Extended Forecast Discussion NWS Weather Prediction Center College Park MD 148 PM 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... Overall, guidance is in good agreement that shortwave energy translating down from Canada to reinforce mean troughing anchored over the western/central U.S. the past several days should also be enough to finally push the trough towards the eastern U.S. this weekend with a ridge building over the West. The pattern change should be welcome relief to portions of the South that will have seen a continuous moist Gulf flow and heavy rain event ahead of this stubborn troughing through the short range period and into the early medium range period. Predictability for the large-scale pattern looks above average through at least the start of the weekend with little notable differences in the latest few runs of the GFS/ECMWF. However, more subtle trends for stronger lead energy/deeper closed low lifting northeastward as this trough initially shifts towards the East Coast, as well as a more northerly/coastal track of accompanying surface low pressure, has led to an upward trend in inland QPF from the Ohio Valley/Lower Great Lakes into the Northeast. Later in the period early next week, main trend is still a consolidation around mean eastern troughing/western ridge. The overall degree of amplification shows the most uncertainty between the guidance, with some embedded shortwave energy within the ridge in the GFS/GEFS mean, and also some disagreement in the degree of troughing and Pacific system track impinging on the Pacific Northwest. Updated WPC forecast follows suit with the prior forecast using a composite blend of the deterministic guidance early in the period given good overall agreement and no particular outlier forecasts, and a contribution from the ECens and GEFS mean in the late period to help smooth out expected increasing differences in smaller-scale shortwave energy. ...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 Day 4/Friday and Day 5/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. There is a strong signal in the guidance for potentially very heavy totals and higher-level impacts for NW California/SW Oregon Day 4, especially given wet pattern and susceptibility to landslides, but very localized nature leads to predictability concerns at this time frame for a targeted Slight Risk. 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 Day 4/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 Day 4/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/Lower Great Lakes 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 for snow along with some windy conditions on the northern edge of the broad precipitation shield into the interior Northeast and New England by Sunday as aided with a deepening low track up/off the East Coast. The trend in the guidance has been for higher QPF further north and inland, with a corresponding increase in the potential for this wintry weather. This storm should become strong enough to be a maritime hazard off the Mid-Atlantic then New England Sunday into Monday. Putnam/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