Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Tue Oct 10 2023 Valid 12Z Fri Oct 13 2023 - 12Z Tue Oct 17 2023 ...Excessive rain threat shifts from the Upper Midwest through the northern Mid-Atlantic/Northeast Friday through Saturday... ...Overview... A dynamic upper trough/closed low and deepened surface low/frontal system will steadily track from the Plains through the Northeast Friday through this weekend. This will foster the spread of heavy precipitation from the Upper Midwest through the northern 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. Out West, an amplifying ridge over the Intermountain West will allow troughing to rebuild the Pacific Northwest, allowing a wet period this weekend well into next week. ...Guidance Evaluation/Predictability Assessment... There is good overall agreement on the progression of the Plains low and the eastern Canada low through this weekend. A key feature responsible for some of the changes is shortwave energy rounding the north of the Canadian low Wednesday that then tracks north of the low over the Plains Thursday. The 00Z ECMWF/UKMET phase this wave into the Plains low, triggering a deeper, slower low over the Great Lakes/Northeast this weekend while the 00Z CMC/06Z GFS left the energy separate/drifting over the Canadian Prairies this weekend. Preference was given to the 00Z EC/UK solution given the 06Z CMC regional trended toward a phasing low. Both the 12Z CMC/GFS have trended this way (but still do not phase) while the 12Z UKMET has some separation and an overall much weaker low. The 12Z ECMWF is delayed today and not available for diagnosis by publish time. The WPC product suite was derived from a blend weighted toward the 00Z ECMWF/UKMET with the 06Z GEFS early on, transitioning to the 00Z ECMWF with the 00Z ECENS/06Z GEFS after Day 5 based on the general pattern of a trough along the northern West Coast, ridge over the west-central, and trough/low over the east-central CONUS. QPF combined the 13Z NBM with the 00Z ECMWF and some UKMET over the eastern half of the CONUS, with a general model blend for the Pacific Northwest. ...Weather/Hazards Highlights... A well developed low pressure system shifting east from the central Plains to the Great Lakes Friday then to the Northeast through Saturday night wraps anomalous moisture into warm/occluded fronts which provide a focus for heavy rains. While longer duration rains and ambient relatively dry soils may partially limit impact, the WPC Day 4/Friday Excessive Rainfall Outlooks (ERO) maintain a Slight Risk over the Upper Midwest with a bit of a northward shift given latest guidance. The Day 5/Saturday ERO has an expanded Slight Risk over the northern Mid-Atlantic given a northward shift in heavy rain focus as Atlantic moisture is concentrated north of the redeveloping system along the Mid-Atlantic coast. 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 night. 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 Marginal Risk was trimmed a bit over South Carolina and maintained south of there into north Florida. Widespread enhanced winds are forecast on the backside of the low first in the Plains/Upper Midwest and then again over the Northeast as the low redevelops along the coast. The associated 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. This cold front crosses all of Florida Saturday into Sunday, bringing relief to a string of near record highs over South Florida Sunday. Strong troughing over the Pacific Northwest/Northwest early next week brings likely moderate rainfall along with some high elevation snow. Jackson 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