Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Wed Jan 24 2024 Valid 12Z Sat Jan 27 2024 - 12Z Wed Jan 31 2024 ...Overview... Organized rainfall to lift up the length of the Eastern Seaboard and vicinity with a deepening low pressure system/coastal low track this weekend may also produce a swath of plowable interior Northeast snows by Sunday. Far upstream, a long duration wet pattern is likely to persist through the next week with focus from northern California through the Pacific Northwest. Much of the rest of our fine nation in between should experience a tranquil break from significant recent bouts of cold air/wintry conditions to inudating wet weather, instead in a relatively warm and dry late January pattern. ...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 Saturday into Monday in an overall pattern with seemingly above normal predictability. This blended solution is also quite compatible with ensembles, the National Blend of Models and WPC continuity. A composite of newer 00 UTC guidance remains in line for this forecast period. Prefer to switch to a forecast based on latest compatible GEFS/ECMWF ensemble means through early to mid next week amid growing model forecast spread and cycle to cycle continuity issues with the timing/amplitude of embedded systems. Model issues and uncertainties remain evident with the newer 00 UTC cycle at these less certain longer time frames. ...Weather/Hazards Highlights... Deep layered southerly flow in advance of a slow moving and amplified eastern Pacific upper trough will lift deep and long fetch moisture up across the eastern Pacific and tangently into northern California and through the Pacific Northwest. This will fuel a protracted wet period over the next week, with conditions eroding over time with the cumulative building of rainfall totals. Lifting systems embedded within the larger scale flow will periodically enhance activity riding the western periphery of an amplified downstream upper ridge over the West, and may over time lead to runoff issues mainly for favored facing terrain. WPC Excessive Rainfall Outlooks (EROs) for Day 4/Saturday and Day 5/Sunday show small Marginal Risk threat areas centered over NW California/SW Oregon and for both days over the Olympics to highlight best guidance signals and topography. An ongoing and significant rainfall episode over the South will finally end Saturday. Deep Gulf of Mexico to Atlantic moisture feed ahead of the last major southern stream shortwave in the series will lift northeastward on the western periphery of an amplified downstream upper ridge. A remaining threat for heavy rain up from the Southeast through the southern Appalachians and vicinity with system approach/passage and deepening low focus has prompted a Day 4/Saturday ERO Marginal Risk area. The lifting of this feature and low also favors a moderate wrapped precipitation swath across the Ohio Valley, Appalachians and Mid-Atlantic through the Northeast this 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 over the interior Northeast by Sunday as aided with a deepening low track up/off the East Coast. This emerging coastal storm will also offer a maritime risk offshore 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