Extended Forecast Discussion NWS Weather Prediction Center College Park MD 301 PM EDT Fri Mar 24 2023 Valid 12Z Mon Mar 27 2023 - 12Z Fri Mar 31 2023 ...Overview... Low pressure both at the surface and aloft are forecast to dive southward across the eastern Pacific early next week before shifting east into the western U.S. and eventually into the central U.S. by late week. These features will likely bring another round of precipitation to California for Monday-Tuesday, with heavy mountain snow and coastal rain. The 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 and provide support for generally warmer than average temperatures there. Farther north, a closed low atop the Hudson Bay region will cause some shortwave perturbations to pivot around its southern periphery across the northern tier U.S. states, possibly helping form western Atlantic surface lows that may come close enough to the New England Coast to bring some rain and inland snow. ...Model Guidance/Predictability Assessment... Some small-scale but impactful model differences in shortwaves are noticeable across the northeastern quadrant of the U.S. even as the medium range period begins Monday and especially going into Tuesday-Wednesday. This affects the surface low and frontal positions and precipitation placement and amounts. The 00Z/06Z model cycle that was available for forecast production was generally showing more focused QPF in the Northeast, and in the newer 12Z guidance a shortwave appears even more amplified (which the 12Z CMC may take to the extreme). The variability amongst models and model cycles leaves low confidence in the forecast there, but a blend of guidance seemed to provide a middle ground solution. In the West, model guidance is rather agreeable on the large scale through around Wednesday for an upper low to dive south/southeastward across the eastern Pacific near the West Coast accompanied by a fairly strong surface low. By the latter half of the week as the trough/low turns eastward, models then diverge in terms of how fast to take the trough and potential embedded low eastward. The GFS suite including the deterministic runs and many ensemble members and its ensemble means have generally shown a faster track east than other solutions. A slower track more like the EC ensemble mean and the ECMWF and CMC suite was favored. A multi-model deterministic blend was favored early on and included some EC mean later in the period, which maintained good continuity with the previous forecast. ...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 strong 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 maintain a Slight Risk area for central portions of the California coast, especially given possible sensitivity in that area from the past events. This pattern should also be overall cooler than some recent ones with lower snow levels. Thus heavy snow is likely for the Sierra Nevada and the northern California mountains. Elsewhere across the contiguous U.S., lingering convection along a frontal boundary could cause localized flash flooding across the Southeast, and a Marginal Risk of excessive rainfall remains valid for the Day 4 period Monday. Recent model trends have this front slower to move south and push offshore, so a Marginal Risk was maintained into day 5 for lingering thunderstorms with high rain rates especially during the day on Tuesday. Farther north on Tuesday-Wednesday, there remains some uncertainty regarding precipitation amounts and types over the Northeast, which depend on positions of smaller-scale shortwaves and surface lows in the western Atlantic. Model guidance has trended wetter in recent cycles there, and accumulating snow is possible especially in higher elevations of the Interior Northeast. Then by Thursday and Friday, the surface low ejecting from the West and the amplified upper pattern should lead to increasing showers and thunderstorms across the Plains and Mississippi Valley. The Storm Prediction Center has already delineated an area of potentially severe convection for parts of the southern half of the Plains on Thursday. Some snow is possible on the backside of the low in the north-central U.S. into later next week, but with considerable uncertainty in placement and amounts of wintry weather for now. 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, aside from a cooldown to near normal around midweek. Tate/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