Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Oct 20 2018

Valid 12Z Tue Oct 23 2018 - 12Z Sat Oct 27 2018

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

A split flow regime will be in place favoring a weak southern
stream trough tracking from the Desert Southwest toward the middle
of the country. Gradually some phasing with the northern branch of
the jet could lead to cyclogenesis across the northern Gulf of
Mexico with the wave curling up along the southeastern U.S. coast.
Across the more northern latitudes, lower heights should
congregate over northeastern North America as an upper low
continues to spin over northern Nunavut. Amplifying flow swinging
through the Great Lakes will ultimately aid in the development of
a coastal wave in the vicinity of Nova Scotia by late
Tuesday/early Wednesday. Back to the northeastern Pacific,
multiple deep cyclones are likely to approach western British
Columbia during the Day 3-5, October 23-25 timeframe. While most
of the action should stay north of the international border with
Canada, the mean flow should turn more zonal across the eastern
Pacific allowing more of an onshore flow regime into the Pacific
Northwest. Toward next weekend, broad ridging is forecast to
gradually encompass the eastern Pacific with longwave troughing
expected to cover the middle of the country. Generally speaking,
outside of the mentioned areas of cyclogenesis, synoptic-scale
gradients should remain on the more washed out across the
remainder of the nation.

Early on, models remain at odds with the position of the surface
low strengthening on approach into the Canadian Maritimes.
Ensemble plots clearly show the dichotomy of solutions with the
12Z GEFS members north of the other global models. Operational
forecasts seem to stick fairly close to their respective camps
with the 00Z guidance offering very little change in these
scenarios. While a wave should spin up, its placement remains
uncertain. As it lifts toward Newfoundland, high pressure will
build over the Great Lakes and northeastern U.S. through Friday.
Back to the northeastern Pacific, rather amped up waves are
anticipated although most of the action should stay over coastal
British Columbia and points offshore. Model agreement exists in
flattening the flow out toward the latter half of the period.
Uncertainty seems to mostly concentrate with the evolution of the
phasing across the middle of the nation leading to modest
cyclogenesis in the Gulf of Mexico. The GEFS and ECMWF ensembles
remain almost a day apart with the 27/1200Z low position being
near the North Carolina coast by the 12Z ECMWF while the 12Z GFS
favors a placement north of Tampa Bay, Florida. The 00Z ECMWF did
make a southern adjustment particularly in light of its prior
solution being on the far northern fringe of its ensembles.
Overall, a multi-model consensus appeared reasonable through Day
4/Wednesday before quickly adding ensemble means to the picture.
Attempted to downplay any individual operational run by the Day
6/7, October 26-27, window given abundant uncertainty with wave
placement and intensity.

Heavy rainfall is likely to impact the Gulf Coast region although
many solutions do keep the most concentrated activity offshore.
The current forecast frontal boundaries keep the system offshore
suggesting any activity inland would be more elevated in nature.
Eventually this precipitation focus will shift north and east with
the potential for wet conditions late in the week over the
southeastern U.S. coast. Across the Pacific Northwest, the
persistence of onshore flow will keep a steady focus for upslope
driven rainfall across the Olympics and Washington Cascades. It
appears the better activity will hold off until later in the week
as the mean flow turns more zonal in nature. Regarding winter
threats, northern Maine could see some snowfall accumulations as
the coastal low takes shape near Nova Scotia by Tuesday/Wednesday.
Looking at the temperature forecast, regions east of the Rockies
are forecast to remain below climatology given the persistence of
upper troughing accompanied by enhanced precipitation chances.
Departures on the order of 10 to 15 degrees are likely over the
Southern Plains through Thursday suggesting highs stay in the 50s
and 60s. Meanwhile, similar anomalies except on the positive side
are noted from the Pacific Northwest eastward into the Northern
Rockies given the influence of maritime air masses during the
period.


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