Extended Forecast Discussion NWS Weather Prediction Center College Park MD 247 PM EDT Sat Jul 18 2020 Valid 12Z Tue Jul 21 2020 - 12Z Sat Jul 25 2020 ...Expect Northeast/Mid-Atlantic heat to moderate after early next week... ...Overview and Weather/Hazard Highlights... To the east of the Rockies a broad mean ridge aloft will persist across much of the southern/south-central tier of the lower 48. Most guidance suggests that a combination of one high center drifting east from the Four Corners states and another ridge building westward from the Atlantic will lead to a slowly strengthening overall ridge centered over the Plains/Mississippi Valley by late next week into the weekend. Meanwhile upper troughing will settle over the West Coast after midweek and flow around the northern side of upper ridging east of the Rockies will promote an active and progressive pattern across the northeast quadrant of the country. The northern tier pattern will promote one or more episodes of rain/thunderstorms (some locally heavy) between the Northern Plains and Mid-Atlantic/Northeast, as a mean frontal boundary oscillates in response to shortwaves aloft. The best-defined surface wave should emerge from the Northern Plains by early Tue and continue into or near New England by Thu. Additional waviness should reach the Plains late in the period. Increasing uncertainty over position of the front late week/next weekend steadily lowers confidence in pinpointing heaviest activity as time goes on. For the full five-day period the best potential for highest rainfall totals currently exists over the Upper Midwest/Upper Great Lakes, parts of the Northeast, and the southern Appalachians/southern Mid-Atlantic. A tropical wave/surface trough and associated energy aloft should generate an area of enhanced rainfall from parts of the Florida Peninsula/Keys around Tue to the western Gulf Coast by late in the week. The Four Corners states will see diurnal showers/storms of varying intensity. Establishment of upper ridging over the central/east-central U.S. by late week/weekend should allow for more moisture to reach the southern Rockies and Arizona/Utah, thus favoring somewhat higher rainfall totals then relative to the first half of the period through midweek. Locations from the Upper Ohio Valley through the central/southern Appalachians, Mid-Atlantic, and southern New England should see highs up to 5-10F above normal into Tue followed by a gradual retreat closer to normal. Upstream flow will bring another area of plus 5-10F anomalies for highs from the Northwest on Tue into the northern High Plains Wed-Thu and spreading eastward thereafter. In-between, the cooler air mass over the Northern Plains on Tue (a few degrees below normal for highs) will moderate as it continues eastward. By late week/weekend a majority of the West should see near to below normal highs due to the upper trough becoming established near the West Coast/leading cold front pushing into the Northwest as well as the moisture across parts of the Southwest into the southern Rockies. ...Guidance/Predictability Assessment... Latest models and ensembles continue to agree fairly well for the large scale pattern evolution through the period. There is some spread for the depth/amplitude of the upper trough progressing from the Northern Plains/Upper Mississippi Valley into the Northeast, with GFS runs tending to be on the deeper side of the spread. This difference along with uncertain convective influences affect the strength and timing of the associated surface wave and southward extent of the trailing cold front. Toward days 6-7 Fri-Sat the GFS is also stronger with an upstream impulse aloft, keeping the Northeast mean trough on the amplified side. Plus the GFS builds upper ridging more over the Plains versus the Mississippi Valley (and possibly extending into the Southeast) as in the ECMWF/CMC and their means. These late-period differences aloft lead to increasing uncertainty over surface front position within and near the area between the Midwest and Mid-Atlantic. For the upper trough settling near the West Coast there has been some question over the portion of energy originating from Alaska but the end result is a similar mean pattern. As with the downstream trough, the GFS is a bit deeper with the West Coast trough compared to most other solutions--mainly during Wed-Fri. Timing and strength of ejecting shortwaves become greater uncertainties after midweek. The updated forecast started with an operational model blend on days 3-5 Tue-Wed and then transitioned to a model/mean blend for days 6-7 Fri-Sat. Keeping the combined total of GFS/GEFS mean weight to about one-third represented the relative distribution of guidance where differences existed, while acknowledging that a decent percentage of the spread was still within typical error bounds for the time frame of interest. Rausch 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