Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Oct 26 2019
Valid 12Z Tue Oct 29 2019 - 12Z Sat Nov 02 2019
...Well below average temperatures expected over much of the
West/Plains next week...
...Potential storm may affect the eastern half of the country
during the latter half of the week...
...Overview and Guidance/Uncertainty Assessment...
Models and ensembles indicate that the northern periphery of the
strong eastern Pacific ridge, extending through western Canada as
of Tue, will quickly flatten. This should change the shape of
downstream troughing over the course of the period--aligned from
near Hudson Bay into the western U.S. during the short range time
frame into midweek followed by a migration into the central U.S.
Within this transition the dominant feature of interest and
forecast problem will involve the energy digging into the Interior
West/Rockies on Tue and then ejecting eastward/northeastward with
time. Guidance thus far has varied considerably with the details
but there seems to be an improving signal for the general idea
that this energy will support a strengthening storm system that
may support areas of heavy rainfall, meaningful snow in parts of
the cold sector, and possibly strong winds.
Over multiple model runs the GFS/GEFS have consistently been on
the progressive extreme with the energy initially digging into the
West, while teleconnections/recent verification in this
pattern/historical biases have favored some variation of a slower
solution such as represented by recent ECMWF runs. Starting with
the 18Z cycle the GFS/GEFS have made a significant adjustment
toward the slower scenario but remain conspicuously fast/east,
especially with the new 00Z UKMET somewhat slower than its
previous run. Meanwhile the 00Z CMC has trended a bit faster than
its past couple of runs that were on the slow edge of the
spread--and is probably a good adjustment in light of approaching
upstream flow and recent tendency for initially slow solutions in
this pattern to adjust a little faster. 12-hourly ECMWF runs have
been oscillating with 12Z runs slower than 00Z runs while the
ECMWF mean has been somewhat more stable. As a result the updated
forecast emphasized an average of the past two ECMWF runs and the
12Z ECMWF mean. For days 3-4 Tue-Wed the blend included the 12Z
UKMET as well as the 12Z CMC before it become noticeably slow.
This kept the forecast well-centered given the ongoing detail
uncertainty. Model runs are providing a steadily improving signal
that the surface low may deepen significantly as it tracks into
the Great Lakes and southern Canada so as confidence improves the
forecast will likely reflect a deeper system.
Behind this system model differences have less significant impact
on sensible weather. However preferences still lean away from the
GFS/GEFS mean as those two solutions (especially the GEFS mean)
have been on the weaker side of the spread for the eastern Pacific
ridge aloft.
...Weather Highlights/Threats...
The forecast remains on track for a large portion of the western
and central U.S. to see much below normal temperatures, with a
broad area seeing one or more days with highs and/or lows at least
20F below normal. Expect the most extreme anomalies over the
northern and central Rockies/High Plains on Tue-Wed with some
readings 30-40F below normal. Such temperatures will likely
challenge daily records for lows and/or cold highs at some
locations. There will be some moderation after midweek but it
will be gradual. Still expect to see strong winds to the south of
the strong surface high building southward over the northern into
central Rockies.
The signal from the guidance is steadily improving for the idea
that ejecting western dynamics will support a strengthening storm
system affecting the eastern half of the country with an array of
significant weather after midweek, with a surface low most likely
tracking through the Ohio Valley/Great Lakes in the Thu-Fri time
frame and then continuing into Canada. Best potential for
meaningful snow will extend from the central Rockies across the
central Plains and then northeastward into the Upper Midwest/Great
Lakes. Areas of heavy rainfall and potentially strong convection
will be possible in the warm sector as low level flow brings Gulf
moisture northward. Some locations over the southern Plains/Lower
Mississippi Valley east/northeast into the Appalachians should see
the highest totals from this event but significant rainfall should
extend well northeastward. Potential strength of the system may
support some strong winds, most likely over the Great Lakes and
Northeast. After this system departs the pattern should be quite
dry over much of the lower 48 with perhaps some scattered light
snow over the extreme northern tier with a cold front dropping
southeast from Canada and a little lingering rainfall over Florida.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are 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