Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1245 PM EDT Wed Jul 15 2020 Valid 12Z Sat Jul 18 2020 - 12Z Wed Jul 22 2020 17Z Update: The global models and the ensemble means are in reasonably good agreement on the synoptic scale through early Tuesday 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. Similar to yesterday, the UKMET and CMC solutions are slightly stronger with the core of the upper ridge. The CMC becomes more progressive with the shortwave energy across south-central Canada by Sunday, and this leads to greater differences across New England by the beginning of the week and thus more out of phase with the model consensus. By the end of the forecast period next Wednesday, the region with the greatest model spread appears to be across the northern Plains and Upper Midwest, with both timing and amplitude differences with a trough arriving from western Canada. The 12Z GFS is somewhat out of phase with the 6Z GEFS mean, and although the 00Z ECMWF is stronger with the trough across the Upper Midwest on Wednesday, it has support from the GEFS and EC mean regarding placement. The first few days incorporated mainly a ECMWF/GFS/CMC/WPC continuity blend, and then more of the ECMWF/EC mean with some of the 6Z GFS for next Tuesday and Wednesday to account for these differences. Overall, no major changes to the existing forecast. /Hamrick ...Hot temperatures for the Great Lakes and Northeast... ...Overview and Guidance/Predictability Assessment... A strong upper high over the northeast Pacific Ocean favors troughing over the Northwest/High Plains and broad ridging over the rest of the lower 48. 500mb heights will be near to above normal CONUS-wide during the warmest time of year for many locations. The main storm track associated with the jet stream will meander along the Canadian border, acting as a focus for more organized showers/storms. 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. For next Mon-Wed, kept a mostly deterministic blend of the GFS/ECMWF (60%) with the GEFS/ECMWF ensemble mean (40%) to retain some details with a couple waves along the surface front straddled from the Rockies to the Great Lakes. ...Weather Pattern/Hazard Highlights... Organized showers and thunderstorms will focus around frontal boundaries as they progress from west to east through the period, or meander north-south (Rockies/Plains). 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 may see an uptick in showers and thunderstorms next week as a mid-level trough moves westward across Mexico. Dry conditions will continue for much of Texas and over much of the West. Warmer than normal temperatures will spread from the Plains into the Great Lakes and then the Northeast this weekend into next week. 80s/90s will be widespread with higher heat indices into the low 100s. Some areas may approach records but many of these are at all-time high values given that it is mid-July. Overnight lows may only fall into the mid/upper 60s along the Canadian border but 70s elsewhere and near 80 in some urban centers. This will continue a very warm start to July for many Midwest/Northeast locations (top 10 warmest start to July from MSP-DTW-CMH-SYR-DCA). By contrast, cooler than normal temperatures are likely for the Northwest into Montana, especially on Sunday where central Montana may see highs 5-10F below normal (70s in the valleys and only 60s in the mountains). Fracasso 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