Extended Forecast Discussion NWS Weather Prediction Center College Park MD 308 AM EDT Sat Sep 14 2024 Valid 12Z Tue Sep 17 2024 - 12Z Sat Sep 21 2024 ...Possible subtropical or tropical low could spread heavy rain to the Carolinas and Mid-Atlantic into Tuesday... ...Overview... Reloading upper troughing will be in place across the western U.S. through much of 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, a persistent upper low will meander while potentially accompanied by surface low pressure systems. NHC and WPC are monitoring one surface low off the Carolina coast for possible tropical or subtropical as the period begins Tuesday, but with ample uncertainty remaining in its evolution and thus the coastal and inland rainfall forecast. ...Guidance/Predictability Assessment... The eastern U.S. is the area of greatest uncertainty during the period, despite general agreement in an upper low south of a east- west oriented ridge that should be eroding over the Northeast Tuesday. This upper low is forecast to expand northward and eventually join with a northern stream upper trough over the Northeast late week. However, details of energy within the upper low and trough will impact the pattern at the surface, including a low pressure system that could show tropical or subtropical development early to midweek. The low may be most likely to be subtropical as cyclone phase space diagrams from various models are generally showing a symmetric warm core low, but borderline at times, while there seem to be some moisture/temperature gradients that would suggest fronts nearby. There is notable uncertainty with the low type and also the low track. Recent model runs/cycles have started to show a bit more persistence for the low to move inland into North Carolina by Tuesday, while guidance diverges in how far inland it could maintain a low. However, AI models for a while now have favored more of an offshore track, so cannot rule that out. The UKMET also has an offshore track. These energy/low differences do create significant sensible weather differences in terms of QPF, and the forecast may continue to fluctuate. Weak lows that may stay frontal may form over the western Atlantic into later in the week as well. In the West, fortunately there is good model agreement in indicating an upper low atop the Great Basin early Tuesday pivoting northeastward Wednesday-Thursday along with a surface low. Meanwhile additional energy digs through the west side of the trough near the West Coast midweek and reinforces the trough, possibly developing another embedded upper low as well. Some uncertainty arises as the base of the trough starts to shift eastward late week. The 00Z CMC is the slowest of recent solutions in progressing it east. The WPC forecast started with a blend of the deterministic 18Z GFS and 12Z ECMWF/CMC/UKMET along with small proportions of the GEFS and EC ensemble means to reduce dependency on any individual model run given the variations especially in the East. As the period progressed, increased the proportion of the ensemble means to about half by Days 6-7. ...Weather/Hazards Highlights... The surface low that may be tropical or subtropical per NHC remains uncertain, but is currently forecast to come into the Carolinas by Tuesday, spreading rain there and into the Mid- Atlantic and Appalachians. Marginal Risks are in place for North Carolina into much of the Mid-Atlantic Day 4/Tuesday and shifting a bit farther north across the Mid-Atlantic toward southern portions of the Northeast Day 5/Wednesday. If the forecast pans out with a relatively deep low coming into these areas, a risk level higher than a Marginal would likely be warranted particularly on Tuesday, but given the lingering uncertainty in the low track maintained the Marginal for now. Showers could be persistent into the latter half of the week as upper troughing remains overhead. Frontal boundaries could also help focus moisture across parts of the Southeast for rain and thunderstorm chances. A Marginal Risk remains in effect for Day 4/Tuesday for the Florida Panhandle particularly due to their wet antecedent conditions. Into Wednesday-Thursday, the urban areas of Southeast Florida may be a focus for convection, potentially prompting Marginal Risk areas in future forecast cycles if guidance agrees. The upper low/trough across the West will provide ample lift/forcing to support rounds of precipitation there next week. For Day 4/Tuesday, a Marginal Risk ERO is in place for parts of northern Idaho to the western half of Montana as the region could see heavy rain rates near the low track that 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. Rainfall amounts may increase in the north-central U.S. by later next week. The West can expect much cooler than normal temperatures next week under the upper trough. High temperatures will be 15-20F below normal across much of the West, gradually moderating closer to normal across the West Coast states but persisting in the Interior West as the week progresses. 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 (until a cold frontal passage in the Northeast late week), 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