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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 2018Z Jul 01, 2020)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
418 PM EDT Wed Jul 01 2020

Valid 12Z Sat Jul 04 2020 - 12Z Wed Jul 08 2020

...Overview and Guidance Assessment...

The large scale pattern over the CONUS during the medium range
period (Sat July 4 - Wed July 8) is expected to feature a mean
trough aloft over portions of western Canada and the far Pacific
Northwest. Meanwhile, for the central/eastern U.S., upper level
weakness/troughing over the Southeast early on is forecast to
shear out or dissolve giving way to mostly zonal/weak flow by the
middle of next week. Model agreement remains average throughout
the period though embedded details are murky, especially with the
evolution of individual shortwave troughs over the Northwest and
potential low development along the Gulf Coast and Southeast. For
the Northwest, there should be some broadening of the overall
trough with some potential amplification by day 6/7 as depicted by
the ECMWF and GFS. The upper ridge in place over the central U.S.
is forecast to retreat toward the Southwest later in the period as
more of a zonal flow sets up over the Great Lakes and Northeast.
Finally, the models still suggest potential low pressure
development along the Gulf Coast that then transitions off the
Southeast coast before lifting northeast away from the east coast
or shear out.

For the WPC blend, the day 3/4 blend utilized the latest GFS and
ECMWF followed by some inclusion of the UKMET and CMC given the
relatively good agreement across the deterministic guidance.
However, for the day 5-7 periods, higher components of the ECENS
and GEFS was incorporated while maintaining some of the
deterministic ECMWF. For the Southeast low, a blend of the
deterministic ECMWF/GFS was utilized though the forecast
confidence here is below average.


...Weather/Hazard Highlights...

Low predictability and confidence in the evolution of the
potential low development along the Gulf Coast and Southeast
exists and while there is some signal for heavy rainfall given the
higher PWs and stalling front in place, exact details and areas
for potential excessive rainfall are not clear at this time.
Future updates may be able to pinpoint details better. The
Northern Plains will be the other primary area for heavy rainfall,
thanks to a few waves of low pressure moving along frontal
boundaries within an area of warm/moist air. Elsewhere across the
CONUS, diurnally driven convection will be possible across the
central Rockies as well as the Northeast and Mid-Atlantic but any
large scale area for heavy rainfall is too uncertain at this
point.

Temperatures will largely be summer like for much of the CONUS
with near to above normal temperatures expected virtually
everywhere. The biggest anomalies reside across the Northern
Plains to Great Lakes region where daily highs will top 10-15F
above normal. Meanwhile, the combination of heat and humidity will
produce excessive heat across portions of southern Florida and the
Southwest with heat indices near 110F.

Taylor


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

Hazards:
- Heavy rain across portions of the Upper Mississippi Valley and
the Northern Plains, Sat-Sun, Jul 4-Jul 5.
- Heavy rain across portions of the Southeast, Sat-Mon, Jul 4-Jul
6.
- Flooding possible across portions of the Middle Mississippi
Valley and the Ohio Valley.
- Flooding occurring or imminent across portions of the Middle
Mississippi Valley and the Northern Plains.
- Excessive heat across portions of California and the Southwest,
Sat-Tue, Jul 4-Jul 7.
- Excessive heat across portions of the Southeast, Sat-Wed, Jul
4-Jul 8.
- Much above normal temperatures across portions of the Central
Plains, the Middle Mississippi Valley, and the Southern Plains,
Wed, Jul 8.
- Much above normal temperatures across portions of the Great
Lakes, the Mid-Atlantic, the Northeast, the Central Appalachians,
and the Upper Mississippi Valley, Sat-Wed, Jul 4-Jul 8.
- Much above normal temperatures across portions of the Upper
Mississippi Valley, Sat-Sun, Jul 4-Jul 5.
- Much above normal temperatures across portions of the Northeast,
Tue-Wed, Jul 7-Jul 8.

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