Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
229 AM EDT Wed Sep 22 2021
Valid 12Z Sat Sep 25 2021 - 12Z Wed Sep 29 2021
...Heavy rainfall will slowly exit the Northeast/New England
Saturday...
...Overview...
Upper ridging over the Labrador Sea and off the West Coast into
the Great Basin will favor troughing over the Great
Lakes/Northeast and a small closed low over the Southwest.
Troughing over the North Pacific this weekend will dive
southeastward early next week toward the Pacific Northwest. This
favors a mostly dry pattern once the lead eastern system moves
into Atlantic Canada and generally moderating temperatures.
...Guidance Evaluation/Preferences...
Upper level troughing will progress through the Great Lakes and
Northeast Saturday and Sunday with a couple vort maxes pushing
through. This will act to sweep a pair of cold fronts cleanly off
the coast later Sunday into Monday. Meanwhile upstream heights
building over the central/northern Plains will continue into early
next week ahead of a digging shortwave trough for the Pacific
Northwest Tuesday/Wednesday. The fast flow supports relatively dry
conditions across much of the CONUS aside from increasing moisture
for the Pacific Northwest by the middle of next week.
For the Northeast/New England, guidance has finally come together
on the timing and depth of the upper low features as they push
through the region. Earlier differences between the GFS and other
models have lessened such that a consensus approach can be used
there. For the Northwest early to mid next week, considerable
uncertainty in how the upper pattern evolves as the energy moves
onshore toward the Intermountain West by the middle of next week.
The GFS is fast and flat while the ECMWF slows and close off the
upper low over the interior West (likely tied to downstream
slowing/closing off of another low over the Great Lakes). Given
the much above normal spread, higher amounts of the ECENS/GEFS
means were used. This favored well with continuity while adjusting
toward the latest trends in the guidance.
...Weather and Potential Hazards...
Expect lingering rain, some of which could be locally heavy, along
and ahead of the slow-moving cold front across the Northeast and
New England on Friday into Saturday. The second system in its wake
will bring additional showers and possibly thunderstorms to the
Great Lakes region through the weekend. Meanwhile, a
stationary/stalled frontal boundary draped across central/south
Florida will bring daily chances of diurnally-driven
thunderstorms. Much of the rest of the Lower 48 will be relatively
dry, though some isolated showers and storms will be possible
across portions of CA/NV/AZ associated with the weak closed upper
level low late this weekend into early next week while return flow
moisture will bring increasing chances of storms to Texas by the
middle of next week.
Cooler, but still near normal temperatures will prevail across the
central/eastern U.S. this weekend in the wake of the cold front
passages. Meanwhile, as ridging aloft asserts itself across the
central U.S. and troughing develops on each coast, above normal
readings are likely with daily highs 10 to 15 degrees above normal
Sunday into next week across the central/northern Plains. A front
passing through the Northwest early next week will bring below
normal readings in its wake with highs 5 to locally 15 degrees
below normal.
Taylor/Fracasso
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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