Extended Forecast Discussion NWS Weather Prediction Center College Park MD 221 AM EST Tue Jan 23 2024 Valid 12Z Fri Jan 26 2024 - 12Z Tue Jan 30 2024 ...Excessive Rainfall/Flooding Threat over the South to continue late week from the Gulf Coast states to the Southern Appalachians... ...Overview... A significant and protracted rainfall event is expected today through saturday with heavy rain/convection and runoff threats amid a slow to translate and amplified pattern, with primary focus to reform and spread from southeast Texas through the Lower Mississippi/Tennessee Valleys and Gulf Coast states through the southern Appalachians and vicinity. ...Guidance/Predictability Assessment... The WPC medium range product suite was mainly derived from a composite blend of reasonably well clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET/Canadian for Friday into Sunday. Forecast spread in this period has decreased, bolstering forecast confidence. This blended solution is overall quite compatible with ensembles, the National Blend of Models and WPC continuity. Opted to include some input from the GEFS/ECMWF ensemble means into early next week to build a baseline forecast amid gradually growing forecast spread. A similar composite of latest guidance from the 00 UTC cycle overall seems to favorably remain in line. ...Weather/Hazards Highlights... An amplified upper ridge will build over the West Coast by Friday that should prove slow to shift eastward across a warmer and stabilized West through next Tuesday. A series of amplified eastern Pacific systems will lift offshore and bring an axis of long fetch moisture up on the western periphery of the shielding ridge, with main focus into favored terrain within coastal areas from the Pacific Northwest to northern California. WPC Excessive Rainfall Outlooks (ERO) for Day4/Friday and Day5/Saturday offer Marginal Risk threat areas over NW California/SW Oregon and the Olympics to highlight best guidance signals and leads into additional wet coastal weather potential later weekend into early next week. A significant rainfall episode over the South will linger into Friday/Saturday as deep moisture feed in advance of the last major southern stream shortwave in the series works across the region and lifts energetically northeastward on the western periphery of a slow to recede Southeast/East downstream upper ridge. A remaining threat for leading heavy rain and thunderstorms through the Gulf Coast states into the Southeast/southern Appalachians and vicinity with system approach/passage and enhanced frontal wave/deepening low focus has prompted issuance of a Day 4/Friday Slight Risk area ERO over the central Gulf Coast States with a surrounding Marginal Risk Area Day4/Friday extending up through/lee of the southern Appalachians Day 5/Saturday. Areas in vicinity of the track of the initially closed upper center may also see locally heavier downpours as highlighted by the Day4/Friday Marginal Risk area ERO over Oklahoma/north Texas. This lifting/shearing of this feature favors a moderate precipitation swath northeastward across the Mid-South and Ohio Valley through the central Appalachians, Mid-Atlantic and New England through the weekend. While within a widespread and significantly warmer pattern, enough cooled air may remain in place to allow potential for some snow along with some windy conditions on the far northern edge of the broad precipitation shield into the interior Northeast by Sunday as aided with a deepening low track up/off the East Coast. This storm should become strong enough to be a maritime hazard off the Mid-Atlantic then New England Sunday into Monday. 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