Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 AM EDT Mon Oct 22 2018

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

...Guidance/Predictability Assessment/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 which spawns a wave of low pressure
traversing the northern Gulf of Mexico on Thursday/Friday before
the parent shortwave attains a negative tilt. During the weekend,
a strengthening coastal low will likely lift poleward in the
vicinity of the Gulf Stream with potential impacts to the Eastern
Seaboard. 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.

Recent guidance is initially in decent agreement with much of the
uncertainty looming around the phasing shortwaves and eventual low
pressure development/progression. Ensemble low plots depict
continued scatter although this is likely an artifact of a pair of
waves initially in the picture. Operational models continue to
waver with the ultimate timing of a deepening surface low off the
southeastern U.S. coast Friday evening. On the western side of the
spread are the 00Z UKMET as well as the 00Z/12Z ECMWF runs while
the GFS solutions and their ensembles stay a bit farther offshore.
Given the run-to-run variability and spread shown in the ensemble
low plots, confidence still remains on the lower side with
timing/position details. Moving westward, height falls dipping
down from Saskatchewan will lead to another surge of colder air to
the Mississippi River Valley region with the shortwave likely
joining the downstream mean trough. Ensemble spaghetti plots show
numerous spatial differences which somewhat hinges on the timing
of the lead impulse. Finally, across the eastern Pacific and
adjacent western coast of North America, ensemble means are at
odds with one another by next Monday, October 29. The 12Z ECMWF
ensemble mean has trended toward a longwave trough in this region
while the 18Z GEFS/12Z NAEFS means favor quasi-zonal to weak
ridging. Ensemble spaghetti plots reflect these differences with
possible changes likely in future model cycles.

Regarding the preference, utilized a combination of the 18Z/12Z
GFS and 12Z ECMWF with their respective ensemble means (including
the 12Z NAEFS) throughout the period. While initially weighting 80
percent of these operational runs into the mix on Day 3/Thursday,
incrementally decreased their influences through the weekend
before a full ensemble approach by the following week.


...Weather Highlights/Threats...

The most active precipitation patterns will focus along the
central/eastern Gulf Coast and along the Eastern Seaboard as well
as back over the Pacific Northwest. Across the former regions, the
mentioned surface low and accompanying tropical moisture fluxes
will maintain a wet period from southern Louisiana eastward into
the southeastern U.S. coast. As the coastal cyclone deepens over
the Gulf Stream, comma-head precipitation should extend back into
the Eastern Seaboard with impacts driven by the proximity of the
wave to the coast. There may even be some wintry implications over
the northeastern U.S. interior mountains. 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. Considering the temperature forecast, below average
temperatures are in the forecast from the Great Plains eastward
given the persistent of troughing. Some of the largest departures,
on the order of 10 to 15 degrees, will congregate along 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. If the GEFS/NAEFS ensemble solutions
were to verify, this mild weather could extend into the following
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