Extended Forecast Discussion NWS Weather Prediction Center College Park MD 342 PM EDT Mon Aug 03 2020 Valid 12Z Thu Aug 06 2020 - 12Z Mon Aug 10 2020 18Z Update: The forecast period begins on Thursday with generally above average model agreement on the main synoptic scale features, and this also holds true going into Friday. By that time, the main differences are related to the amplitude of the West Coast upper trough, with the UKMET and GFS/GEFS mean indicating a slightly greater degree of height falls. By the second half of the forecast period going into the weekend, the CMC is slightly stronger with the southern U.S. ridge compared to the other guidance and becomes out of alignment with the ensemble means by Sunday and Monday across the Pacific Northwest in building a much stronger trough. There has been considerable run-to-run differences in recent GFS runs across western Canada and into the northwestern U.S., with the 00Z run indicating a highly amplified solution by the end of the week, and the 6Z and 12Z runs more progressive. Taking these factors into account, the fronts and pressures forecast was derived mainly from a deterministic blend through Day 5, with slightly greater weighting towards the ECMWF. By the weekend and into Monday, some WPC continuity was maintained along with some of the ECENS and GEFS means, and less of the GFS and no CMC. The previous discussion follows below for reference. /Hamrick ...Overview and Guidance/Predictability Assessment... Models and ensembles show the mean flow settling into a pattern fairly representative of summer. An upper ridge will remain anchored over the southern parts of the Plains and Rockies. The western periphery of an Atlantic ridge may extend into Florida and portions of the Gulf of Mexico/Gulf Coast region as an eastern U.S. trough gradually weakens/departs, ultimately leading to broader southern tier ridging with time. Meanwhile North Pacific flow will bring a fairly strong upper trough into the Northwest U.S./western Canada Thu-Fri. The core of dynamics will eject into Canada but height falls will continue across the northern tier U.S. through the period, pushed along by upstream flow. The succession of features within this band of westerlies should bring one or more fronts across the Northwest and into the Northern Plains. A separate upper trough near the California coast may close off a low that could retrograde a bit later in the period. The most prominent forecast problem involves the details of North Pacific into northern U.S./southern Canada flow. For the leading shortwave reaching the Northwest U.S. and western Canada there are still typical strength/timing differences but overall decent agreement for about the first half of the period. After that time the guidance rapidly diverges for the timing and strength of advancing height falls in response to differences that develop in the upstream flow. These differences originate from a sensitive evolution to the south of the Aleutians around days 4-5 Fri-Sat. Guidance is essentially evenly split between a more progressive scenario (ECMWF/ECMWF mean/CMC) and a slower progression (GFS/GEFS mean/CMC mean) that leads to deeper troughing over the northwestern states late in the period. The ECMWF mean has made a pronounced trend toward the progressive idea over the past couple days while operational ECMWF runs have tended to be fairly progressive during that time. GFS/GEFS runs have been stable in the slower camp. The sensitive/low-predictability nature of the evolution that serves as the source of guidance divergence as well as trends leading to greater spread versus yesterday favor a blended/intermediate approach. An operational model blend represents consensus and continuity well for the lower 48 and vicinity during the first half of the period. This includes northern tier flow, a front advancing from the High Plains, and modest wave development near the Mid-Atlantic coast in response to the weak upper trough crossing the East. Then a trend to a model/ensemble mean blend provides the desired intermediate solution across the northern tier U.S. while depicting the most common ideas of guidance for other features. ...Weather/Hazard Highlights... The upper trough reaching the Northwest at the start of the period may be strong enough to produce rainfall over some areas. The leading front should provide a focus for showers/thunderstorms once it reaches the Northern Plains. Specifics become increasingly uncertain late in the period but this first front could stall for a time while another front approaches, which could enhance amounts over and east of the Northern Plains. Meanwhile a warm front advancing across the remainder of the Plains may help produce one or more episodes of convection with locally heavy rainfall. The forecast has been consistent in advertising periods of rain between the central-southern Appalachians and East Coast in association with the weak upper trough crossing the East and wavy front over the southern Mid-Atlantic into the western Atlantic Ocean. Any localized downpours may be problematic over areas that previously receive significant rainfall from Isaias. The southern High Plains and vicinity will see temperatures around 5-10F above normal through the period under the ridge aloft. Isolated locations on the high end of that range and with a relatively shorter period of record could challenge one or more daily record values. Initial below normal readings over the Mississippi/Ohio Valleys will trend warmer with time as moderately above normal temperatures spread from the Rockies into the Plains and then the northeastern quadrant of the lower 48. The separate upper troughs affecting the Northwest and California late this week will lead to cool highs at least 5-10F below normal. Expect near to modestly below normal readings over these areas Sat-Mon. The forecast has lower confidence over northern areas. Rausch Hazards: - Flooding possible across portions of the central and southern Appalachians, the Mid-Atlantic, the Northeast, as well as the northern Plains. - Flooding occurring or imminent across portions of the Mid-Mississippi Valley and the northern Plains. - Flooding likely across portions of the northern Plains. 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