FXUS02 KWNH 030635 PREEPD Extended Forecast Discussion NWS Weather Prediction Center College Park MD 234 AM EDT Thu Apr 3 2025 Valid 12Z Sun Apr 06 2025 - 12Z Thu Apr 10 2025 ...Overview... The frontal boundary responsible for significant and potentially historic flooding and flash flooding during the short range period should finally push east by Sunday. This happens from a gradually amplifying Great Lakes to Northeast trough and eventual ejection of southern stream energy. Some moderate to heavy rainfall potential along the central Gulf Coast and parts of the Southeast with this front. A shortwave reaching the West on Sunday-Monday could bring mostly light precipitation to parts of the Northwest. After Monday, the medium range period should trend much drier across the country as the overall pattern briefly becomes flatter and more progressive on the way toward a western ridge/eastern trough configuration. ...Guidance/Predictability Assessment... Overall, the latest guidance shows good agreement on the larger scale pattern, but plenty of uncertainty still in the details of smaller scale systems/energies. There are still some initial questions on how fast southern stream energy ejects eastward on Sunday-Monday, with the GFS still on the slightly faster side of consensus. Elsewhere, latest GFS runs remain most aggressive with the degree to which the digging Great Lakes trough closes off an upper low by Tuesday, leading to a stronger/more wrapped surface system. The CMC also shows this to some extent as well, but the better consensus of models would favor more of an open trough, with some potential for a neutral or negative tilt at times. Mean troughing over the northeast Pacific should send a couple of shortwaves into the West, which could eventually reach the Plains by midweek. Both the GFS and ECMWF show a modest reflection of a stronger system with a defined surface low, but the CMC is much weaker. Ensemble means are also weak/flat which indicates there is still a lot of uncertainty with this. The WPC forecast for tonight favored a deterministic model blend for the first half of the period, increasing the ensemble means to half the blend by Day 7 to mitigate differences in the guidance. This maintains good continuity with the previous WPC forecast, at least through Day 6. ...Weather/Hazards Highlights... By the start of the period on Sunday, the front responsible for significant rainfall over the Lower Mississippi Valley to Ohio Valley region should finally begin to shift eastward. Moderate to heavy rain and strong storms are possible along the front from the Gulf Coast states into the Tennessee Valley/Southeast, and farther north into the Mid-Atlantic and Northeast near the front. The Sunday ERO shows a Marginal Risk for this region with an embedded Slight Risk farther south across parts of southern Alabama and Mississippi where recent heavy rainfall has primed soils and the greatest instability should be located. Guidance shows potential for a band of somewhat heavy QPF to extend farther northeast across northern Georgia into or near the far southern Appalachians, but latest first-guess fields plus neutral ground moisture (and likely drier by then given no rain forecast in the shorter term) favor keeping this region in Marginal Risk for the time being. The front may slow for a period across parts of the Southeast into Monday, and anomalous moisture and instability favor just a marginal risk for the Day 5 ERO at this point since current QPF signals are modest at best. Elsewhere by Monday, a cold upper trough reaching the Great Lakes should lead to unsettled weather over the region with rain or snow. This precipitation should extend into the Northeast as well. Latest guidance shows significant differences for surface/upper evolution over the Northeast early- mid week, with the more extreme side of the spread producing some significant snowfall over parts of New England in contrast to much lighter and scattered snow in the greater proportion of solutions. Probabilities of 0.25 inch liquid in the form of snow reach no higher than 10-15 percent or so during the Tuesday-Tuesday night period, reflecting the guidance majority. Meanwhile upstream, the upper shortwave/front expected to reach the West Coast into early next week will be accompanied by a brief increase in moisture but the system will be fairly progressive. Thus expect mostly light precipitation but with some terrain- enhanced moderate activity over western Washington/Oregon and northwestern California, with decreasing amounts inland over the Northwest. Above normal temperatures will linger along the East Coast into Sunday with daytime highs 10 to 20 degrees above normal.Meanwhile, well below normal temperatures will be in place across parts of the southern Rockies and Plains where daytime highs 10 to 20 degrees below normal may linger into Sunday. Moderated below normal temperatures will also shift into the central U.S. and Midwest Monday and the East by Tuesday-Wednesday. Highs over and near the Ohio Valley could be up to 15 to 20 degrees below normal on Tuesday. Warmer than average temperatures initially over the Northwest this weekend should expand into the remainder of the West and the northern/central Plains by next week. A strengthening upper ridge will likely expand coverage of plus 10 to 20 degree anomalies over the West on Wednesday and Thursday. Santorelli 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 $$