Extended Forecast Discussion NWS Weather Prediction Center College Park MD 248 PM EDT Tue Apr 25 2023 Valid 12Z Fri Apr 28 2023 - 12Z Tue May 02 2023 ...A couple of rounds of heavy rain are possible in the East... ...Amplified pattern causes cooler than normal temperatures in the central/eastern U.S. and warmth in the West... ...Overview and Guidance/Predictability Assessment... Models and ensembles generally agree upon an amplified pattern across the contiguous U.S., with multiple rounds of energy coming in late this week forming a mean closed upper low over the Great Lakes early next week, while a strong upper ridge over the West Coast late week gets squeezed a bit eastward into the Interior West as eastern Pacific troughing develops. There are still some model discrepancies in the details of the flow that then influence model QPF and other sensible weather elements, but generally within range of normal spread for the medium range period. One such model difference was that the 00Z GFS showed a faster track of energy dropping south through the central High Plains Friday and ejecting eastward as a southern stream trough or closed low. Stuck closer to the 00Z ECMWF and CMC and 06Z GFS better consensus. Leaning this way seems good since the newer 12Z guidance has generally come in with an even slower southern stream low/trough by the weekend. Did prefer to use these deterministic models through day 5/Sunday as they showed this feature separate from the northern stream upper low formation over the Midwest/Great Lakes, unlike the ensemble means. But incorporated more ensemble mean proportion for days 6-7 as spread increased in the West in particular, and leaned toward the slower trough axis of the CMC/ECMWF suites as at least the 00Z GEFS mean and especially the GFS itself were east of consensus including most ensemble members. There remains quite a bit of variability in terms of closed low potential and position within the overall trough. ...Weather/Hazards Highlights... The progressive ejection of a leading southern stream upper trough and an associated main low/frontal system will tap moisture from the Gulf of Mexico into the Southeast then increasingly from the western Atlantic up across the Mid-Atlantic Friday into Saturday with transition to moderate triple point/coastal low development. A swath of convection with the cold front across the Southeast/southern Mid-Atlantic and more protracted wrap-back rains with the lows may produce some local runoff issues. A WPC experimental Day 4 Excessive Rainfall Outlook Marginal Risk area remains in place for much of the Mid-Atlantic and parts of the Carolinas. Rain totals may be fairly high but relative lack of instability may limit rain rates. Wedged high pressure dammed into the region from the Northeast may prove slow to retreat in this pattern. Meanwhile, upper trough energies rounding the top of the upper ridge over western Canada should dig sharply southward through the Plains to produce a separate to closed southern stream system whose subsequent track and wave development would bring another swath of convective rains across the southern Plains and perhaps (if not offshore) into the Gulf Coast states Friday into the weekend. This activity may need a risk area for excessive rainfall if models start to agree better on position of heavy rain totals, given instability should be in place that could help create high rain rates. Potent energies by then also dig more earnestly to the lee of the amplified ridge to carve out a massive and slow-moving closed upper low/trough to settle over the north-central U.S then Great Lakes/Midwest this weekend-Tuesday, with troughing extending into the east-central U.S. and producing a cool and unsettled pattern. Cold Canadian air surging down through the central to east-central U.S. should provide some much below normal temperatures. Meanwhile, the ejecting lead southern stream system and enhanced moisture and instability would likely lift robustly up the East Coast around the now main Midwest closed system to support heavy rainfall that may linger/wrap-back across the Northeast into early next week in wet flow that could produce widespread runoff issues given multi-day deep Atlantic inflow and slow pattern translation. Upstream, there remains a strong guidance signal for the pattern to significantly amplify as a stabilizing and warming upper ridging builds and shifts gradually eastward from the West Coast to across the Intermountain West Friday-Tuesday. This will support much above average temperatures with some record values possible. Upper trough energies and most precipitation/cooling on the western periphery of the ridge should mainly hold over/off the West Coast during this period. Tate/Schichtel 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, experimental excessive rainfall outlook, winter weather outlook probabilities and heat indices are at: 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