Extended Forecast Discussion NWS Weather Prediction Center College Park MD 244 PM EDT Sat Oct 28 2023 Valid 12Z Tue Oct 31 2023 - 12Z Sat Nov 04 2023 ...Synoptic Overview... The strong cold front that will deliver the coldest air mass so far this season is expected to clear the southeast U.S. coast by Tuesday morning, and then clear the Florida Peninsula by late Wednesday, with a strong surface high building in behind the front. A couple of reinforcing upper troughs will give this colder airmass some staying power across much of the central and eastern states, with perhaps the first notable lake effect snow event of the season, and a low pressure system developing off the New England Coast. The airmass should gradually modify some going into the end of next week, and a Pacific storm system approaching the northwestern coast will increase the prospects for rain and mountain snow to close out the work week and into next Saturday. ...Guidance Evaluation/Predictability Assessment... The upper level pattern at the start of the period (Tuesday) is expected to feature a strong embedded shortwave over the Upper Midwest within the broad troughing advancing through the central U.S.. The strength and amplitude of that feature as it reaches the Ohio Valley and Eastern U.S. remains very uncertain with rather large spread between the GFS and non-NCEP guidance. The GFS solution advertises a weaker shortwave that passes quickly through the central/eastern U.S. and results in an offshore low well away from the Northeast coast while high pressure quickly overtakes the region in its wake. Meanwhile, the ECMWF and other non-NCEP guidance shows a bit slower solution, with a more amplified shortwave that induces low pressure closer to the Northeast coast on Wednesday. The ensemble means including the GEFS would favor the slower solutions, so the WPC blend included higher weights of the ECMWF/CMC/UKMET for Day 3-5. Elsewhere, guidance has the more typical biases for the upcoming Pacific Northwest atmospheric river event for mid-week where a blend of the GFS/ECMWF and the ensemble means was used. ...Weather/Hazards Highlights... The main weather highlights will be the arrival of some of the coldest air so far this fall across much of the central to eastern U.S., in the wake of a strong cold front and as high pressure settles over the central to eastern United States. High temperatures Tuesday and Wednesday will be 10 to near 25 degrees below normal across much of the central U.S. and upwards of 10 degrees below normal for the Carolinas northward through New England. The only warm and above normal spots will be across the Florida Peninsula as well as portions of southern Arizona and parts of California. Overnight lows will be similarly below normal, with near to sub freezing temperatures overspreading a large portion of the country. In fact, Wednesday and Thursday morning lows may dip into the upper 30s as far south as the Gulf Coast from Texas through the western Florida Panhandle. Further to the north across northern Texas through the Mid-Mississippi Valley, numerous locations may set record lows for Wednesday morning as readings fall well into the 20s. Southern stream energy interacting with an approaching shortwave out of the Great Lakes may develop a coastal low off the Mid-Atlantic/Northeast late Tuesday through Wednesday. If the low ends up developing closer to the coast, rainfall will be more widespread along the coastal areas of the Northeast before ending later Wednesday and Wednesday night. Otherwise, the next main precipitation event will be an atmospheric river event for the Pacific Northwest late Wednesday through Thursday where widespread rainfall of 1-2 inches will be common for the west-facing slopes of the Olympics and Cascades and northern California coastal ranges. Some lake effect snow could also make headlines from Michigan to Upstate New York as a strong surge of cold air advection rushes across the warmer lake waters in the wake of the cold front mid-week, with a few inches of accumulation possible for some areas. Taylor 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