Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Thu Jan 25 2024 Valid 12Z Sun Jan 28 2024 - 12Z Thu Feb 1 2024 19Z Update: The 12Z model guidance suite maintains good overall agreement across the continental U.S. on Sunday with good overall continuity from the 00Z guidance. By Monday, the guidance is still in good agreement, but the CMC is less amplified and faster with the trough settling south across the Upper Midwest compared to the ECMWF/GFS, and this holds true as this secondary trough exits the East Coast by mid-week. The 12Z GFS is not nearly as strong as the 6Z run, and it is slower to exit the coast compared to the ECMWF. At the time of fronts/pressures composition, the 00Z GFS served as a good substitute for the 6Z run since the former agreed better with the overall model consensus for the second half of the forecast period. Much more substantial differences come into play by next Thursday with the GFS much stronger with a closed low over south-central Canada and does not have much support from it ensemble mean, whereas overall model agreement is decent across the West Coast region through this time. Ensemble means accounted for about 50% of the forecast by next Thursday. The previous forecast discussion is appended below for reference. /Hamrick --------------------- ...General Overview... A storm system forecast to exit an inundated South Saturday will lift to the Mid-Atlantic Coast around Sunday night before exiting as a coastal storm. For Sunday into early Monday, the deepening system seems likely to produce a wrapped swath of well organized precipitation from the Upper Ohio Valley/Central Appalachians through the Mid-Atlantic into New England, with interior snows possible from the central Appalachians through interior Pennsylvania/New York/New England. Far upstream, portions of the West Coast will also be rather unsettled at times with multiple rounds of rain expected for the Pacific Northwest well into next week, leading into a larger atmospheric river event signal next midweek farther southward down the coast into California to monitor for severity and impact. Meanwhile, a relatively quiet weather pattern should be in place for much of the rest of the lower 48. ...Guidance/Predictability Assessment... The last few model cycles have been in good agreement on the synoptic scale into next week, albeit with an overall slower guidance trend. There is generally good agreement that a large eastern Pacific upper trough with a lead atmospheric river focus into coastal Washington/Olympics into early next week will work more in earnest toward California through next midweek. There are lingering differences with weather focus of a main surface low to track up/off the Eastern Seaboard Sunday into Monday, but the system seems deep enough to warrant a moderate increase in QPF versus WPC continuity. Later, quite a bit of uncertainty develops over the central to eastern U.S. with a series of upper impulses dropping from Canada, albeit in a relatively tranquil weather regime. Accordingly, the WPC forecast was primarily derived from a multi-deterministic model blend valid for Sunday and Monday, transitioning Tuesday and for next Wednesday and Thursday to the more compatible ensemble means amid growing forecast spread and model run to run consistency issues as noted. ...Weather/Hazards Highlights... Deep layered southerly flow in advance of a slow moving and amplified eastern Pacific upper trough will lift and channel deep and long fetch moisture up across the eastern Pacific, with early next week focus likely shifted northward over the Pacific Northwest on the western periphery of an amplified upper ridge downstream over the West. This will fuel a protracted wet period next week, with conditions eroding with the cumulative building of rainfall totals. Systems embedded within the larger scale flow will periodically enhance activity, and may over time lead to runoff issues mainly for favored facing terrain. The WPC Excessive Rainfall Outlooks (EROs) for Day 4/Sunday and Day 5/Monday now show small Marginal Risk threat areas centered over the Olympic Peninsula to highlight best model guidance signals and topographical forcing, and this has been maintained for the 12Z update. It is important to again note that the slow approach of a large eastern Pacific mean upper trough is expected to act to refocus a lead atmospheric river more in earnest toward California around next midweek with the potential for heavy rainfall and growing runoff issues. An ongoing and significant rainfall episode over the Gulf Coast region will finally taper off Saturday. 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. 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 from the central Appalachians through the interior Northeast 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