Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM 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, dynamic upper trough/closed low energies and surface frontal system will work out from the West to favor enhanced mountain snows for especially the northern Rockies into 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... Model and ensemble forecast spread remains on the low side through medium range time scales in a pattern with seemingly better than average predictability for flow evolution and main embedded weather focusing storms/systems over the next week in active flow. The WPC product suite was primarily derived from a composite blend of well clustered guidance Thursday into Saturday from the 18 UTC GFS/GEFS, 00 UTC ECMWF/Canadian/UKMET/ECMWF ensemble mean along with the compatible 01 UTC National Blend of Models. Shifted WPC guidance blend emphasis toward the somewhat more amplified/less progressive ECMWF/Canadian/ECMWF ensemble mean cluster versus the GFS/GEFS by Sunday/next Monday given upstream amplitude/guidance trends. WPC product continuity is well maintained with this forecast plan and the latest 00 UTC guidance overall remains in line. ...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 out from the Intermountain West to especially the northern Rockies. 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 soils may partially limit impact, the WPC Day 4/Thursday and Day 5/Friday Excessive Rainfall Outlooks (EROs) are now planned to 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. Meanwhile, a separate warm front lifting through the Gulf of Mexico and the Florida Peninsula brings 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 and possible remnant tropical moisture that is now being monitored by NHC. ERO Day 4/5 marginal risk areas shift out from the central Gulf Coast to the Southeast. 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 and deepening as a maritime risk along with widespread enhanced backside low winds and post-frontal falling temperatures back inland with system passage. 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