Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Sat Mar 25 2023 Valid 12Z Tue Mar 28 2023 - 12Z Sat Apr 1 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 Wednesday morning, and may come close enough to the New England Coast to bring some rain and inland snow. ...Model Guidance/Predictability Assessment... The 00Z model guidance suite is in a little better agreement with the synoptic scale pattern on Tuesday compared to similar time scales yesterday. There has been a trend for a more offshore solution with the developing low off the East Coast Wednesday compared to the 12Z guidance, and this would keep more of the precipitation away from New England. The CMC made the biggest adjustment in track with the 00Z run compared to its 12Z run Friday. Looking ahead to the end of the week, the GFS and GEFS mean are still on the progressive side of the guidance with the trough/surface low ejecting east across the Plains, although the GEFS mean does not differ as much as it did yesterday. Taking these factors into account, the forecast was primarily derived from a CMC/ECMWF/GFS blend with some ECENS through Wednesday, and still keeping some of the GFS/CMC/ECMWF through the end of the forecast period while increasing the percentage of the ECENS and none of the GEFS mean in line with previous WPC continuity. ...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 1 to 3 inches of total rainfall, perhaps locally higher, across the windward terrain of both the coastal mountain ranges and the foothills of the Sierra. The Day 4 excessive rainfall outlook maintains a Slight Risk area for central portions of the California coast, and a broader Marginal Risk area from Cape Mendocino to the greater Los Angeles metro area. Wet antecedent conditions from recent heavy rain make these parts of California more vulnerable to potential flooding issues. Heavy snow is also likely for the higher terrain of the Sierra and northern California mountains, where 1-2 feet of accumulation is likely during this time period. There is not a strong signal for organized heavier rainfall for Day 5, so no excessive rainfall areas are currently needed for next Wednesday/Wednesday night. 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 Tuesday. On Wednesday, 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 the northern periphery of the precipitation may reach portions of central and southern New England, with light snow possible for parts of Upstate New York, Vermont, New Hampshire, and Maine, and mainly rain for southern New England. By the end of the week, showers and storms are expected to increase ahead of a developing storm system across the Plains and Midwest. Some of these storms could be severe on Thursday across portions of the southern Plains, and windy conditions are likely to develop across much of the Plains in general. Temperatures are expected to be well below late March averages across much of the northern Intermountain West, the Rockies, and the Dakotas on Tuesday, with highs running 15 to 25 degrees below average in many cases. This will equate to readings topping out in the 20s to middle 30s from Wyoming to North Dakota. The chilly conditions are likely to continue through much of the week from the Rockies to the northern Plains, and also the West Coast where highs of 10 to 20 degrees below normal are likely, which will lead to lower snow levels for the event coming early in the week. Meanwhile, the presence of the Gulf Coast upper ridge will tend to favor temperatures running up to 15 degrees above average from Texas to the Southeast U.S. and Florida for the end of the week. 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