FXUS02 KWBC 260700 PMDEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Apr 26 2024 Valid 12Z Mon Apr 29 2024 - 12Z Fri May 03 2024 ...Potentially severe thunderstorms and heavy rain causing flash flooding are forecast to continue in the Lower Mississippi Valley Monday... ...Overview... Upper troughing will support a surface frontal system with widespread thunderstorms ahead of it across the central U.S. into Monday. Heavy rain is likely to cause flash flooding in the Lower Mississippi Valley in particular, while severe weather may also be a threat. The upper pattern looks to become more zonal by Tuesday- Wednesday, but uncertain shortwaves in the northern stream should continue to push frontal systems across the Lower 48 with additional showers and thunderstorms. More amplified troughing may come into the West by late next week, ahead of which near normal to warmer than normal temperatures will be the norm across much of the country from the Intermountain West eastward. ...Guidance/Predictability Assessment... Model guidance is reasonably agreeable at the start of the period Monday, with compact upper and surface lows in the north-central U.S. and upper ridging in the East while additional energy comes into the Northwest. By Tuesday-Wednesday, though the overall flow becomes more zonal, the northern stream jet shows some notable differences in the timing of those potent shortwaves coming into the northwestern to north-central U.S., leading to some out of phase differences. There are generally two camps for these features. The 12/18Z GFS and 12Z ECMWF, in a departure from their previous runs, indicate some energy feeding faster into the Northwest while some energy is held back over the Pacific. Most of the ECMWF-based AI/machine learning models are generally similar to this 12Z trend. However, some guidance like the CMC, UKMET, and the AI FourCastNet stick with a solution like the older GFS/ECMWF runs in holding most energy back and creating a deeper upper low in the eastern Pacific. The 06Z GFS-based AI Graphcast was also in this second camp, but the 12Z run flipped to more progressive initial energy. Ensemble members vary somewhat but do generally stick with their parent models more than preferred. The incoming 00Z CMC and UKMET still indicate more energy hanging back west than the GFS/EC camp, but are not as deeply aggressive with the eastern Pacific upper low as their 12Z runs. Overall, evidence was more compelling to prefer the GFS/ECMWF type of solution, even though this was a new trend. Models do still indicate troughing coming into the West by Friday with ridging ahead of it in the central U.S. and some troughing to the east of that, but with amplitude and exact positioning dependent on earlier forecast problems. Thus the WPC forecast was based on a blend heavily favoring the 12/18Z GFS and 12Z ECMWF, but gradually reducing the deterministic runs in favor of the GEFS and EC ensemble means especially by Day 7 given increasing model spread. ...Weather/Hazards Highlights... Ample moisture and instability will be present across much of the central U.S. into Monday ahead of the upper trough and a surface frontal system. The most likely focus for the heaviest convection and rainfall is across the Lower Mississippi Valley, where a Slight Risk is maintained in the Monday/Day 4 Excessive Rainfall Outlook. This may be a higher-end Slight Risk if the heaviest model QPFs of 3-5 inches with rain rates over 2 inches per hour end up verifying. By Tuesday the upper- level support is forecast to weaken, so overall lower rainfall amounts and rain rates are expected. However, some additional convection could develop, which could lead to flash flooding concerns especially if storms develop over areas that saw excessive rainfall amounts the previous day. A Marginal Risk will serve as a starting point to cover this threat on Tuesday/Day 5. There is also some potential for convection farther north from generally the central Plains into the Mid- Mississippi and Ohio Valleys. Model guidance is not agreeable with where any heavier rainfall might occur and these elements may be progressive anyway, precluding any ERO risk area there at this point. By midweek and beyond, additional rounds of rain and thunderstorms are likely for the central/southern Plains and Mid-/Lower Mississippi Valley as moisture surges ahead of frontal systems again. Elsewhere, multiple shortwaves will lead to rounds of precipitation across the northern tier. In the Northwest, some higher elevation snow is possible with this activity, with the most widespread coverage on Monday before snow levels may rise even further. Mainly rain is expected farther east across the northern Plains to Midwest/Great Lakes/Ohio Valley and into the Northeast/Mid-Atlantic at times. With the exception of the cooler West Coast and Northwest especially Monday-Tuesday, the bulk of the lower 48 will have near to warmer than average temperatures next week. An upper ridge over the East Monday-Tuesday should help produce temperatures around 10-20F above normal, with temperatures in the 80s possibly as far north as the Great Lakes region. Scattered daily record highs are possible. Temperatures are likely to gradually moderate but stay above average in the East until late next week. Meanwhile, parts of the central U.S. will also see above normal temperatures with anomalies around +10F Monday-Wednesday before cooling. The Intermountain West to Rockies on the other hand should gradually warm and see its greatest warm anomalies Thursday-Friday. 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 $$