Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Mon Sep 16 2024 Valid 12Z Thu Sep 19 2024 - 12Z Mon Sep 23 2024 ...Overview... Mean upper troughing across the West into later this week will promote cooler weather with rain and high elevation snow. Rounds of rain and thunderstorms are likely in the central U.S. in the vicinity of frontal systems ahead of this troughing, which are likely to become more widespread Friday and especially Saturday as the trough's dynamics move east. Meanwhile in the East, upper troughing will linger for late week into the weekend, which along with weak surface frontal systems over the western Atlantic could lead to showers across parts of the Eastern Seaboard. ...Guidance/Predictability Assessment... Model guidance is agreeable with the large scale pattern at the beginning of the period Thursday, showing a closed upper low atop California and vicinity and a positively tilted trough toward a compact upper low in south-central Canada, with ridging from the southern Plains to Midwest and troughing atop the East. There is some spread with the evolution of troughing in the East, but the most uncertain aspect of the forecast is with the West trough moving eastward and how it is affected by the northern stream flow. By late week, there is already spread with the timing of the trough's eastward movement. The 12Z CMC was on the slower side of the deterministic guidance, with the 12/18Z GFS faster and the 12Z EC in between. However, in looking at the EC-based AI models, the bulk of them were faster than the deterministic EC and more like the GFS runs, at least in part due to the timing of northern stream troughing. The ensemble means were also on the faster side. Thus leaned more toward the GFS/AI models/ensemble means for the forecast. However the 00Z models have generally come in slower than their previous runs, including the GFS and the EC (EC still slower than the GFS). So there is still considerable uncertainty with the trough's timing and this causes sensible weather differences like with the QPF, for low confidence in the details. Then the flow upstream across the eastern Pacific into the Northwest is also quite uncertain over the weekend into early next week. 12/18Z deterministic models were really favoring ridging moving into the Northwest Sunday into Monday, but meanwhile the ensemble means were completely out of phase and indicating some troughing (especially the CMC mean that was considered overdone with the troughing). The AI models showed a lot of spread with the pattern as did the individual ensemble members, not increasing confidence. Leaning toward the ensemble means with flatter flow but with some troughing seemed most prudent. The newer 00Z models are generally at least flatter with the flow coming into the Northwest, but expect the forecast to continue to change there, with precipitation chances and frontal positions being affected. The WPC forecast used a blend of mainly the deterministic guidance favoring the 12/18Z GFS runs early on, with gradual decrease of the deterministic models in favor of the ensemble means from the EC and GFS as the period progressed, reaching half means by Day 6 and more by Day 7 given the increasing spread. ...Weather/Hazards Highlights... An upper low to start the period over the West will provide lift/forcing to produce precipitation across parts of California into the Great Basin on Thursday and shifting into the Rockies Friday. The higher elevations can expect snow given the depth of the upper low. Farther east, scattered rain and thunderstorms are likely in the Plains to Upper Midwest for multiple days later this week into the weekend near and ahead of frontal systems, and as the mean trough moves eastward. Model guidance shows quite a bit of spread with the placement and amounts of heavy rain that may occur, making confidence in the details low. Considered drawing a Marginal Risk into the ERO for Day 5/Friday around the central Plains to parts of the Midwest, but with some recent models (00Z UKMET and ECMWF) trending down in rainfall amounts and the overall spread, held off at this point, but some ERO risk could certainly be needed in future issuances. It appears that Saturday (Day 6) may be the wettest day with the best dynamical support. Upstream, the questions with the pattern aloft create considerable uncertainty with the QPF in the Northwest into the weekend. In the East, remnant (partially tropical) moisture will promote showers persisting into late week and the weekend given troughing remaining overhead and weak Atlantic fronts/surface lows. The area that appears to have the greatest risk of impacts from potential heavy rain into Thursday will likely be portions of Virginia into North Carolina especially given what may be wet antecedent conditions by then, so a Marginal Risk remains in place there for Day 4. It remains in question how far north into the Northeast the showers reach. The Florida Peninsula could also see some focused convection due to a frontal boundary in a moist environment. This front stalling into the weekend could increase rain totals, which may create flooding problems especially for the typically sensitive urban corridor of Southeast Florida. Underneath the upper trough in the West, much cooler than normal temperatures are expected. High temperatures will be 15-20F 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 part 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