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Extended Forecast Discussion
 
(Latest Discussion - Issued 1600Z Apr 21, 2019)
 
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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
1200 PM EDT Sun Apr 21 2019

Valid 12Z Wed Apr 24 2019 - 12Z Sun Apr 28 2019

...Overview and Guidance Preferences...

Models and ensembles continue to show a lot of spread for possible
solutions within the progressive northern stream and slower
southern stream, and degree of interaction between the two
streams.  This type of scenario lends itself to low predictability
at extended time frames.  In addition larger scale pattern
differences that develop over parts of the northern/central
Pacific and Bering Sea contribute to differences that arise
downstream late in the period.  These issues keep forecast
confidence below average--especially by the Fri-Sun.

Especially with the arrival of the 06Z GFS and agreement from
latest GEFS means, there is better clustering for the
trough/possible embedded upper low tracking eastward across the
southern tier states.  The old 12Z ECMWF had jumped somewhat
toward more phasing than prior runs but the new 00Z run has
reversed back to more separation and now the 00Z CMC is the only
model to show faster progression.  Ensemble means offer reasonable
support for the 06Z GFS/00Z ECMWF in principle for most of the
period.  Note that while the probability for maintaining
separation per latest majority cluster is greater, it is a close
enough call whether northern stream flow affects the southern
feature that significant changes in future guidance are still
possible.  Meanwhile guidance for the midweek system crossing
southeastern Canada and New England has gravitated toward a
concentrated upper low and fairly vigorous surface system.

There are two major question marks in the forecast over the
eastern Pacific into western North America.  First is the degree
of separation that develops well offshore the West Coast during
the latter half of the week.  ECMWF/ECMWF ensemble runs still show
considerable separation leading to a southern stream closed low
but are a bit faster than 24 hours ago.  The 00Z UKMET/CMC provide
added support for the ECMWF idea.  The 06Z GFS adjusted to a
solution about halfway between the ECMWF cluster and faster 00Z
GFS.  These considerations provide greater support for tilting the
forecast more toward the ECMWF cluster.  The second issue is
rapidly increasing spread that occurs over the Bering Sea (ranging
between deep GFS troughing and strong ECMWF ridge) and farther
south over the Pacific (differences in closed low position). 
Respective ensemble systems have tended to reflect hints of their
operational counterparts but actually agree remarkably well on
broad cyclonic flow over the northwestern states--in contrast to
the deep trough/upper low which 00-06Z GFS runs show over the West
Coast by next Sun.  Thus prefer to emphasize the ensemble
mean/ECMWF ideas for this part of the forecast.

Based on above considerations the updated forecast started with a
blend of operational guidance for days 3-5 Wed-Fri and then
transitioned toward primary weight on the 00Z ECMWF/ECMWF mean and
06Z GEFS mean.


...Weather Highlights/Threats...

Widespread showers and thunderstorms are likely with the system
crossing the Southern Plains and Lower Mississippi Valley mid-late
week and models/ensembles continue to show a strong signal for
heavy rainfall from portions of central Texas east toward the
Mississippi River.  Severe thunderstorms are also possible. 
Please see convective outlooks issued by the Storm Prediction
Center for further details on the severe weather threat.  Expect
scattered to numerous showers and thunderstorms to progress
eastward into the Southeast as well as the Tennessee/Ohio Valleys
(where Gulf moisture may interact with a front) by late Thu into
Fri.  The heavy rainfall signal is less clear over these areas--as
well as the Southeast Coast/Florida into the weekend--but would
expect continued potential for at least locally heavy rainfall
given the recent gravitation of guidance toward the slower half of
the spectrum.  Farther west, the energetic upper-level trough
entering the northwestern U.S. late in the week should produce
rain and mountain snow from the Northwest to the northern Rockies.
 Rain and possibly even snow (though mostly along/north of the
Canadian border) may spread into areas from the Northern Plains to
the Upper Midwest/Great Lakes as energy aloft and associated low
pressure move into the central U.S. by next weekend.  Multiple
uncertainties over details of flow aloft by next weekend lower
confidence in coverage of scattered precipitation over parts of
the West next weekend.

Above average temperatures are likely through much of the period
across central and southern portions of the western U.S.  Expect
broad coverage of high temperatures 10 to 20 deg F above average
from California to the Great Basin Wed through Fri or Sat.  The
most likely daily records will be for warm lows Wed-Fri but a few
record highs are possible as well.  The warmth should spread into
the Rockies and portions of the High Plains by Fri-Sat while
cooler temperatures overspread the Northwest/northern Rockies and
eventually northern Plains in association with the arriving upper
trough/cold front.

Rausch/Ryan


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

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