Extended Forecast Discussion NWS Weather Prediction Center College Park MD 317 PM EDT Wed May 15 2024 Valid 12Z Sat May 18 2024 - 12Z Wed May 22 2024 ...Southern tier heavy rainfall threat possibly lingering into the weekend over the Southeast... ...Overview... Behind an upper trough and surface system progressing eastward from the Mississippi Valley Saturday onward, guidance shows a system crossing southern Canada and the northern tier of the lower 48 during the weekend. Then guidance is consistent in principle regarding establishment of what may become a fairly amplified mean trough aloft over the West. While a lot of embedded details are still uncertain, the forecast pattern evolution should support a transition of rainfall emphasis from the South/East to start the weekend toward development from the northern half of the Plains into the Midwest and continuing eastward between Sunday and the middle of next week. Meanwhile the western trough will likely bring cooler temperatures to the region and at least some scattered precipitation, including high elevation snow in the northern Rockies. ...Guidance/Predictability Assessment... Even as the period begins Saturday, there are some model/ensemble differences across the east-central U.S. with the upper trough and weak surface low. Among an overall slower trend in the guidance over the past day or two, GFS runs remain on the slower side of the envelope in tracking these features east compared to non-NCEP models and AI/ML models. The CMC may be a good middle ground in position as the ECMWF is on the eastern side. Into next week, there is an increasing model signal for this energy and the surface low to linger in the western Atlantic near the Outer Banks or so. The most recent 12Z ECMWF now shows this as well after more progressive earlier runs. With the Saturday-Sunday southern Canada/northern tier U.S. system resolved fairly well, the next feature of interest is the developing western U.S. upper trough. Multiple rounds of shortwave energy should arrive into the Northwest during the weekend into next week, with one or more upstream features possibly helping to deepen/reinforce the overall trough next week. This brings into play the typical low-predictability issue of when/how individual shortwaves will eject from the mean trough and generate well- defined surface waves east of the Rockies. Adding to this, a southern stream upper low will likely reach into the Southwest by early in the week and may contribute to low pressure development downstream. At least generally, models show a couple of rounds of central U.S. surface low development Monday and Tuesday with a northeast track by Wednesday but with low- moderate confidence in the details. Timing and strength of upstream energy diving through the west side of the trough next week is also uncertain. The 12Z ECMWF now clusters with the GFS runs in showing a closed upper low nearing or atop the Pacific Northwest by midweek. The WPC forecast was based on a blend of the 06Z GFS and 00Z ECMWF and CMC early in the period, and gradually increased the ensemble means component to half by Day 6 and over half Day 7 as the spread increased. This generally maintained good continuity with the previous forecast though with a slightly slower/lingering western Atlantic low. ...Weather/Hazards Highlights... As of Day 4 Saturday-Saturday night, the upper trough/surface system moving through the East will spread an area of rainfall across the southern two-thirds or more of the East. The Excessive Rainfall Outlook covering this period depicts a Marginal Risk area extending from northern Florida into the central Appalachians and parts of the Mid-Atlantic. There should be favorable moisture and instability within this area but so far the guidance, even as simple as ensemble probabilities of exceeding an inch, has not clustered particularly well for heaviest activity. The primary region of interest for the possibility of an eventual Slight Risk upgrade would be in the vicinity of northern Florida into southern Alabama/Georgia if guidance shows better clustering, given wet antecedent conditions and what signals currently exist for high rain rates. Some rainfall could linger along parts of the East Coast into Sunday if the slower side of the envelope verifies for the system affecting the region, but details remain very uncertain. Meanwhile the greater rainfall emphasis will shift into the central Plains/Midwest region around the start of the week, in association with shortwave energies ejecting from the mean trough and developing surface low pressure systems. During the Day 5 (Sunday- Sunday night) ERO period, guidance shows mixed signals regarding the magnitude/location of convective development within the central Plains/Midwest region. As some guidance indicates maximum amounts from Nebraska/southeast South Dakota to Minnesota while some are well south from Kansas to Missouri, continue to depict no risk area in the ERO at this point but expect that a Marginal Risk would be needed if a more coherent guidance signal arises. Another round of perhaps heavier rainfall amounts is likely over portions of the northern half of the Plains and Mississippi Valley into Monday-Tuesday of next week, after the ERO period. Meanwhile, portions of the West, especially northern areas, may begin to see scattered precipitation by the weekend or early next week and continuing into midweek as upper troughing settles over the region. Some precipitation may be in the form of snow in the high elevations of the northern Rockies. Expect South Texas to see multiple days of hazardous heat during the period with highs persistently running 10-15F above normal with max heat index values possibly reaching at least 110F. Highs near or over 100 degrees could stretch farther north across the southern High Plains at times. Southern Florida may also experience hazardous heat during the weekend with highs locally up to 5-10F above normal and similar max heat index values, followed by continued above normal but slightly less extreme readings. Both areas may see daily record highs. The southern two-thirds of the West will be quite warm on Saturday but then the developing upper trough will lead to a cooling trend, with northern parts of the West seeing near to below normal highs from the weekend onward and areas to the south trending to a mix of slightly above/below normal highs. Areas from the central/southern Rockies into the Great Lakes will tend to see above normal temperatures during the period. Clouds and rainfall should keep highs on the cool side over the Mid-Atlantic during the weekend. Tate/Rausch 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