####018006596#### FXUS02 KWBC 041831 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 231 PM 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... Latest 00z and 06z guidance continues to show good agreement for the overall synoptic pattern, though plenty of uncertainty still in the details which has impactful implications on QPF and surface features. A general compromise of the deterministic solutions offered a good starting point for the Tuesday-Thursday/Day 3-5 forecast. After this, there remain significant differences between model guidance and run to run continuity on how far south and east the lingering northern Plains energy gets into the Great Lakes/Northeast. Also uncertainty out west with the evolution of energy into the Great Basin and a blocky upper ridge/high to the north. The WPC blend trended towards more ensemble mean influence late period to mitigate these concerns. This is consistent both with previous WPC continuity as well as the ECMWF-initialized AI/ML models. ...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 into Day 5/Wednesday for this area given the potential for protracted heavy rain, but likely without widespread significant flooding impacts. 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 remains 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 remains in the vicinity of the Mid-South Day 5/Wednesday as energy interacts with a wavy and stalling front and combines with pooling moisture and instability. The Storm Prediction Center also highlights a broad area of the Midwest to Mississippi Valley/parts of the Southern Plains for severe weather potential Tuesday and Wednesday. Beyond Wednesday, a favorable pattern will continue 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, likely 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 Tuesday-Thursday before moderation by Friday and Saturday. Santorelli/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 $$