Extended Forecast Discussion NWS Weather Prediction Center College Park MD 443 PM EDT Sun Sep 15 2024 Valid 12Z Wed Sep 18 2024 - 12Z Sun Sep 22 2024 ...A coastal storm could spread heavy rain into the Mid-Atlantic states while locally heavy rain could impact northwestern Montana on Wednesday into early Thursday... ...Overview... Deep upper troughing will be in place across the western U.S. through this week to promote cooler weather with rain and high- elevation snow. Rain on the backside of a deepening low could impact northwestern Montana on Wednesday into early Thursday. There appears to be a good chance for bands of heavy rain and strong thunderstorms across the northern and central Plains ahead of a slowing front late this week into the weekend with warmer than average temperatures. Meanwhile in the East, a broad upper low initially accompanied with tropical moisture from a coastal storm will be slow to exit the East Coast with marginal heavy rain potential mainly across the northern Mid-Atlantic Wednesday into Thursday. ...Guidance/Predictability Assessment... The most uncertain portion of the medium-range forecasts continues to be along the East Coast where the track and forward speed of a coastal storm remain in question into the short-range, even down to the current time with the analysis. The GFS has been the fastest guidance, taking the system north then northwestward into South Carolina and then pretty much dissipating it well inland by the time medium-range begins on Wednesday. The ECMWF has flipped its eastern track since a couple of days ago, now tracking the system slowly westward for a day or so before swinging it onshore into North Carolina on Tuesday. Present satellite loop indicates that a northward motion might have begun after initially being stationary. The WPC medium-range forecasts are based on a consensus of the guidance with a general northward track bringing the system well inland by Day 3. For the rest of the country, agreement among models remain acceptable through Day 7. There is increasing spread regarding the forward speed of the upper trough that is forecast to head into the northern Plains by the weekend. This required some modifications to the frontal positions and the associated QPF distribution. The WPC medium-range forecasts were based on a consensus of the 00Z EC/EC mean, 06Z GEFS, the 00Z CMC/CMC mean, and the 00Z Ukmet while avoiding 06Z GFS on Days 3 and 4 owing to its fast forward motion of the coastal storm near the Carolinas. ...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. Remnant moisture could linger into Thursday/Day 5, over portions of the Mid-Atlantic to allow some locally heavy rain where a Marginal Risk of lash flooding is maintained 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 on the backside of a deepening low pressure system across northwestern Montana. 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. Kong/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