####018006418#### FXUS02 KWBC 040646 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EDT Sat May 4 2024 Valid 12Z Tue May 07 2024 - 12Z Sat May 11 2024 ...Northern Plains to Midwest Heavy Rain Threat early next week... ...Mid-South Excessive Rainfall Threat into next Midweek... ...Overview... It remains the case that recent models and ensembles indicate a synoptic pattern that swings upper-level troughs/lows onshore across the western U.S. that lead to and subsequently favor downstream upper focus and cyclogenesis over the northern mid- section of the country followed by gradual lifting of the troughs toward the Northeast. The beginning of the medium-range period (Tuesday morning) will likely feature one such cyclogenesis over the northern High Plains. This will support cooler than normal temperatures, gusty winds, and precipitation including higher elevation snow in the West early in the week. Rain that could be locally heavy shifts into the northern Plains and downstream through the Midwest. Meanwhile farther south, mean upper ridging will lead to warming pre-frontal temperatures for much of the central and eastern U.S. that could yield some record values. Upstream, Pacific upper trough energy will work inland over an unsettled West and to the south-central U.S.. This is likely to interact with a wavy and stalling lead front and combine with pooling moisture and instability to fuel rain and thunderstorms over the south-central states/Mid-South and onward/eastward into later next week, still also under some influence from the lead upper low/trough. ...Guidance/Predictability Assessment... 18 UTC GFS and 12 UTC ECMWF/Canadian model solutions were well clustered for Tuesday-Thursday and generally in step with ensembles and National Blend of Models (NBM) guidance. This bolsters forecast confidence in a composite model blend to provide maximum system detail as consistent with a period with above normal predictability. Gravitated guidance preference toward ensemble means into later next week, and opted to edge toward the tad more robust ECMWF ensemble mean. WPC product continuity is decently maintained with this plan. Latest 00 UTC guidance remains in line. ...Weather/Hazards Highlights... As a main/closed upper low tracks out of the West with additional energies reaching the West Coast, widespread precipitation will include some enhanced snow potential given cooled temperatures. The higher elevations of the Great Basin into the Intermountain West/Rockies will see May snow as the medium-range period begins Tuesday morning. Vigorous troughing aloft in conjunction with a potent cold front will lead to windy conditions and a threat of high winds from much of the Rockies to High Plains as the cyclogenesis should be well under way across the northern High Plains into Tuesday. Precipitation is forecast to be ahead of the system as it spreads across the northern Plains to Midwest Tuesday into midweek. Rainfall on the west side of the strong surface low could be heavy across eastern Montana into the Dakotas given strong upper-level support for lift with a slowdown of the storm track. However, this region is not particularly sensitive to flooding concerns, and instability should be limited wrapping around the low. A WPC Excessive Rainfall Outlook (ERO) Marginal Risk area remains in place Day 4/Tuesday and now into Day 5/Wednesday for this area given the potential for protracted heavy rain, but likely without widespread significant flooding impacts. The Marginal Risk extends southeast into the Upper/Mid-Mississippi Valley, where more instability will be in place for higher rain rates, but storms will likely be faster-moving early week. The Storm Prediction Center also has portions of the Plains and Mississippi Valley delineated with potential for severe weather early through midweek. Convective rainfall is forecast to shift east into the Great Lakes and Ohio Valley Tuesday into Wednesday with system progression and intercepting return flow. The Marginal Risk ERO was maintained farther east into the Great Lakes, Ohio Valley and down toward the mid-Mississippi Valley for Day 4/5 as the fronts push farther toward the east and northeast. A Slight Risk threat area has been introduced for areas in vicinity of the Mid-South Day 5/Wednesday as energy interacts with a wavy and stalling front and combines with pooling moisture and instability. This will fuel a favorable pattern for rain and thunderstorms across the south-central states/Mid-South and broadly onward to the East where downstream energy track may support moderate late next week coastal low development, likley off New England. Much cooler than average maximum temperatures will slowly moderate while spreading next week from the West to the northern Plains as the closed upper low ejects eastward. But farther southeast, mean upper ridging will meanwhile spread quite warm pre-frontal temperatures from the South through the Appalachians/Mid-Atlantic to threaten record seasonal values. 90s are forecast to become more widespread across the southern tier, with temperatures over 100F (and even higher heat indices) into south Texas and the possibility for the first excessive heat days of the year. Schichtel 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 $$