Extended Forecast Discussion NWS Weather Prediction Center College Park MD 424 PM EDT Thu Oct 19 2023 Valid 12Z Sun Oct 22 2023 - 12Z Thu Oct 26 2023 ...Rain chances increase for the south-central U.S. next week... ...Overview... The medium range period begins Sunday with a strong Canadian Maritimes low producing gusty winds and some lingering precipitation over the Northeast, followed by a cool and drier pattern as the supporting upper trough departs in favor of ridging at the surface and aloft. Meanwhile, an upper low is likely to dive southeast across the West early in the week, producing some precipitation including snow in the Northern Rockies. By Tuesday or so, additional energy could track initially south across the northeastern Pacific and then potentially pivot east into the Northwest. But model guidance shows some considerable differences in the strength, track, and even the existence of this energy, as well as its potential interaction with the upper low over the Southwest at that point. Regardless it seems likely that eastern Pacific moisture (possibly lingering from what is currently Hurricane Norma) and Gulf moisture could combine ahead of the upper-level systems and surface fronts to spread potentially heavy rainfall into parts of the central U.S., while snow chances increase once again in the Northern Rockies and perhaps into the northern High Plains, but the model differences keep confidence low in the details for now. ...Guidance Evaluation/Predictability Assessment... Model guidance is in reasonable agreement for the early part of the period, and a model blend favoring the 00Z ECMWF, 06Z GFS, and 00Z CMC was favored. But model spread quickly increases with the two features mentioned above. By Day 5/Tuesday there are minor variations with the placement of the upper low in the Southwest U.S./northwestern Mexico, including the 00Z UKMET slightly farther west of consensus. But greater differences are already present with the energy/potential upper low coming through the northeastern Pacific toward North America, with variations from model to model and cycle to cycle. In terms of the 00/06Z model cycle, the ECMWF was most aggressive with British Columbia/U.S. Northwest energy in the form of a strong closed low compared to other guidance. The 00Z GFS had backed off considerably from this type of pattern in a change from yesterday's solutions, but the 06Z GFS run at least had some energy making its way into the Northwest. The 12Z ECMWF and GFS runs fortunately have converged somewhat, with the GFS strengthening while the ECMWF weakened somewhat. But meanwhile the 00Z and newer 12Z CMC have hardly any energy in the Northwest midweek. Ensemble members show considerable spread. Given that the pieces of energy stem from the uncertain Arctic, additional model changes are likely, and this is a rather low confidence aspect of the forecast. These northern stream differences also have effects on the evolution of the southern stream upper low atop the Southwest midweek. Recent deterministic models have been maintaining the southern low as a separate feature (not tracking northeast and getting absorbed into the northern stream troughing) for longer. So the WPC forecast did trend in this direction, a change from the previous forecast. Another big question is the possibility of ridging coming into the West by Wednesday-Thursday. This was most evident in CMC and older GFS runs that were weaker with the Northwest energy. This still seems like a lower probability solution but would yield very different sensible weather in the West, so will continue to monitor. Regardless, troughing in the West by midweek and beyond has been trending weaker in general, as even solutions like the ECMWF that have strong energy in the Northwest have it tracking more eastward than digging as much as model runs from a day ago. Thus by the latter half of the medium range period, the WPC forecast quickly ramped up the percentages of the GEFS and EC ensemble means and included a bit of the previous forecast, with the deterministic runs becoming so questionable. This allowed for an intermediate solution. Expect the details to change with time. ...Weather/Hazards Highlights... The Days 4-5 Excessive Rainfall Outlooks that cover Sunday through Monday night depict no risk areas at this time. On Sunday guidance shows signals of some lingering Northeast precipitation that could produce localized enhanced totals, but lack of instability and fairly rapid departure of anomalous moisture should temper short-term rainfall rates. GFS runs have been persistent in depicting the onset of locally heavy rainfall over the southern Plains Sunday-Monday due to its fast progression of Norma, but the GFS is on its own with other models much drier. Otherwise moisture will spread across the West with the central West Coast and northern Rockies likely seeing relatively higher but not exceptionally heavy totals. Probabilities for meaningful snow at high elevations of the Rockies should start increasing early in the week. By late Monday into Monday night, some instability could come into Arizona/New Mexico with moisture increasing as well ahead of the upper low, so there may be some convection in the Southwest to consider, but probabilities for flash flooding still remain low for now. The western and central U.S. forecast during Tuesday-Thursday will depend in part on the uncertain pattern aloft. A general weakening trend for troughing in the West has led to a later arrival of/less anomalously cool temperatures than the previous forecast showed. But there is room for these temperatures to decrease again confidence builds. Northern High Plains locations could see highs at least 15F below normal though by Wednesday-Thursday, with increasing snow coverage in the Rockies and some higher elevation snow in the Cascades as well. More questionable is how much snow could spread into lower elevations of areas like Montana, so continue to monitor upcoming forecasts. Meanwhile the pattern should become favorable for increasing coverage and intensity of rainfall over the central U.S. by Tuesday or so, with lingering moisture from Hurricane Norma likely contributing to this activity. The current most likely pattern would produce a fairly broad area of moderate to heavy rainfall in the southern half of the Plains, with uncertainty in how much stretches into the Midwest. Expect the Northeast to see gusty winds on Sunday behind the departing Canadian Maritimes system. Cool temperatures over the East Sunday-Monday should moderate to near or above normal by midweek. The early week cool spell may bring the first frost or freeze of the season to some locations from the Great Lakes into the Appalachians. Warm temperatures over the Rockies and then Plains early in the week, with some highs at least 10-15F above average, should moderate thereafter but morning lows will remain quite warm (10-20F above normal) across the Plains into the Great Lakes through Thursday in the moist flow around the western side of East Coast/Atlantic high pressure. Tate/Rausch 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 Hazards: - Heavy rain across portions of the Central/Southern Plains and the Mississippi Valley, Tue-Wed, Oct 24-Oct 25. - Heavy snow across portions of the Northern Plains and the Northern Rockies, Tue-Wed, Oct 24-Oct 25. - Flooding possible across portions of the Southeast. - Frost/freeze across portions of the Northeast, the Appalachians, the Tennessee/Ohio Valley, the Mid-Atlantic, the Southeast and the Great Lakes, Mon-Tue, Oct 23-Oct 24. WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: 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