Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 PM EDT Mon Mar 27 2023 Valid 12Z Thu Mar 30 2023 - 12Z Mon Apr 03 2023 ***Strong low pressure to produce some severe thunderstorms, episodes of heavy rainfall, and windy conditions across portions of the Plains and Midwest late week into the weekend*** ...Synoptic Overview... Latest guidance shows fairly progressive upper level mean flow across most of the lower 48 between Gulf of Mexico/northwestern Caribbean ridging and a Hudson Bay upper low, while gradually amplifying North Pacific/Gulf of Alaska flow should ultimately reload the long-term West Coast mean trough after departure of an initial western U.S. trough late this week. This leading western trough should support lee cyclogenesis over eastern Colorado Thursday night and a vigorous low pressure system will likely continue northeastward through the Great Lakes and southeastern Canada during the weekend. Meanwhile, the next upper level trough/closed low amplifying from the Gulf of Alaska should reach the Pacific Northwest by Saturday-Sunday and support a multi-day period of enhanced rain/mountain snow from northern/central West Coast through the northern half of the Rockies. A leading cold front crossing the western U.S. will help to reinforce the cool weather pattern, with the northern part extending farther east in wavy fashion across the northern tier and eventually central Plains. ...Model Guidance/Predictability Assessment... The primary guidance discrepancy continues to involve latest GFS runs leaning to the slow side of the envelope as the system ejecting from the West/Plains tracks through the Great Lakes by early Saturday and continues across southeastern Canada into Sunday. The GEFS mean, as well as other means and operational models, generally support the upper low opening up a little earlier and the surface system progressing somewhat faster than seen in the GFS. The new 12Z GFS does nudge a tad faster than the 00Z/06Z runs from about late Saturday onward. Behind this system, guidance trends are starting to resolve a better defined northern Plains into Upper Great Lakes wave around Sunday-Monday but with a fair degree of spread for the track. The ensemble means provide a reasonable intermediate solution. Finally, the latest operational models and means compare reasonably well for the amplifying West Coast trough from the weekend into early next week relative to typical spread/error ranges with forecasts 5-7 days out in time. A model/mean blend offers a good representation of this upper trough and leading surface front. ...Weather/Hazards Highlights... The ejecting western U.S. upper trough along with its low pressure system tracking northeastward from the central High Plains late Thursday onward will produce a variety of significant weather late this week into the weekend. Precipitation will expand across the central and then eastern U.S. with strong to severe thunderstorms possible in the southern half of the Plains Thursday and into large portions of the Mississippi Valley in the warm sector of the low on Friday based on recent outlooks from the Storm Prediction Center. The Day 4 (12Z Thursday-12Z Friday) Excessive Rainfall Outlook shows a Marginal Risk area across portions of the Upper Midwest, with the daytime update expanding the northern extent to accommodate latest guidance but keeping in mind the occasional northward bias that guidance can have (favoring maintenance of the existing southern part). Guidance shows both moisture and instability pushing northward in the vicinity of a leading warm front and trailing wavy front with cold/stationary segments with multiple rounds of convection possibly leading to some instances of flash flooding. Swaths of moderate to heavy rain should reach the east-central U.S. by late week, though guidance continues to disagree with precise locations of highest totals. Currently the best signal for locally enhanced QPF (with a reasonable combination of moisture and instability) exists from near Arkansas into portions of the Ohio/Tennessee Valleys, with sufficient confidence to enlarge the prior Marginal Risk area during the Day 5 period (12Z Friday-12Z Saturday). Improved guidance clustering for heaviest totals in future runs, especially if overlapping areas that have had heavy rain recently, would help to increase confidence enough to resolve any possible embedded Slight Risk area. Meanwhile a band of snow is possible in the deformation zone on the northwest side of the surface low, most likely from South Dakota through parts of Minnesota and eventually the northern Great Lakes. Positioning and magnitude of snow accumulations will have to be refined with time as models get a better handle on the system's evolution. The tight pressure gradient around this storm system will also produce breezy to windy conditions from the southwestern to south-central U.S. and then progressing eastward to varying degrees across a decent portion of the eastern U.S. Reloading of upper troughing along the West Coast by the weekend and early next week will support multiple days of rain/mountain snow (some locally moderate to heavy) from the Pacific Northwest and northern California into the Rockies, with the moisture shield pushing gradually farther southeast with time. Persistent upper troughing across much of the western U.S. will lead to cooler than normal temperatures across the region for most of this forecast period. Best potential for highs around 10-20 degrees below normal will be Thursday into Friday as the first upper trough passes through and then over north-central areas by Sunday-Monday as the next trough settles in. The northern Plains will also be quite cold versus normal with some highs 15 to 25 degrees below average depending on the placement and day, equating to highs in the 20s to lower 30s, and lows in the 0s to low 10s across much of North Dakota for the end of the week. Upper level ridging extending from a Gulf of Mexico upper high should result in warm and humid conditions to rebuild across the southern/central Plains Thursday and spread into the Mississippi Valley Friday and the East by Saturday, with highs generally 5-15F above average and lows around 15-25F warmer than normal. A few record warm low temperatures could be tied or set. Rausch/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