Extended Forecast Discussion NWS Weather Prediction Center College Park MD 316 AM EDT Tue Oct 10 2023 Valid 12Z Fri Oct 13 2023 - 12Z Tue Oct 17 2023 ...Excessive rain threat to shift from the Midwest/Great Lakes to the northern Mid-Atlantic/Northeast Friday into Saturday as heavy rains exit the Southeast... ...Overview... A dynamic upper trough/closed low and deepened surface low/frontal system will steadily track from the Plains to the Northeast Friday through next weekend. This will foster the spread of heavy precipitation from the Midwest/Great Lakes through the Mid-Atlantic/Northeast as inflow moisture increases and is sustained with addition influx from the Atlantic. To the south, a frontal boundary enhanced with tropical moisture will lift from the Gulf of Mexico to spread heavy precipitation across the Southeast through Friday before lifting offshore. Well upstream, a slow translation of an amplified and complex eastern Pacific upper/surface system seems set to 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 Friday into the weekend from the 18 UTC GFS and 12 UTC ECMWF/Canadian/UKMET along with the compatible 01 UTC National Blend of Models. Shifted WPC guidance blend emphasis toward ensembles at longer time frames amid slowly growing forecast spread/uncertainty, but favored the more amplified/less progressive ECMWF ensemble mean versus the GEFS given upstream amplitude and overall guidance trends. WPC product continuity is reasonably well maintained with this forecast plan. Latest 00 UTC guidance remains in line overall, but there remains pesky run to run variances with the track of the main systems. ...Weather/Hazards Highlights... Dynamic upper trough support and deep cyclogenesis/wrapping anomalous moisture into warm/occluded fronts across the central to eastern U.S. will shift focus for emerging heavy rains from the Midwest/Great Lakes to the northern Mid-Atlantic/Northeast Friday into the weekend. While longer duration rains and ambient relatively dry soils may partially limit impact, the WPC Day 4/Friday Excessive Rainfall Outlooks (ERO) shows collaborated Marginal and Slight Risk areas given potential for training and model and ensemble signals for some higher rain totals. The Day 5/Saturday ERO shows a marginal risk area centered over the northern Mid-Atlantic with system approach and enhancing Atlantic moisture inflow and possible coastal development that will present a protracted maritime risk. A Slight Risk area was also issued for eastern PA and portions of NJ where moisture influx and instability may be maximized/prolonged. 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. Meanwhile, a separate front meandering through the Gulf of Mexico and the Florida Peninsula lifting in advance of the aforementioned main storm will bring spread a threat for heavy rainfall across the Southeast 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. The Day 4/Friday ERO shows a marginal risk area. The warm front lifting late week may increase convection areal coverage there as instability lifts inland. Well upstream, guidance signals are growing that the slow translation of an amplified and complex eastern Pacific upper/surface system seems set to renew wet flow into the Pacific Northwest/Northwest into early next week, with likely moderate totals to focus over the Pacific Northwest to include some higher elevation snow. 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