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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1546Z Oct 19, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Fri Oct 19 2018

Valid 12Z Mon Oct 22 2018 - 12Z Fri Oct 26 2018

...Guidance/Predictability Assessment and Weather
Highlights/Threats...

Latest models and ensembles are well clustered through midweek in
a pattern with above normal forecast confidence. Prefer a
composite guidance solution to smooth out lingering small scale
variance. Prefer to lean more on ensemble guidance and continuity
by days 6/7 as variance gradually grows.

The pattern is highly amplified at higher latitudes with a mean
mid-upper level trough and surface low/storm solution into Alaska
down to the Northeast Pacific...including powerful surface lows
crashing into coastal British Columbia. Lead height falls into
western Canada and the Pacific Northwest along with an
increasingly precipitation amount and chance of moderate
precipitation. Downstream, highly amplified flow will be in place
with a mean upper low spinning over the upper reaches of Nunavut.
Shortwaves embedded in this flow are primed to intensify and
eventually become negatively-tilted near New England. Many
solutions show a rather deep surface cyclone over the Canadian
Maritimes by midweek.

Well south in split flow that initially sets up across North
America, a weak and stretched out southern stream trough anchors
from the Desert Southwest. This feature should gradually press
eastward in response to a lowering of upstream heights.
Downstream, toward the Gulf of Mexico, a lingering frontal zone
and eventual wave of low pressure should impact the Gulf Coast
states then Southeast with a threat for heavy repeat rainfall.
Expect more 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.

A heavy rainfall threat will remain along the western/central Gulf
Coasts with the axis moving eastward in time accompanying the weak
area of low pressure. It remains to be seen how much instability
will approach the coast but hefty QPF is noted in the guidance
just offshore. The forecast itself tends to keep this axis right
along the coast but the ultimate placement with be heavily driven
by where the instability sets up. Across the Pacific Northwest,
semi-persistent onshore flow will keep conditions wet over the
Olympics and Cascades throughout the period. Pacific dominated air
masses will preclude any notable winter weather threats. Across
New England, some snow may fall over northern sections if the low
pressure were to deepen enough to pull in colder air down from
eastern Canada. Regarding temperatures, below average numbers will
prevail over much of the central/eastern U.S. During the early to
middle part of next week, readings could be around 10 to 15
degrees below climatology over the Southern Plains with persistent
northerly flow and active precipitation. Over the northwestern
U.S., air masses of Pacific origin will keep mild air in place
through at least the middle of next week.


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