Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Tue Apr 23 2024 Valid 12Z Fri Apr 26 2024 - 12Z Tue Apr 30 2024 ...Severe thunderstorms and heavy rain possible for the Plains and Mississippi Valley late this week through this weekend... ...Overview... The upper level pattern through this weekend will feature mean troughing over the western U.S. while a ridge builds into the East Coast. This pattern will result in unsettled weather in the West and fairly benign weather for the East Coast. A couple of potent shortwaves/embedded upper lows will travel through the base of the western trough and eject northeastwards across the Central U.S. towards the Upper Midwest/Great Lakes, which will trigger rounds of active, and potentially hazardous, weather across portions of the Plains and Mississippi Valley. Early next week, a deep upper low will slide south along the western coast of Canada towards the Pacific Northwest, and the ridge that was situated over the East Coast will shift offshore into the Atlantic. ...Guidance/Predictability Assessment... Model guidance continues to show good agreement on the overall upper level pattern, but there is still some disagreement on finer scale details, which is to be expected in the medium range time period. The main features of interest will be the two upper level shortwaves that are forecast to bring impactful weather to the Central U.S. through the weekend. Agreement is high on the timing and location of the first shortwave Friday into Saturday, but model solutions show a bit more divergence with the second shortwave Sunday into Monday. The 12Z ECMWF is the quickest with the second wave, the 12Z UKMET is the slowest, and the 18Z GFS and 12Z CMC fall in between, closer to the ensemble means. A blend of the deterministic GFS/ECMWF/CMC/UKMET was used for the WPC forecast for Friday through Sunday to get a middle of the road solution close to the GFS and ensemble mean solutions. The 00Z GFS and ECMWF were not available at the time that the forecast was made; however, they remain consistent with the trends shown in their previous runs. After Sunday, there are some more significant differences in how the models handle the upper low moving down the West Coast, which has an impact on the evolution of downstream features as well. One way to mitigate these differences was to add ensemble means to the WPC blend to smooth out some of the differences. The CMC seemed to be an outlier compared to the other available guidance, so it was removed from the WPC model blend for days 6 and 7 (Monday and Tuesday). ...Weather/Hazards Highlights... Late this week, an upper level shortwave will move northeast across the Central U.S. and will be accompanied by a surface low pressure system. The surface low will move across the Central Plains and Upper Midwest on Friday, lifting a warm front north across the South-Central U.S. and pushing a cold front across the Central/Southern Plains and Middle/Upper Mississippi Valley. This system is forecast to produce showers and thunderstorms over portions of the Plains and Mississippi Valley, and the ingredients (instability and moisture) will be in place for locally heavy rainfall totals. A broad Marginal Risk area remains in place for portions of the Plains and Middle/Upper Mississippi Valley in the Day 5 (Friday) ERO. The southern tail of the trailing cold front is forecast to stall across the Southern Plains on Saturday while another surface low develops over the Southern/Central Plains, which will focus another round of heavy precipitation across the Southern Plains and Mid-Mississippi Valley. A Marginal Risk area was introduced for these regions in the Day 5 (Saturday) ERO. In addition to heavy rainfall, severe thunderstorms will be possible ahead of the dryline in the warm sector of this system, and the Storm Prediction Center has highlighted portions of the Plains and Mississippi Valley in the Severe Weather Outlook for Friday and Saturday. Isolated to scattered severe thunderstorms will be possible and may bring a threat of damaging winds, large hail, and tornadoes. On the western side of the dryline, strong winds and low humidity will create and increased wildfire risk for portions of New Mexico, west Texas, the Texas and Oklahoma panhandles, and far southeastern Colorado. Over the weekend, a second upper level shortwave will develop over the Southwest then push across the Central U.S. towards the Upper Midwest early next week. This system will bring another round of showers and potentially severe thunderstorms to portions of the Plains and much of the Mississippi Valley. Precipitation chances will also spread into the Ohio Valley and Northeast over the weekend as the leading system moves over the top of the East Coast ridge. By Monday-Tuesday, the East Coast ridge will move offshore into the Atlantic, and the trailing cold front associated with the second low pressure system will bring precipitation chances to the Gulf and East Coasts. In the West, mean upper troughing will allow precipitation chances to persist across much of the region through the end of this week, but drier conditions will likely develop over the Southwest early next week. Precipitation will fall as rain at lower elevations and as snow at higher elevations. Daily high temperatures will likely remain slightly below normal across the West, but highs may return to near normal in the Southwest Sunday through Tuesday after precipitation comes to an end. Temperatures will trend above normal across much of the south- central and eastern U.S. as a warm front lifts north later this week, and high temperatures will likely reach the 70s and 80s for much of these regions. Highs will likely reach the 90s and potentially over 100 degrees for some locations in south Texas by Saturday. Dolan 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