Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 PM EDT Sun Apr 21 2024 Valid 12Z Wed Apr 24 2024 - 12Z Sun Apr 28 2024 ...Low pressure systems in the Central U.S. should lead to severe weather and heavy rain for the latter part of the week... ...Overview... During the forecast period from midweek into next weekend, rounds of upper troughing and surface lows will lead to a wet pattern for the Western and Central U.S., including potentially severe thunderstorms and heavy rain across the Plains and Mississippi Valley and springtime snow for the higher elevations of the Intermountain West. Meanwhile in the East, some upper troughing will push through a cold front or two on Wednesday and cause some rain/snow before surface high pressure behind dries out the region. These features will lead to below normal temperatures in the northeastern quadrant of the country Wednesday-Friday before warming next weekend. ...Guidance/Predictability Assessment... For Wednesday-Friday in the East, the main feature of note will be a trough pushing through with an embedded upper low. Fortunately models have converged better with the timing and depth of this feature, as recent GFS runs have transitioned to a less amplified and faster feature. Thus there was less QPF (including snow) lingering in the Northeast compared to the previous forecast. In the Western to Central U.S., there are multiple upper-level and surface features of concern. Southern stream troughing pushing from the eastern Pacific across California into the Four Corners states and then northeastward into the central Plains and Upper Midwest late week shows reasonable agreement. The associated surface low track is also in okay alignment, with the ensemble means in between the slightly farther northwest 00Z ECMWF/CMC and slightly farther southeast GFS runs, within typical spread for the medium range period. More model differences arise with secondary energy/troughing diving into the West Friday into the weekend. Though the 00Z/06Z operational GFS and 00Z ECMWF looked pretty agreeable with each other with a deep and west trough axis, the full suite of guidance suggested that these were too amplified. Ensemble means were less amplified, and looking at the individual members showed that these deterministic models were both deeper than most if not all the members. Additionally, the available AI/machine learning models showed a less amplified and farther east trough axis. The CMC was more reasonable with the trough position. The 12Z models fortunately have come in with a faster and shallower trough as expected. Upstream across the northeast Pacific, 12Z models are also more agreeable in indicating an upper low by next Sunday after a lot of spread in the 00Z/06Z runs. Thus the WPC forecast began with a blend of the deterministic guidance, and transitioned to a blend favoring the ensemble means over half by the late period, while reducing the deterministic components especially from the ECMWF and GFS. ...Weather/Hazards Highlights... The upper trough and cold front passing through the Northeast Wednesday will be accompanied by widespread light to moderate rain along with high elevation snow for portions of northern New England. Farther west, warm air advection across the south-central U.S. will bring a return to scattered showers and storms across this region, with some potential for enhanced rain totals. Recent models indicate the axis of heaviest rain (1-2 inches) crossing from northeast Oklahoma into northwest Arkansas. However, did not feel there was enough run-to-run consistency in placement nor confidence in impacts to delineate an Excessive Rainfall Outlook for Day 4/Wednesday. More widespread showers and storms are then expected to develop across portions of Kansas, Nebraska, Iowa, and Missouri going into Thursday (Day 5) as moisture and elevated instability increase with surface low pressure system development across the Central Plains. A Marginal Risk area in the ERO seems prudent and there were minimal changes to the previous issuance. The Storm Prediction Center is also watching for severe weather ahead of the dryline with the low pressure system, with outlooks for parts of the southern half of the Plains on Thursday and parts of the central Plains and Mississippi Valley on Friday. On the backside of the dryline, conditions will be favorable for fire development in the southern High Plains Thursday-Friday. By late week into the weekend, showers and storms should progress into the Ohio Valley and Great Lakes and portions of the Eastern Seaboard, while also being renewed in the Plains/Mississippi Valley. Multiple rounds of energy moving through the West will help to produce precipitation chances there. With cooler air in place given upper troughing, snow will be possible across the higher elevations including the Cascades/Sierra Nevada across parts of the Intermountain West into the northern/central Rockies. Much of the West should have above normal temperatures on Wednesday, before returning to near or slightly below normal levels for much of the West Coast and Intermountain West to close out the week, as the mean upper trough encompasses the region. Warmer temperatures and increasing humidity are forecast for the central/southern Plains by Friday and beyond as the upper ridge axis builds back, and southerly flow from the western Gulf advects moisture northward ahead of the developing surface low. This spreads northeastward to include the Ohio Valley and the Appalachians, with more widespread above normal readings for overnight lows (+10 to +20 degrees) with conditions more typical of May or early June by next weekend. Highs could get over 100 degrees for portions of southern Texas by Saturday. Tate/Hamrick 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