Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Wed Aug 18 2021
Valid 12Z Sat Aug 21 2021 - 12Z Wed Aug 25 2021
...Weather Pattern Overview...
A well defined upper level trough should be in place across the
Pacific Northwest and the Upper Midwest to begin the forecast
period Saturday, and an upper level ridge situated over the
Northeast U.S. and also along the Gulf Coast. This Gulf Coast
ridge will keep Grace tracking westward and remaining well south
of the Texas/Mexico border.
...Model Analysis and preferences...
Much of the model discrepancy continues to be focused across the
Pacific Northwest and extending eastward to the northern Plains
through the medium-range period, where multiple shortwaves are
forecast to interact with a synoptic scale trough. The 18Z GFS
remained more progressive than the model consensus with these
shortwave passages, however the 12Z GFS aligned better and it was
incorporated into the forecast in lieu of the 18Z run. With the
next trough/upper low arriving by Sunday night/Monday across the
Pacific Northwest, the 00Z GFS accelerates across southern Canada
by Tuesday, whereas the ensemble means and the ECMWF/CMC suggest a
slower progression. In terms of Tropical Storm Henri, the 00Z
guidance has continued to adjust westward with the track, bringing
the storm closer to eastern New England by early next week,
particularly with the UKMET and GFS, so this is something that
will need to be monitored very closely in the upcoming days.
The WPC fronts/pressures forecast was derived from a blend of the
12Z ECMWF/ECENS/CMC/GFS was used as a baseline, and then a greater
portion of the ensemble means for Tuesday and Wednesday.
...Sensible weather and potential hazards...
Moderate to heavy rainfall is likely across portions of northern
Minnesota and the northern Great Lakes on Saturday ahead of the
developing low pressure system, with the majority of the rain
happening before this forecast period. The next upper trough
dipping into the Pacific Northwest should bring another round of
rain/storms across the northern Plains into the Upper Midwest this
weekend into early next week. Meanwhile, another corridor of
enhanced showers and storms may materialize across portions of the
Mid-South to the Tennessee Valley this weekend. Depending on the
eventual track of Henri, some rain and wind may be possible for
portions of New England early in the week.
In the temperature department, expect rather warm conditions
across the upper Midwest and northern New England while some high
elevation snow for Wyoming and Montana is within the realm of
possibility. Meanwhile, the upper high that is forecast to extend
across the Deep South and Gulf Coast region will likely result in
temperatures running a few degrees above average. Pleasantly cool
conditions are expected from the northern Plains to the Upper
Midwest with the trough in place.
Hamrick
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