Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Wed Oct 16 2024 Valid 12Z Sat Oct 19 2024 - 12Z Wed Oct 23 2024 ...A Four Corners upper low will bring the first snow of the season to parts of the central/southern Rockies and heavy rain/flooding concerns to the south-central High Plains this weekend... ...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 or broadly cyclonic flow should set up over the lower 48, spreading the coverage of above normal temperatures from the north-central to northeastern tier this weekend while Gulf of Mexico/southern Texas/Mexico upper ridging also assists in bringing warmer readings to the southern states by next week. ...Guidance/Predictability Assessment... The primary forecast issues in latest guidance continue to be with the timing of the upper low ejecting from the Southwest/eventual influence of incoming northern stream flow, and then details within northeastern Pacific troughing that the majority of guidance suggests will split by next Tuesday-Wednesday. Solutions continue to vary somewhat for the ejecting Southwest upper low. By Sunday the 12Z UKMET briefly strays a bit east of consensus before returning to the middle of the spread. GFS runs have tended to be faster from Suday onward, though the 00Z and 12Z runs compare better to other guidance than the 06Z version. ECMWF/CMC runs have been leaning to the slower side of the envelope. Most machine learning (ML) models favor an intermediate timing, perhaps with a slight tilt in the ECMWF direction. By around Tuesday the upper low's timing may also depend on the arrival of northern stream energy, which again shows an intermediate timing in the ML guidance. Regarding the eastern Pacific/western North America pattern toward next Tuesday-Wednesday, the majority of dyanmical models and ensemble means have recently trended toward the idea of fairly rapid closed low formation over the east-central Pacific, while by next Wednesday there would be some upper ridging along/offshore the West Coast with flat or northwesterly mean flow over the northwestern U.S. as part of broader zonal to cyclonic flow pattern over at least the northern half of the lower 48 and southern Canada. ML models and the ECMWF AIFS mean suggest the upper ridge axis could be a bit farther offshore the West Coast, with typical spread for how lower 48 shortwave energy may look. Aside from these details, the dynamical/ML majority offers a comparable theme overall and is preferred by the latter part of the forecast. In contrast, latest ECMWF runs show less flow separation over the East Pacific and thus maintain more ridging or southwesterly flow over the West. Regardless of how agreeable the majority cluster may be at the moment, these flow separation cases can still produce significant variability during the extended range. The first half of the updated forecast incorporated a composite of 00Z dynamical models, given less favorable comparisons for the Southwest upper low in the 06Z GFS. By next Tuesday-Wednesday the blend increased 06Z GEFS/00Z ECens input (reaching half total weight by Wednesday) while reducing ECMWF influence as it strayed from the majority for the overall Pacific/North America pattern. ...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. The upper low that stalls during Saturday will provide strong forcing and lead to slow movement of rain and storms. Most models show 4-6 inches of rain centered around northeast New Mexico, exceeding the 5-10 year ARI if not higher. ECens/CMCens probabilities also highlight this exact area, while the GFS/GEFS have been skewed a little north- northeast and the 12Z UKMET shifts the upper low and rainfall axis a bit east. The Day 4/Saturday ERO maintains continuity for the Slight Risk area depicted over northeastern New Mexico and vicinity in accordance with the majority guidance cluster, while the surrounding Marginal Risk area accounts for current guidance spread/lower probability solutions. Sensitive burn scar areas will be particularly vulnerable during this potential heavy rain event. By Sunday the upper low should start moving northeastward, spreading moderate to heavy rain with it. This activity will likely focus over the Texas/Oklahoma Panhandles into the western half of Kansas. Given the moisture/instability and forcing in place, heavy rain may cause isolated flooding threats especially if areas of high rain rates train in the same locations. The Day 5/Sunday maintains a Marginal Risk area due to these flooding concerns. 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, but these areas are not typically sensitive to flooding from the expected 2-4 inches of rain and in particular with the low stream flows observed at this early point in the season. With time, expect upstream shortwave energy to expand the precipitation shield southward and eastward but with gradually lesser max amounts. There is still uncertainty about specifics of eastern Pacific/West Coast flow aloft by next Tuesday-Wednesday, but most guidance currently suggests that the Northwest will see a drier trend by then. 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 promote 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 and to some extent into Monday as highs reach the 70s. Forecast transition of the upper pattern toward broadly cyclonic or nearly zonal mean flow would moderate temperatures over the northern Plains and vicinity by midweek, while a subtropical upper high across the Gulf/northern Mexico/Texas will allow for warmer temperatures to expand across the the southern Plains into Southeast. Rausch/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