Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Sat Oct 12 2024 Valid 12Z Tue Oct 15 2024 - 12Z Sat Oct 19 2024 ...Overview... As the medium range period begins Tuesday, upper troughing will settle in across the eastern half of the U.S., leading to below normal temperatures along with frost/freeze concerns for the north- central U.S. to Northeast, along with lake-enhanced precipitation. Meanwhile upper ridging (aside from embedded energy) will set up atop the Interior West, and this ridge is forecast to shift eastward into the central U.S. through the week with warmer than average temperatures underneath it. The pattern in the West is forecast to flip to troughing by midweek to late week with a couple rounds of reinforcing energy. This will cool temperatures there and lead to increasing precipitation chances, including higher elevation snow. ...Guidance/Predictability Assessment... Model guidance starts the medium range period in relatively good agreement, with a broad trough across the Midwest to East while general ridging in the West is broken up a bit by an embedded southern stream upper low atop the central Great Basin Tuesday. The trough in the East should gradually sweep eastward Wednesday- Friday, with some differences in the timing. Recent models have not indicated energy breaking off in the southern part of the trough to form a closed low like some runs from a day or two ago, so at least that aspect is in better agreement now. Then upstream, ridging in the west-central to central U.S. and troughing coming into the West behind it start to show spread in timing as soon as Wednesday-Thursday. Generally the 12Z EC suite, including the deterministic run and ensemble members and mean, is more consolidated with the West trough and produces a deeper eastern Pacific surface low (all this creating a stronger atmospheric river), and thus is slower/farther west with the trough axis and the ridge axis to the east. GFS runs and ensemble members show a Western trough with shortwaves coming through at different times to make it broader, and the ridge shifts east more quickly. Often a middle ground solution in between the slow EC and fast GFS is preferred, and the bulk of the 12Z AI/ML models are generally in between with the trough and ridge positions, increasing confidence in an intermediate solution. The 12Z CMC was in between and thus could be utilized some. The newer 00Z GFS and EC have similar positions as their older runs. Then by late week, some models show an upper low closing off in the Southwest, specifically GFS and CMC runs. The ECMWF does not show this and neither do the EC-based AI/ML models. Will continue to monitor this uncertain potential. The WPC forecast was based on a blend favoring the 12Z ECMWF/18Z GFS/12Z CMC early, gradually reducing the proportion of the deterministic runs (especially the EC and GFS) as the period progressed in favor of the ensemble means, reaching over half means by Day 6 and well over half by Day 7 as spread increased. ...Weather/Hazards Highlights... Behind a surface low pressure system pulling north of New England by Tuesday and underneath the upper trough causing cool westerly flow, lake-enhanced and terrain-enhanced precipitation is likely from the Great Lakes region into the Interior Northeast and into the central Appalachians. Colder temperatures should allow for snow especially in the higher elevations. Precipitation should taper off Wednesday, 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. The developing and reloading upper trough and multiple surface fronts moving into the West will gradually push precipitation into the northern/central West Coast by Tuesday and through the northern Rockies into Wednesday-Thursday. While there is good confidence in the overall event, unresolved specifics of the upper trough lead to greater uncertainty with respect to the southward/eastward extent of precipitation as well as magnitude. Expect relatively higher totals over favored coastal areas and the Cascades. Higher elevation snow is likely by midweek, which may be the first snow of the season in some locations given the warm Fall in the West thus far. While details vary, model guidance generally shows precipitation spreading east through the Interior West/Rockies into Thursday, and by Friday Gulf moisture may stream in just ahead of the upper trough for some increasing rain chances in the Plains to Upper Midwest. An upper ridge aligned over the Rockies into Tuesday and then shifting eastward by mid and later week will promote well above normal temperatures in its vicinity. The biggest anomalies are forecast across the northern Rockies and High Plains Tuesday- Wednesday, where highs into the 70s and 80s are 15-25 degrees above average for mid-October. These warm temperatures will shift into much of the Plains and Midwest by Thursday-Friday. For much of the eastern U.S., temperatures are forecast to be below average by 10-15 degrees Tuesday-Wednesday underneath cool high pressure. Morning lows may cause frost/freeze concerns from the north-central U.S. through the Midwest/Great Lakes and Northeast/Appalachians. Expect some moderation there by Thursday- Saturday. Meanwhile, 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