Extended Forecast Discussion NWS Weather Prediction Center College Park MD 212 PM EST Sun Nov 06 2022 Valid 12Z Wed Nov 09 2022 - 12Z Sun Nov 13 2022 ...Heavy snow over the West on Wednesday, with an increasing winter storm threat in the Northern Plains on Thursday... ...Tropical disturbance could bring heavy rainfall to parts of Florida and the Southeast mid to late week... ...Overview... An amplified upper trough over the West Coast to start the period Wednesday will translate inland over the Rockies by Thursday and strengthen into a possible closed low over the Upper Midwest/Great Lakes by early Friday. There is an increasing threat for a winter storm to develop over parts of the Northern Plains with possibly appreciable accumulating snowfall Thursday into Friday, though still plenty of uncertainty on the specifics. Meanwhile, upper level ridging will prevail across the Eastern U.S. through about Thursday before a tropical disturbance (that remains quite likely to develop into a subtropical or tropical storm per the National Hurricane Center) over the Bahamas/Florida is likely to get wrapped up into the approaching trough from the West. Moderate to heavy rainfall is expected for Florida and should spread up the Eastern Seaboard during the latter half of the week. ...Guidance and Predictability Assessment... Models agree on the presence of amplified troughing over the West by the middle of next week and that it should have a general track eastward, with a trend over the past day or so toward an upper low closing off in the Midwest by Friday especially among the GFS and ECMWF runs. The WPC forecast continued to indicate this closed low. More uncertain is the exact track of the associated surface low and thus the positioning of the heaviest wintry precipitation in the cold air on the backside of the low. The 00/06Z model/ensembles seemed to have a bit of a trend south compared to the previous forecast and the WPC fronts/pressures plots reflected this, as did the QPF (less precipitation from North Dakota to northern Minnesota and more precipitation from South Dakota to southern Minnesota. These are somewhat minor changes for the medium range timeframe but could have considerable impacts. Overall confidence continues to increase for a winter storm, but the placement and timing may continue to shift. Farther south, models also continue to trend towards the potential for a tropical or subtropical storm to form likely in the next couple of days per the National Hurricane Center. Models generally track this feature westward potentially over the northern Bahamas and into at least eastern Florida. Models/ensemble members diverge notably after that. Most show the tropical system accelerating northeastward near the Eastern Seaboard as it gets picked up by the midlatitude trough. But some (mainly GFS runs and GEFS members) show the feature lingering over Florida for longer, tracking it west and potentially even into the eastern Gulf. So its track remains uncertain at this point with phasing differences. Monitor the latest outlooks and forecasts from the NHC regarding possible tropical development of this system. Upstream, a brief period of ridging is forecast for the West behind the main trough late this workweek, but there is some uncertainty with the timing of another trough/possible elongated upper low approaching this weekend. The updated WPC forecast utilized a blend of the 00Z/06Z deterministic models led by the GFS and ECMWF through day 5, attempting to maintain some strength of systems like the upper/surface lows in the Midwest, and going for middle ground positions. Introduced and increased the proportion of the GEFS and EC ensemble means on days 6-7 given increasing large scale uncertainties. ...Weather/Hazard Highlights... Heavy mountain snow and valley rain will be ongoing across parts the West on Wednesday, with the heaviest totals across the higher terrain of the Great Basin. As the trough moves inland and strengthens, a defined surface low looks to emerge out of the central High Plains deepening as it moves into the Upper Mississippi Valley by Friday. With model runs continuing to show the potential for accumulating and possibly significant snowfall across parts of the Dakotas and far northern Minnesota Thursday into Friday, the probabilities in the Winter Weather Outlook for Day 5 continue to increase, and focus a little farther south given the recent model trends. Depending on strength of the low, gusty winds could accompany snowfall and lead to possible blizzard conditions. Again, while a meaningful winter storm over parts of the northern tier continues to become more likely, the specifics in the placement of the heaviest snow remain in question and are dependent on the low placement and strength. In the warm sector of this storm, moderate to heavy rainfall is possible across parts of the Upper Midwest/Great Lakes. Additionally, there could be an axis of freezing rain and/or sleet between the snow and rain where temperatures at the surface and aloft support it, but the placement remains uncertain. Despite plenty of uncertainty in the details still, the tropical disturbance moving towards Florida later this week continues to show an increasing threat for coastal and maritime impacts, including coastal flooding, tropical-storm-force winds, heavy rainfall, rough surf, and beach erosion. A Slight Risk of excessive rainfall was added to the Day 5 experimental Excessive Rainfall Outlook for this forecast to show the potential for flash flooding with the heavy rain. The area may have to shift in future forecasts depending on the track of the tropical system. Moisture from this system should move up the East Coast later this week. Amounts and coverage remain uncertain. But in this forecast, there happened to be good model agreement for heavier rain to come onshore in the Carolinas, Mid-Atlantic, and Northeast, so the forecast reflected higher amounts along the Eastern Seaboard. Much below normal temperatures are expected to expand across the West through the latter part of this week associated with the deepening upper trough. The coldest air will likely be across the northern High Plains and Plains with recent forecasts showing highs in that area up to 20-30F below normal Wednesday-Friday. Many locations over the West could see daily records for cold highs through Wednesday. In contrast, well above normal temperatures will progress from the Central Plains to the East Coast Wed-Fri with widespread daily records for warm lows possible. Wednesday should be the warmest day for the Plains and Thursday should be for the Upper Midwest with readings 15-25F above normal. A strong cold front through this region by Thursday will usher in a much colder airmass, with high temperatures by Friday and the weekend in the Plains and Midwest as much as 10 to 20 degrees below normal. The cooler temperatures will reach the East this weekend, and Sunday looks to be near or below normal for almost the entire CONUS. Tate/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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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