Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Fri Aug 23 2024 Valid 12Z Mon Aug 26 2024 - 12Z Fri Aug 30 2024 19Z Update: The 12Z model guidance suite is still indicating some timing differences with the shortwave trough crossing from the Northern Rockies to the Great Lakes region for early next week. The past few runs of the GFS have been considerably more progressive and generally weaker with the trough by Tuesday across the Upper Midwest, with better overall agreement among the CMC/UKMET/ECMWF and the machine learning guidance supporting a non-GFS solution for this region. Going into the second half of the week, the guidance is in better overall agreement with the next synoptic scale trough building over the Northern Rockies and Northern Plains, and the GFS is more in line with the consensus for this event, even though it is generally weaker than the CMC/ECMWF solutions. Upper level ridging then tries to build back in across the Intermountain West by next Friday. Taking these factors into account, the updated WPC fronts/pressures forecast was hedged more in the direction of the non-NCEP guidance through early Wednesday, and then kept some of the GFS for the second half of the forecast period and gradually increased the ensemble means to about 40%. The previous forecast discussion is appended below for reference. /Hamrick ----------------------- ...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. After better consolidation in the 12Z model guidance suite, a Marginal Risk will be added to the Day 4 (12Z Monday-12Z Tuesday) excessive rainfall outlook for portions of the eastern Dakotas and into northern Minnesota, where a potential MCS could develop. Farther south, both Monday and Tuesday EROs have maintained 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. Hamrick/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