Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Fri Mar 24 2023 Valid 12Z Mon Mar 27 2023 - 12Z Fri Mar 31 2023 ...Synoptic Overview... The weather pattern early next week will be featured with a deepening low pressure system just off the West Coast that eventually moves inland across the interior western U.S., and eventually emerges over the Plains and Midwest by the end of the week. This trough over the western U.S. will serve to amplify the downstream upper ridge over the south-central U.S. and the Gulf Coast region. To the north will be a closed low over the Hudson Bay region with shortwave perturbations pivoting around its southern periphery across the northern tier states. One of these shortwaves should be strong enough to spur surface cyclogenesis off the East Coast by late Wednesday, and may come close enough to the New England Coast to bring some rain and inland snow. ...Model Guidance/Predictability Assessment... Model differences become apparent as early as Monday in regard to timing and position of a couple of weak shortwaves crossing the eastern U.S., and the UKMET has a stronger low crossing the Ohio Valley that doe not have much support from the other guidance. The 18Z GFS develops a low over the northern Great Lakes region by Wednesday that also does not have much model support, but is relatively close with the large low pressure system across the western U.S. and remains relatively close to the CMC/ECMWF as the low emerges across the Plains late in the forecast period. The 00Z GFS is stronger with the offshore low that develops off the New England coast on Wednesday. The biggest difference noted, surprisingly, is a much more progressive 18/00Z GEFS mean with the western U.S. trough and this trend is substantial as early as Wednesday night, and well ahead of the ECENS and the operational GFS by the end of the week. Taking these factors into account, the forecast was primarily derived from a CMC/ECMWF/GFS blend with some ECENS through Tuesday, less GFS for Wednesday owing to the low over the Great Lakes, and still keeping some of the GFS/CMC/ECMWF for Thursday and Friday while increasing the percentage of the ECENS and none of the GEFS mean. ...Weather/Hazards Highlights... The next major storm system to affect the West Coast is expected to arrive Monday night and persist into early Wednesday as a well organized low pressure system advects a plume of deep moisture from the Pacific inland across much of central and northern California. Although this upcoming event does not appear to be as heavy as some recent atmospheric river events, the potential does exist for 2 to locally 4 inches of total rainfall across the windward terrain of both the coastal mountain ranges and the foothills of the Sierra. The Day 4 excessive rainfall outlook maintains a Marginal Risk area for much of central and northern California, and for Day 5, there is enough model agreement to support a Slight Risk area for central portions of the California coast. It is likely that this Slight Risk area will expand in coverage as the event gets closer in time. Heavy snow is also likely for the higher terrain of the Sierra and northern California mountains. Elsewhere across the continental U.S., lingering convection along a quasi-stationary frontal boundary may be locally heavy at times from Louisiana to northern Florida, and a Marginal Risk of excessive rainfall remains valid for the Day 4 period Monday. Looking ahead to Wednesday and Thursday, a surface low is likely to develop off the East Coast, but uncertainty remains in its placement and how far inland its precipitation gets. With this forecast cycle, it appears most of the precipitation should remain offshore, but any northward adjustment could bring accumulating snow for parts of the Northeast states. By the end of the week, showers and storms are expected to increase ahead of a developing storm system across the Plains and Midwest. Temperatures are expected to be well below late March averages across much of the Intermountain West, the Rockies, and the Northern Plains to start the work week, with highs running 15 to 25 degrees below average. This will equate to readings topping out in the high 10s to low 30s from Wyoming to North Dakota. The chilly conditions are likely to continue through much of the week for the western half of the nation, including the West Coast where highs of 10 to 20 degrees below normal are likely. Meanwhile, the presence of the Gulf Coast upper ridge will tend to favor temperatures running 5 to 15 degrees above average from Texas to the Southeast U.S. and Florida. 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices 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