Extended Forecast Discussion NWS Weather Prediction Center College Park MD 255 PM 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 remains in reasonably good agreement through Day 3/Saturday before greater than typical spread increases through the remainder of the medium range period, particularly beyond Days 4/Sunday and 5/Monday. Regarding the evolution of the Four Corners/Central U.S. system this weekend into early next week, quite a bit of variability from cycle to cycle continues to stem from small scale changes within several low predictability northern stream shortwaves across the northern Plains and Upper Midwest. How models handle these waves and their interaction with the trough/closed low ejecting out of the Four Corners and southern Plains late Saturday into Sunday is directly impacting the downstream placement/track of a surface low/frontal system and its resulting QPF. The 00Z CMC, ECMWF, and UKMET actually showed decent agreement with the large scale pattern while the 00Z and 06Z GFS (and even to some degree the GEFS) were distant outliers. After Day 5/Monday, the pattern largely becomes quasi-zonal and dominated by more fast-moving shortwave troughs in the northern stream, which again lends to lower predictability than usual. Thus, for the first few days of the medium range period, a multi-model blend consisting mostly of the non-NCEP deterministic guidance from the 00Z/16 cycle was favored before transitioning to a majority of non- NCEP ensemble means for the latter half of the period. ...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 from eastern Oklahoma to the Ozarks, to extreme northeast Texas where the heaviest rain is currently forecast. Only minor adjustments were made to the Slight with this package given at least modest agreement of where the heaviest rain is expected to fall. Otherwise, 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 a fair amount of 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. Given the continued uncertainty, decided to maintain the Marginal Risk centered over Missouri and surrounding states for the Day 5/Sunday ERO period as the delineated area is still a solid compromise until models come into better agreement. 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. Miller/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