Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sun Oct 25 2020
Valid 12Z Wed Oct 28 2020 - 12Z Sun Nov 01 2020
...Heavy precipitation threat from the Plains/Mid-Mississippi
Valley eastward to the Mid-Atlantic later this week...
Deep closed low will exit out of New Mexico early Wed and travel
ENE to the Mid-Atlantic by Friday as it weakens into an open wave.
Just ahead of its path, newly formed Tropical Storm Zeta will
likely make landfall on Wednesday per the National Hurricane
Center just ahead of the upper low and its surface fronts. The two
sources of moisture will merge into/along the frontal boundary
over the Tennessee Valley Thursday and move to the East Coast
Friday. The GFS runs remained quicker with the timing of both the
upper low and Zeta while the UKMET was slowest. Preferred the more
stable guidance near the 12Z ECMWF/Canadian and ECMWF ensemble
mean along with the slightly quicker GEFS mean for the upper low.
Zeta lied closer to the GFS but also noted an unusually high
degree of uncertainty not uncommon in nascent storms. How the
interaction evolves will dictate the axis/orientation of heavy
rain but the event itself looks quite probable. Pattern becomes
much quieter by next weekend CONUS-wide.
Very cold airmass in the Plains will moderate but many areas will
see see temperatures 10-30 degrees colder than normal on
Wednesday. To the east of the upper low, near to above normal
temperatures will trend toward near to below normal as the system
progresses eastward. In the Pacific Northwest, California and into
the Great Basin region, temperatures will be 5-15 degrees above
normal through most of the period.
Over the Southeast, moisture from Zeta will stream northward
toward the in situ frontal boundary on Wednesday into Thursday
just ahead of the closed low. Heavy rain and flooding are possible
from the mid-Mississippi Valley to the Tennessee Valley. As the
system moves eastward en masse, rainfall will continue to focus
along the boundary to the Mid-Atlantic. Guidance remains
inconsistent where this may be and will depend on the interaction
of the two features at the start of the period. On the northwest
side of the upper low, snow is likely over New Mexico to the TX/OK
panhandles and southwestern KS. With marginally cold air in parts
of the Northeast, snow is possible in higher elevations as the
precipitation streams eastward (primarily in a warmer air mass).
The Pacific Northwest will see a few bouts of mainly light rain
and higher elevation snow.
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: