Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Thu Jul 18 2019
Valid 12Z Sun Jul 21 2019 - 12Z Thu Jul 25 2019
...Widespread Excessive Heat Threat Through The Weekend...
...Overview and Weather Highlights/Threats...
A hot and humid airmass is expected to affect a large portion of
the country into the beginning of this coming week as a broad
ridge strengthens. Many locations within this excessive heat area
will likely reach, or set new daily high minimum records and some
maximum temperature records into Monday. Heat indices are forecast
to be 105 to 115F+; therefore heat headlines will persist. This
heat wave may become problematic for those working or recreating
outdoors, individuals sensitive to heat, especially the elderly
and animals.
The intensity of the heat wave will reduce by early next week for
a large portion of the country. As the ridge builds over the
west-central portions of the country, a trough develops over the
eastern one-third of the U.S. A cold front will pass through and
eventually stall over the southern Mid-Atlantic, Southeast, Gulf
Coast and Texas. This boundary will become a focus for widespread
convection Tuesday and beyond. Moisture and instability pooling
along this boundary may result in enhanced rainfall amounts and
increase the risk for flash flooding.
A last of a series of impulses will track east from the Canadian
Prairies and into the northern tier of the U.S. this weekend. This
pattern will be favorable for developing showers and thunderstorms
capable of producing locally heavy convection/MCS activity over
the northern Great Plains/Great Lakes Saturday night before
activity focuses more on the cold front Sunday into next week.
Locally excessive rainfall may result from the multiple rounds of
rain/high rain rates over the northern tier. Guidance continues to
suggest the max QPF occurs over the Upper Mississippi Valley and
Great Lakes this weekend that spreads into the east-central and
southeast states early next week with the trough
amplification/cold front. The tail end of this cold front is
likely to bring enhanced upslope flow to the central/southern
Rockies as it sags south Sunday into next week. Expect renewed
monsoonal convection over the Southwest under the developing ridge
starting Monday.
...Guidance/Predictability Assessment...
The greatest uncertainty continues to be with the strength and
positioning of a western Canadian ridge this weekend and the
associated effects on the strength and timing of the downstream
trough over the eastern U.S. early next week. This forecast
consisted of the 12Z ECWMF, 12Z ECWMF/NAEFS ensemble mean and the
18Z/00Z GFS, with the weighting of the means increasing day 5
through 7.
Campbell/Jackson
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
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