Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Wed Apr 16 2025 Valid 12Z Sat Apr 19 2025 - 12Z Wed Apr 23 2025 ...Heavy rainfall/flooding and severe weather in parts of the central U.S. this weekend... ...Overview... At the start of the medium range period Saturday, positively tilted upper troughing will be in place from the north-central U.S. to the Four Corners, pivoting east into Sunday. Ample moisture inflow ahead of this trough and an elongated frontal system will allow for heavy rain with potential flooding as well as strong to severe thunderstorm chances stretching from the southern Plains into the Lower/Middle Mississippi Valley and portions of the Ohio Valley this weekend. Snow is likely to linger in the Colorado and New Mexico Rockies Saturday on the backside of the system, while warm temperatures are likely in the East ahead of the cold front. Modest rainfall should shift into the east-central U.S. by Monday as the front moves through, and by Tuesday some moisture may return to the central U.S. for additional rain. ...Guidance/Predictability Assessment... Model guidance starts reasonably agreeable on Saturday with the north-central to southwestern U.S. troughing. However, models quickly diverge with the timing of the trough(s) pivoting east even by Sunday. Interactions of energy in the northern and southern streams of the trough play a part in this. For example, the 12Z ECMWF was quite aggressive in showing a small closed low in the northern stream trough by Sunday atop northern Minnesota, shaping the trough differently and speeding up the surface low track a bit. Meanwhile the 12/18Z GFS are fast to pivot the trough and resultant surface low (and QPF). Despite the dynamical GFS and ECMWF on the faster side, the AI models continue to be considerably slower. Somewhat unusually, preferred the CMC/UKMET upper trough and surface low track in terms of the deterministic models, as they tended to agree with the ensemble means the best, and were in between the faster EC/GFS and slower AI models. The 18Z GFS in particular then showed the upper trough center/possible closed low tracking farther east across the Ohio Valley by Monday rather than the pivot northeast that non-NCEP guidance showed. The newer 00Z GFS remains fast and then farther east than consensus with the upper and surface low, though it evolves that way somewhat differently with a northern stream connection. The 00Z CMC and UKMET still seem reasonable, while the 00Z ECMWF looks better than the 12Z run with the trough and low since it no longer has the unusual northern stream upper low. Most guidance shows these features lifting north and weakening into the workweek. The upper pattern looks to flatten out overall, but there could be some troughing in the Northwest by Monday with some model spread in its evolution of tracking more south or more east into Tuesday-Wednesday. The 12Z CMC was the strongest with diving energy southward compared to consensus that skirted the trough eastward. So did not prefer the 12Z CMC in the West, and the 00Z CMC looks more consistent with other models. The WPC forecast used a multi-model blend favoring the CMC for the early part of the period, but quickly increased the proportion of GEFS/EC ensemble means as the period progressed. The blend was half means by Day 5 and all means by Day 7 as model spread increased. ...Weather/Hazards Highlights... On Saturday, the slow pivot of the base of the upper trough, a consolidating frontal system with some west-east orientation to promote possible training storms, and precipitable water anomalies over the 90th percentile are likely to cause widespread heavy rain of 3+ inches, with locally higher amounts. A Slight Risk of excessive rainfall stretches from north-central Texas across portions of Oklahoma, Arkansas, Missouri, and far southeast Kansas and southwestern Illinois in the Day 4/Saturday ERO. This is considered a higher end Slight Risk in eastern Oklahoma to the Ozarks where the heaviest rain is currently forecast. A broader Marginal Risk surrounds the Slight and is drawn across much of the Ohio River where streamflows remain high. The Storm Prediction Center forecasts severe thunderstorm potential across the southern Plains through Arkansas into the Lower Ohio Valley on Saturday as well. By Sunday there remains some model spread in the timing and placement of the upper and surface lows and thus where rainfall will focus, but broadly rain and thunderstorm chances are likely from the southern and central Plains into much of the Mississippi Valley. Have delineated a Marginal Risk centered over Missouri and surrounding states for the Day 5/Sunday ERO period, but stay attuned to the forecast as this could shift around if and when models converge better on heavy rainfall placement. At least it seems likely that rainfall totals will be lower on Sunday compared to Saturday as the low/frontal system start to move more. Severe weather is possible from eastern Texas into the Lower Mississippi Valley on Sunday per SPC. With this weekend's rain event, the heaviest rainfall totals are likely to be west of the most sensitive areas of the Mississippi Valley and Ohio Valley from recent flooding. However, some rain atop recently flooded areas, and runoff from other areas into these rivers, could exacerbate ongoing flooding issues in some areas, so continue to monitor. By Monday, rain chances are forecast to spread into the east-central U.S. as the front pushes east, but with generally lower amounts. Then by Tuesday, there remains uncertainty in the pattern, but generally an increase in rain chances seems possible in the central U.S. again in the vicinity of fronts. Farther west, snow is likely to continue in the central/southern Rockies on Saturday before precipitation pulls east on Sunday. Farther north, a possible upper trough could support precipitation in the Northwest this weekend and persisting into Monday, including higher elevation snow in the northern Rockies. Above normal temperatures by 10-20 degrees will be likely from the southeast into the Eastern Seaboard on Saturday with upper ridging ahead of the main trough, raising temperatures well into the 70s into parts of New England. On the other hand, colder than normal temperatures especially in terms of highs are expected for the Four Corners states into the central Plains Saturday. Temperatures should warm closer to normal Sunday in the central U.S., and meanwhile cool in the Northeast where a cold front comes through. Above average temperatures could continue across the southeastern U.S. into early next week though. Periods of slightly below normal temperatures in the Northwest and slightly above normal temperatures from the central Great Basin to central U.S. are possible into next week, but temperatures in most places should be pretty close to average. 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