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Extended Forecast Discussion
(Latest Discussion - Issued 1559Z Oct 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
1158 AM EDT Mon Oct 22 2018

Valid 12Z Thu Oct 25 2018 - 12Z Mon Oct 29 2018


Deep eastern troughing and a coastal storm will be the focus for
the medium range period as the remnants of Hurricane Willa in the
Pacific combine with northern stream energy to bring a widespread
rainfall to much of the east later this week into this weekend.
This will maintain a cool pattern for the central/eastern states
and mild temperatures for the west.

...Guidance/Predictability Assessment and Preferences...

The synoptic-scale pattern ahead will initially feature a sprawled
out upper trough extending from the Canadian Maritimes back up
toward northern Nunavut. The southern extension of these lower
heights should gradually gain latitude in response to the next
system taking shape across the central U.S. A phasing of streams
continues to be advertised between a wave of low pressure
traversing the northern Gulf of Mexico on Thursday/Friday
(remnants of Willa) before the parent shortwave attains a negative

Farther upstream, quasi-zonal flow sets up over the northeastern
Pacific through Friday which favors a series of perturbations to
track into the Pacific Northwest and British Columbia coasts. A
broad positive height anomaly emerges for the weekend across the
western U.S. while migratory shortwaves traverse the top of the
ridge with a track from British Columbia/Alberta toward the
north-central U.S. Many solutions support significant
amplification with a couple of these waves as they move toward the
Mississippi River Valley over the weekend. However, given the
uncertainty with the tropically-infused nor'easter and a lack of
upper level anomaly centers to guide the flow, opted to continue
with a middle ground solution nearest the 00Z ECMWF ensemble mean.

With the developing Gulf and western Atlantic system, the 00Z
ECMWF/UKMET/Canadian lied farther west than the ensemble means and
the recent GFS. Preferred a solution just west of the ensemble
consensus given the likely amplified pattern and the tendency for
the GFS to be too weak/east with east coast systems beyond
~100hrs. Recent FV3-GFS runs were closer to the ECMWF-led cluster
but there remained significant timing differences as the low lifts
up the coast. This also maintained continuity from the overnight
forecast. By the end of the period, with so much spread in all
directions as the low heads toward/into Canada and the secondary
system rotates through the Ohio Valley, relied on the ensembles
(mostly the ECMWF ensemble mean) to form a consensus with some
artistic manipulations.

...Weather Highlights/Threats...

Precipitation will focus along the central/eastern Gulf Coast and
along the eastern seaboard with some modest precipitation over the
Pacific Northwest as a couple frontal boundaries waver through
WA/OR/ID. The eastern coastal cyclone will deepen over the Gulf
Stream and spread comma-head precipitation back into the coastal
plain as its tropical connection sustains itself through the
period. Impacts will be driven by the proximity of the low to the
coast. To the north, marginally cold air would support some wintry
precipitation over the northeastern U.S. interior mountains
(central/northern Appalachians). Back over the northwestern U.S.,
persistent onshore flow will allow for multiple days of
orographically-driven precipitation. The usual focus should be
over the upslope terrain of the Olympics and Washington Cascades.

Below average temperatures are forecast from the Great Plains
eastward given the persistent troughing. Some of the largest
departures, on the order of 10 to 15 degrees, will congregate over
much of the Eastern Seaboard given the extended period of cloud
cover/precipitation as well as post-frontal cold advection.
Meanwhile, milder weather will prevail over sections of the West
Coast into the Intermountain West given the influence of Pacific
air masses on the forecast and upper ridging moving into
California, perhaps continuing into next week.


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