Prognostic Meteorological Discussion
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000
FXUS02 KWBC 311852
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 PM EDT Sat Oct 31 2020

Valid 12Z Tue Nov 03 2020 - 12Z Sat Nov 07 2020

...Heavy precipitation for the Pacific Northwest...

...Overview...

After cold upper-level troughing exits the Northeast Tue,
generally zonal large-scale flow can be expected across the CONUS
through about Thu, with the exception of some embedded shortwaves.
Then, troughing comes into the West by the end of the week,
spreading cooler temperatures and precipitation to a larger
proportion of the West after some potentially heavy precipitation
in the Pacific Northwest around midweek given moist inflow and a
series of frontal passages. Other than that, most of the CONUS
should stay dry through the medium range period, with the
exception of rain over the Florida peninsula under easterly
post-frontal flow well to the north of a possible tropical system
that is being monitored by NHC for development in the Caribbean.

...Guidance Evaluation/Preferences...

For the most part, 00Z and 06Z model guidance was well clustered
with the overall pattern over the CONUS described above, including
a shortwave moving across the central U.S. Wed/Thu. Thus a
multi-model deterministic blend was able to be used for the first
half of the forecast period, while transitioning to more usage of
the GEFS and EC ensemble means for the mass fields by the end of
the period to reduce influence of small-scale differences from the
deterministic models. This method maintained good continuity with
the previous WPC forecast.

Recent GFS runs (00, 06, and 12Z) have been aggressive with
bringing the possible tropical system from the Caribbean toward
Florida by next weekend. At this point, there is little support
for this solution from the 00Z EC ensemble members and other
guidance, which push the potential system into central America
earlier on. So these GFS solutions were not favored at this point.
But this will continue to be monitored for future forecasts, as
the tropical system evolution has been inconsistent in most
models. Uncertainty remaining with a possible upper low/trough
around the Gulf Coast next weekend also contributes to the
uncertainty with the tropical system`s movement.

...Weather/Hazard Highlights...

The initial troughing in the East will help cause cold
temperatures with potential for record cold highs Tue, while
lake-enhanced snows linger downwind of Lake Ontario. Otherwise,
widespread above normal temperatures are likely to spread across
the western and central U.S. (where some record highs are
possible) toward the eastern U.S. later in the week. The most
anomalous temperatures are expected in the Northern/Central
Plains, where highs should be 15 to 25 degrees above normal into
the 60s and 70s. Temperatures are forecast to cool to below normal
in the West Coast toward the Northern Rockies by Sat as the main
upper trough comes in.

Higher than normal moisture values with an atmospheric river
coming into the Pacific Northwest Tue, as well as a series of
frontal systems passing through, should lead to modest to heavy
precipitation totals especially in the Cascades and Olympics.
Given the troughing coming in late in the week, rain and lowering
elevation snows are forecast to spread toward the Northern Rockies
Thu/Fri and farther south by Sat. Rain is also forecast for
southern and eastern parts of the Florida peninsula for the second
half of the week, due to easterly flow to the north of a frontal
system that may stall over Cuba during the period.

Tate

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

$$




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