Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
252 AM EDT Sun Sep 26 2021
Valid 12Z Wed Sep 29 2021 - 12Z Sun Oct 03 2021
...Overview...
An amplified upper pattern is expected especially early in the
medium range period, as ridging in the east-central U.S. separates
a likely closed low over the Northeast and mean troughing over the
West. Southern stream troughing/energy over the Four Corners
states and moist inflow will lead to heavy rainfall potential over
southern portions of the Rockies and Plains toward the end of the
week.
...Guidance Evaluation/Preferences...
Model guidance from the 12/18Z cycle begins the medium range
period Wednesday fairly well clustered with the pattern described
above, and has trended more agreeable than earlier model cycles
with the formation and placement of a closed low in the East
around Wednesday-Thursday. Most deterministic guidance produces
the closed low over the Northeast U.S., continuing the general
northward trend. The main outlier by early Friday was the 12Z
UKMET, which splits the energy and closes off the low centered
around Cape Hatteras, while other recent guidance has trended away
from the placement that far south. On Friday and beyond, quite a
bit of model variability arises quickly with the upper/low trough
feature. Trough energy moving southward spins southwest in the 12Z
ECMWF, creating a farther south (around Cape Hatteras) upper low
by the weekend, but spins southeastward in the GFS runs while
keeping the core of the upper low to the north. Meanwhile the 12Z
CMC maintains most of its energy within the low rather than
splitting it off. Hurricane Sam may also be a factor to consider
as it moves northward through the Atlantic late in the week.
Overall confidence is low with the upper low placement and
evolution by the weekend. At this point the WPC forecast tried to
take a middle ground approach, with upper low placement around
Downeast Maine on day 6/Saturday as a compromise center point.
Ensemble mean guidance was only somewhat helpful as the
differences in the upper low position in the ensemble members led
to more of an open trough than individual models. Overall
preferred the 12Z ECens mean compared to the GEFS means as the
former maintained a closed low at least through Friday.
Meanwhile in the West, an upper trough with shortwaves moving
through for midweek has general agreement along the guidance.
Models show shortwave energy splitting with some moving into the
north-central U.S./south-central Canada, and some forming a
southern stream trough or potential closed low centered in the
Desert Southwest around Thursday. Some differences in the exact
track and magnitude of both these features remain. But guidance
all has similar ideas in lifting the Southwest energy
northeastward through the Four Corners states for the end of the
week that a model blend worked well to depict the feature.
The WPC forecast began with a 12/18Z deterministic model composite
blend, eliminating the 12Z UKMET by Friday, and incorporating the
GEFS and especially the EC mean guidance for the latter part of
the period and increasing the weight of the means by the weekend
to minimize individual model differences.
...Weather and Potential Hazards...
Central parts of the Plains will see showers and thunderstorms
around midweek as moisture pools ahead of a shortwave and frontal
boundary, and some rain could spread to the Upper Midwest on
Thursday and Friday. But the greater chance for multi-day
accumulations of multiple inches of rain will be across the
southern High Plains beginning Thursday and lasting into the
weekend, as the upper low in the Southwest provides moist inflow
from both the Pacific and the Gulf of Mexico. The Pacific
Northwest can expect precipitation on Wednesday into Thursday,
with the highest totals in the Olympics and Cascades, and some
snow possible at the highest elevations. Scattered showers and
thunderstorms are expected across southern Florida as a couple of
fronts move across, while areas of light rain are possible across
the Eastern Seaboard due to the upper low/trough, mostly likely
over the Northeast.
Warmer than average conditions are forecast underneath the upper
ridge from the Dakotas into the Upper Midwest through the period.
Temperatures 10-20F above average will mean highs in the 70s and
80s there. Meanwhile, much of the West should see below normal
temperatures given the upper trough. The most anomalous
temperatures of 10-20F below normal are forecast in the Great
Basin on Wednesday, while shifting east and slightly moderating as
the week progresses. Cool high temperatures could linger in the
Four Corners states into the weekend. Near to slightly below
average highs are forecast for the East.
Tate
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