Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Sun Oct 13 2024 Valid 12Z Wed Oct 16 2024 - 12Z Sun Oct 20 2024 ...Overview... A pattern change is forecast by midweek in the West, as upper troughing and then reinforcing energy come in from the eastern Pacific, leading to much cooler temperatures and increasing precipitation chances, including higher elevation snow. Upper ridging ahead of this trough will promote warmer than average temperatures in the northern/central Plains to Midwest as it gradually moves east, eventually displacing the chilly upper trough and surface high in the East by late week into next weekend. ...Guidance/Predictability Assessment... Recent model guidance remains rather agreeable in the amplified and somewhat progressive pattern for the latter half of this week. Models are more agreeable than a day ago regarding the timing of the western U.S. trough, central U.S. ridge, and eastern U.S. trough moving gradually eastward with time. Additionally, latest models are not as deep with a eastern Pacific surface low and as strong with the atmospheric river coming into the Pacific Northwest compared to some older models (e.g. 12Z EC from the 11th and 00Z EC from the 12th). With those aspects of the forecast showing better consensus, the main forecast issue on the larger scale is with the evolution of the western trough late week into the weekend. GFS and CMC runs have been persistent now over several runs in showing an upper low cutting off on the southern side of the trough, with differences in position for the cutoff low but somewhere in the southwestern to south-central CONUS. The 12Z ECMWF split some energy into the Four Corners region but did not close off a low, and now the newer 00Z EC just keeps a phased trough. However, most AI/ML guidance that is EC-based does indicate a closed low forming. The WPC forecast does not explicitly depict a closed low at this point but does show some southern stream troughing separating from the northern stream. Whether or not a low closes off in the southern stream will impact timing of troughing progressing eastward (as the cutoff low would tend to stall but the northern stream trough can move more quickly east if the flow splits). The WPC forecast was based on a multi-model deterministic blend favoring the 12Z ECMWF and 18Z GFS early in the period, but as spread increased quickly in the western/central U.S. into the later period, used the GEFS and EC ensemble means as half the blend by Day 6 and over half by Day 7. ...Weather/Hazards Highlights... The developing and reloading upper trough and multiple surface fronts moving into the West will push precipitation across the Northwest Wednesday, with enhanced precipitation totals in terrain like the coastal ranges and Cascades into the northern Rockies. Higher elevation snow is likely, which will be the first snow of the season in some locations given the warm Fall in the West thus far. Precipitation is forecast to spread across the Intermountain West and Rockies Thursday and Friday. By Friday and into the weekend, Gulf moisture could stream in just ahead of the upper trough for increasing rain chances in the Plains to Upper Midwest. The forecast details vary, but models generally show some moderate to locally heavy rain totals in the south-central Plains. In the northeastern U.S., cool westerly flow underneath the upper trough to start the period Wednesday will lead to some continued lake-enhanced and terrain-enhanced precipitation across the central/northern Appalachians and the Great Lakes region into the Interior Northeast. Colder temperatures should allow for snow especially in the higher elevations. Precipitation should taper off by Thursday, and the East is likely to be dry for later week as a cool surface high sets up behind a secondary cold front. The exception could be across South Florida, which is forecast to see return flow of moisture by late week for increasing rain and thunderstorm chances. An upper ridge aligned over the central U.S. midweek and shifting eastward later week will promote well above normal temperatures in its vicinity. The biggest anomalies are forecast across the northern Rockies and High Plains on Wednesday, where highs into the 70s and 80s are 15-25 degrees above average for mid-October. These warm temperatures will shift into the central/northern Plains and Midwest by Thursday-Friday. Meanwhile, temperatures are forecast to be below average by 10-20 degrees on Wednesday in much of the eastern U.S. under cool high pressure. Morning lows may cause frost/freeze concerns from the north-central U.S. through the Midwest/Great Lakes and Northeast/Appalachians. Some gradual moderation/warming is likely there Thursday-Friday and even more so into the weekend when upper ridging comes overhead, for temperatures around 5-10 degrees above normal in the northeastern tier. On the other hand, the upper trough and frontal surges coming into the West will lead to below normal temperatures spreading gradually inland for the latter half of the week. These colder temperatures may also lead to additional frost/freeze concerns for still susceptible growing areas. 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