Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Tue Oct 29 2019
Valid 12Z Fri Nov 01 2019 - 12Z Tue Nov 05 2019
1530 UTC Update...
Overall, changes to the ongoing forecast from overnight were
relatively minor. The forecast was updated to reflect a blend of
the latest deterministic guidance (ECMWF/GFS/UKMET) during days
3-5 (Fri-Sun), with days 6-7 (Mon-Tue) based more heavily on the
ECENS/NAEFS ensemble means along with continued deterministic
ECMWF (which fit reasonably well within the ensemble-based
consensus). The one change of note involved the clipper system
crossing the northern Plains/Midwest/Great Lakes Mon-Tue, where
models and ensembles showed a trend toward a slower progression.
This trend was reflected in the updated WPC forecast.
Ryan
Previous Discussion (issued at 0657 UTC)...
...Overview and Guidance/Uncertainty Assessment...
Latest models and ensemble means continue to show an amplified
pattern whose large scale tends to yield better than average
predictability, featuring a northeastern Pacific ridge and
downstream trough whose multi-day average axis should be over
east-central North America. In contrast to the short-range system
affecting the lower 48 and forecast to be over southeastern
Canada/New England as of early Fri--which has been plagued by high
spread and variability in the guidance for multiple
days--solutions cluster remarkably well in most respects through
the middle of the extended period. Not surprisingly shortwave
details become more uncertain toward days 6-7 Mon-Tue.
Based on guidance through the 18Z cycle, a blend of the 18Z GFS
and 12Z ECMWF/UKMET provided a good representation of the system
near New England early Fri with a position near an average of the
18Z GEFS/12Z ECMWF means. Review the Model Diagnostics Discussion
PMDHMD for updated model preferences incorporating the new 00Z
guidance in the short range time frame through 12Z Fri. Behind
this system the aforementioned blend also reflected consensus for
a couple upstream features, a central/eastern U.S. front with
anchoring surface low tracking over southern Canada/Lake Superior
and an upstream wave reaching just north of the northern Plains by
early day 5 Sun. The rest of the forecast trended toward an even
model/mean blend consisting of the 18Z GFS/12Z ECMWF and their
ensemble means, with model details beginning to diverge somewhat
but still in the proverbial ballpark relative to the GEFS/ECMWF
means that were quite similar to each other through day 7 Tue.
This solution would have the southern Canada wave on Sun track
over/near the Upper Great Lakes Mon and northeast thereafter while
the trailing front pushes into the central U.S. in a fashion
similar to the Fri-Sat front.
The relative agreement of the GEFS/ECMWF means conceals some of
the individual member spread that becomes evident over the
northwestern U.S. and western Canada toward the end of the period.
The spread becomes more dramatic when adding in some CMC
ensembles that develop more troughing than most GEFS/ECMWF
members. Over the past couple days the multi-day means by D+8
have been differing over exactly where the strongest positive
height anomaly center(s) will be within an area covering the
northeastern Pacific/Gulf of Alaska through the eastern Bering
Sea. These differences lower confidence in determining exactly
how shortwaves rounding the ridge may amplify downstream late in
the period and beyond.
Along and offshore the California coast there is a converging
trend regarding potential for a weak upper low, whether from one
individual feature or consolidation of multiple impulses.
GFS/GEFS runs are still somewhat deeper than other guidance but an
even blend between them and the ECMWF/ECMWF mean is reasonable
given some recent trends toward the GFS idea in principle.
...Weather Highlights/Threats...
The East will see a rapid drying trend while brisk to strong winds
over the Northeast on Fri will slacken as low pressure lifts
northeastward away from New England. Northern tier areas will see
periods of mostly light rain and/or snow with a couple
waves/frontal passages, including some enhancement to the lee of
the Great Lakes. Specifics become more uncertain after this
weekend. The front extending south from the departing Fri system
will settle over the Florida Peninsula, providing some focus for
occasional rain (mostly light) as easterly low level flow becomes
established.
Some areas over the Rockies and Plains will see temperatures
10-20F or so below normal Fri-Sat followed by readings closer to
normal. Another front dropping south from Canada may bring
modestly below normal temperatures during the first half of next
week. Meanwhile during the weekend a modified form of the chilly
Plains air will reach the East with highs/lows generally 5-15F
below average. Eastern areas should return close to normal by
Mon-Tue. The West will see a gradual increase in coverage of
above normal highs but with anomalies tending to be around plus
10F or less.
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