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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0647Z Oct 19, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 AM EDT Fri Oct 19 2018

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

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

Split flow will initially set up across North America with a weak,
stretched out southern stream anchoring the Desert Southwest. This
feature should gradually press eastward in response to a lowering
of upstream heights. Across the higher latitudes, highly amplified
flow will be in place with a mean upper low spinning over the
upper reaches of Nunavut. A pair of shortwaves embedded in this
flow are primed to intensify and eventually become
negatively-tilted over the vicinity of New England. Many solutions
show a rather deep surface cyclone over the Canadian Maritimes by
mid-week. Back to the northeastern Pacific, a very active pattern
sets up consisting of at least a pair of powerful surface lows
crashing into coastal British Columbia. Some solutions deepen such
waves below 965-mb in some cases while shearing into the
downstream ridge. Ultimately the pattern favors the eastward
migration of a mean ridge toward central Canada while the
synoptic-scale flow flattens across the northeastern Pacific.
Several perturbations in the flow are primed to intensify in this
regime with uncertainty looming with each instance. Looking to the
Gulf of Mexico, a lingering frontal zone and eventual wave of low
pressure should impact the Gulf Coast with a threat for heavy
rainfall.

Models have a reasonable grasp on the pattern at hand through
roughly Day 4/Tuesday before uncertainty looms on both coasts. The
12Z UKMET remained south of the consensus off the New England
Coast by mid-week with 12Z ECMWF ensemble spread improving
relative to yesterday. Across the Pacific, the conveyor belt of
shortwaves traversing the active flow will remain an issue given
the depth of many such features. Decent clustering exists with the
initial wave off the British Columbia coast on Tuesday with more
uncertainty looming offshore with the next amplifying shortwave.
Run-to-run spatial differences are noted with GFS solutions with
this issue noted among other global models. At the very least,
ensemble means are juxtaposed atop one another which will be the
preferred route late in the period (Day 6/7, October 25/26).
Before then, a multi-model blend of the 18Z GFS and 12Z
ECMWF/UKMET seemed reasonable early on.

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 Maine if the low
pressure were to deepen enough to pull in colder thicknesses 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.


Rubin-Oster


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