Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Fri Oct 11 2024 Valid 12Z Mon Oct 14 2024 - 12Z Fri Oct 18 2024 ...Overview... Latest guidance still indicates that 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... Recent model guidance has shown reasonably good agreement early in the medium range 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. The trough in the East should gradually sweep eastward Tuesday-Wednesday, but there has been 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 showed this type of solution, including yesterday's 12Z GFS (but not the 18Z or 00Z run), yesterdays's 12Z CMC, and an older 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. This was in particular championed by the 00 UTC UKMET, but the newest 12 UTC UKMET has dramtically backed away from that closed low etc. solution to most other guidance. However, the majority of guidance (especially when considering the newer 00/12Z cycles) 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 composite of today's 06/12Z GFS, 00/12Z ECMWF/Canadian runs and their means seemed reasonable at this point. 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, differences like these shortwaves and overall downstream progression into later next week have sensible weather differences, like the strength of the associated atmospheric river (recent ECMWF runs showing somewhat stronger than the GFS) and timing of precipitation. For now prefer a middle ground approach by these longer time frames closer to a composite of more compatible ensemble means than rather than leaning toward any particular model solution. Overall, the WPC med-range product suite was mainly derived from a blend of reasonably clustered guidance of the GFS/ECMWF/Canadian, the NBM and WPC continuity Monday into Wednesday in a period with seemingly above normal predictability. Switched mainly to their associated ensemble means at longer time frames amid slowly growing forecast spread as predictailty settles closer to average levels. ...Weather/Hazards Highlights... It generally remains the case that 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/Northwest 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. Much colder temperature on tap for the Northwest/Northern Intermountain West later next week may also lead to additional frost/freeze concerns for still susceptible growing areas. This would be due to upper trough onset and frontal surges. Elsewhere, 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. Tate/Schichtel 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