Extended Forecast Discussion NWS Weather Prediction Center College Park MD 235 PM 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... The latest suite of model guidance are in reasonably good agreement regarding the overall synoptic pattern across the CONUS during the early stages of the medium range period (Wednesday--Thursday). The deterministic Euro's progressive and phased trough pattern in the West continues to be an outlier, compared to the UKMET, GFS and Canadian suites which have persistently developed a cut-off low/split flow upper-level pattern. Elsewhere, the deterministic 06z GFS develops a closed mid-level low over the Mid- Atlantic Coast on Wednesday night, which deviates significantly from the rest of the other pieces of guidance and is inconsistent with past runs, which maintain a ridge over the East Coast next weekend. A non-06z GFS blend, consisting of the 00z GFS/EC/CMC/UKMET were utilized on days 3 and 4 due to reasonable agreement across the CONUS. The 00z CMCE and 06z GEFS were introduced to the blend on day 5, but the deterministic guidance still carried most of the weighting in that blend. By day 6, the 00z ECE is incorporated into the blend while the deterministic Euro is removed. The 06z GEFS and 00z CMCE are favored through 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