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Extended Forecast Discussion
 
(Latest Discussion - Issued 1529Z Dec 09, 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
1029 AM EST Sun Dec 09 2018

Valid 12Z Wed Dec 12 2018 - 12Z Sun Dec 16 2018

...Heavy rain threat for parts of the east Fri-Sun...


...Overview and Guidance Evaluation/Preferences...

The weather pattern over the next week is expected to remain quite
active across the Northwest and become increasingly wet for much
of the eastern third of the nation later in the week. The models
continue to waver (sometimes significantly) with the details in an
otherwise rather predictable longwave upper pattern that is
well-represented in the latest ensemble means and ongoing WPC
continuity. Accordingly, the WPC medium range product suite was
primarily derived from a blend of the latest GEFS/ECMWF ensemble
means and continuity with minority weighting applied to the 00Z
ECMWF/UKMET early in the period (favoring the slower solutions)
but the 06Z GFS later in the period (favoring the best ensemble
clustering well away from the much farther south ECMWF in the
Southeast). Confidence is no better than average with the track of
the system next Saturday in the east, but the signal for modest to
locally heavy rain remains focused on the southern Appalachians.


...Weather Highlights and Hazards...

Precipitation/wind periodically increases into the Northwest
midweek and again into the weekend as heights fall with a
succession of approaching shortwaves/frontal systems. Higher
Pacific Northwest terrain will observe the heaviest rain/mountain
snow (with varying snow levels as upper temperatures rise and fall
between each system), with activity then working to the
north-central Great Basin/Rockies. Precipitation may be enhanced
with stronger-than-forecast surface lows that may track near/over
northwestern Washington Friday but these waves have little
predictability at this time range.

By Thursday, a Pacific shortwave will dive from the Rockies into
the southern Plains to reinforce upper troughing while spawning
surface cyclogenesis over Oklahoma. Gulf moisture/warm advection
increasing ahead of the deepening system will help to expand the
precipitation shield to the east and then wrap around to the north
of the surface low. Relatively slow movement of the system will
support a heavy rain threat and some cold sector (marginally cold
at best) snows through the Midwest/Great Lakes and
central/northern Appalachians to the northwest of the uncertain
low track. Rainfall may be heavy from the east side of the
Appalachians to the coastal plain with the infusion of Atlantic
moisture as well, which may fall over areas currently receiving
appreciable snowfall (especially over the southern Appalachians).
Temperatures will trend milder in the east (supporting a mostly
rain event next weekend) but the track of the surface low will
determine the areal extent of wintry weather (most uncertainty in
the Northeast).


Fracasso/Schichtel


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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