Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1153 AM EDT Tue Jun 23 2020 Valid 12Z Fri Jun 26 2020 - 12Z Tue Jun 30 2020 ...Guidance/Predictability Assessment... The medium range begins Friday with a transient shortwave traversing the northern tier of the country from the Northern High Plains into the Midwest/Northeast this weekend. Early next week, the guidance indicates the pattern may transition to more of a high amplitude/blocked regime with a likely closed low developing over the Northern Rockies/high Plains and ridging building downstream across the Central U.S. to the Great Lakes. Deterministic model solutions continue to struggle with timing of the upper shortwave exiting the Northern High Plains on day 3/Friday and continuing into the Upper Midwest this weekend and the Northeast early next week. The 00z UKMET and Canadian sped up to join the ECMWF faster timing with the upper shortwave and the associated surface low and front, so the GFS and GEFS are on the slow side of the distribution. Given the good clustering of the ECMWF/ECMWF Ensemble Mean, UKMET, and Canadian global, a blend of these solutions was used. Differences also develop in the timing of the 500 mb trough in the eastern lakes, New York, and New England, with the 0z ECMWF breaking continuity and forming a closed low that moves east more slowly than other guidance across the northeast/adjacent Canada. The 00z ECMWF and 06z GEFS Mean/00z Canadian/00z UKMET agree well on faster timing than the operational 00z ECMWF, so a blend of the means is preferred on Days 6-7 in the northeast. Troughing over the Northwest U.S. amplifies on Sunday, potentially closing off an upper low over the Northern Rockies by Day 6/Monday. The ECMWF/Canadian/GFS/UKMET all show a closed low developing, but differences persist on timing/location. The dominant cluster consists of yesterday's 12z ECMWF/00z ECMWF/00z ECMWF/00z UKMET. The 00-06z GFS move the low southeast a bit faster than the dominant cluster. The 06z GEFS Mean is slower than the 00-06z GFS and even slower than the ECMWF/UKMET, so confidence is good adjusting the GFS to be slower, given its long history of moving closed lows too quickly. The 00z Canadian moved the closed low faster south than the other models, but like with the GFS, the Canadian ensemble mean was slower than the operational run, and clustered better with the UKMET/ECMWF. A blend of the primary cluster of solutions of the ECMWF/UKMET/ECMWF Ensemble mean/00z Canadian Ensemble Mean was used. ...Overview and Weather/Hazard Highlights... A cold front should focus showers and potentially strong storms across the Upper Midwest/Upper Mississippi Valley on Friday, moving eastward into the Great Lakes and New England on Saturday. There remains a signal for locally heavy rainfall threat with this system. Showers and storms should linger through the weekend across the Southern states as well along a weakening frontal boundary and within a region of favorable dynamics aloft. A lingering sfc trough in south Texas should focus more showers/storms through Day 4, with locally heavy rain indicated in a few of the models. Upper ridging builds in behind this system across the Central and Eastern states bringing a warming trend to the region this weekend into early next week, especially across the Northern Plains, Upper MS Valley, and upper Great Lakes. The next cold front dives into the Northwest by Saturday morning with showers and storms likely developing along and near it this weekend as it shifts into the Rockies. The upper low over the northern Rockies early next week brings a threat for locally moderate to heavy precipitation across Montana into northern Wyoming. Significant height falls and cooling temperatures could allow precipitation to fall as snow across the higher elevations of the northern Rockies. On Tuesday, 30 June, showers and storms may break out along the front in the central to northern Plains. Temperatures across the West near the upper trough trend from above normal on Friday, to below or well below normal in spots by early next week. By Tuesday afternoon 30 June, as the East Pacific ridge builds onshore in WA, temperatures will moderate and may even climb to above normal in western WA. Petersen/Santorelli 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