AND THE GREAT LAKES/NORTHEASTERN U.S., WITH ONE AREA OF LOW PRESSURE EXPECTED TO DEEPEN OVER THE NORTH ATLANTIC SOUTH OF THE CANADIAN MARITIMES EARLY IN THE MEDIUM RANGE, AND ANOTHER SURFACE LOW POTENTIALLY DEVELOPING ALONG THE MID-ATLANTIC COAST BY SAT. TWO CLOSED UPPER-LEVEL LOWS ARE EXPECTED IN THE SOUTHERN STREAM FLOW ACROSS THE CONUS DURING THE PERIOD. THE FIRST OF THESE IS FORECAST TO BE CENTERED ACROSS THE SOUTHERN/CENTRAL PLAINS ON DAY 3 (THU), WITH THE SECOND EXPECTED TO DEVELOP AS A VIGOROUS SHORTWAVE MOVES ONSHORE IN THE PAC NW ON DAY 3 AND QUICKLY AMPLIFIES AS IT MOVES SOUTHEASTWARD, WITH A CLOSED UPPER-LOW ACROSS THE FOUR CORNERS REGION ON DAY 4 (FRI). ONE OF THE MAIN SOURCES OF UNCERTAINTY IN THE FORECAST THIS PERIOD EMANATES FROM HOW QUICKLY THESE UPPER LOWS BEGIN THE PROCESS OF PARTIALLY PHASING WITH THE NORTHERN STREAM, AND HOW QUICKLY THEY WEAKEN AND MOVE EASTWARD. THE ECMWF HAS SHOWN A NOTABLE BIAS FOR THE PAST FEW DAYS OF PHASING THESE SYSTEMS, AND MOVING THEM EAST TOO QUICKLY. THE ECMWF HAS ALSO SHOWN SOME DEGREE OF RUN-TO-RUN INCONSISTENCY, WITH A GENERAL TREND TOWARD KEEPING THE UPPER LOWS UNPHASED AND DEEPER FOR A LONGER PERIOD OF TIME. THE GFS HAS BEEN REMARKABLY CONSISTENT WITH RESPECT TO THESE FEATURES FROM RUN-TO-RUN OVER THE PAST FEW DAYS, AND THE SAME HOLDS TRUE TODAY. THE 00Z ECMWF ENSEMBLE MEAN WAS A BIT SLOWER WITH THE PROGRESSION OF THESE SYSTEMS RELATIVE TO THE DETERMINISTIC 00Z ECMWF, BUT THE GFS ALONG WITH THE CMC, GEFS MEAN, AND NAEFS MEAN ALL ARE ON BOARD WITH THE SLOWER AND LESS QUICKLY PHASED SOLUTION. ADDITIONAL UNCERTAINTY ARISES BY DAYS 6-7 (SUN-MON) AS ANOTHER PACIFIC SHORTWAVE APPROACHES/REACHES THE NORTHWEST AND AMPLIFIES. THE GFS/CMC/GEFS/NAEFS ALL SHOW A SLOWER PROGRESSION OF THIS SYSTEM RELATIVE TO THE 00Z EC/EC MEAN. NOTABLY, THE ECMWF HAS TRENDED GRADUALLY SLOWER WITH THIS WAVE, WHILE THE GFS HAS REMAINED FAIRLY CONSISTENT. THE WPC FORECAST LEANED HEAVILY TOWARD THE 06Z GFS/06Z GEFS/00Z NAEFS SOLUTIONS, WHICH SHOW A SLOWER PROGRESSION OF THE UPPER LOWS AS THEY CROSS THE CENTRAL U.S., AND A SLOWER ARRIVAL OF THE SHORTWAVE TO THE PAC NW DAYS 6-7 RELATIVE TO THE 00Z EC/EC MEAN. MINORITY COMPONENTS OF THE 00Z EC MEAN WERE INCLUDED, TO ACCOUNT FOR SOME DEGREE OF UNCERTAINTY IN THE SOLUTIONS. ...SENSIBLE WEATHER HIGHLIGHTS/POTENTIAL HAZARDS... GIVEN THE ANOMALOUS AND ENERGETIC NATURE OF THE SYSTEM MOVING FROM THE CENTRAL TO THE EASTERN U.S. ON DAYS 3-5, MODELS SHOW THE POTENTIAL FOR SUBSTANTIAL LOW/MID-LEVEL MOISTURE FLUX AHEAD OF THE SYSTEM. THUS, THE POTENTIAL FOR AREAS OF DEEP CONVECTION APPEARS ENHANCED, ESPECIALLY ACROSS PORTIONS OF THE LOWER MS VALLEY/TN VALLEY/SOUTHEAST. THE STORM PREDICTION CENTER IS HIGHLIGHTING A POTENTIAL THREAT OF SEVERE THUNDERSTORMS ON THU ACROSS THOSE AREAS. THE SYSTEM AMPLIFYING ACROSS THE WESTERN U.S. INITIALLY MAINTAIN A POOL OF ANOMALOUS MOISTURE ORIGINATING IN THE TROPICAL PACIFIC, WHICH ALONG WITH THE RAPIDLY FALLING HEIGHTS COULD SUPPORT AREAS OF HEAVY RAIN AND SNOW ACROSS THE INTERIOR WEST ON THU AND FRI. ADDITIONALLY, A PERIOD OF EASTERLY LOW-LEVEL UPSLOPE FLOW NORTH OF A DEVELOPING SURFACE CYCLONE MAY PRODUCE A CONCENTRATED AREA OF HEAVY PRECIPITATION FOCUSED ALONG THE EASTERN SLOPES OF THE CO ROCKIES FRI NIGHT INTO SAT. AS THE SECOND UPPER LOW MOVES INTO THE SOUTHERN/CENTRAL PLAINS BY DAYS 6-7, VIGOROUS MOISTURE RETURN FROM THE GULF OF MEXICO WILL ONCE AGAIN COMMENCE, AND AREAS OF INTENSE CONVECTION WILL BE POSSIBLE FROM PORTIONS OF THE SOUTHERN PLAINS TO THE LOWER MS VALLEY. WITH RESPECT TO TEMPERATURES, THE SPLIT FLOW PATTERN WILL KEEP MUCH OF THE CONUS GENERALLY MILD WITH NEAR OR ABOVE AVERAGE TEMPERATURES. THE NORTHEAST AND PORTIONS OF THE MID-ATLANTIC WILL SEE THE POTENTIAL FOR THE INCURSION OF COOLER MARITIME AIR BY LATE IN THE WEEK INTO THE WEEKEND AS HIGH PRESSURE BUILDS IN TO THE NORTH AND LOW PRESSURE POTENTIALLY DEVELOPS NEAR THE MID-ATLANTIC COAST BEFORE MOVING OFFSHORE. BELOW AVERAGE TEMPERATURES WILL ALSO ACCOMPANY THE UPPER LOW ACROSS THE INTERIOR WEST THU AND FRI AND INTO PORTIONS OF THE SOUTHERN/CENTRAL PLAINS BY THE WEEKEND. RYAN $$

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