Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 AM EDT Tue Oct 15 2024 Valid 12Z Fri Oct 18 2024 - 12Z Tue Oct 22 2024 ...Overview... Deep upper troughing with a developing closed low in the Interior West will lead to a cool and wet pattern there, including higher elevation snow and frost/freeze concerns in some of the valleys. Heavy rain chances could increase in the southern/central Plains by the weekend. Upper ridging ahead of this trough/low will promote warmer than average temperatures in the northern/central Plains to Midwest as it meanders. The ridge should gradually warm the East as well and promote dry conditions for the most part, except for South Florida. Additional rounds of precipitation may come into the Pacific Northwest late week into the weekend. ...Guidance/Predictability Assessment... Model guidance is reasonably agreeable regarding the large scale pattern during much of the period. Models agree an upper trough digging into the Great Basin Friday will split off a closed upper low into the southern stream atop the Southwest by the weekend. There is modest spread among deterministic models/ensemble members/AI models with the timing and placement of this upper low, with spread increasing by early next week as it ejects northeast and may join back up with the mean flow by Tuesday. CMC runs have been on the slower side of the guidance envelope though interestingly the 12Z CMC ensemble mean was faster than other means. Preferred the 12Z GEFS mean over the 18Z GEFS mean as it matched better with the 12Z EC mean (12Z means both slower). But overall the model spread for this feature is well within typical ranges for a closed upper low like this in the middle to late medium range period. Upper ridging is agreeable ahead of this trough/low building from the east-central U.S. Friday toward the Eastern Seaboard Saturday displacing initial troughing, but more uncertain shortwaves could disrupt this ridging in the East by Sunday. A ridge is forecast to rebuild atop the Mississippi Valley and vicinity instead by Sunday, downstream of the Southwest upper low, pushing east into the workweek as the upper low does too. Energy is likely to dive southward through the eastern Pacific early next week and perhaps form a closed low offshore. This leads to some notable uncertainties in the flow pattern reaching the West without much agreement in the details yet. The WPC forecast was based on a blend of the deterministic models early in the period, with some ensemble means introduced by Day 5 and increasing to about half by Day 7 for the mass fields given the increasing spread. ...Weather/Hazards Highlights... The upper low closing off across the Southwest late week into the weekend will push precipitation into the Four Corners states, Rockies, and High Plains. Expect enhanced precipitation totals in terrain with higher elevation snow, which will be the first snow of the season in some locations given the warm Fall in the West thus far. Some precipitation could continue across the central/southern Rockies into the weekend. By later Friday into the weekend, Gulf moisture may stream in just ahead of the upper low for increasing rain chances in the central/southern High Plains. While forecast details vary, models have been fairly persistent in showing some moderate to heavy rain potential focused in northeastern New Mexico and nearby, with some modest instability. The Day 5/Saturday ERO proposes a Marginal Risk there and heavy rain would be most concerning atop sensitive burn scar areas. The current forecast indicates that the south-central Plains should continue to be a focus for enhanced precipitation on Sunday before finally moving eastward Monday. Additionally, more rounds of precipitation are likely for the Pacific Northwest into the weekend with additional impulses coming through. Meanwhile, the eastern U.S. is forecast to be generally dry late week into early next week, with the exception of parts of the Florida Peninsula. There, return flow of moisture near a meandering front could increase rain and thunderstorm chances especially for South Florida by Friday and into the weekend. There could be a nonzero chance for flash flooding with this activity especially if heavy rain rates occur in the sensitive urban areas. However, as models continue to vary with how much rain may fall onshore versus offshore, no areas are currently delineated in the Days 4-5/Friday- Saturday EROs awaiting hopefully better model agreement. An upper ridge aligned over the east-central U.S. Friday will promote above normal temperatures in its vicinity. The biggest anomalies of plus 10-20 degrees are forecast across the Midwest. Meanwhile another round of chilly morning lows may cause frost/freeze concerns for the Interior Northeast and Appalachians. Gradual moderation/warming is likely to continue there into the weekend when upper ridging comes overhead, allowing for temperatures around 5-10 degrees above normal in the northeastern tier. Then the ridge rebuilding over the central U.S. early next week will lead to warmer than average temperatures in the northern Plains to Upper Midwest once again. On the other hand, the upper trough and frontal surges in the West will cause below normal temperatures to spread across the Great Basin/Intermountain West and Southwest on Friday with slow moderation into the weekend. These colder temperatures may also lead to additional frost/freeze concerns for any still susceptible growing areas/valley locations. 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 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