Prognostic Meteorological Discussion
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000
FXUS02 KWBC 200517
PMDEPD

EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
117 AM EDT FRI APR 20 2018

VALID 12Z MON APR 23 2018 - 12Z FRI APR 27 2018

...HEAVY RAINFALL LIKELY FOR PORTIONS OF THE SOUTHEAST INTO THE
MID-ATLANTIC NEXT WEEK...


...OVERVIEW...

AN UPPER LOW IS FORECAST TO MEANDER AROUND THE CANADIAN
ARCHIPELAGO NEXT WEEK WITH ANOTHER NEAR BRISTOL BAY ALASKA WHICH
SUPPORTS NEAR TO SLIGHTLY ABOVE AVERAGE HEIGHTS OVER THE LOWER 48
AMID PROGRESSIVE BUT STILL WAVY FLOW. AN INITIAL UPPER/SFC LOW
WILL LIFT THROUGH THE EAST COAST NEXT MON-WED WHILE ANOTHER OFF
THE WEST COAST WILL EVENTUALLY (PROBABLY) MOVE INLAND BY LATER IN
THE WEEK.


...GUIDANCE/PREDICTABILITY ASSESSMENT...

IN THE WEST, MODELS AN ENSEMBLES CONTINUE TO STRUGGLE WITH THE
EVOLUTION OF THE DEVELOPING UPPER/CUTOFF LOW NEAR 35N/130-135W BY
NEXT TUESDAY. BY NEXT WED-FRI, LATEST 12Z/18Z GUIDANCE EITHER
TAKES THE UPPER LOW INTO CALIFORNIA AS EARLY AS LATE WED/EARLY THU
(12Z CANADIAN) OR A WEEK FROM SUNDAY (12Z GFS) WITH THE 12Z ECMWF
IN BETWEEN. GEFS ENSEMBLES HAVE BEEN INCONSISTENT AND USUALLY
FOLLOW THE LEAD OF THE DETERMINISTIC GFS BETWEEN QUICKER/SLOWER
WHILE THE ECMWF ENSEMBLE MEAN HAS BEEN MUCH STEADIER THOUGH STILL
WITH LARGE E-W SPREAD IN THE 51 MEMBERS. WITH THE SUPPORT OF THE
12Z CANADIAN ENSEMBLES, OPTED TO RELY MOSTLY ON THE BROAD
CONSENSUS OF THE 12Z ECMWF/CANADIAN/ECMWF ENSEMBLE MEAN AND 18Z
GEFS MEAN BY THE END OF NEXT WEEK WHICH WOULD BRING THE CLOSED LOW
VERY NEAR THE CENTRAL CALIFORNIA COAST.

IN THE EAST, INITIAL CLOSED/SFC LOW WILL TAKE A DEFINED SYSTEM UP
THE COAST IN WINTER-LIKE FASHION BUT WITHOUT THE COLD AIR SINCE I
AM TOLD IT IS LATE APRIL. MODELS FORM A GOOD CONSENSUS THROUGH
ABOUT WEDNESDAY MORNING BEFORE UPSTREAM DIFFERENCES WITH THE
KICKER SYSTEM PUTS THE KIBOSH ON THAT. GIVEN THE LACK OF
CONSISTENCY WITH THAT SYSTEM OPTED TO RELY ON THE GOOD CONSENSUS
OF THE ENSEMBLE MEANS WHICH WERE CLOSER TO THE 12Z CANADIAN AND
EVENTUALLY THE GFS RATHER THAN THE 12Z ECMWF.


...WEATHER/THREATS HIGHLIGHTS...

HEAVY RAINFALL LOOKS PROBABLE/LIKELY IN THE SOUTHEAST INTO THE
MID-ATLANTIC EARLY IN THE WEEK AS THE SFC LOW TRACKS UP THE COAST.
MOISTURE FROM THE GULF OF MEXICO AND THE CENTRAL CARIBBEAN WILL
SURGE AHEAD OF THE COLD FRONT AND AROUND THE LOW -- PRECIPITABLE
WATER ANOMALIES AROUND +1 TO +2 STANDARD DEVIATIONS ALONG THE
ENTIRETY OF THE I-95 CORRIDOR (FROM S TO N) MON-THU AS IT WRAPS
INTO NEW ENGLAND MIDWEEK. 12Z GEFS M-CLIMATE QPF SHOWED 48-HR
VALUES IN THE "MAX" PERCENTILE WHICH MEANS THAT IT`S QUITE UNUSUAL
FOR THE GEFS MEAN TO FORECAST THIS MUCH RAINFALL IN THE
SOUTHEAST/MID-ATLANTIC FOR THIS TIME OF YEAR. STRONG RETREATING
HIGH PRESSURE TO THE NORTHEAST WILL ENSURE STRONG EASTERLY FLOW AT
THE SURFACE WITH WINDY CONDITIONS ALONG THE COAST AND POSSIBLE
COASTAL FLOODING, DESPITE THE RELATIVELY MODEST LOW PRESSURE.

THE WEST MAY ESCAPE THE WEEK WITHOUT MUCH PRECIPITATION, IF ANY,
DEPENDING ON HOW QUICKLY THE CLOSED LOW APPROACHES THE COAST. FOR
NOW, WILL LIMIT QPF TO THE IMMEDIATE COAST AND SIERRAS GIVEN LOW
POPS OF 10-20%. TEMPERATURES SHOULD REMAIN NEAR TO ABOVE NORMAL
WITH ANOMALIES REACHING +10 TO +15F. ISOLATED DAILY RECORDS FOR
HIGHS AND/OR WARM MINIMA MAY BE POSSIBLE. CLOUDS/PRECIP WILL KEEP
HIGHS WELL BELOW NORMAL OVER THE SOUTH AND THEN EAST (BUT
OVERNIGHT LOWS NEAR TO ABOVE AVERAGE) AS LOW PRESSURE TRACKS UP
THE COAST. HIGHS WILL BE MORE TYPICAL OF MID-MARCH IN MANY AREAS.
REINFORCING COOL AIR WILL SURGE OUT OF E MT AND THE HIGH PLAINS ON
MONDAY WITH THE KICKER TROUGH INTO THE MIDWEST AND THEN TO THE
EAST COAST LATER IN THE WEEK.

ASSOCIATED WPC MEDIUM RANGE FORECASTS OF 500 MB HEIGHTS...SURFACE
SYSTEMS...WEATHER GRIDS...QUANTITATIVE PRECIPITATION FORECASTS
(QPF)...AND WINTER WEATHER OUTLOOK SNOW/SLEET PROBABILITIES CAN BE
FOUND AT...

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4


FRACASSO


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