Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sat Aug 24 2024 Valid 12Z Tue Aug 27 2024 - 12Z Sat Aug 31 2024 ...Overview... Expect the strong upper ridge over the Middle Mississippi Valley as of early Tuesday to drift southeast with time, gradually weakening as it drifts off the Southeast coast toward the end of the week. This should promote a similar drift and eventual moderation of Tuesday's hot temperatures over the east-central U.S. Meanwhile a northern Plains shortwave should round the top of the ridge but still with a fair degree of uncertainty for specifics as it brushes the Great Lakes/Northeast. Most guidance still has a strong upper trough coming into the Northwest U.S. and western Canada by Tuesday and continuing onward thereafter, bringing some cool daytime highs to areas from the Northwest into northern High Plains Tuesday- Thursday. Upstream energy may lead to some broadening of the overall mean trough by next Saturday. An upper ridge will likely build over more of the West by late week after the initial trough's departure. The leading northern tier shortwave/surface reflection and then the cold front associated with the advancing western North America trough will produce areas of rainfall over the eastern half of the country. Diurnal convection should continue over the southern Rockies and along the Gulf Coast. ...Guidance/Predictability Assessment... Based on 12Z/18Z guidance, the updated forecast maintained a preference for recent ECMWF/UKMET/CMC runs and the ECens/CMCens means. GEFS means offer decent comparisons for the overall pattern toward the end of the period though. GFS runs have continued to have difficulty in resolving the leading shortwave tracking out of the northern Plains and eventual evolution of the vigorous trough tracking eastward from western North America. The 18Z GFS had a fairly novel solution of combining its excessively progressive northern tier shortwave energy with amplifying eastern Canada dynamics to produce a deep and slow moving upper low near New England and Nova Scotia late in the week. Other dynamical and machine learning (ML) models offered minimal support for that scenario. The new 00Z GFS has finally adjusted a lot closer to other guidance in principle. Still, there is quite a bit of spread for shortwave specifics as it continues east/southeast--ranging between shearing out to staying fairly well defined as it crosses New England. The GFS had been starting to get closer to other guidance for the ultimate evolution of the southern Canada/northern tier U.S. upper trough and embedded low by the latter half of the week, but the new 00Z GFS has returned to splitting energy in a fashion not suggested by other dynamical and ML models or the ensemble means (including the GEFS). This issue also affects some finer details of the pattern over the Rockies by Thursday. ...Weather/Hazards Highlights... The strong upper ridge initially over the Mississippi Valley will support a broad area of anomalous heat on Tuesday, followed by a very gradual southeastward suppression corresponding to the ridge's movement and eventual weakening. Some areas may see one or more days with highs up to 10-15 above normal (and a few daily records) Tuesday into Thursday or Friday, with greatest anomalies finally declining to plus 5-10F range by next Saturday. Experimental HeatRisk continues to show a fairly broad area of Moderate to Major level impacts with small embedded pockets of Extreme. In contrast, the strong upper trough progressing eastward from the Northwest produce below normal highs from the Northwest into the northern High Plains Tuesday-Thursday with some minus 10-15F anomalies. Expect temperatures to rebound to above normal levels over the Northwest by late week as upper ridging builds into the region. During the 12Z Tuesday-12Z Wednesday period covered by the Day 4 Excessive Rainfall Outlook, a Marginal Risk area remains in place over portions of the southern Rockies given adequate moisture/instability, GFS/ECMWF QPF signals, and likely existence of shortwave impulses aloft. By Day 5 (Wednesday-Wednesday night) most guidance suggests that convection over the region should trend somewhat lighter. The exception comes from the 00Z GFS whose more favorable upper pattern has little support from consensus. Thus no risk area is depicted for now in the Day 5 ERO. Farther north, a leading northern tier shortwave may still produce some convection near the Upper Great Lakes early Tuesday but without a sufficiently strong signal for a risk area in the Day 4 ERO. Some convection will likely extend eastward/southeastward into parts of the Northeast/Mid-Atlantic by Wednesday, but at this time guidance signals for location and magnitude of rainfall are too incoherent to merit a risk area in the Day 5 ERO. On the other hand, the Day 5 ERO has introduced a Marginal Risk area covering most of Minnesota/eastern North Dakota and far northeastern South Dakota as the strong western North America upper trough approaches, with the associated cold front as well as a leading warm front helping to focus convection. Continued eastward progression of the Plains cold front after early Thursday should bring areas of rain and thunderstorms into the eastern part of the country by late week into the weekend. Some convection may persist over and near the southern Rockies late in the week. Expect episodes of showers and thunderstorms along the Gulf Coast through the period, while southern Florida could see some increase of moisture toward the end of the week. 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