Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Fri Apr 30 2021
Valid 12Z Mon May 3 2021 - 12Z Fri May 7 2021
...Heavy rainfall becoming more likely across the Deep South
Tuesday into Wednesday...
...Guidance/Predictability Assessment and Preferences...
An active weather pattern is forecast to continue going into the
medium range period next week as two significant troughs cross the
southern tier states and result in heavy rain and strong
thunderstorms. Meanwhile, an upper level ridge becomes
established over the Intermountain West towards the end of the
week ahead of the next Pacific storm system that will likely reach
the West Coast by next Friday.
The 00Z model suite is in above average agreement through Tuesday
across the continental U.S., and for this time period a general
blend of the UKMET/GFS/ECMWF/CMC sufficed as a starting point in
the forecast process. By Wednesday, the guidance diverges more on
timing of smaller individual shortwaves in the northern stream
flow, and thus precipitation coverage and placement across the
northern Plains and Upper Midwest later in the forecast period is
less certain. By Friday, there is good agreement that an
organized Pacific storm system will affect the West Coast, with
the GFS a bit more progressive than the slower ECMWF/CMC
solutions, so leaning towards the slower timing seems prudent for
now. There is also some indication of a potential coastal low
near the Mid-Atlantic region to close out the week, although
details remain uncertain. More of the NAEFS/EC means were
incorporated for the Thursday through Friday time period.
...Weather/Hazards Highlights...
Multiple rounds of moderate to heavy rainfall are likely across
portions of the central and southern tier states next week as
multiple disturbances cross the region. The region that appears
most favored for excessive rainfall is the Deep South, mainly from
northern Louisiana to the southern Appalachians, where 3 to 5
inches of rain may be realized in some areas. Most of this should
fall during the Tuesday-Wednesday time period ahead of the slow
moving cold front. This may be enough to cause some flooding,
particularly where convective training develops or across areas
that will have already received noteworthy rain prior to Monday.
Across the Wyoming and Colorado Rockies, a late season snow event
is likely for the higher elevations west of Denver and Cheyenne,
with several inches of accumulation possible.
In terms of temperatures, highs are expected to be about 5-15
degrees above average for much of the eastern U.S. ahead of the
front for early next week, with some 90 degree readings likely
across portions of the Virginia and North Carolina Piedmont on
Tuesday. This will be followed by a return to more seasonal
temperatures to close out the week behind the cold front, and
perhaps slightly below normal. Across the western U.S., heat is
expected to build in response to the upper level ridge, with highs
10-20 degrees above normal possible for the middle to end of the
week for portions of the Intermountain West and the Desert
Southwest. This will equate to some 100+ degree high temperatures
for the lower elevations of eastern California and southern
Arizona.
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