Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Oct 11 2024 Valid 12Z Mon Oct 14 2024 - 12Z Fri Oct 18 2024 ...Overview... During the early part of next week, 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. Precipitation will be likely early week in the northeastern U.S., while more uncertain light rain may fall in the Southeast, and southern Florida should see rounds of thunderstorms. Meanwhile the western U.S. can expect mean upper ridging (aside from embedded energy) early next week, which 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 shows reasonably good agreement early in the forecast period, with broad troughing across the Midwest to East anchored by a Great Lakes upper low while general ridging in the West is broken up by an embedded southern stream upper low atop the central Great Basin Monday. A surface low in the Northeast continues to have some placement spread, but a more inland track like the 18Z GFS/12Z ECMWF seemed most agreeable with the ensemble means. The trough in the East should gradually sweep eastward Tuesday-Wednesday but there is continued uncertainty with whether or not energy breaks off in the southern part of the trough to form a possible closed low. A minority of models show this type of solution, including the 12Z GFS (but not the 18Z or 00Z run) and the 12Z CMC, and an AI/ML model run or two. A solution like that would bring more rainfall into the Southeast and spin up a Southeast or western Atlantic surface low. However, the majority of guidance (especially when considering the newer 00Z cycle) depicts an open trough, and WPC continues to favor that for a single deterministic forecast. There is also some question regarding the width of the trough dependent on shortwave positions, but a blend of the 18Z GFS/12Z ECMWF and their means seemed reasonable at this point though the trough may end up narrower per many AI/ML models. In the West, there is better than normal large scale agreement for the upper trough to move gradually inland Tuesday and beyond with reinforcing shortwave energies. However, detail differences like these shortwaves and their timing and placement have sensible weather differences like the strength of the associated atmospheric river (ECMWF showing stronger than the GFS) and timing of precipitation. For now prefer a middle ground approach rather than leaning toward any particular model solution. The WPC forecast used a blend favoring the deterministic 18Z GFS and 12Z ECMWF with smaller proportions of the CMC and ensemble means early in the period. Gradually increased the proportion of the ensemble means to over half by Days 6-7 amid increasing uncertainty (while also eliminating the 12Z CMC given its eventual outlier status across the East). ...Weather/Hazards Highlights... Low pressure in the Northeast as the period begins on Monday will lead to some light to moderate precipitation there, and behind the system passage lake-enhanced and terrain-enhanced precipitation is forecast across the Great Lakes region to Interior Northeast Tuesday-Wednesday. Colder temperatures should allow for snow especially in the higher elevations. Farther south, there may be some rain along a front in the Southeast on Monday-Tuesday depending on the evolution of the upper trough and surface features. Then across South Florida, some instability and high PWs near a frontal boundary will lead to rain and thunderstorms in the vicinity. There is still some uncertainty in rainfall amounts and how much rain falls offshore versus onshore, but there may be a nonzero chance of flash flooding especially if areas get hit with heavy rainfall multiple times, and considering initially wet ground conditions. There should be lessening precipitation coverage across the East behind a secondary cold front as the week progresses. 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 will be possible by midweek, which may be the first snow of the season in some locations given the warm Fall in the West thus far. An upper ridge aligned over the Rockies into early next week 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 Monday- 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 gradually cool Monday- Wednesday as a cool surface high pressure system settles in behind a cold front. 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- Friday. The Gulf Coast states can expect above average temperatures into Monday, and a few record highs are possible into southern Texas, before the cold front pushes through. Later week upper troughing will produce cooler than normal temperatures first for the West Coast around Wednesday and spreading into the Great Basin and Rockies Thursday-Friday. 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