Prognostic Meteorological Discussion
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FXUS02 KWBC 251553
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1153 AM EDT SAT MAR 25 2017

VALID 12Z TUE MAR 28 2017 - 12Z SAT APR 01 2017

...OVERVIEW AND GUIDANCE EVALUATION/PREFERENCES...

THE PATTERN ACROSS NORTH AMERICA THROUGH THE MEDIUM RANGE WILL
PERSIST OF A RELATIVELY 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 FARTHER EAST, WITH A NUMBER OF
RELATIVELY LOW AMPLITUDE WAVES PROPAGATING THROUGH THE NORTHERN
STREAM ACROSS CANADA, AND A MORE AMPLIFIED SOUTHERN STREAM ACROSS
THE CONUS. PARTIAL PHASING OF THE TWO STREAMS WILL OCCUR
PERIODICALLY ACROSS SOUTHEASTERN CANADA AND THE GREAT
LAKES/NORTHEASTERN U.S.

SUCH PHASING WILL BE IN THE PROCESS OF OCCURRING ON DAY 3 (12Z
TUE) ACROSS THE NORTHEASTERN U.S. AS A FRONTAL SYSTEM WITH A
NORTHERN COMPONENT ACROSS SOUTHERN QUEBEC AND A SOUTHERN COMPONENT
ACROSS THE OHIO VALLEY MOVE EAST, WITH THE EVENTUAL DEVELOPMENT BY
DAY 4 (12Z WED) OF A DEEPENING SURFACE LOW EAST OF NEW ENGLAND
WHICH SKIRTS THE CANADIAN MARITIMES. MODELS SHOW RELATIVELY LITTLE
SPREAD ON DAY 3 PRIOR TO THE COASTAL LOW DEVELOPMENT, BUT SPREAD
INCREASES BY DAY 4 WITH THE 00Z ECMWF/UKMET/CMC SHOWING A QUICKER
PROGRESSION WHILE THE 06Z GFS ALONG WITH THE FEW PREVIOUS GFS RUNS
HAVE BEEN PERSISTENTLY SLOWER.

FARTHER WEST, A CLOSED UPPER LOW WILL BE CROSSING THE FOUR CORNERS
REGION ON DAY 3, WITH MODEL SPREAD RELATIVELY LOW INITIALLY. AS
THE UPPER LOW MOVES EAST INTO THE PLAINS ON DAYS 4-6, DIFFERENCES
EMERGE WITH RESPECT TO THE DEGREE OF DEAMPLIFICATION THAT OCCURS
AS THE WAVE MOVES INTO A MID/UPPER-LEVEL RIDGE AXIS ACROSS THE
SOUTHEASTERN STATES. THE 00Z/06Z RUNS OF THE GFS HAVE CONTINUED TO
KEEP THE WAVE MORE AMPLIFIED, WHICH RESULTS IN A MORE SOUTHERN
TRACK ACROSS THE SOUTHEAST. ON THE OTHER HAND, THE 00Z EC/EC MEAN
ALONG WITH THE 06Z GEFS MEAN WEAKEN THE SYSTEM A LITTLE QUICKER
WHICH RESULTS IN A LESS AMPLIFIED WAVE TRACKING A LITTLE FARTHER
NORTH.

MODELS ARE IN GENERAL AGREEMENT ON ANOTHER UPPER TROUGH MOVING
INTO THE PACIFIC NORTHWEST ON DAY 5. CONSENSUS IS HIGH IN THE
GENERAL EVOLUTION OF THIS SYSTEM, AS IT IS FORECAST TO QUICKLY
AMPLIFY AND DIG SOUTHWARD ACROSS THE INTERIOR WEST, WITH A CLOSED
UPPER LOW POSSIBLE BY DAY 6 (12Z FRI) ACROSS THE SOUTHWESTERN U.S.
THE 00Z ECMWF IS ON THE FASTER SIDE OF THE SPREAD THROUGHOUT THE
EVOLUTION OF THIS SYSTEM, WHILE THE GFS HAS SHOWN GOOD RUN-TO-RUN
CONTINUITY WITH A SLOWER SOLUTION, WHICH IS ALSO SUPPORTED BY THE
00Z ECMWF ENSEMBLE MEAN. FURTHER, THE EC MEAN HAS SHOWN GOOD
RUN-TO-RUN CONTINUITY ON A SOLUTION MORE LIKE THE GFS. THUS, WILL
LEAN AWAY FROM THE DETERMINISTIC 00Z ECMWF BY DAYS 6-7.

