Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Fri Oct 27 2023 Valid 12Z Mon Oct 30 2023 - 12Z Fri Nov 3 2023 ***Major pattern change to result in abruptly colder conditions compared to the recent September-like warmth across much of the Southern U.S.*** ...Synoptic Overview... A major pattern change will continue to be in progress across much of the central and eastern portions of the nation as a strong cold front heralds the arrival of the coldest airmass thus far this season. This front should 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 and surface cold fronts 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. 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. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite is in good overall synoptic scale agreement for the beginning of the week, and have come into better focus with a potent northern stream shortwave dropping south from northern Canada for the middle of the week. This feature will determine the extent of any coastal low cyclogenesis off the New England coast, and the consensus right now is for the low to develop far enough offshore to keep the heaviest precipitation away from New England, but this will continue to be monitored for future trends. Right now, the GFS remains the most progressive and weakest with this feature, whilst the non-NCEP guidance favors a stronger system. For the Pacific storm system moving in late week, the GFS is the most progressive with the trough, and the 12Z ECENS slower, with the GEFS mean representing a reasonable compromise. A more zonal flow pattern is likely across the nation by Friday, and the greatest model spread is across the Intermountain West and the northern Rockies as the trough moves inland. Therefore, the ensemble means accounted for about half of the model blend going into next Friday. ...Weather/Hazards Highlights... In contrast to the heavy rainfall expected in the short range forecast period across the south-central U.S., the extended forecast going into early next week features a return to quieter conditions in terms of rainfall across the entire country. The rainfall expected along and behind the strong cold front is forecast to become lighter from Texas to the Mid-Atlantic, and should be below the thresholds of any excessive rainfall headlines. There may be some locations that get 0.5 to 1 inch of rainfall on Monday across parts of Texas, but most areas affected should be under half an inch. Depending on the potential evolution of an offshore low around midweek, some moisture is expected to be advected inland across portions of the Northeast U.S., with rain for the big cities and perhaps some snow for the higher inland terrain. 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, with several inches of accumulation for some areas. Looking ahead to the end of the week, showers and mountain snow is likely to make a return from northwestern California to western Washington, with some of this moisture reaching the northern Rockies by late Friday. A temperature dichotomy will exist across the Eastern U.S. on Monday with warm and humid conditions across the southeast states ahead of the front with highs well into the 80s, and much colder from Texas to the Ohio Valley where highs will struggle to reach 50 degrees in many cases. The cold airmass then engulfs all of the central and eastern U.S., with the exception of the Florida Peninsula, by Tuesday. Widespread anomalies on the order of 15 to 25 degrees below average for this time of year are expected, perhaps setting some daily record lows. The first frost/freeze of the season is expected for a vast expanse of the south-central U.S. and across much of the Mid-Atlantic going into the middle of the week. This airmass should then modify going into the end of the week as a more zonal flow pattern evolves. 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