Extended Forecast Discussion NWS Weather Prediction Center College Park MD 442 PM EDT Mon Aug 10 2020 Valid 12Z Thu Aug 13 2020 - 12Z Mon Aug 17 2020 ...Overview and Guidance/Predictability Assessment... Latest guidance continues the established pattern of closed lows aloft progressing across Canada with each associated trough extending into the northern U.S. and pushing along temperature moderating and convection focusing fronts. The leading front that settles into the Mid-Atlantic and extending back into the Plains will provide a multi-day focus for rain/thunderstorms, with a weak shortwave aloft drifting over the eastern states possibly helping to enhance activity at times. The trailing part of the front will lift northeastward as a warm front ahead of the next cold front that should travel from the Plains into the East by Sat-Mon. Within persistent southern tier upper ridging expect the strongest upper high to drift a bit to the west, from the southern Rockies into Arizona. The ridge aloft will promote some triple-digit highs and potential daily records from the southern High Plains to the Desert Southwest. Models/ensembles agree that a developing Northeast Pacific upper trough will begin to approach the West Coast early next week but with only modest rainfall potential into the Pacific Northwest. Based on data available through the 06Z cycle the updated forecast--which preserves large scale continuity--started with an operational model blend from 00Z/06Z runs early in the period and then transitioned to a model/mean blend later in the period (eventually reaching 60 percent total 06Z GEFS/00Z ECMWF means) given the typical increase of detail uncertainties. Operational ECMWF input switched from the 00Z run to the prior 12Z/09 run for about the latter half of the forecast as there was little support from other solutions for the 00Z run's more concentrated upper trough and corresponding Great Lakes/southern Canada surface low development. The new 12Z ECMWF has returned toward consensus. Otherwise there are ongoing timing/amplitude differences with the northern tier trough that will take more time to resolve, while details of wave development along the Mid-Atlantic front have low predictability due to small scale of supporting shortwave energy and possible dependence on convective evolution. One other 12Z model note is that the CMC eventually becomes more amplified than consensus with the entire Northeast Pacific to western Atlantic pattern. ...Weather/Hazard Highlights... There will be two areas of particular interest within a broad area of rain/shower and thunderstorm potential from about the northern half of the Plains through much of the eastern U.S. One will be over the Mid-Atlantic into the Carolinas as the combination of a wavy Mid-Atlantic front and weak shortwave energy aloft help to focus daily showers/storms with some heavy downpours, likely to be most numerous during the late afternoon to evening hours. The other will be over the Midwest/Upper Mississippi Valley and vicinity. The aforementioned front extending back through the north-central U.S. will act to focus convective development as impulses and favorable upper jet support interact with lower level return flow and pooling moisture and instability. By late week/weekend the draped front will be lifting northward ahead of a cold front approaching from the West. Thus expect additional episodes of strong to severe thunderstorms with locally heavy downpours. Meanwhile the Southeast/Florida will continue to experience diurnal convection of varying intensity with some areas of heavy rainfall contingent on less predictable local flow details. Corresponding to the slow westward drift of strongest ridging aloft, expect the emphasis for highest max temperature anomalies up to plus 10-15F and possible daily records to shift from the southern High Plains to the Desert Southwest over the course of the period. Parts of southern Arizona may already see such anomalies late this week. Coverage of above normal temperatures will also expand across the remainder of the West during the weekend/early next week as the upper ridge builds northward. Ahead of this warming trend, cold frontal progression out of the Northwest will bring an area of moderately below normal highs from the Northwest into the central U.S. Much of the northeastern quadrant of the lower 48 should see above normal min temperatures while daytime highs will be more dependent on clouds/precipitation. Rausch/Schichtel Hazards: - Heavy rain across portions of North Dakota into northern Minnesota, Thu-Fri, Aug 13-Aug 14. - Heavy rain across portions of the Mid-Atlantic into the central Appalachians, Thu-Sun, Aug 13-Aug 16. - Flooding occurring or imminent across portions of the Middle Mississippi Valley. - Excessive heat across portions of the Desert Southwest, Fri-Mon, Aug 14-Aug 17. - Excessive heat across portions of the southern Plains, Thu-Fri, Aug 13-Aug 14. - Excessive heat across portions of the lower Mississippi Valley, Sat-Sun, Aug 15-Aug 16. - Much above normal temperatures across portions of the southern High Plains, Thu-Fri, Aug 13-Aug 14. 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