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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1507Z Apr 22, 2018)
 
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Abbreviations and acronyms used in this product
 
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White


EXTENDED FORECAST DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1107 AM EDT SUN APR 22 2018

VALID 12Z WED APR 25 2018 - 12Z SUN APR 29 2018

15 UTC UPDATE...

THE FORECAST UPDATE THIS MORNING INCORPORATED THE LATEST
DETERMINISTIC GUIDANCE, BUT OVERALL REMAINED VERY CLOSE TO
CONTINUITY. A BLEND OF THE 00Z ECMWF/CMC/UKMET AND 06Z GFS SERVED
AS A STARTING POINT DURING DAYS 3-5 (WED-FRI), WITH MAJORITY
ENSEMBLE MEANS (00Z ECENS/NAEFS) DURING DAYS 6-7 (SAT-SUN). A
COUPLE AREAS OF SIGNIFICANT UNCERTAINTY REMAIN LATER IN THE
FORECAST PERIOD, INCLUDING THE EVOLUTION OF NORTHERN STREAM ENERGY
CROSSING THE PLAINS/MIDWEST THU-FRI, AND WHETHER A CLOSED UPPER
LOW DEVELOPS ACROSS THE OH VALLEY BY FRI-SAT. THE EVOLUTION AND
TRACK OF THE UPPER LOW INITIALLY WELL OFF THE WEST COAST BECOMES
PROBLEMATIC BY DAYS 6-7, WITH LATITUDINAL DIFFERENCES IN THE
POSITION AS WELL AS SIGNIFICANT TIMING DIFFERENCES SHOWN AMONG THE
DETERMINISTIC GUIDANCE. THE BETTER AGREEMENT SHOWN AMONG ENSEMBLE
MEANS JUSTIFIED STEERING THE FORECAST TOWARD THE MEANS BY LATE IN
THE FORECAST PERIOD.

RYAN


PREVIOUS DISCUSSION (ISSUED AT 0540 UTC)...

...OVERVIEW AND GUIDANCE PREFERENCES...

MODELS AND ENSEMBLES CONTINUE TO SHOW PERSISTENT TROUGHING IN THE
EAST AND A CLOSED LOW OFF THE CALIFORNIA COAST EVENTUALLY COMING
ASHORE LATE IN THE WEEK INTO NEXT WEEKEND WITH RIDGING IN BETWEEN,
CENTERED OVER THE ROCKIES. A LEAD AND TRAILING SYSTEM WILL PUSH
THROUGH THE EAST WED-THU WITH A THIRD ON ITS HEELS FRI-SAT, THOUGH
THERE WAS INCREASED TIMING SPREAD WITHIN THE GUIDANCE -- GFS
GENERALLY QUICKER THAN THE ECMWF/UKMET WITH THE ENSEMBLE MEANS IN
BETWEEN. OPTED TO RELY MORE ON THE ENSEMBLE GUIDANCE WITH TIME TO
HELP MITIGATE FUTURE CHANGES TO FRONTAL TIMING. TO THE WEST,
CLOSED LOW APPEARS MUCH BETTER HANDLED UNTIL ABOUT FRIDAY WHEN
THERE BECOMES A N-S DIFFERENCE IN TRACK (THROUGH OREGON OR INTO
CALIFORNIA PER THE ECMWF VS CANADIAN VS FARTHER SOUTH GFS).
ENSEMBLES WERE CLUSTERED NEARER TO THEIR DETERMINISTIC PARENT AND
GIVEN THE MULTI-DAY CHANGES A POSITION IN BETWEEN THE ECMWF/GFS
ENSEMBLE MEANS OFFERED A PRUDENT POSITION, TAKING THE UPPER LOW
THROUGH NORTHERN CALIFORNIA INTO SE OREGON NEXT WEEKEND. 


...WEATHER/THREATS HIGHLIGHTS...

EXITING LEAD AND TRAILING UPPER/SFC LOWS WILL SPREAD RAINS THROUGH
THE NORTHEAST AND THEN THROUGH THE SOUTHEAST NEXT WED-FRI WITH
ANOTHER ROUND WITH THE THIRD SYSTEM NEXT FRI/SAT BEFORE THE FRONT
EXITS THE NORTHEAST BY SUNDAY. TEMPERATURES WILL GENERALLY BE
BELOW AVERAGE WITH THE EXCEPTION OF MAINE.

IN THE WEST, UPPER RIDGING WILL ENSURE WELL ABOVE AVERAGE
TEMPERATURES (+10 TO +20F ABOVE CLIMO) WITH SOME RECORDS POSSIBLE
UNTIL THE CLOSED LOW FINALLY PUSHES INLAND. PRECIPITATION WILL
GRADUALLY INCREASE ALONG THE COAST OF NORTHERN CALIFORNIA INTO
OREGON LATE IN THE WEEK THEN THROUGH THE NORTHERN SIERRAS INTO THE
GREAT BASIN WITH AMOUNTS DEPENDING ON THE TRACK OF THE UPPER LOW.
FAVOR THE CONSENSUS SLOWER THAN THE GFS BY NEXT SUNDAY WHICH WOULD
KEEP MOST/ALL OF THE PRECIPITATION WEST OF THE DIVIDE THROUGH THE
CENTRAL ROCKIES.

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