Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Wed Sep 11 2024 Valid 12Z Sat Sep 14 2024 - 12Z Wed Sep 18 2024 ...General Overview... The remnant low associated with what is now Hurricane Francine will continue to dissipate across the Ohio Valley over the weekend, though with some lingering potential for locally heavy rainfall to the southeast. Meanwhile, a closed upper high expected to drift over the Great Lakes/Northeast will promote above average temperatures near its path into early next week. Consolidating energy aloft south of the upper ridge may interact with an initial frontal wave off the Southeast coast to develop a surface system (possibly becoming subtropical/tropical for a time) that tracks back onto land and produces some enhanced rainfall. Across the Western U.S., an amplified mean trough will likely set up near the West Coast with an embedded low possibly reaching near California by early Monday and then ejecting northeastward as additional energy digs into the trough by Wednesday. The ejecting upper low may produce an area of significant precipitation depending on its exact track. ...Guidance/Predictability Assessment... The updated forecast based on 00Z/06Z guidance started with a blend of the 00Z ECMWF/UKMET/CMC and 06Z GEFS mean for about the first half of the period and then quickly transitioned to a half model (more ECMWF than CMC) and half ensemble mean (06Z GEFS and 00Z ECens/CMCens) mix by Tuesday-Wednesday. Primary considerations for replacing the GFS with the GEFS mean in the forecast were that the 00Z/06Z GFS runs were a combined deep/northward extreme with the system that may develop over the western Atlantic and then track into the Carolinas and beyond, while GFS runs also strayed somewhat to the fast side with the upper trough/low ejecting from West next week. The evolution over the western Atlantic into Eastern U.S. still has below average confidence in specifics given dependence on smaller scale energy underneath the Northeast upper ridge, potential convective influences while offshore, and how much time it spends over water. The new 12Z CMC is gradually raising the probability of some variation of the GFS/ECMWF scenario, though the UKMET/ICON have very different ideas and the machine learning (ML) models are also mixed regarding how much surface development may occur. Cyclone phase guidance suggests potential for this feature to become subtropical/tropical for a time, so the coordinated WPC/NHC depiction depicts such an idea after Sunday. New 12Z model trends generally show faster progression from the western Atlantic westward/northwestward. Within the western mean trough, most of the ML models provided added support along with the ensemble means and ECMWF/CMC for slower Monday-Wednesday system ejection versus the 00Z/06Z GFS, with the new 12Z GFS adjusting somewhat slower. The ML models also reflect most dynamical guidance in digging additional energy into the trough by next Wednesday, though with typical uncertainty for the time frame regarding existence/location of a possible closed low. ...Weather/Hazards Highlights... The remnant low from Francine dissipate by Saturday and evolve into a surface trough, while the majority of the remaining rainfall should be located to the east and southeast of the low. There will likely be a remaining inflow band across central Tennessee and extending into northern Alabama, where locally higher higher rainfall totals could be observed. Orthogonal low level flow across the southern Appalachians could result in some higher rainfall totals here as well, and both of these regions are encompassed in a broad Marginal Risk area for the Day 4/Saturday Excessive Rainfall Outlook. By Day 5/Sunday, a lingering band of enhanced rainfall potential may exist over parts of the Tennessee Valley/South with a Marginal Risk area in the ERO. In addition, while the current deterministic forecast for the system that may develop off the Southeast coast is not as fast, trends in the 12Z models offer enough support from probabilistic perspective to introduce a Marginal Risk area in the Day 5 ERO near the coast of the Carolinas. Beyond early Monday, some areas of heavy rainfall my extend farther west/northwest into the Appalachians, with northward extent being sensitive to strength of ridging at the surface and aloft. Showers and thunderstorms are likely this weekend for parts of the Southwest with an increased moisture connection ahead of a possible tropical disturbance lifting north over the southern Baja Peninsula, and a weak shortwave passing through. To account for the possibility of enhanced rainfall, Marginal Risk areas are maintained for southern portions of Arizona on Saturday, and extending farther north into Arizona and western New Mexico on Sunday. Elsewhere across the West, increasing showers and storms are likely by early next week as upper troughing amplifies near the West Coast and ejects northeastward. Based on the current forecast of the ejecting embedded low, the northern Rockies may be a focus for heavy rainfall around Tuesday. The southern Plains may also see scattered showers/storms as tropical moisture gets wrapped in ahead of the Western trough. Near to slightly below normal high temperatures over the West on Saturday will trend colder with time heading into the start of next week and then persist through Wednesday as mean troughing aloft becomes established. A fairly broad area of highs 10 to perhaps 20 degrees below normal will be possible by next Monday-Wednesday from California and parts of Oregon into the Intermountain West and northern Rockies. Meanwhile, expect warm conditions to continue from most of the Plains into the Great Lakes and parts of the Northeast where upper level ridging will tend to govern the overall weather pattern. Rausch/Hamrick 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