Extended Forecast Discussion...UPDATED NWS Weather Prediction Center College Park MD 329 AM EDT Fri Mar 22 2024 Valid 12Z Mon Mar 25 2024 - 12Z Fri Mar 29 2024 ...Heavy snow likely to continue into Monday for parts of the north-central Plains and Upper Midwest... ...Heavy rain threat across the Lower Mississippi Valley to Southeast early to midweek... ...Overview... A strong low pressure system is forecast to intensify and track across the central Plains to the Upper Midwest Monday-Tuesday, bringing heavy snow to the north-central U.S. to the low's north and west. Strong winds and very cold temperatures are also forecast. On the warm side of the system, heavy rain associated with strong thunderstorms ahead of a relatively potent cold front will likely lead to flooding issues across the Lower Mississippi Valley and Southeast Monday-Tuesday, with some uncertainty beyond that in how much rain may fall for the East Coast. Meanwhile, a low pressure system from the Pacific is forecast to move into the northwestern U.S., bringing rain and mountain snow to the region. ...Guidance/Predictability Assessment... 12/18Z model guidance was reasonably consistent with the surface low position in the north-central U.S. Monday-Tuesday, with good agreement across model suites and run-to-run consistency. The incoming 00Z suite actually shows a little more spread with the timing of the low, with the GFS trending faster while the ECMWF trended slower. Fortunately the 12/18Z average position was in between. There is high confidence in the impactful winter storm occurring, though with typical uncertainty in mesoscale banding of placement of heaviest amounts as well as the timing. By Wednesday, spread increases with the low placement. The 12Z/18Z GFS runs were aggressive in showing a low along the front down in the Lower Mississippi Valley that zooms north toward the Upper Great Lakes and interacts with the main low. A secondary low that potent seems less likely and fortunately the 00Z GFS has backed off, with still some variations by Wednesday among guidance in the low but within typical spread. Farther south, there remains notable model spread regarding the heavy rain area in the south-central to southeastern U.S., with the GFS/GEFS keeping the heaviest rainfall farther north than the ECMWF cluster by Tuesday. This is probably partly due to that secondary frontal low that the GFS was showing. At least there is some overlap in the heavy rainfall potential to indicate a Slight Risk. The differences aloft and at the surface and with rainfall placement only grow from mid- to late week. The 12Z ECMWF is slow with moving southern stream energy developing into a closed low over the Southeast compared to other guidance and even the EC-based AI/ML models. The EC ensemble mean was slower too but not to the same extent of showing a slow closed low. Thus favored a GFS/GEFS mean/EC mean blend for the East by Days 6-7. The 00Z GFS has slowed a bit while the 00Z ECMWF is faster, so hopefully there will be some continued model convergence there. In the West, the first model that looks to diverge from consensus was the 12Z CMC by Wednesday, which showed upper and surface lows much farther west in the eastern Pacific than other guidance. Then the GFS ended up bringing the potent energy quickly east into the Intermountain West Thursday-Friday. The ECMWF seemed like a good middle ground in tracking the trough eastward and generally matched well with the AI/ML models. With this and considering the GFS's fast bias, favored the EC more. The WPC forecast used a blend of the deterministic guidance early in the forecast period. Given the aforementioned issues with the ECMWF in the East, the GFS in the West, and the CMC everywhere, gradually increased the proportion of GEFS/EC ensemble means to the entire blend by Day 7. ...Weather/Hazards Highlights... The deep surface low tracking from the central Plains to Upper Midwest will be the driver (along with upper-level support) for multi-hazard impactful weather for early-mid next week. A widespread late season winter storm should be ongoing Monday with heavy snow accumulation (8+") likely from northeastern Nebraska into eastern parts of the Dakotas, much of Minnesota, and far northwestern Wisconsin, as this region will be on the colder side of the low with a deformation zone developing. High winds due to the deep surface low could lead to blizzard conditions in some areas. Timing/track uncertainty may change the eventual location of the impactful winter weather, but the risk for disruptions to daily life and hazardous travel is likely. High winds are also possible southward across the Plains on Monday behind the cold front, though with drier weather. Farther south ahead of the cold front, ample moisture streaming in and a strong jet aloft will support heavy rain and potentially severe thunderstorms for the Lower Mississippi Valley Monday, shifting into the Southeast on Tuesday. Slight Risks are in place in the Day 4-5 Excessive Rainfall Outlooks due to the potential of a likely squall line causing rain rates of 2+" per hour. The areal coverage of heavy QPF is somewhat less on Tuesday compared to Monday, but still have a Slight Risk considering that the ECMWF/GFS are agreeable in indicating the squall line lasting into the early part of the day on Tuesday into MS/AL. By midweek, the stormy weather then reaches the Florida Panhandle north into the Carolinas up the Eastern Seaboard, but with considerable spread in rainfall amounts by Wednesday-Thursday due to the model differences. Some lingering moderate snow is possible across parts of the south-central Rockies on Monday, with light amounts elsewhere across the Intermountain West. The Pacific Northwest could see some moderate lower elevation rain and higher elevation snow amounts Monday, with increasing totals by Tuesday-Wednesday and expanding into California as troughing and low pressure approach. Some moderate to heavy precipitation should expand into the Intermountain West midweek, and will continue to monitor the potential for further expansion of the precipitation possibly in the form of snow to affect the north-central Plains once again by Thursday. Much colder than average temperatures (more comparable to January) are likely for the northern and central High Plains in the wake of the strong cold front. Highs will only be in the 20s and 30s for these areas on Monday, but a moderating trend will commence going into the middle of the week, even though Montana and North Dakota will still remain chilly by late March standards. The West should see generally cooler than average highs and near average lows next week, while the East can expect bouts of above and below normal temperatures averaging out to near normal. Tate 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 (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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