Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Wed Nov 1 2023 Valid 12Z Sat Nov 4 2023 - 12Z Wed Nov 8 2023 ...Synoptic Overview... The amplified upper level pattern during the short range forecast period is forecast to evolve into a more quasi-zonal flow across the continental U.S. going into the weekend, with broad cyclonic flow aloft over southern Canada. Shortwave perturbations originating from a large upper level gyre over the northeastern Pacific will quickly track eastward, and the trough that develops across the Plains by Monday will likely spur surface cyclogenesis across the Ohio Valley and Great Lakes region by Tuesday, and then reaching the East Coast by Wednesday. Unsettled conditions are expected to continue across the Pacific Northwest with multiple rounds of rainfall. ...Guidance Evaluation/Predictability Assessment... With the transition to a more zonal upper level flow pattern and the relatively fast progression of individual shortwaves, there is typically a reduction in forecast confidence and this holds true in this case. Models have struggled with low and frontal placement with the storm system approaching the Pacific Northwest on Saturday, and earlier runs of the ECMWF have appeared displaced to the east of the model consensus, so little in the way of ECMWF was used during the weekend time period. However, the 00Z ECMWF is in much better agreement here. Looking ahead to early-mid next week, the GFS is quite fast with taking the surface low across New England and then offshore, and well ahead of its own ensemble means, so the GFS was not favored by this time. To account for the uncertainties in the model guidance, about 25% of the previous WPC pressures was incorporated for days 3 through 6, and ensemble means were gradually increased, and by next Wednesday up to 70% of the means were used. ...Weather/Hazards Highlights... Significant rainfall hazards are not expected for most of the continental U.S. through next weekend. Similar to the past few days, the only region where there will be a risk of excessive rainfall leading to potential flooding concerns will be in the coastal ranges of the Pacific Northwest with the enhanced onshore flow that will continue into the weekend. There is a Marginal Risk of Excessive Rainfall planned on Day 4 (Saturday) from northwestern California to the Oregon Cascades and coastal ranges. 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. Looking ahead to Day 5 (Sunday), additional moderate to heavy rain is likely for areas slightly farther north as another potential atmospheric river event develops with 1-2 inches of additional rainfall during that time period. Across the eastern U.S., widespread light to moderate rain will likely develop from the Midwest to the Ohio Valley and Great Lakes region as the next low pressure system gets better organized, and this rainfall should reach the East Coast by late Tuesday into Wednesday. Temperatures will continue a slow warming trend over the weekend as the airmass modifies across the Eastern U.S. and readings return to near seasonal averages. Some slightly above average temperatures are likely by Monday across a large portion of the central and eastern U.S. ahead of the next front, and then cooling back down by mid-week. 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, with the coldest weather over northern North Dakota. 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