Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM 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 new 00Z CMC adds a new wrinkle of showing more extreme southwestward elongation of eastern trough energy, leading to a different southern Plains pattern than other guidance. Meanwhile, the model spread is reasonable for the eastern Pacific upper low that may eject into the Northwest by the first half of next week, thus favoring an intermediate solution for that part of the forecast. Based on the array of guidance, the updated forecast started with the standard 12Z/18Z operational model composite for the first half of the period and then transitioned to a model/ensemble mean mix (GEFS/ECens/CMCens) with operational input consisting of more 12Z ECMWF than 18Z GFS plus a small 12Z CMC component. ...Weather/Hazards Highlights... During the Friday-Saturday period covered by the Days 4-5 Excessive Rainfall Outlooks, there are currently two regions of focus for Marginal Risk areas. One is over and just south of the eastern Great Lakes (Friday) extending south/east into the Appalachians and East Coast (Saturday) as the initial Midwest/Plains cold front continues eastward. Guidance shows sufficient moisture and instability to support the potential for some decent rain rates, but frontal progression should temper overall totals somewhat. Model/ensemble signals for specifics are still not as agreeable as desired though. The other region with a Marginal Risk each day is the southern Rockies and in Saturday's case extending into parts of Arizona. Expect some episodes of convection to occur over sensitive terrain, while on Saturday some shortwave energy may approach from the west. Elsewhere, the aforementioned front may produce areas of locally heavy rainfall between the East and southern Rockies but without enough focus (or wet enough antecedent conditions) to merit a risk area so far. 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. 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 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