Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Wed Apr 26 2023 Valid 12Z Sat Apr 29 2023 - 12Z Wed May 03 2023 ...Multi-day heavy rain threat to focus this weekend into next week over the Northeast and vicinity as a major closed low settles over the Midwest... ...Amplified pattern causes cooler than normal temperatures in the central/eastern U.S. and warmth in the West... ...Overview and Guidance/Predictability Assessment... Guidance still show differences with embedded system timings/strengths and interactions, but increasingly agree with the larger scale development and evolution of an highly amplified upper level flow pattern for North America, bolstering forecast confidence to above normal levels overall. The WPC medium range product suite was primarily derived from a composite blend of best clustered guidance from the 12 UTC ECMWF/UKMET/Canadian and GEFS/ECMWF ensemble means along with the 01 UTC National Blend of Models and WPC continuity. These guidance pieces overall now offer a bit more flow amplitude and the 12 UTC GFS/GEFS seems to be trending a bit more favorably versus the earlier 18 UTC versions. The broad blending process tends to mitigate the smaller scale guidance variances consistent with individual system predictability. ...Weather/Hazards Highlights... The progressive lifting of a leading southern stream upper trough and an associated low/frontal system will increasingly tap Atlantic moisture lifting from the Mid-Atlantic to the Northeast for the weekend with transition to moderate triple point/coastal low development. Expect a surge of wrap-back rains with the lows may produce some local runoff issues. A WPC experimental Day 4 Excessive Rainfall Outlook "Marginal Risk" area will be issued. 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 this weekend. Plan to issue an experimental Day 5 WPC Excessive Rainfall Outlook "Marginal" threat area for north-central Florida and vicinity, given instability should be in place that could help create high rain rates as strong Gulf convection ahead of the main wave slams inland. 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 through at least early-mid next week, with troughing extending into the east-central U.S. and producing a very 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, this ejecting southern stream system and enhanced moisture and instability would likely lift robustly up the East Coast around the now major Midwest closed low/trough to support heavy rainfall that may linger/wrap-back across the Northeast well into next week in wet flow that could produce widespread runoff issues given multi-day deep Atlantic moisture influx and slow pattern translation. Accordingly, WPC will also issue a Day 5 "Marginal Risk" ERO for the northern Mid-Atlantic/Northeast. This has also been collaborated with local NWS WFOs and will be monitored for any ERO risk upgrade given favorable ingredients and duration. 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 toward the Rockies Saturday-Wednesday. This will support much above average temperatures with some record values possible. Amplifying eastern Pacific upper trough energies are expected to mainly reinforce/dig southward so the bulk of moderate precipitation/cooling on the western periphery of the ridge should primarily hold over/off the West Coast. 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