Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 404 PM EST Wed Jan 24 2024 Valid 12Z Sat Jan 27 2024 - 12Z Wed Jan 31 2024 ...General Overview... The storm system that is forecast to be over the south-central U.S. on Saturday is expected to reach the Mid-Atlantic Coast by Sunday night. This will result in locally heavy rainfall across the southern Appalachians, and inland snow from portions of Pennsylvania to central New England. The West Coast will also be rather unsettled at times with multiple rounds of precipitation expected and a larger atmospheric river event towards the middle of next week. Meanwhile, a relatively quiet weather pattern should be in place across the Rockies and into the Central U.S. and Midwest states with high pressure in place. ...Guidance/Predictability Assessment... The 12Z model guidance suite is in good overall agreement on the synoptic scale, including the potent southern stream shortwave trough for this weekend. There has been an overall slower trend in the guidance compared to the 00Z cycle, with the surface low departing the East Coast later Sunday evening compared to earlier forecasts. Looking ahead to early next week, the GFS/GEFS mean are farther west with the axis of the upper ridge across the northern Rockies, whereas the CMC/ECMWF are more displaced to the east. There is generally good agreement that a large upper trough with a potential atmospheric river will approach the West Coast towards the middle of next week, but more uncertainty resides across the Upper Midwest with a trough dropping southward from central Canada. The fronts/pressures forecast was primarily derived from a multi-deterministic model blend through Sunday, followed by gradually increasing percentages of the ensemble means going into the first half of next week. ...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 into northern California and also western Oregon and the Olympic Peninsula. 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. The 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 model guidance signals and topographical forcing. An ongoing and significant rainfall episode over the Gulf Coast region will finally taper off 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. For the 12Z update, the existing Day 4 Marginal Risk was expanded some to account for model variations in higher QPF placement. There was also a modest upward trend in QPF across portions of the southern Appalachians, and it's possible a Slight Risk area could eventually be needed in later forecasts. 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/Hamrick 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