Extended Forecast Discussion NWS Weather Prediction Center College Park MD 211 PM EDT Sun Oct 08 2023 Valid 12Z Wed Oct 11 2023 - 12Z Sun Oct 15 2023 ...Excessive rain threats for the Midwest to Northeast and central Gulf Coast/Southeast/northern Florida mid to late week... ...Overview... A closed upper low will shift slowly eastward across eastern Canada mid-later week to end Great Lake effect rains and shift unsettled conditions under/around this amplified feature away from the Northeast. Far upstream, dynamic upper trough energies will work onshore and dive southeastward through the West. Early week rains will ease over the Pacific Northwest with system/frontal progressions to favor enhanced mountain snows from the Cascades to especially the northern Rockies with closed trough/low development and passage. This potent upper system will steadily track from the Plains to the Northeast Friday into 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 by mid to later this week before lifting off the East Coast. ...Guidance Evaluation/Predictability Assessment... Model and ensemble guidance agreement continues to be above average through the medium range period, and the WPC blend utilized a deterministic model blend for days 3-5. By days 6 and 7, there are some timing differences with the deep upper low through the Midwest/Great Lakes, and some lingering run to run discontinuities. The CMC is a little slower/displaced to the northwest of the better consensus. Also some timing differences with the next trough entering the West late in the period. Incorporation of the ensemble means seemed to mitigate these differences and the WPC blend late period used a 50/50 operational (ECMWF and GFS) and ensemble mean blend. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... Dynamic upper trough/height falls and surface system passage across the West into mid-later week will favor a swath of autumn mountain snows from the Cascades through the Intermountain West and especially the northern Rockies. Downstream, precipitation chances will expand east throughout the week as a frontal system moves across the nation, reaching the Central U.S. by midweek. 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 Central Plains through the Midwest into Wednesday. The WPC Day 4/Wednesday Excessive Rainfall Outlook (ERO) shows a Marginal Risk over the Mid-MS Valley given risk for training and growing model and ensemble signals. Guidance suggests this heavy rainfall area will likely expand across the Midwest with continued system development and track/inflow that is covered at this point by a Marginal Day 5/Thursday ERO risk area. Meanwhile, a separate warm front lifting through the Gulf of Mexico and the Florida Peninsula brings a threat for heavy rainfall from the central Gulf Coast to the Southeast Wednesday/Thursday. This may be aided by an area of low pressure in the Gulf of Mexico and possible remnant tropical moisture to be monitored. An ERO marginal risk area continues for Day 4/Wednesday from the central Gulf Coast to the Florida Peninsula. A Day 5/Thursday ERO marginal risk area was also included downstream to span parts of the Southeast. These two main systems may increasingly interact to some degree to spread enhanced rainfall and runoff issues possible across the East/Northeast Friday into Saturday, with widespread enhanced backside winds and post-frontal falling Autumn temperatures. Santorelli/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