Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 PM EDT Mon Oct 09 2023 Valid 12Z Thu Oct 12 2023 - 12Z Mon Oct 16 2023 ...Mid-late week excessive rain threat for the Midwest/Great Lakes to Northeast as well as the Southeast... ...Overview... Guidance still offers a strong signal that a leading closed upper low will shift slowly eastward across eastern Canada late week and ease unsettled conditions under/around this amplified feature away from the Northeast. Meanwhile, another dynamic upper trough/closed low and surface low/frontal system will work out from the West to favor enhanced mountain snows for the northern/central Rockies and even into the Black Hills and perhaps the lower elevations of Nebraska Thursday. This potent and well-organized upper system will then steadily track from the Plains to the Northeast Friday through next weekend. This will foster enhanced precipitation chances to spread in earnest from the central U.S./Midwest through the Northeast as inflow moisture increases ahead of deep cyclogenesis. To the south, a frontal boundary enhanced with tropical moisture will lift through the Gulf of Mexico to increase heavy precipitation chances across the central Gulf Coast and Florida/Southeast into later this week before lifting off the East Coast. Next upstream, the slow translation of an amplified and complex eastern Pacific upper/surface system may renew wet flow into the Pacific Northwest into early next week. ...Guidance Evaluation/Predictability Assessment... As the leading upper low lifts away from the CONUS, the main concern will be for the next upper and surface lows coming out of the West tracking east through late week/weekend. There is good agreement for this system to exist. Guidance does differ in the details of the placement of the surface low; these differences are somewhat small in terms of position but create notable differences in the QPF. This forecast cycle slowed the surface low and the associated frontal system a bit through Thursday-Friday, with the QPF track also slowing compared to continuity and the 13Z NBM starting point, as it incorporated the slower ECMWF and CMC. The newer 12Z models support this as well, with a pretty persistent ECMWF low position and the 12Z GFS/GEFS mean lows trending west a bit through Friday. By the weekend into early next week, the 00Z CMC differed in terms of the evolution of the upper low (splits it with some held back in the northern Plains and some moving east, unlike other guidance), so it was not favored. The surface low ends up with some variability in model/ensemble guidance especially as it shifts into the western Atlantic early next week. Then farther west, ridging is likely across the interior West up into central Canada, while energy may enter the Northwest from the eastern Pacific. The strength of this energy and the onshore flow and troughing it brings is still in question, with the GFS less amplified compared to other guidance. Given these considerations for this timeframe, preferred a blend of the 06Z GFS and GEFS mean and 00Z ECMWF and EC mean. ...Weather/Hazards Highlights... Dynamic upper trough/height falls and surface system passage across the West into Thursday will favor a swath of autumn mountain snows that could be heavy across the northern/central Rockies, with some farther east into the High Plains possible. Meanwhile downstream, precipitation chances will expand east as this main system works increasingly into the Central U.S. by later week. Deep cyclogenesis and wrapping anomalous moisture into a leading warm front lifting through the east-central U.S. will shift focus for emerging heavy rains from the north-central Plains through the Midwest/Great Lakes. While longer duration rains and ambient relatively dry soils may partially limit impact, the WPC Day 4/Thursday and Day 5/Friday Excessive Rainfall Outlooks (EROs) show Marginal and Slight Risk areas from the north-central Plains/Midwest/Great Lakes given potential for training and model and ensemble signals for some higher rain totals. Risk areas were not changed too fundamentally in this update, but adjusted to show the slower trend of the QPF axis. Meanwhile, a separate front meandering through the Gulf of Mexico and the Florida Peninsula could bring a threat for heavy rainfall to the Southeast Thursday into Friday. This may be aided by an area of low pressure in the Gulf of Mexico that is now being monitored by NHC and possible eastern Pacific remnant tropical moisture. ERO Day 4/5 marginal risk areas shift out from the central Gulf Coast to the Southeast. The warm front lifting late week may increase the areal coverage of convection there as instability treks farther inland. These two main systems may increasingly interact to some degree to spread enhanced rainfall and runoff issues possible across the East/Northeast this weekend with coastal low development/deepening as a maritime risk. Widespread enhanced winds are possible on the backside of the low first in the Plains and tracking eastward into the Atlantic. The system's cold front will also spread cooler than average temperatures from west to east across the lower 48, with a first frost/freeze of the fall season possible in the central Plains. Tate/Schichtel 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