Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sun Oct 21 2018

Valid 12Z Wed Oct 24 2018 - 12Z Sun Oct 28 2018

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

At the start of the period, a strengthening wave of low pressure
will be in the process of exiting New England toward the Canadian
Maritimes. As these height falls adjoin the persistent negative
height anomaly across Nunavut, the focus shifts toward a phasing
pair of streams across the middle of the country. This process
will ultimately lead to a development of a surface wave which
tracks eastward across the northern Gulf of Mexico before making a
northward turn toward the Gulf Stream next weekend. Considering
the pattern across the eastern Pacific, after another powerful
cyclone lifts toward the Gulf of Alaska, the mid/upper-level flow
turns more zonal. Eventually marked height rises take shape toward
the western U.S. which establishes a higher amplitude pattern into
the following weekend. Northwesterly flow on the eastern extent of
this mentioned ridge could lead to an impressive clipper-like
system over the Mississippi River Valley by Day 6/7, October 27/28.

Model spread has vastly improved with the initial coastal low
exiting New England mid-week. As the pattern amplifies across the
middle of the country, the 12Z UKMET stood out as an outlier as
its solution favored a building ridge over the western Atlantic
which is not shown in other models. Shifting focus to the Gulf of
Mexico development, ensemble low plots still exhibit a great deal
of spread with multiple waves possible. The suggested negative
tilt of the shortwave indicates a coastal low is likely to form
over the western Atlantic but details are quite nebulous. After
being slower in recent model cycles, the quicker trend shown by
the 18Z GFS has continued with its latest 00Z run. Run to run
continuity is lacking in most of the global guidance which
suggests diminished confidence with any forecast details. As this
wave nears the Gulf Stream next weekend, renewed height falls in
the form of a clipper system may impact the Middle Mississippi
Valley as shown by the 18Z GFS as well as some ensemble members of
various global models. Overall, the preference favored a consensus
of the 12Z GFS/ECMWF/CMC through Day 4/Thursday before quickly
adding ensemble means to the picture. The 12Z GFS/ECMWF
combination seemed to cluster well but the inconsistency of
operational runs suggests ensemble approaches are necessary into
the latter half of the period.

The wettest conditions during the period should congregate along
the Gulf Coast region given the eastward migrating wave of low
pressure. Recent guidance continues to favor the heaviest rainfall
just offshore where the better instability should set up. However,
it would not take much to move some of these convective elements
inland. The focus should gradually move northeastward with the
coastal Carolinas getting in on the action Friday onward. Across
the Pacific Northwest, somewhat persistent onshore flow will keep
conditions wet during much of the period. Local orographics will
dominate with most of the activity focusing over the Olympics and
Cascades. Considering wintry precipitation during the forecast,
the best threat should be over northern Maine with the initial
coastal low mid-week. Some chances may occur next weekend with the
next coastal system but too much uncertainty exists to pinpoint
more precise details. Regarding temperatures, well below average
readings are anticipated from the middle of the country eastward.
On Wednesday, departures from climatology could sit in the 15 to
20 degree range over the Southern Plains with similar numbers
likely on Thursday up into New England given prevailing
northwesterly flow. In terms of above average temperatures,
Wednesday/Thursday could prove to be rather mild days over the
Upper Intermountain West and Northern Rockies as highs move well
into the 60s.


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