Extended Forecast Discussion NWS Weather Prediction Center College Park MD 350 PM EDT Fri Jul 17 2020 Valid 12Z Mon Jul 20 2020 - 12Z Fri Jul 24 2020 ...Hot temperatures for the Northeast/Mid-Atlantic through early next week... ...Overview and Weather/Hazard Highlights... A relatively strong and broad subtropical ridge is forecast to remain in place across the CONUS southern tier during the extended forecast period, while an active and progressive setup persists across the northern tier. Then troughing is expected to enter the West Coast late in the week. The strongest positive 500 hPa height anomalies are initially expected to be in place across the Mid-Atlantic and Northeast Mon/Tue, shifting into the Atlantic Wed. The ridge will promote above average temperatures, with highs forecast to be 5 to 15 deg F above normal across portions of the Mid-Atlantic and Northeast continuing from the short range Sun into Mon, and to a lesser extent Tue/Wed. Parts of the southern Mid-Atlantic region may see temperatures approach or reach 100 deg (with heat indices in the 105-110 range), especially on Mon. Isolated record high temperatures are possible during that time frame, with record high minimum temperatures likely more commonplace. Farther west, a wave of low pressure is forecast to develop along a frontal boundary and move across the Central Plains, Midwest, and Great Lakes Mon-Wed. This low pressure system along with a mid/upper-level shortwave are expected to produce an area of enhanced shower/thunderstorm potential. Model guidance shows the strongest signal for areas of heavy rainfall across the Middle/Upper Mississippi Valley Mon into Tue, where 1-2 inches of rain are possible (with locally higher amounts). As the system reaches the eastern U.S. Wed-Thu, scattered to numerous showers and thunderstorms are expected along much of the Eastern Seaboard, with locally heavy rain possible. One area to watch for potentially heavy rain is eastern North Carolina/Virginia, where moisture with high PW values should advect toward the frontal zone to the north, which could provide a focus for convection. By Thu-Fri the next shortwave is likely to be moving across the Rockies, accompanied by another potential wave of low pressure, and areas of convection are possible across the Midwest along a lingering surface front. Lastly, a broad area of moisture and convection is forecast to drift westward across the Gulf of Mexico next week, and may bring some increase in shower/thunderstorm activity to southwestern portions of Florida and then to the central/western Gulf Coast by late next week. ...Guidance/Predictability Assessment... Overall, model/ensemble guidance showed relatively good consensus at the larger scales. Overnight GFS runs seemed to come into better alignment with the wavy jet in central and eastern portions of the country, especially the 06Z run. A deterministic model blend was able to be used early in the period. By day 5, the 00Z CMC became a progressive outlier with its positioning of the trough entering the West, already over the Four Corners region. It was also farther east than consensus with the wavy flow from the Plains eastward, so removed any use of the CMC from the blend. Emphasis on ECENS/GEFS ensemble means was gradually increased through time to account for gradually increasing spread/uncertainty on the details, with majority weight placed on ensemble means during days 6-7 (Thu-Fri). One such detail was with the flow coming out of the Gulf of Alaska Thu-Fri--the 00Z deterministic ECMWF had a fairly deep upper low dropping south toward British Columbia during that time, and the 00Z GEFS mean actually had some indication of this low as well. But at this time, a blend favoring the ensemble means was preferred to minimize these more uncertain features. Tate/Ryan 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/Middle Mississippi Valley and Central Plains, Mon, Jul 20. - Flooding possible across portions of the Northern Plains. - Flooding occurring or imminent across portions of the Upper/Middle Mississippi Valley, the Great Lakes, and the Northern Plains. - Flooding likely across portions of the Central Plains. - Excessive heat across portions of the Mid-Atlantic, the Middle/Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Northeast, the Southeast, and the Ohio Valley, Mon-Wed, Jul 20-Jul 22. 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