Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Fri Aug 23 2024 Valid 12Z Mon Aug 26 2024 - 12Z Fri Aug 30 2024 ...Overview... Guidance shows strong Plains/Mississippi Valley/Southeast upper ridging as of early Monday weakening somewhat as it becomes confined more over the Southeast after midweek. Guidance agrees that an initial New England upper low/trough will depart, but guidance differs for important details of other northern stream features, namely shortwave energy ejecting from the northern Rockies/High Plains and a vigorous trough moving into the Northwest U.S. and western Canada by Tuesday-Wednesday and continuing onward thereafter. The Northeast upper low and northern stream systems to the west will produce areas of rainfall while diurnal convection should continue over the southern Rockies and along the Gulf Coast. The upper ridge will focus above normal temperatures over the east- central U.S. into Southeast, while the combination of systems farther west will generally promote below normal highs from parts of the Interior West into the northern High Plains. ...Guidance/Predictability Assessment... The primary forecast issues involve the initial energy ejecting from the northern Rockies/High Plains and the ultimate evolution of upstream western North America trough energy after midweek. In both cases, the majority of other dynamical guidance as well as a fair cluster of machine learning models suggest that latest GFS runs offer lower probability solutions for these features. Regarding the leading shortwave, the overwhelming majority of guidance shows a greater split of energy versus the GFS as of early Monday, leading to a slower progression of the northern Rockies/Plains shortwave. Beyond that point most guidance diverges for how this energy will evolve as it possibly tracks across southeastern Canada and nearby parts of the Great Lakes/Northeast, so confidence is minimal in any specific solution there. Solutions are fairly similar for the incoming Pacific trough into Wednesday. However by the latter half of the week, latest GFS runs have been lifting the core of the trough well north of other guidance while sometimes leaving weak energy behind over the Rockies and overall yielding less troughing than consensus over the northern tier states--with a corresponding influence on surface details. GEFS mean runs are also somewhat north of the non-GFS models for the Canada upper low but at least better resemble the majority for the northern U.S. trough. Guidance comparisons led to updating the forecast with an operational model composite early with modest inclusion of the 12Z ECens/CMCens means later in the period, all days with somewhat lower than usual GFS weight due to lower confidence in its northern stream features. ...Weather/Hazards Highlights... Upper ridge evolution will lead to the east-central U.S. seeing the most anomalous heat early in the week, followed by gradual suppression into the South/Southeast. Some areas may see one or more days with highs up to 10-15 above normal (and a few daily records) Monday through Wednesday or Thursday, with anomalies tending to be in the plus 5-10F range by Friday. Experimental HeatRisk shows a fairly broad area of Moderate to Major level impacts with even some very isolated pockets of Extreme showing up by midweek. In contrast, the combination of features over the West will yield below normal highs from parts of the Interior West into northern High Plains, with best potential for locally minus 10-15F anomalies on Monday and Wednesday-Thursday. Leading shortwave energy and one or more wavy surface fronts may produce rainfall of varying intensity over the northern Plains and vicinity early in the week. Thus far the combined clustering for location and magnitude are not sufficient to merit a risk area in the Days 4-5 Excessive Rainfall Outlook, but this region is worth monitoring in case future trends gravitate toward the heavier side of the broad guidance spread. Farther south, both Monday and Tuesday EROs have introduced a Marginal Risk area over portions of the southern Rockies given adequate moisture/instability, GFS/ECMWF QPF signals, and likely existence of shortwave impulses aloft. The next system pushing in from the Pacific should not produce much rainfall as it reaches the Northwest early in the week, but will likely generate more widespread rainfall over parts of the Plains/Midwest after midweek. Elsewhere, the Northeast upper low should produce some diurnally favored rainfall early in the week while some episodes of showers/thunderstorms will be possible near the Gulf Coast. Some diurnal convection may persist over the southern Rockies/Arizona beyond midweek. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw