Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EDT Fri May 3 2024 Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024 ...Northern Plains to Midwest Heavy Rain Threat early next week... ...Possible heavy rain across the interior South later next week... ...Overview... Models and ensemble means continue to indicate a synoptic pattern that swings upper-level troughs/lows onshore across the western U.S. that would favor cyclogenesis over the mid-section of the country followed by gradual lifting of the troughs toward New England. The beginning of the medium-range period (Monday morning) will likely feature one such cyclogenesis over the central to 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 may 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 into later next week, still also under some influence from the lead upper low/trough. ...Guidance/Predictability Assessment... Model and ensemble solutions overall seem reasonably clustered for Monday-Wednesday and generally in step with National Blend of Models (NBM) guidance. This bolsters forecast confidence in a composite GFS/ECMWF/Canadian model blend to provide maximum system detail as consistent with feature predictability. Opted to edge toward the ECMWF and EC mean by Days 6 and 7 in favor of the more southern placement of the heavier rainfall across the interior South for next Thursday and Friday. WPC product continuity is decently maintained in this manner. The WPC forecast package was based on 40% blend of the 00Z ECMWF/EC mean, 40% from the 06Z GFS/GEFS, and 20% from the 00Z Canadian/GEM mean, increasing to 55% from the EC solutions by Day 7. ...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 Monday 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 Monday and Tuesday with some timing differences among models (faster with the ECMWF). 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 gradual 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 in the Day 4/Monday and into Day 5/Tuesday 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. The Storm Prediction Center also has portions of the Plains delineated with potential for severe weather on Monday. 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 5 as the fronts push farther toward the east and northeast. Upstream, aforementioned Pacific upper trough energy will work inland over an unsettled West and over the south-central U.S. into later next week, riding under the base of the lingering main upper low/trough. These will interact with wavy and stalling fronts and combine with pooling moisture and instability to fuel a favorable pattern for rain and thunderstorms across the south-central states/Mid-South and onward to the East where downstream energy track may support moderate late next week coastal low development. 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. Kong/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