Extended Forecast Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Thu Jul 16 2020 Valid 12Z Sun Jul 19 2020 - 12Z Thu Jul 23 2020 19Z Update: No major changes were necessary to the existing forecast for the continental U.S. for the medium range period. The forecast was mainly derived from a blend of the GFS/ECMWF/UKMET, along with some previous WPC continuity for the first half of the forecast period, and then more weighting towards the GEFS and EC means for days 6 and 7 given the increased amount of model spread across the northern tier states and southern Canada. /Hamrick ...Hot temperatures for the Northeast/Mid-Atlantic next Sun-Tue... ...Overview and Guidance/Predictability Assessment... Strong upper high/ridging over the northeast Pacific Ocean as well as near the Davis Strait favors troughing over the Northwest/High Plains and broad ridging over the rest of the lower 48. The main storm track associated with the jet stream will meander along the Canadian border, acting as a focus for more organized showers/storms with additional areas from the Gulf of Mexico westward as inverted troughs push through. Ensembles remained in excellent agreement overall but with expected timing/amplitude differences in the shorter wavelength features to the north among the deterministic models. Blend of the 12Z/18Z guidance served well as a starting point as most runs were within the ensemble spread. Kept a mostly deterministic blend of the GFS/ECMWF/Canadian/UKMET to start, trending toward increased GEFS/ECMWF ensemble mean weighting for next Tue-Thu due to differences in the northern tier. To the south of the broad 588dm height line, ensembles mostly aligned in their progression of the inverted mid-level trough from east to west. ...Weather Pattern/Hazard Highlights... Organized showers and thunderstorms will focus around the cold front Sunday across the Great Lakes then into the Northeast. Front should settle to about 40N Mon/Tue and ebb/flow as weak waves pass along it to the northeast. Some thunderstorms may carry heavier downpours and cause flash flooding/runoff issues. Garden-variety showers/storms will dot the Southeast as is typical of mid-summer. More numerous afternoon showers/storms may persist over Florida amid continued easterly flow. Parts of the Southwest and southern/central Rockies will see an uptick in showers and thunderstorms next week as the lead mid-level trough moves westward across Mexico. Higher rainfall totals may stay over Mexico but could easily sneak into AZ/NM as moisture levels increase through the week. Dry conditions will continue for much of Texas and over the interior West/Great Basin aside from isolated dry thunderstorms that may later have rainfall reach the ground given an increase in moisture into NV/UT. Warmer than normal temperatures over the Great Lakes/Northeast Sunday will gradually shrink in size over the Mid-Atlantic, which will see readings well into the mid or upper 90s early in the week. Heat indices will climb into the low 100s given dew points in the 70s. Some areas may approach records but those are a high bar for mid-July. Overnight lows may only fall into the 70s to around 80 in some urban centers south of the front. By next Wed/Thu, heat will build across the Plains ahead of a cold front. Until then, cooler than normal temperatures are likely from Montana to the northern Plains Sun-Tue as Canadian high pressure moves through. Fracasso 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 Northern/Central Plains and the Upper/Middle Mississippi Valley, Sun-Mon, Jul 19-Jul 20. - Flooding occurring or imminent across portions of the Upper Mississippi Valley and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley. - Excessive heat across portions of the Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, the Ohio Valley, and the Central Appalachians, Sun, Jul 19. - Excessive heat across portions of the Middle/Lower Mississippi Valley, the Ohio Valley, the Tennessee Valley, and the Southeast, Sun-Mon, Jul 19-Jul 20. - Excessive heat across portions of the Mid-Atlantic, the Northeast, and the Central Appalachians, Sun-Wed, Jul 19-Jul 22. - Much above normal temperatures across portions of the Great Lakes, the Central Appalachians, the Ohio Valley, the Mid-Atlantic, and the Northeast, Sun, Jul 19. 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