Extended Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EDT Wed Oct 16 2024 Valid 12Z Sat Oct 19 2024 - 12Z Wed Oct 23 2024 ...Overview... A closed upper low near the Four Corners this weekend will lead to a cool and wet pattern there, including higher elevation snow and frost/freeze concerns in some of the valleys. Heavy rain is possible just ahead of this upper low in the south-central High Plains, pushing eastward but lessening in amounts with time. Meanwhile an atmospheric river coming into the Pacific Northwest could lead to locally heavy precipitation. As the Four Corners upper low tracks northeast next week, quasi-zonal flow should set up over the lower 48, spreading the coverage of above normal temperatures from the north-central to northeastern tier this weekend farther south across much of the U.S. into next week. ...Guidance/Predictability Assessment... Most guidance still agrees with the large scale pattern this weekend, as an upper low slowly moves across the Four Corners with ridging ahead of it. More uncertain shortwaves over the Southeast or so could affect the mean ridge pattern over the East, but look to have fairly minimal weather impact. For the early part of the period, the WPC forecast consisted of a multi-model deterministic blend of the 18Z GFS and 12Z ECMWF/CMC/UKMET. Model spread increases somewhat as the upper low ejects northeast, with the track and timing partly dependent on upstream shortwave energy. GFS runs remain the fastest for the feature to move east and get absorbed into the northern stream. The ECMWF and the CMC are slower, though many of the 12Z EC-based AI/ML models are a bit faster than the operational EC. For the newer 00Z cycle of deterministic runs, most models trended slower but still have the GFS on the faster side and EC/CMC slower. An intermediate solution still seems like the best course of action. There is good agreement for an upper ridge to set up in the southern stream. Upstream, models vary considerably with the energy distribution within a western Canada to eastern Pacific mean trough especially into Tuesday-Wednesday. Models show the potential for an upper low to pinch off in the Pacific but with varying placement. This spread begins to influence sensible weather over the Northwest by Tuesday but has the potential to produce more significant spread and run- to-run variability over more of the lower 48 thereafter. Thus for the latter part of the period, the forecast used the 18Z GEFS/12Z EC ensemble means in the blend and increased their proportion to half by the end of the period amid increasing spread. ...Weather/Hazards Highlights... The closed upper low over the Southwest this weekend will help focus precipitation in the Four Corners states, Rockies, and High Plains. Expect enhanced precipitation totals in terrain with higher elevation snow, which will be the first snow of the season in some locations given the warm Fall in the West thus far. The central Rockies/San Juans can expect notable snow Saturday with some perhaps lasting into Sunday. Farther east into the High Plains, heavy rain will be a concern in a moist environment (PWs over the 90th if not 95th percentile) with some instability. A Marginal Risk is maintained for Day 4/Saturday for the south-central High Plains also considering the slow movement of the upper low and thus the rain and thunderstorms. Heavy rain would be most concerning atop sensitive burn scar areas. Into Sunday, the upper low is forecast to move somewhat more quickly northeastward and spread moderate to heavy rain with it, likely focused in the Texas/Oklahoma Panhandles into the western half of Kansas. This rain generally looks to be beneficial rather than cause a notable flooding threat, as showers/storms are forecast to move more quickly, but rain rates may be high enough for perhaps a nonzero flood risk. Some showers will shift into the central Plains/Mid-Mississippi Valley by Monday, though with decreasing intensity. Meanwhile, the Pacific Northwest will likely see more precipitation over the weekend as an atmospheric river coincides with the southern periphery of a strong jet pushing into British Columbia. Some locally heavy rain (and higher elevation snow) are possible in the Olympics/Cascades Saturday, areas that are not typically sensitive to flooding from the expected 2-4 inches of rain. With time, expect upstream shortwave energy to expand the precipitation shield southward and eastward but with gradually lesser max amounts. Uncertainty about specifics of eastern Pacific/West Coast flow aloft decreases confidence in precipitation details over the Northwest by Tuesday-Wednesday. The eastern U.S. is forecast to be generally dry this weekend into early next week, with the exception of parts of the Florida Peninsula. There, return flow of moisture near a meandering front could increase rain and thunderstorm chances especially for South Florida on Saturday. Rain amounts and rain rates do not look to be heavy enough to cause flooding concerns at this point, even in the sensitive urban corridor. Easterly flow could spread some light showers across the Peninsula into early next week. Cooler than average temperatures will be in place over the Four Corners/Great Basin this weekend underneath the upper low. Highs around 15-20 degrees below normal are likely Saturday, with some gradual moderation Sunday and especially into the workweek. Meanwhile, upper ridging ahead of the upper low will help cause above normal temperatures across the northern Plains to Upper Midwest/Great Lakes and Northeast. The Midwest can expect the greatest anomalies of 15-25 degrees above average on Sunday as highs reach the 70s. The upper pattern is forecast to relax somewhat into next week for developing quasi-zonal flow, though a subtropical upper high across the Gulf/northern Mexico/Texas will allow for warmer temperatures to expand southward through much of the lower 48 into Tuesday-Wednesday, though cooling across the northern tier behind a cold front. 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