Extended Forecast Discussion NWS Weather Prediction Center College Park MD 313 PM EDT Tue Aug 27 2024 Valid 12Z Fri Aug 30 2024 - 12Z Tue Sep 03 2024 ...Overview... Guidance expects a slow eastward drift of the mean pattern over the course of the period, with a ridge crossing the West and settling over the Rockies/High Plains by early next week while the downstream Canada/northern tier U.S. trough pushes into eastern North America. Meanwhile an eastern Pacific upper low may eject into the Northwest by early next week. The forecast pattern evolution will serve to moderate the hazardous heat initially over parts of the East, with increasing coverage of near to below normal highs east of the Rockies by next week. A leading front crossing the East will produce showers and thunderstorms late week into the weekend followed by a drying trend, while more persistent chances for rain of varying intensity will exist across portions of the southern tier--especially from the southern Rockies to west-central Gulf Coast. ...Guidance/Predictability Assessment... Latest dynamical and machine learning (ML) models are fairly agreeable with important features for about the first half of the period. From about Sunday onward, guidance begins to differ with specifics of lower-predictability energy that may reach/evolve over the southern Rockies and vicinity. Over the past couple days the GFS has been the most aggressive with the depth of this energy, and while recent trends of ML models have raised the potential for the existence of this energy aloft, they suggest that it should be weaker and more progressive than depicted by the GFS. The main outlier from the 00/06Z cycle was the CMC, which showed more extreme southwestward elongation of eastern trough energy, leading to a different southern Plains to East pattern than other guidance. The newer 12Z CMC seems in better alignment. Then, model spread begins as pretty reasonable for the eastern Pacific upper low, perhaps higher confidence than normal for that region, until Monday-Tuesday when spread increases at least partly due to additional troughing up near Alaska. Preferred the ensemble means by that point as an intermediate solution. Based on the array of guidance, the updated forecast started with the standard 00Z/06Z operational model composite for the first half of the period and then transitioned to a model/ensemble mean mix (GEFS/ECens) for the latter period as spread increased. ...Weather/Hazards Highlights... As a first front tracks across the eastern to south-central parts of the country, ample moisture and instability pooling along and ahead of it could lead to locally heavy rain causing a flash flooding threat. The biggest change to the Day 4/Friday ERO was to combine the Marginal Risks from the Great Lakes region southwestward into the Ohio Valley and Mid-Mississippi Valley through the southern Plains to join with the New Mexico one. Models show some variability in the placement/focus, but for the central part of the country, the front starting to lay out more west to east could lead to training convection with heavy rain rates. Antecedent conditions are not very wet though, which could temper effects. The northern part of the risk area could see quicker movement of the convection but tends to be more sensitive to rain. By Day 5, locally heavy rain looks to affect parts of the Northeast to Mid-Atlantic back into the Appalachians and perhaps the Tennessee/Ohio Valleys as the front progresses, supporting a Marginal there. The Southwest/southern Rockies/southern High Plains should also continue to see thunderstorms into Saturday as shortwave energy approaches from the west. Some episodes of convection there could occur over sensitive terrain. Meanwhile, guidance is persistent with the potential for convection along parts of the Louisiana/Texas coast but with continued differences for greatest amounts and exact location, including whether inland or just offshore. Since notable rain may be limited to the immediate coast over sandy soils, and currently looks to stay east of the Houston metro area, continued to hold off on any EROs there. Also, most but not all guidance shows precipitable water values gradually increasing over the Florida Peninsula during Friday- Saturday which could enhance localized amounts there, but again would like to see a more pronounced guidance signal before introducing a risk area at this distant time frame. After Saturday, uncertain details of shortwave energy that may evolve/progress over and east of the southern Rockies will play a role in convection over the southern Rockies/Plains, with some locally heavy amounts possible. Rainfall may continue along the western half of the Gulf Coast. Eastern U.S. rain will settle into the southern tier along a couple cold fronts as trailing high pressure promotes a drying trend farther north. The Northwest may see scattered rainfall early next week as an eastern Pacific upper low ejects inland. Hazardous heat will linger over parts of the eastern U.S. into Friday ahead of a cold front advancing from the Midwest. Best potential for highs up to 10-15F above normal will be over and just south of the Ohio Valley, with a few locations possibly approaching daily records. Experimental HeatRisk shows Moderate to Major (and isolated Extreme) level impacts on Friday. Continued erosion of the eastern upper ridge and approach/arrival of the cold front should suppress the heat more into the South and with only single- digit anomalies. The central U.S. will see near to moderately below normal highs late this week. A second cold front dropping through the central/eastern states Sunday-Tuesday should bring more widespread below normal highs. Meanwhile, expect an area of above normal temperatures to drift from the Pacific Northwest into the northern High Plains from late this week into early next week, corresponding to movement of an upper ridge. Rausch/Tate 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