Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Sun Sep 15 2024 Valid 12Z Wed Sep 18 2024 - 12Z Sun Sep 22 2024 ...Possible subtropical or tropical low could spread heavy rain to the Mid-Atlantic into Wednesday... ...Overview... Reloading upper troughing will be in place across the western U.S. through this week, promoting cooler weather and rain plus higher elevation snow. Some rain will spread into the Plains as well, becoming moderate to heavy at some locations into late week, but with warmer than average temperatures. Meanwhile in the East, a persistent upper low will meander while potentially accompanied by surface low pressure systems. In particular, NHC and WPC are monitoring possible surface low development in the short term off the Southeast coast (may become tropical or subtropical) that could be inland over the Mid-Atlantic at the start of the period Wednesday. However there has been considerable uncertainty with this system's evolution and path along with the coverage/intensity of associated rainfall. ...Guidance/Predictability Assessment... The eastern U.S. is the area of greatest uncertainty during the early part of the medium range period, despite general agreement in an upper low meandering there, likely sinking south by next weekend. 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. By the start of the period Wednesday, this surface low seems most likely to be onshore with a middle ground consensus possibly over Virginia. Whether the low will be tropical/subtropical or extratropical remains in question. By Wednesday it may be acquiring fronts even if it approaches the U.S. as tropical or subtropical, which is certainly a possibility per the NHC outlook probabilities and a look at the cyclone phase space diagrams from various models. The structure/type of the low will also impact QPF. The 12Z ECMWF seemed like a southwestern outlier in its QPF focus compared to the 12Z EC mean and other guidance like the CMC and UKMET, so for QPF did blend in some of these solutions to the NBM for a trend north. The forecast will likely continue to fluctuate given the continued model spread. Later in the week, weak lows that may stay frontal may form over the western Atlantic supported by the troughing aloft. In the West, there is generally better model agreement for at least the early part of the period, as a first upper low and its associated surface low pivot northeast from Montana into southern Canada Wednesday-Thursday while trailing energy digging across California may create another embedded upper low. By Friday and beyond though, models show increasing spread regarding North Pacific flow entering the western North America mean trough and its influence on Thursday's California/Nevada trough or upper low. The flow pattern in the 18Z GFS led to an Interior West to Central U.S. trough ejecting east much more quickly than consensus. The 12Z GFS showed better positioning with this trough but was faster/flatter than favored with the northern stream. The newer 00Z GFS fortunately now seems to have better agreement with the EC/CMC and ensemble means. The WPC forecast began with a blend of the deterministic models (12/18Z GFS and 12Z ECMWF/CMC/UKMET). Gradually decreased the proportion of the deterministic models, particularly the GFS runs (and the time-limited UKMET), as the period progressed in favor of the GEFS and EC ensemble means. The ensemble mean proportion reached half by Day 6 and more by Day 7 given the increasing spread. ...Weather/Hazards Highlights... The surface low that may be tropical or subtropical per NHC/WPC coordination remains more uncertain than desired, but for now is most likely to be located in Virginia or vicinity by Wednesday, spreading rain into the Mid-Atlantic and Appalachians. For Wednesday/Day 4, a Marginal Risk area remains in place across much of the Mid-Atlantic region and stretches into southern parts of the Northeast given a slightly faster trend in the rain spreading north and east. Above normal moisture looks to linger into Thursday/Day 5, prompting a continued Marginal Risk in the Mid- Atlantic especially after a wet previous couple of days. Showers are likely to persist into late week along parts of the Eastern Seaboard as upper troughing remains overhead. The Florida Peninsula could also see some focused convection due to a frontal boundary in a moist environment. Currently this thunderstorm activity seems to be sub-Marginal Risk for flash flooding, but if guidance is agreeable in trending upward with amounts in future cycles, future Marginals cannot be ruled out, especially for the typically sensitive urban corridor of Southeast Florida. The mean trough aloft over the West will provide ample lift/forcing to support rounds of precipitation there this week. Heavy rain may linger across parts of Montana on Day 4/Wednesday for a continued Marginal Risk, as the region could see heavy rain rates for multiple hours near the track of a leading upper low ejecting through the northern Rockies, with potential for some instability to be in place. The secondary upper low should then spread precipitation from the West Coast states midweek into the Great Basin and Rockies. The higher elevations can expect snow with both of these systems given the depth of the upper low. Farther east, scattered rain and thunderstorms are likely in the Plains for multiple days this week near and ahead of frontal systems. Rainfall amounts may increase in the north- central U.S. by later week, as the front trailing from the initial Montana into Canada system stalls for a period of time while awaiting some combination of northern stream and western U.S. dynamics (with possible surface low development). Underneath the upper trough in the West, much cooler than normal temperatures are expected. High temperatures will be 15-25F below normal across much of the West, gradually moderating closer to normal across the West Coast states but with some minus 10-15F anomalies persisting in the Interior West and reaching the northern High Plains over the weekend. 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, through the latter half of the week. The scope of above normal temperatures will decrease into the weekend and mainly focus across the Upper Great Lakes to the Ohio Valley. The Northeast can expect warmer than average temperatures until a cold frontal passage late week, while temperatures south of there are forecast to be near to below normal. 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