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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