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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1606Z Oct 18, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1206 PM EDT Thu Oct 18 2018

Valid 12Z Sun Oct 21 2018 - 12Z Thu Oct 25 2018

...Guidance/Predictability Assessment and Weather

Model and ensemble forecasts seem reasonably well clustered Sun
into Tue, bolstering forecast confidence to above normal levels
with preference of a composite guidance solution. The pattern is
highly amplified at higher latitudes with a mean mid-upper level
trough and surface low/storm solution into Alaska and the
Northeast Pacific. Slow translations leads to an infringement of
lead height falls into western Canada and the northwest U.S. along
with an increasingly precipitation amount and chance of moderate
precipitation days 5-7.

Rapidly increasing forecast spread later Tue into next Wed/Thu in
a transitioning pattern downstream lowers forecast confidence and
suggests preference for an ensemble mean blend and continuity. In
this later time frame recent GFS runs become much less amplified
and more progressive with northern stream flow across Canada and
the northern tier of the lower 48 than the ECMWF. I'd favor
decently amplified flow inertia and lean at least as amplified as
the ensemble means if not quite the ECMWF. At the same time,
recent GFS runs also bring much more organized southern stream
energy with origin from east Pacific tropical activity than other
guidance to produce widespread heavy precipitation well inland
over the southern Plains in stark contrast to alternate guidance
that holds the bulk of precipitation more into the lower
Mississippi Valley and in general closer to the coast and offshore
into the Gulf of Mexico. The GFS seems way overdone considering
latest NHC input and considering that any tropical system influnce
would likely be more disrupted moving inland over the moutains of
Mexico. WPC medium range QPF focuses a heavy rainfall threat over
the western then central Gulf Coast states. Guidance is in better
agreement to bring modest precipitation on the backside up
northward through the southern and central rocky states with
progression of a main southern stream trough from CA/NW Mexico.


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