Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed May 1 2024 Valid 12Z Sat May 04 2024 - 12Z Wed May 08 2024 ...Renewed heavy rain threat in the south-central U.S. this weekend... ...Overview... An upper low that is strong for this time of year is forecast to move into the West this weekend and track east into the Plains as next week progresses. This will support cooler than normal temperatures, gusty winds, and precipitation including higher elevation snow in the West. Ahead of this, a subtropical jet coming into the south-central U.S. and the western side of a front stalling will lead to rain and thunderstorms in the southern half of the Plains and Mississippi Valley this weekend, potentially in some areas that have received ample rainfall recently and/or will during the short range period. Rain and storms are also possible farther east. Then mean upper ridging downstream will lead to warming temperatures in much of the central and eastern U.S. into next week. ...Guidance/Predictability Assessment... The first model diagnostics concern during the medium range period is with the upper low that drops through the eastern Pacific and enters the West this weekend. Though models have been gradually converging over time toward a middle ground, there are still some remaining differences in the position and depth of the upper low. For multiple days now, GFS runs have showed the farthest southwest and deepest low/trough diving south and then east, while ECMWF runs have been shallow and generally fast. This continued in the 12/18Z cycle though with less dramatic differences than in earlier cycles. Given the convergence toward a middle solution and AI/machine learning models generally in the same ballpark, a blend of the 12Z GFS (which was less extreme than the 18Z GFS) combined with the 12Z ECMWF/CMC and some GEFS and EC ensemble means seemed to create the preferred intermediate solution. Fortunately the incoming 00Z model suite seems to finally be in better alignment with the timing and depth of the low at least through the weekend. The upper low is forecast to pivot east/potentially northeast into the north-central U.S. during the workweek, along with a reasonably deep surface low. Models continue to show spread with those features with the main outlier the 12Z CMC which was farther east. Additionally, upstream, yet another round of energy and troughing may dive through the Pacific Monday-Wednesday. The 12Z CMC was also quite aggressive with this energy. Overall favored a blend heavy on the GEFS and EC ensemble means along with some lingering components of the GFS and ECMWF late in the forecast period. ...Weather/Hazards Highlights... As the upper low tracks into and through the West, widespread precipitation is likely. Moisture streaming perpendicular into the foothills of the Sierra Nevada could cause a heavy rain and flooding threat there, and a Marginal Risk remains in place for Day 4/Saturday. Meanwhile the higher elevations of the Sierra, north into the Cascades, and east into the Intermountain West/Rockies will see some May snow. Precipitation is forecast to move steadily eastward through the weekend into early next week, reaching the northern Plains to Midwest Monday and Tuesday. The trough/low aloft and frontal boundaries could also produce some high winds across the Great Basin to Rockies. Meanwhile this weekend, the subtropical jet looks to be active coming into the south-central U.S., focusing convection in a moist and unstable environment. Many areas in the south-central U.S. will have quite saturated antecedent conditions because of recent and upcoming short range events, so this will be a factor for flooding as well. On Saturday and Sunday, Marginal Risks are delineated in the ERO across southern/central portions of the Plains and Mississippi Valley. Will also maintain an embedded Slight Risk on Day 4/Saturday for portions of southwest Oklahoma and into portions of northwest Texas where potential exists for an MCS with heavier rainfall rates, though the position varies somewhat between models. Farther east on Saturday, an axis of instability is forecast across the southern/central Appalachians and into the Piedmont between the synoptic cold front and backdoor front in the East as a shortwave aloft moves through. These ingredients could produce heavy rainfall and a Marginal Risk is in place there for Day 4 as well. Showers and thunderstorms should continue for parts of the eastern U.S. on Sunday. For the weekend, portions of the east-central U.S. are forecast to see above average temperatures, but the Northeast and Mid-Atlantic could be cooler than average at least for highs due to a backdoor front. Temperatures this weekend should warm in the Intermountain West to northern Plains ahead of the West upper low, while cooler than normal temperatures by 10-20 degrees for highs will impact the West. As the upper low/trough shifts east, this should cool down temperatures toward the north-central U.S. while staying below average in the Northwest. On the other hand, mean upper ridging will cause warm temperatures to spread across the south-central and east-central U.S. into the Eastern Seaboard. 90s are forecast to become more widespread across the southern tier, with temperatures over 100F into southwest Texas. 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