Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Thu Aug 29 2024 Valid 12Z Sun Sep 01 2024 - 12Z Thu Sep 05 2024 ...Overview... An upper trough crossing the Great Lakes/Northeast around the start of the week will give way to rising heights thereafter as a portion of initial Rockies ridging drifts into the Plains/Midwest. The remainder of the ridge should settle into the Southwest as an eastern Pacific upper low tracks through the northwestern states. There has been a lot of spread for details of incoming Pacific flow and its possible interaction with ejecting upper low energy after Tuesday, but the most common theme is for some degree of ridging to rebuild over the Northwest U.S. into western Canada. Across the southern tier of the lower 48 from the High Plains eastward, guidance still differs in resolving lower-predictability shortwave features that will influence rainfall coverage coverage and amounts, with northern stream flow uncertainties possibly playing a role as well. At the very least, a couple fronts settling into the South and a persistent surface trough/embedded waves near the western Gulf Coast should provide a focus for some of the activity. An area of well above normal temperatures should progress from the Northwest into northern High Plains Sunday- Tuesday followed by a rebound in temperatures over the West Coast states by Wednesday- Thursday. Moderately below normal daytime highs will expand from the Plains toward the East Coast early in the week, followed by moderation over most areas except for the Southeast. ...Guidance/Predictability Assessment... Forecast issues can be most broadly broken down into two aspects, one being uncertain specifics of incoming Pacific flow by midweek/potential interaction with ejecting Pacific-Northwest U.S. upper low energy, and possible evolution/progression of southern Plains energy and relative dominance of northern stream flow (with some dependence on developing spread to the north). Regarding the Pacific/northern tier part of the forecast, dynamical and machine learning (ML) models continue to show significant disagreements among each other and in consecutive runs. Ensemble spaghetti plots develop a lot of spread as well. Stray runs like the 18Z GFS and old 00Z/28 ECMWF were on the extreme side of the spread for how much incoming flow amplifies into the northern tier behind the ejecting Northwest upper low. At least for the moment there is relatively greater clustering toward the idea of less northern stream influence on the Northwest upper low, ultimately corresponding to the feature reaching the northern Plains/north- central Rockies by around next Thursday. The 12Z CMC was a slow extreme in leaving the energy stuck over the Northwest. In addition, a developing forecast problem upstream by Thursday involves whether Pacific trough energy splits or remains phased. As for the southern tier, there is a continued signal for some amount of energy aloft to evolve over or near the southern Plains but in a variety of ways and differing degrees of progression. An additional complication is that different dynamical and ML models vary in how much northern stream influence there may be. By late in the period this leads to a range of solutions from a progressive southern tier shortwave that brings a lot of moisture through the southeastern quadrant of the lower 48 and perhaps extending even a little northward (00Z CMC/ICON, 18Z GFS) to greater northern stream dominance over the East and a much more suppressed rainfall pattern (latest ECMWF runs and 00Z GFS). ML models do not favor the northward extreme of rainfall coverage but otherwise do not fall into a particular side of the envelope. Ensemble means/blended solutions with max amounts tempered in the northern part of the rainfall shield seem to provide a reasonable account for the high uncertainty at this time. The latest forecast started with a 12Z/18Z operational model composite early in the period. With the 18Z GFS becoming more unfavorable mid-late period, the GFS component switched to the 12Z run. Also 18Z GEFS/12Z ECens input was introduced for the latter half of the period, reaching half weight by next Thursday. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks covering the Sunday through Monday night period focus on three primary regions of interest, all with Marginal Risk areas both days. One is over the southern Mid-Atlantic west into the Appalachians where a couple of fronts will be sagging southward over the course of the period, with associated instability and moderately anomalous moisture. Meanwhile rainfall should continue along parts of the Louisiana/Texas coast, with prior days of rainfall providing wet ground conditions heading into the Days 4-5 period. There is still uncertainty in the coverage of showers and thunderstorms over the southern High Plains and vicinity but still enough of a signal to favor maintenance of the Marginal Risk. At the moment there seems to be a little better agreement on Day 5 relative to Day 4, with better consensus with respect to anomalous moisture. Precipitation coverage and totals across the southern tier after early Tuesday remain very uncertain due to significant differences in upper flow details. Some areas of locally heavy rainfall appear likely given the fronts settling over the South helping to serve as a focus, but with very low confidence in resolving the details of coverage and totals. Expect the Northwest to see scattered rainfall early next week as the eastern Pacific upper low ejects inland. Some scattered rainfall could reach the northern Plains by midweek or so. Upper ridging will support an area of well above normal temperatures drifting from the Northwest into the northern High PLains during Sunday-Tuesday, with some locations seeing highs 10-15F or so above normal. This warm air should quickly moderate farther eastward. A return of upper ridging over the West Coast states by Wednesday-Thursday should promote above normal temperatures with anomalies for highs again reaching plus 10-15F at some locations. Below normal highs over the central U.S. on Sunday will expand toward the East Coast early next week, suppressing what heat lingers over the Southeast to start the week. Most of the central/eastern U.S. should trend closer to normal by next Thursday but the southern Mid-Atlantic/Southeast may remain below normal for highs. 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