Extended Forecast Discussion NWS Weather Prediction Center College Park MD 256 AM EDT Tue Oct 31 2023 Valid 12Z Fri Nov 3 2023 - 12Z Tue Nov 7 2023 ...Synoptic Overview... An expansive surface high will continue to encompass much of the eastern U.S. for the end of the week, with the high slowly moving offshore over the weekend. This airmass will slowly moderate after the widespread frost and freeze conditions earlier in the week. However, cold temperatures will have staying power for the Northern Plains and Upper Midwest through early next week. A storm system moving inland across the Pacific Northwest and Northern Rockies is expected to increase precipitation across these areas for the end of the week, and then a renewed surge of moisture following that as a second storm system approaches the region. An organized low pressure system is expected to develop across the Ohio Valley and Great Lakes region by next Tuesday. ...Guidance Evaluation/Predictability Assessment... Model agreement is generally good across the central/eastern U.S. early in the period, but there are some differences noted across the eastern Pacific by Friday and then across the Rockies and western High Plains going into Saturday regarding the eventual amplitude of the trough that develops. The overall pattern transitions to a much more zonal flow, and these situations tend to have lower overall forecast confidence regarding shortwave specifics, and this holds true going into the Sunday through Tuesday time period with limited confidence in any particular model run. It appears the GFS is too strong with the first shortwave crossing the Ohio Valley on Sunday compared to the model consensus, whereas the ECMWF appears too weak. Looking ahead to early next week, there is a slightly better signal for the second and potentially stronger low pressure system across the Ohio Valley and Great Lakes region, but still significant model spread on its location and magnitude. Ensemble means accounted for 60% of the forecast blend by next Tuesday. ...Weather/Hazards Highlights... Significant rainfall hazards are not expected for most of the continental U.S. through Saturday. The only region where there will be a risk of excessive rainfall leading to flooding concerns will be in the Pacific Northwest with the enhanced onshore flow that will unfold late this week and also into Saturday. There is a Marginal Risk of Excessive Rainfall planned on Day 5 (Saturday) from northwestern California to the Olympic Peninsula in Washington. Locally heavier rainfall on the west facing slopes may just be enough to cause some minor flooding concerns, especially in urban and burn scar areas. There should be enough of a lull on Day 4 to avoid having a Marginal Risk at this time. Looking beyond Saturday, additional moderate to heavy rain is likely for these same areas as another potential atmospheric river event develops with a few inches of additional rainfall during the Sunday-Monday time period. Showers will likely also increase in coverage across portions of the Ohio Valley and Great Lakes region as the next low pressure system gets better organized. Temperatures will start off cold Friday morning across much of the central and eastern U.S. with a slow warming trend ensuing over the weekend as the airmass continues to modify. Pleasantly mild conditions will also return to the Rockies and adjacent portions of the central and southern Plains this weekend. However, it should continue to remain quite chilly for parts of the northern Plains and Upper Midwest with highs generally ranging from the upper 20s to lower 40s in most cases. Hamrick 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, 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