Extended Forecast Discussion NWS Weather Prediction Center College Park MD 330 AM EDT Fri Sep 20 2024 Valid 12Z Mon Sep 23 2024 - 12Z Fri Sep 27 2024 ...Guidance/Predictability Assessment... The 12 UTC guidance suite was overall in better than average agreement through early-mid next week for the mid-larger scale pattern evolution and main systems. However, the 06 UTC GFS/GEFS introduced additional outllier variance. Opted for a composite of best clustered guidance from the 12 UTC GFS/ECMWF and NAEFS/ECMWF ensemble means that seemed to offer a good forecast basis in conjucture with the National Blend of Models. The majority of blend weighting was applied to these models in this period, and then to the ensemble means later next week amid growing uncertainty and in an effort to best emulate NHC preferences for depiction of possible Gulf Tropical activity and as per WPC system continuity. ...Weather/Hazards Highlights... Energy over the West will splits southward over the Rockies, then southwestward toward the Southwest through early-mid next week. This may provide a period favorable for enhanced rainfall over the southern High Plains and southern Rockies Tuesday into Wednesday in upper diffluent flow and with frontal proximity. A WPC Marginal Risk area has been introduced. Meanwhile farther north, upper trough energy and wavy frontal systems will be kicked from the Plains to the Midwest/East early next week along with an associated threat of showers/thunderstorms with some local runoff issues. WPC Excessive Rainfall Outlook (ERO) Marginal Risk areas are depicted to shift eastward Day4/Monday and Day/5 Tuesday across the Great Lakes/Ohio Valley to the northern Mid-Atlantic. Upper trough reinforcement may support additional heavy rainfall potential to monitor over the East mid-later next week in amplifying flow. This occurs as far upstream development of a potent and slowly progressive upper trough from the northeast Pacific early next week provokes downstream upper ridge amplification and warming, with an eastward shift from the NOAM West Coast to mid-continent next week. Meanwhile NHC and WPC will continue to monitor possible tropical development in the Caribbean that may slowly move northward next week. The ultimate position and southward breath of the upper trough over the lower 48 will determine extent of northward lifting. Latest 00 UTC models have trended westward and split. Farther west, most areas should stay dry under mean ridging. The Washington coastal ranges/Cascades could be an exception and offer growing potential over time with subsequent ample Pacific energy approach. Then in South Florida, showers and storms may focus near a frontal boundary lingering in a moist environment. These do not look to be too heavy next week until around late week, when there may be some surge of tropical moisture northward ahead of any potential tropical development farther south in the Caribbean that may slowly make its way north into the Gulf during next week. There is still considerable uncertainty in the track and timing of any possible tropical system, so continue to check updated forecasts. Schichtel 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