Extended Forecast Discussion NWS Weather Prediction Center College Park MD 250 PM EDT Sat Oct 07 2023 Valid 12Z Tue Oct 10 2023 - 12Z Sat Oct 14 2023 ...Increasing heavy rain threats for the Midwest and Gulf Coast/northern Florida by mid to late next week... ...Overview... It remains the case that a deep upper level low is forecast to park itself over southeastern Canada and the Great Lakes and churn in place through most of next week with some indications it may begin to push east later in the week. A nearly stationary surface low will create precipitation chances near the Great Lakes early in the week with some lake effect enhancement possible. Out West, a trough will move onshore early in the week diving through the West with possible closed upper low development over the Rockies around Thursday. This feature will drive a surface frontal system east across the country through the week. Precipitation chances will be greatest into Tuesday in the Pacific Northwest where locally heavy downpours may be possible, then precipitation will spread into the Central U.S. midweek and to the East Coast/Northeast by Friday. A frontal boundary with deepened tropical moisture lifting through the Gulf and Florida will increase precipitation chances across Florida/the Gulf Coast then Southeast as well by mid to later in the week before lifting up the East Coast. ...Guidance Evaluation/Predictability Assessment... Model and ensemble solutions remain overall well clustered with the large scale pattern through about Thursday and a general deterministic model blend worked well. There continues to be a trend towards a definitive closed low developing over the Rockies around Wednesday moving into the Plains/Midwest late week. Latest model runs offer additional uncertainties with whether this low phases with the bigger low over southeast Canada or remains a separate feature. Through the 00z/06z model runs, both the ECMWF and CMC (along with the ECENS mean) suggest these features stay separate and the new Midwest upper low simply pushes the southeast Canadian low to the east. The GFS/GEFS seemed alone in suggesting a much slower/more northward progression of this low, but was more consistent with WPC continuity. GFS was also much too fast with the surface system through the East compared to the better consensus. A blend towards the ECMWF, CMC, and ECENS mean late period broke WPC continuity in some ways, but the new 12z GFS (available after forecast generation time) did come in much better agreement with the ECMWF and CMC which increases confidence in this shift. Also some timing differences in the next trough as it reaches the West Coast next Friday-Saturday. ...Weather/Hazards Highlights... The threat for excessive rainfall is forecast to remain low Tuesday. However, moisture working into the Pacific Northwest ahead of an approaching frontal system will combine with upper level dynamic support and terrain upslope to enhance rainfall for the coastal mountain ranges, Olympics, and Cascades that will likely see the highest rainfall totals. A Marginal Risk of Excessive Rainfall was not introduced as the system seems progressive and much of the Pacific Northwest has lingering drought conditions, so much of this rainfall could be more beneficial than hazardous. Higher elevations from the Cascades through the northern Rockies will see snow with upper trough/height fall approach and passage. Precipitation chances will expand east throughout the week as a frontal system moves across the nation, reaching the Central U.S. by around midweek. Deep cyclogenesis and wrapping moisture into a leading warm front lifting through the east-central U.S. will shift focus for possibly heavy rain from the Central Plains through the Midwest into Wednesday. The WPC Day 5 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 Thursday across the Midwest with continued system development and track/inflow and a possibly heavy to excessive rainfall threat. Meanwhile well south, 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 in the coming days. Latest model guidance prompted a marginal risk being introduced by the Day 5/Wednesday ERO from the central Gulf Coast to the Florida Peninsula, with this risk likely to continue into Thursday as well. These two main systems may increasingly interact 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