Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Mon Oct 30 2023 Valid 12Z Thu Nov 02 2023 - 12Z Mon Nov 06 2023 ...Synoptic Overview... An expansive surface high will encompass much of the eastern U.S. through this weekend with the high gradually weakening through the period. Underneath the high, calm winds and clear skies will allow temperatures to drop low enough for widespread early morning frost and freeze conditions to form mid-week. Temperatures in the east will moderate as the high weakens through the weekend. Cold conditions are also in the forecast for the north-central U.S. with a renewed surge of cold air expected to surge into the region early next week. In the West, a couple low pressure systems will approach the coast throughout the medium range period that will aim an atmospheric river at the Pacific Northwest. ...Guidance Evaluation/Predictability Assessment... Models are in generally good agreement on the overall pattern with a deep upper-level low initially located near Hudson Bay in Canada that will slowly drift northeast and a couple of upper-level shortwaves/lows that will move down from Alaska towards the Pacific Northwest. There are some differences in timing of shortwaves moving around the Hudson Bay low and of the shortwaves/lows moving into the Northwest. No model was a clear outlier, though the CMC did stray farther away from the solutions of the other available guidance. For this reason, the CMC was given less weight in WPCs model blend this afternoon. WPCs model blend for this forecast package is purely deterministic for the first half of the period and ensemble means were introduced for the second half to smooth out some timing differences. ...Weather/Hazards Highlights... Significant rainfall hazards are not expected for most of the continental U.S. through the end of this week. The only place where there will be a risk of excessive rainfall leading to flash flooding will be in the Pacific Northwest with the atmospheric river event that will unfold mid-to-late this week. There is a Marginal Risk of Excessive Rainfall on Day 4 (Thursday) for parts of western Washington and Oregon where a few inches of rainfall will be possible. 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 5 to avoid having a Marginal Risk at this time, but another surge of Pacific moisture is expected to arrive over the weekend. Temperatures will start off cold Thursday morning with potentially record-breaking low minimum temperatures from the South to the Mid-Atlantic, and the central and eastern U.S. will likely see widespread frost and freeze conditions. A slow moderation trend in temperatures will persist through the end of the week with temperatures returning to near normal values by Friday and reaching slightly above average by Saturday. 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. Hamrick/Dolan 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