Extended Forecast Discussion NWS Weather Prediction Center College Park MD 304 PM EDT Mon Apr 29 2024 Valid 12Z Thu May 02 2024 - 12Z Mon May 06 2024 ...Overview... A heavy rain and flash flooding threat persists over the southern Plains Thursday night as an advancing trough over the northern Plains shifts the heavy rain focus to to the Midwest. The heavy rain risk then spreads to the Gulf Coast and Deep South Friday. The next heavy bout of heavy rain overspreads the southern Plains Saturday into Sunday. Low pressure moves toward or over the West Coast this weekend, though great uncertainty on track/progression persists on if this impacts the Northwest only, or will California receive significant precipitation. ...Guidance/Predictability Assessment... The exit of low pressure from the northern Plains with its effects to the northeast through the weekend and the closed low that swings southeast from the Aleutians Wednesday to a point off the PacNW late Friday are the key upper level low pressure features for Days 4-7. The Northwest low has the greatest uncertainty with the 00Z ECMWF tracking a more open wave east over the northern tier to MN through Monday, while the 06Z GFS and 00Z CMC kept a much deeper low off the coast, tracking south to northern CA by late Saturday with reinforcing troughs keeping longwave troughing over the West into the weekend. The 00Z UKMET was more in between with this feature with a low tracking into the Great Basin. However, the 00Z UKMET was much different with the central U.S. low/trough and was not included in the forecast blend from this morning. Now onto 12Z guidance for today, GFS still tracks the low into CA, remaining the farthest west/slowest with this feature while the CMC took a more eastward track, not digging as far south over the weekend. The UKMET is a bit farther west/dug a little farther south by 00Z Sunday (it still only has a 5-day forecast). And the 12Z ECMWF coming in is farther west/slower with the low as it approaches/moves into the Northwest. 12Z guidance generally supports the GFS/CMC solution of a more cutoff/stalled low. The morning WPC blend favored the 06Z GFS/00Z CMC by Day 5 for the forecast pressure/fronts and especially for QPF, though continuity with the previous forecast was achieved with the GEFSmean for QPF in the PacNW. Also for QPF, the late Saturday into Sunday southern Plains rains which were closer to a 06Z GFS/00Z CMC blend than the 13Z NBM, providing focus for repeating heavy rain concerns there. The coverage of the precip footprint from the central through eastern U.S. Thursday night through Sunday in the 00Z ECMWF-AIFS (AI guidance tends to feature the footprint of QPF well, though it is often way low for magnitude in that footprint) lined up well with the 06Z GFS, so the 06Z GFS was incorporated into the QPF for that time and place. ...Weather/Hazards Highlights... The Day 4 (12Z Thursday to 12Z Friday) Slight Risk over the Central U.S. was trimmed a bit from the west given timing confidence increase in the global consensus while allowing for ongoing heavy rain from Wednesday to be accounted for and an expansion in to the Chicago area given the 12Z GFS increase there. A focus for more significant impacts remains from North Texas through eastern Oklahoma and western Arkansas for heavy rain on top of wet antecedent conditions. Should this trend continue, embedded upgrades within the Slight Risk may be needed. By Friday, the cold front progressing east in the northern two- thirds of the country or so will shift rain chances to the Great Lakes and Ohio/Tennessee Valleys, and toward the Appalachians to Eastern Seaboard on Saturday. However, the front lingers over the southern Plains and produce additional rounds of rain and storms through the weekend. A Day 5 (12Z Friday to 12Z Saturday) Marginal Risk is expanded from far east Texas into the Deep South based on the latest QPF consensus for heavier rainfall and (at least in Texas) wet antecedent conditions. Higher elevation snow can be expected wherever this low tracks into the West this weekend. The eastern third of the country will be warmer than average by 10-15 degrees through Friday, with 80s persisting over the Ohio Valley and Mid-Atlantic. Meanwhile cooler than average temperatures are forecast across the Rockies and northern Plains. These cooler temperatures will push east behind a cold front and help moderate temperatures a bit in the East, though generally remaining a few degrees above normal. Temperatures across the Rockies to Plains are forecast to warm up into early next week due to upper ridging ahead of the Pacific trough while temperatures on the West Coast likely become cooler than normal this weekend, particularly under the upper low. Jackson 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