Extended Forecast Discussion NWS Weather Prediction Center College Park MD 323 AM EDT Sat Mar 23 2024 Valid 12Z Tue Mar 26 2024 - 12Z Sat Mar 30 2024 ...Heavy rain threat across the Southeast Tuesday-Wednesday... ...Heavy rain and snow possible across much of the West by midweek and beyond... ...Overview... As snow starts to exit the Upper Midwest on Tuesday, the main focus for potentially hazardous weather will be much farther south, as heavy rain and thunderstorms are likely ahead of a cold front tracking across the southeastern U.S. Tuesday-Thursday. Meanwhile, one or more Pacific systems should bring increasing totals of rain and mountain snow to the West Coast states and then into the Rockies mid-late week. Some of this moisture may reach the northern Plains by Friday. ...Guidance/Predictability Assessment... While model guidance is more agreeable across the CONUS than a day ago, there are certainly still some differences that impact sensible weather. In the East, minor variations are seen in the models through Thursday or so, though the details do impact QPF axes and amounts in the Southeast. There are some increasing differences by Friday-Saturday with southern stream energy developing low pressure in the western Atlantic. These are generally within normal spread for the Day 6-7 timeframe. The 12Z GFS surface low position seemed to match best with the ensemble means, while other models were slower. The incoming 00Z suite shows lessening spread in the upper/surface lows. In the West, the main outlier was the 12Z UKMET, showing much less energy coming from the Pacific and a displaced surface low position by Wednesday. The CMC was in much better alignment compared to a day ago, with an ECMWF/GFS/CMC blend generally reasonable. By Day 7/Saturday there become more differences in ejecting energy into the central U.S., so quickly transitioned to the ensemble means comprising just over half the blend by Day 7 after maintaining more deterministic models through Day 6. ...Weather/Hazards Highlights... Some snow may linger across northern parts of the Upper Midwest on Tuesday as the surface low tracks across the Upper Great Lakes. Meanwhile farther south, ample moisture and a strong jet aloft will support heavy rain. This is likely to be ongoing in parts of the Southeast as the period begins early Tuesday, with model guidance indicating a squall line crossing Mississippi and Alabama. This could produce rain rates that support Slight Risk levels in the Excessive Rainfall Outlook for Tuesday, though areal coverage of heavy QPF seems to be less so compared to Monday (now in the short range). The main change with the Tuesday Slight Risk from the previous issuance was to expand to the central Gulf Coast where instability should be highest. Some rain extending into Tennessee and Kentucky and over to the southern Appalachians is covered by a Marginal Risk. Then on Wednesday, the potential for heavy rain continues to shift eastward as the cold front(s) press toward the Eastern Seaboard. Models vary tremendously on the placement and amounts of rainfall on Wednesday in the East. The first take at the Wednesday Excessive Rainfall Outlook indicates a Marginal Risk across portions of Florida across Georgia and into the Carolinas. If some models like some GFS runs verify, with multi-inch totals across a swath inland in those states, this would need to be upgraded. But some models show much lighter QPF with a swath more coastal or offshore. Given the spread, will not go for more than a Marginal Risk at this point. Some potentially heavy rain could linger into Thursday, currently most likely over the eastern Carolinas, but with continued uncertainty. Rain totals are also uncertain for the Mid-Atlantic/Northeast Wednesday-Thursday. Farther west, some snow is likely in the Intermountain West on Tuesday. Deeper moisture starts to stream into the West Coast late Tuesday and especially Wednesday as upper troughing and low pressure approach. Rain rates could be high enough to be concerning for flooding across portions of California into western Oregon, so have a Marginal Risk delineated there for Wednesday. Heavy snow is likely in higher elevations midweek and beyond, stretching into the Intermountain West as well. There is a general signal for precipitation to expand into the north-central U.S., possibly in the form of snow again, by Friday. Much colder than average temperatures (more comparable to January) are likely for the northern and central High Plains in the wake of the strong Plains/Midwest storm. Highs will only be in the 20s and 30s for these areas on Tuesday. The central U.S. will see a warming trend going into the middle and late part of next week, though Montana and the Dakotas will still remain chilly by late March standards. The West should see generally cooler than average highs and near average lows next week. The East may see mixed above and near normal temperatures until late week when the relative warmth from the central U.S. expands east. 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