Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
350 PM EDT Fri Jul 30 2021
Valid 12Z Mon Aug 02 2021 - 12Z Fri Aug 06 2021
...Lingering monsoonal moisture and heavier rain for parts of the
Interior West on Monday..
...Heavy rainfall threat for parts of the South and the
Southeast...
...Overview...
Positive height anomalies over western/central Canada and
southwest of California will favor continued troughing in the
eastern states next week. This will promote cooler than normal
temperatures east of the Plains with highest rainfall chances
along and ahead of a slowly southward-moving cold front into the
Southeast. The Interior West will see diminishing rainfall,
focused along the Rockies and perhaps shifting to the Plains later
next week.
...Guidance/Predictability Evaluation...
Latest models and ensembles remain in good agreement on the large
scale pattern through the week. The differences lie in the details
difficult to resolve at this time scale. Recent deterministic runs
continue to waver on how quickly (or slowly) to bring height falls
into the Pac NW later in the week with ECMWF/ECENS mean a little
bit slower than the GFS/GEFS mean. The lack of a clear upstream
kicker over Alaska would favor the slower solutions, but a general
blend of the ECMWF/GFS with the means gets us a good starting
point. The CMC seems too quick to bring lower heights into the
West and also quicker to erode troughing over the Ohio Valley and
push it eastward the latter half of the week. Thus, the CMC was
excluded from the blend during the late periods. This cycle of the
WPC forecast favored a blend of the deterministic guidance days
3-5, with modest weighting towards the means by days 6 and 7.
Continued more than usual blending of the ECMWF/GFS later in the
period just for some added strength and definition to individual
features typically washed out by the means. This approach also
maintains good continuity with the previous WPC forecast.
...Weather Highlights/Hazards...
With the upper high around the lower CO River Monday, mid-level
energy will spur afternoon convection over the northern/central
Rockies, some of which may contain heavier rain. As heights build
a bit over the Southwest, the moisture source will decrease and
rain coverage/intensity will decrease through the week. In the
East/Southeast, troughing will push a cold front close to or into
the Gulf of Mexico by midweek. Abundant moisture ahead of the
boundary will support heavier rain chances anywhere from Texas
eastward to the Carolinas. As the upper pattern remains mostly in
place, the front may wobble about due, in part, to areas of low
pressure that may act to enhance rainfall into the Mid-Atlantic.
Temperatures will slowly moderate from below normal in much of the
Interior West (owing to clouds and rainfall) early in the week to
near or slightly above normal by the end of next week. Areas along
the Canadian border may see the warmest anomalies of +5 to +10
from eastern Montana to the western Great Lakes. Much of the
East/Southeast will see near to below normal temperatures along
and behind the cold front, perhaps as much as 10 degrees below
normal for early August on some days.
Santorelli/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
Hazards:
- Heavy rain across portions of the Southeast and the Lower
Mississippi Valley, Mon, Aug 2.
- Heavy rain across portions of the Southeast and the
Mid-Atlantic, Mon-Thu, Aug 2-Aug 5.
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