Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Fri Sep 13 2024 Valid 12Z Mon Sep 16 2024 - 12Z Fri Sep 20 2024 ...Overview... Reloading upper troughing will be in place across the western U.S. through next week, promoting cooler weather and rain and higher elevation snow. Some rain may spread into the Plains as well, but with warmer than average temperatures. Meanwhile in the East, early in the week a Rex Block type pattern will set up with an upper high situated over the Northeast and a weak upper low farther south, accompanied by a coastal surface low pressure system. The evolution of the pattern in the East is rather uncertain as the week progresses, including the surface low's possible tropical or subtropical evolution and the coastal and inland rainfall forecast. ...Guidance/Predictability Assessment... Model guidance remains persistent in indicating a deep upper low centered over northern California as the period begins Monday, which along with an accompanying surface low should lift northeast with time. There is some minor increasing spread with these upper/surface lows by Wednesday-Thursday. The 18Z GFS/ECMWF were pretty agreeable in their positions just north of Montana, but these were south of the ensemble mean positions and now south of/slower than the 00Z model positions. Meanwhile the 12Z CMC seemed to get rid of this compact upper low too quickly, but the 00Z run looks in better alignment. Another round of vorticity is forecast to dig south along the west side of the trough atop the West Coast states during Tuesday-Thursday before moving the trough east by Friday. Once again the 00Z CMC seems more agreeable with consensus than the 12Z run did. Upstream by Friday there is uncertainty with the flow pattern coming into British Columbia and potentially affecting Pacific Northwest fronts, which will have to be monitored as currently models are out of phase, GFS runs with ridging and CMC/EC with troughing. The eastern U.S. pattern shows plenty of uncertainty in the details of the energy embedded within the broader upper low gradually drifting north with time, affecting the surface pattern with a coastal stationary front with waves of low pressure, one of which has some potential for subtropical or tropical development. Some GFS runs remain on the aggressive side with the surface low strengthening, but ensemble members do show some support for a low that deep. The new 00Z ECMWF shows any notable QPF staying offshore of the East Coast through the first half of the week, unlike the GFS in particular. So these energy/low differences do create significant sensible weather differences, and the forecast is likely to continue to fluctuate. The WPC forecast was based on a blend of deterministic models (18Z GFS and 00Z ECMWF/CMC/UKMET) early in the period, introducing and increasing the proportion of the EC and GEFS ensemble means as the period progressed given increasing spread. ...Weather/Hazards Highlights... The upper low/trough across the West will support rounds of precipitation there next week, providing ample lift/forcing. On Monday, lingering moisture from the Pacific Tropical Storm Ileana (that will be dissipated by then) could also join and lead to heavy rain for portions of the Four Corners states, prompting a Marginal Risk there. Precipitation may also be somewhat heavy on Monday around northern Nevada, but held off on any excessive rainfall risk due to uncertainty if the rain would be impactful there. By Tuesday, the pivoting upper and surface lows should spread heavy precipitation into the northern Rockies/High Plains. Did introduce a Day 5/Tuesday Marginal Risk of excessive rainfall for the western half of Montana as heavy rain rates near the low track could persist over the same areas for multiple hours, and there could be some instability in place. The higher elevations can expect snow with this system given the depth of the upper low. Another round of energy will cause the Pacific Northwest to see precipitation on Tuesday, shifting into much of the Great Basin midweek and then into the northern Rockies. Farther east, scattered rain and thunderstorms are likely in the Plains for multiple days next week near and ahead of frontal systems. A stationary front stretching from the far western Atlantic and back across the Gulf Coast with waves of low pressure along it could maintain rain chances across the Southeast to Carolinas for the early part of next week at least, perhaps into mid and later week. The QPF remains highly uncertain given the model differences, including how far west and north rain could reach. At this point, WPC shows a Marginal Risk for Day 4/Monday across eastern portions of the Carolinas near where a surface low is most likely to be just offshore. Held off on any risk area for Day 5/Tuesday because of the increasing spread in position of any heavy rainfall, but in general there could be some rain in the Carolinas into the Mid- Atlantic Tuesday-Thursday. The West can expect much cooler than normal temperatures next week under the upper trough. High temperatures will be 15-25F below normal across much of the West, gradually moderating closer to normal across the West Coast states but persisting in the Interior West. Some areas may reach the freezing mark in the higher elevations of the Great Basin/Rockies. Meanwhile, the central U.S. should see warmer than normal highs by 10-15F and lows by 15-20F, including highs in the 90s in the southern half of the Plains. Warmer than average temperatures will stretch into the Great Lakes and Northeast, with more typical temperatures south of there. 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