Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1256 PM EDT Tue Jul 14 2020 Valid 12Z Fri Jul 17 2020 - 12Z Tue Jul 21 2020 17Z Update: The global models and the ensemble means are in good agreement synoptically through the forecast period with the broad 588+ dm ridge centered over the southern 2/3 of the continental U.S., and active westerlies across the northern tier states. The upper ridge peaks on Friday and Saturday across the central U.S., with the UKMET and CMC slightly stronger with it. By the end of the forecast period, the region with the greatest model spread appears to be across the Northeast U.S. and adjacent portions of eastern Canada, where the CMC becomes more out of phase compared to the model consensus, and the 12Z GFS is faster with the surface low and the frontal passage. The first few days incorporated a ECMWF/GFS/CMC/UKMET blend, and then more of the ECMWF/EC mean for next Monday and Tuesday and slightly less of the GFS/GEFS mean and no CMC to account for these differences. Overall, no major changes to the existing forecast. /Hamrick ...Hot temperatures pushing into the Great Lakes this weekend... ...Overview and Guidance/Predictability Assessment... A strong upper high over the northeast Pacific Ocean favors troughing over the Northwest and broad ridging over the rest of the lower 48. Around the upper high, a tropical wave may pass across Florida and through the Gulf of Mexico while the storm track meanders along the Canadian border. Ensembles remained in excellent agreement overall with expected timing/amplitude differences in the shorter wavelength features to the north. Blend of the 12Z/18Z guidance served well as a starting point. For next Sun-Tue, kept a mostly deterministic blend of the GFS/ECMWF/Canadian (60%) with the GEFS/ECMWF ensemble mean (40%) to retain some detail to the system moving through the Great Lakes/Ontario. ...Weather Pattern/Hazard Highlights... Showers and thunderstorms will focus around and precede frontal boundaries as they progress from west to east through the period, some of which may carry heavier downpours and cause flash flooding/runoff issues. Garden-variety shower/storms will dot the Southeast as is typical of mid-summer. More numerous afternoon showers/storms may persist over Florida in response to a tropical wave and continued easterly flow. Parts of the southern/central Rockies may see an uptick in showers and thunderstorms next week on the west side of the upper ridge. Dry conditions will continue for much of Texas and over much of the West. Temperatures will trend hotter in the Great Lakes and then Northeast this weekend into next week, with 80s/90s widespread and higher heat indices. Some areas may approach records. Overnight lows may only fall into the mid/upper 60s along the Canadian border but 70s elsewhere and near 80 in some urban centers. Max temperature anomalies of +5 to +10F will migrate from the Plains on Friday to the Great Lakes this weekend and Northeast/Mid-Atlantic next week. Min temperature anomalies may be a couple degrees more (+7 to +13F). By contrast, cooler than normal temperatures are likely for the Northwest into Montana, especially on Sunday where central Montana may see highs 5-12F below normal (70s in the valleys and only 60s in the mountains). 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 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