BASED ON THE ABOVE ANALYSIS, THE WPC FORECAST WAS INITIALLY BASED
ON A 50/50 BLEND OF THE 06Z GFS AND 00Z ECMWF ON DAYS 3 AND 4.
AFTER DAY 4, THE WPC FORECAST TRENDED GRADUALLY TOWARD A MORE
ENSEMBLE BASED APPROACH, WITH THE 00Z ECMWF ENS MEAN WEIGHTED
SLIGHTLY MORE THAN THE 06Z GEFS MEAN BY DAYS 6-7 (FRI AND SAT).


...SENSIBLE WEATHER HIGHLIGHTS/POTENTIAL HAZARDS...

THE CENTRAL U.S. WILL SEE A RATHER STORMY PATTERN THROUGH THE
MEDIUM RANGE DUE TO REPEAT UPPER-LEVEL LOWS EMERGING FROM THE
WESTERN U.S. WED AND THURS WILL BRING THE POTENTIAL FOR WIDESPREAD
CONVECTION ACROSS THE SOUTHERN AND CENTRAL PLAINS AS THE FIRST
UPPER LOW AND THE ASSOCIATED SURFACE FRONTAL SYSTEM SLOWLY MOVE
EAST. IMPRESSIVE LOW-LEVEL MOISTURE FLUX AND INSTABILITY SUGGEST
THE POTENTIAL FOR EXCESSIVE RAINFALL/FLASH FLOODING ACROSS
PORTIONS OF THE SOUTHERN AND/OR CENTRAL PLAINS ON WED/THURS.
ADDITIONALLY, THE STORM PREDICTION CENTER IS FORECASTING THE
POTENTIAL FOR SEVERE THUNDERSTORMS FROM PORTIONS OF THE SOUTHERN
PLAINS TO THE LOWER MISSISSIPPI VALLEY DURING THAT TIME. THIS
SYSTEM WILL MOVE EAST ON FRI AND SAT WITH THE POTENTIAL FOR
SHOWERS AND THUNDERSTORMS TO AFFECT AREAS FROM THE MS/OH VALLEYS
TO THE EASTERN U.S. GIVEN INCREASING MODEL SPREAD, SIGNALS ARE
MUCH LESS CLEAR FOR ANY INTENSE CONVECTION ON DAYS 6-7 ACROSS
THESE AREAS. THE PACIFIC NORTHWEST WILL ALSO SEE THE POTENTIAL FOR
HEAVY PRECIPITATION EARLY IN THE MEDIUM RANGE AS THE EVENTUAL
SECOND WESTERN U.S. CLOSED LOW MOVES ONSHORE AS A SHORTWAVE LATE
ON DAY 3 INTO DAY 4, ACCOMPANIED BY A POTENTIAL ATMOSPHERIC RIVER
PRODUCING HEAVY RAINS AND MOUNTAIN SNOWS.

WITH RESPECT TO TEMPERATURES, THE SPLIT FLOW PATTERN AT THE
MID/UPPER LEVELS WILL KEEP MUCH OF THE CONUS GENERALLY MILD WITH
NEAR OR ABOVE AVERAGE TEMPERATURES THROUGH THE MEDIUM RANGE. THE
NORTHEAST AND PORTIONS OF THE MID-ATLANTIC WILL SEE THE POTENTIAL
FOR THE INCURSION OF COOLER MARITIME AIR BY THE SAT-MON TIME
PERIOD AS NORTHEASTERLY FLOW DEVELOPS AT THE LOWER LEVELS. BELOW
AVERAGE TEMPERATURES ARE ALSO EXPECTED ACROSS THE INTERIOR WESTERN
U.S. BY THU-FRI AS THE SECOND TROUGH/UPPER LOW AMPLIFIES OVERHEAD
AND HEIGHTS QUICKLY FALL.

RYAN

$$





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