Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
216 AM EDT Sat May 22 2021
Valid 12Z Tue May 25 2021 - 12Z Sat May 29 2021
...Record heat likely for portions of the Southeast/southern
Mid-Atlantic next week...
...Overview...
An upper low near the Davis Strait will retrograde toward the
Canadian Archipelago next week, which favors a low-amplitude
trough/ridge pattern across the lower 48. Heights will generally
be near to above normal over all of the CONUS by late next week
which will translate to surface temperature as well. Over the
East, record highs well into the 90s to around 100F are likely for
portions of the Southeast into the Mid-Atlantic. A backdoor front
may cool some areas briefly in the otherwise very warm pattern.
Rainfall will be focused over the Plains to the Midwest/Great
Lakes.
...Guidance Evaluation and Preferences...
Through the 12Z/18Z cycle, the deterministic models still had
trouble forming a consensus on timing the incoming cold front and
upper trough or weak upper low into the Northwest Tuesday. The GFS
runs remained a bit quicker but a blended/consensus starting point
was favored given a bit quicker trend. Upstream over the Gulf of
Alaska, much larger differences emerged in the 12Z ensembles
regarding the handling of a couple features from the North Pacific
and off the mainland, leading to large differences between the
ECMWF ensembles and the GEFS/Canadian. However, there was some
overlap between some GEFS members and the ECMWF idea with the
deterministic models in principle, leading to favor the ECMWF
ensembles over the GEFS into mid week. By later in the week, ECMWF
ensembles seemed to suffer from a smearing of 50 members with
differences in timing smaller-scale features into the West,
leading to a broad trough of low amplitude. In the East, cold
front may dip into the southern Mid-Atlantic on Friday with
perhaps a wave of low pressure along the front draped back to the
Plains. Low confidence in the details as the models have little
consistency and the ensembles were rather undefined. Maintained a
wavy front astride the Ohio Valley that may lift back into the
Northeast as upper ridging attempts to build just offshore.
...Weather Highlights/Hazards...
Highest chances of rainfall next week will likely be over the
central Plains to the Midwest and Great Lakes along a wavy frontal
boundary. Parts of the interior Northwest/northern Rockies will
see some light rain and high mountain snow with the Tuesday
system. This front may touch off light to modest showers and
perhaps some thunderstorms east of the Rockies as it
weakens/dissipates. Some rainfall amounts have the potential to be
locally heavy in some convectively-enhanced areas (generally over
the Plains) along the frontal boundary. Backdoor cold front in the
East may bring only light showers to some areas on its way south
or back north as a warm front.
Temperatures will be cool in the West to start, warm/hot in the
East, and within several degrees of normal in between. Coldest
anomalies of about 5 to 10 degrees below normal will be centered
from the Great Basin/Northwest to the High Plains Tue-Wed. In the
East, ahead of the backdoor cold front (or behind the warm front
as it advances back to the northeast), temperatures may climb to
10-20F above normal. This will bring 80s/90s into the
Northeast/Mid-Atlantic and 90s to near 100F over the Southeast.
Records highs are likely for some areas and could be widespread
over several states Tue-Wed along the I-75 and I-95 corridors,
respectively. Heat indices may be kept in check by dew points in
the 50s/60s through the week.
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