Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Mon Oct 23 2023 Valid 12Z Thu Oct 26 2023 - 12Z Mon Oct 30 2023 ...A heavy to excessive rainfall threat across parts of the Upper Midwest on Thursday... ...Heavy snow increasingly likely for parts of the Northern Plains on Thursday... ...Overview... The medium range period begins Thursday with shortwave energy moving through the central U.S. which could support a heavy rainfall threat across parts of the Upper Midwest on Thursday along and ahead of a strong cold front. Meanwhile, additional amplified energy through the Northwest will allow for an early season snowstorm across parts of the Northern Rockies into the Northern Plains. Amplified troughing will be renewed into early next week across much of the Western U.S. as the next shortwave drops into the Pacific Northwest on Friday. This would also allow for a strong upper ridge to build over the Southeast quadrant of the U.S. through the period. ...Guidance Evaluation/Predictability Assessment... As has been the case for several days, the GFS continues to be fast with the shortwave through the Midwest on Thursday and slightly more amplified with the initial system through the West. However, compared to recent runs, models have come into much better agreement across the board in the early periods and so a general deterministic model blend was usable for the day 3-5/Thursday-Saturday period for tonights WPC progs. Models agree the next shortwave should arrive in the Pacific Northwest around Friday, with some hints that a closed low could eventually form out of this. The GFS is most excited about this evolution showing complete separation from northern stream energy (which should slide east through the northern tier) and bringing the upper low much farther south and west into California/the Southwest next Sunday-Monday. The ECMWF and CMC, with support from the ensemble means, suggest a little bit weaker southern stream system and more phased troughing overall. The days 6 and 7 forecast blend incorporated more of the ensemble means to help temper the detail differences in the deterministic solutions, with minor contributions still from the ECMWF for some added system definition. This evolution has implications for another round of heavy rainfall across the Central Plains/Midwest region late period, with possibly snow on the backside so details will need to be monitored in the coming days. ...Weather/Hazards Highlights... Highly anomalous moisture, with tropical connections to what is currently Tropical Storm Norma, will interact with a frontal boundary through the north-central U.S to fuel a heavy to possibly excessive rainfall threat on Thursday across much of the Upper Midwest/Great Lakes. There was enough model agreement to introduce a slight risk from northeast Iowa into central Wisconsin on the Day 4/Thursday Excessive Rainfall Outlook. Despite dry antecedent conditions across this region, slow moving storms along the frontal boundary with plenty of moisture and instability to work with, could pose a flash flood risk. On the north and west of this system, heavy snow threats continue to increase across parts of the north-central Rockies into the Northern Plains on Thursday. This would be the first accumulating snow event of the season for the lower elevations of eastern Montana/North Dakota where several inches of snow could fall. Elsewhere, showers will continue across parts of the southern/central Plains on Thursday along a weakening frontal boundary. Out West, additional precipitation/higher elevation snows will move eastward across the Region this coming weekend. An additional heavy rain threat could develop across parts of the Central Plains/Middle Mississippi Valley into the Upper Midwest early next week, with some snow potential for the northern fringe of the precipitation shield. This remains still very uncertain but is worth monitoring. Well above normal temperatures across the Central U.S. and the Midwest will shift east and settle this weekend across the East/Southeast as upper ridging builds over this region. Meanwhile, renewed and amplified troughing across the West will continue a prolonged period of below normal temperatures, with daytime highs 15-30 degrees below normal to spread across much of the interior West and the Northern/Central Plains. By next Monday, much below normal temperatures could sink as far south as the southern Rockies and northern Texas. Santorelli 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