Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
147 AM EDT Thu May 6 2021
Valid 12Z Sun May 9 2021 - 12Z Thu May 13 2021
***Heavy rain and more storms for portions of the South and the
Ohio Valley***
***Unseasonably cool for the western High Plains with some snow
for the Rockies***
...Weather Pattern and Model Guidance...
A split flow upper level pattern is expected across the U.S. and
southern Canada for the medium range period, with embedded
shortwave impulses in the southern stream providing an active
weather pattern across the south-central U.S. with multiple rounds
of showers and storms. Farther north, a broad trough over the
Intermountain West and northern Rockies, and a large Canadian
surface high sinking southward across the north-central U.S., will
result in below average temperatures from the Rockies to the East
Coast. A moderation trend may commence by the middle of next week
across the West as an upper level ridge tries to build back in
after the trough lifts out of the region.
The deterministic model guidance is in decent synoptic scale
agreement for the Sunday-Monday time period, with the exception of
the 12Z CMC across the northwestern U.S. where it is farther north
with the placement of the upper trough, and it is slower with the
progression of the surface low across the Ohio Valley. By Monday
night, the UKMET becomes stronger than the other guidance with a
shortwave tracking towards the Pacific Northwest, and considerably
slower and stronger with the trough crossing the Great Lakes
region when compared to the ensemble means. Towards the end of
the forecast period, the GFS/GEFS is slower with the ejection of
the western U.S. trough across the Plains and Midwest, whereas the
ECMWF/ECENS are more progressive, which is normally opposite of
what is typically seen at day 5-7 time scales.
...Weather Highlights/Hazards...
The best rainfall prospects will likely be from the Ohio Valley to
the central Appalachians on Mothers Day, associated with a surface
low tracking eastward across the region. The potential exists for
1 to 3 inches of rain from southern Indiana to West Virginia.
Another zone of heavy rain, along with strong thunderstorms, is
also probable across portions of the Deep South for next Tuesday
and Wednesday with similar rainfall totals possible. Some
training of thunderstorms may lead to instances of flooding, and
this is something that will continue to be monitored in the days
ahead. Light to moderate rain can also be expected for the
western High Plains early next week, and it will be cold enough
for accumulating snow for the higher terrain of Colorado and
Wyoming. Dry conditions should persist across most of the
Intermountain West and the Desert Southwest.
Temperatures will likely be more reminiscent of March across the
central and western High Plains, and adjacent portions of the
northern Rockies for the Sunday to Tuesday time period, with highs
up to 20 degrees below early May averages. This equates to
readings maxing out in the mid 40s to mid 50s for many of those
areas, and even colder across the central and northern Rockies.
Some of these cooler temperatures should also be realized across
Great Lakes and Northeast U.S. courtesy of the upper trough and
Canadian surface high in place, although not as anomalous. By the
middle of next week, above normal temperatures are likely to make
a return to the West Coast region ahead of the next cold front.
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