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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1558Z Apr 23, 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
1158 AM EDT MON APR 23 2018

VALID 12Z THU APR 26 2018 - 12Z MON APR 30 2018

...OVERVIEW AND GUIDANCE PREFERENCES...

THE LARGE SCALE PATTERN WILL FEATURE AN UPPER LOW MOVING INTO THE
WEST AHEAD OF SOME POSSIBLE UPSTREAM TROUGHING LATE IN THE PERIOD,
AN INITIAL WESTERN RIDGE BUILDING INTO THE PLAINS/MS VALLEY, AND A
RELOADING EASTERN TROUGH THAT SHOULD BECOME SOMEWHAT LESS
AMPLIFIED AS IT DRIFTS EASTWARD BY THE SUN-MON TIME FRAME.  THE
MOST CONTENTIOUS PART OF THE FORECAST INVOLVES THE EVOLUTION OF
POTENTIAL TROUGHING BEHIND THE EASTERN PACIFIC/WESTERN U.S. UPPER
LOW.

REGARDING THE EASTERN PACIFIC/WESTERN U.S. FLOW, RECENT GFS RUNS
ARE MOST AGGRESSIVE WITH AMPLIFYING NORTHEAST PACIFIC ENERGY
BEHIND THE UPPER LOW, LEADING TO MUCH QUICKER EJECTION OF THE LOW
INTO/THROUGH THE WEST VERSUS THE ECMWF/CMC AND THEIR MEANS.  BY
LATE IN THE PERIOD THE GFS BECOMES FAIRLY EXTREME AND IN SOME
CASES AN OUTLIER COMPARED TO THE FULL ENSEMBLE SPREAD FOR WEST
COAST TROUGHING.  TELECONNECTIONS RELATIVE TO POSITIVE HEIGHT
ANOMALIES OVER THE NORTH PACIFIC AND ATLANTIC HINT AT THE
POSSIBILITY OF MODEST TROUGHING BUT NOT THE RESULTING NEGATIVE
ANOMALIES WHICH THE GFS SCENARIO WOULD HAVE NEAR THE WEST COAST. 
GEFS MEANS ARE LESS EXTREME WITH THE OVERALL EVOLUTION THAN THE
GFS BUT STILL SOMEWHAT MORE PROGRESSIVE THAN PREFERRED.  ON THE
OTHER HAND THE ECMWF/CMC PLUS THEIR MEANS ARE TRENDING TOWARD
SOMEWHAT MORE OF A NORTHEAST PACIFIC/BC TROUGH HEADING TOWARD THE
PACIFIC NORTHWEST TOWARD SUN-MON.  THIS LEADS TO GREATER
SIMILARITY WITH THE GEFS MEAN WHICH SUGGESTED THIS IDEA STARTING A
CYCLE OR SO AGO.

WITHIN THE EASTERN MEAN TROUGH, GFS RUNS ARE ON THE FAST SIDE WITH
THE SYSTEM INITIALLY EJECTING THROUGH THE NORTHEAST.  BEYOND THAT
A BLEND/COMPROMISE LOOKS REASONABLE, LEANING AWAY FROM LOWER
CONFIDENCE DETAILS SUCH AS EMBEDDED UPPER LOWS EVIDENT IN 00Z
GFS/CMC RUNS OVER THE EASTERN STATES FOR AT LEAST A PART OF THE
FRI-SUN FORECAST.  UPSTREAM ISSUES LEAD THE GFS TO BECOME MORE
AMPLIFIED THAN CONSENSUS WITH NORTHEASTERN TROUGHING BY LATE IN
THE PERIOD.

BASED ON THE ABOVE CONSIDERATIONS THE UPDATED FORECAST MAINTAINED
A FAIR DEGREE OF CONTINUITY BY WAY OF 00Z ECMWF/ECMWF
MEAN/CMC/UKMET INPUT EARLY (BEFORE THE UKMET STRAYS NORTHWARD WITH
THE EAST PACIFIC UPPER LOW), FOLLOWED BY A TRANSITION TOWARD
GREATER WEIGHT OF THE ECMWF MEAN WITH LESSER INCORPORATION OF THE
NAEFS MEAN/ECMWF.  BY SUN-MON THE PRIMARY CONTINUITY ADJUSTMENT
WAS TO ACCOUNT FOR TRENDS WITH HEIGHT FALLS REACHING THE PACIFIC
NORTHWEST BUT NOT NEARLY TO THE EXTENT OF 00Z ECMWF.


...WEATHER/THREATS HIGHLIGHTS...

UPPER RIDGING OVER THE WEST LATE THIS WEEK AND THEN PROGRESSING
EASTWARD WILL BRING A BROAD AREA OF WARMTH TO WESTERN AND THEN
CENTRAL AREAS DURING THE PERIOD.  GREATEST ANOMALIES OF PLUS
10-20F VERSUS CLIMO ARE MOST LIKELY OVER THE WEST DURING THE
LATTER HALF OF THE WEEK WITH SOME DAILY RECORDS POSSIBLE (MORE
LIKELY FOR WARM MINS).  THE COMBINATION OF UPPER LOW MOVING INTO
THE REGION FROM THE EAST PACIFIC AND POSSIBLE TRAILING TROUGHING
ALOFT SHOULD LEAD TO A COOLING TREND OVER THE WEST LATE IN THE
PERIOD.  AT THE SAME TIME ABOVE NORMAL TEMPS WILL SPREAD INTO THE
PLAINS BUT WITH ANOMALIES GENERALLY SOMEWHAT LESS EXTREME THAN
FORECAST OVER THE WEST LATE THIS WEEK.  AS HEIGHT FALLS COME INTO
THE WEST EXPECT A GRADUAL INCREASE OF PRECIP COVERAGE FROM LATE
WEEK ONWARD, WITH SNOW LEVELS FAIRLY HIGH INITIALLY BUT GRADUALLY
DECREASING WITH TIME.  MOST RAIN THAT FALLS SHOULD BE OF LIGHT TO
MODERATE INTENSITY.  RETURN FLOW FROM THE WESTERN GULF SHOULD BE
SUFFICIENT TO SUPPORT MOSTLY LIGHT/SCATTERED RAIN OVER THE
SOUTHERN ROCKIES/HIGH PLAINS BY THE WEEKEND, SPREADING ACROSS THE
REST OF THE PLAINS EARLY NEXT WEEK AND BEYOND.

OVER THE EAST EXPECT PERIODS OF UNSETTLED WEATHER FROM THE LATTER
HALF OF THE WEEK INTO THE WEEKEND DUE TO FEATURES EMBEDDED WITHIN
THE MEAN TROUGH ALOFT.  GRADUAL DEPARTURE OF THE UPPER TROUGH WILL
LIKELY PROMOTE A DRYING TREND BY SUN-MON.  TEMPS WILL TEND TO BE
BELOW NORMAL BUT LIKELY BEGIN TRENDING CLOSER TO NORMAL BY NEXT
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


RAUSCH