Extended Forecast Discussion NWS Weather Prediction Center College Park MD 422 PM EDT Thu Apr 20 2023 Valid 12Z Sun Apr 23 2023 - 12Z Thu Apr 27 2023 19Z Update: The 12Z model guidance suite is in good agreement on the synoptic scale pattern going into the upcoming weekend with only some minor mesoscale differences noted, so a multi-deterministic model blend suffices as a starting point in the forecast process Sunday and even going into Monday. Going into early next week, differences with a shortwave originating from the northeast Pacific introduce uncertainties across the Pacific Northwest and then across the Rockies and Intermountain West going into Tuesday. The GFS and its ensemble mean have now trended more amplified with this trough and closer to the CMC/ECMWF compared to earlier runs, although the CMC/ECMWF now have a closed low whereas the GFS has more of an open wave aloft. Pattern recognition supports the idea of a more amplified solution across this region, and therefore the forecast was hedged slightly more towards the CMC/ECMWF/ECENS for this trough as it crosses the Rockies and then the Plains, with some previous WPC continuity included for days 6 and 7. Noteworthy model spread is also evident across the Northeast U.S. with the CMC faster in taking the closed low offshore compared to the GFS/ECMWF solutions, similar to yesterday's runs. In terms of the excessive rainfall outlook, a small Slight Risk area was introduced for portions of eastern New England on Day 4 (Sunday) to account for a more unified signal in the 12Z guidance for a concentrated QPF maxima centered near southwestern Maine, with strong onshore flow and orographic ascent. Otherwise, no major changes from the previous WPC forecast were warranted, and the overnight forecast discussion is appended below for reference. /Hamrick --------------------- ...Overview and Guidance/Predictability Assessment... An upper-level low will remain quasi-stationary over the Midwest and Northeast early next week, before weakening and splitting beneath a strengthening downstream ridge. Part of that energy may absorb back into a north-central Canadian trough while the rest may shift into the Canadian maritime mid-to-late next week. There's reasonable agreement between the operational models regarding the evolution of this upper pattern. Where there's more uncertainty is in the West. The latest guidance suggests that a shortwave trough will amplify over the Northwest early next week before propagating into the Plains mid-week and into the East by the latter part of the work week. As with recent forecasts, a model blend weighted more toward the EC/ECENS/CMC was used to mitigate some of the muted and progressive signals displayed by the GFS/GEFS. A ridge will arrive along the West Coast next Tuesday/Wednesday, the strength of which may influence the development of the West/Central U.S. trough. Elsewhere, shortwave energy preceding the amplified western trough may swing across the Gulf Coast states early-to-mid-week. ...Weather/Hazards Highlights... A pair of low pressure systems associated with the quasi-stationary mean upper-low stationed over southeastern Canada will move through the Northeast next Sunday through Tuesday. Rain showers are expected with some areas of focused heavy rainfall possible, particularly over portions of Vermont and New Hampshire, where up-sloping and the presence of the surface wave may support enhanced rates. A pair of marginal risk areas, encompassing much of New England, were drawn to account for this threat, although the progressive nature of the system should keep any flash flooding concerns low. Both systems will move into the Canadian maritime and Atlantic by next Tuesday, leaving slightly cooler temperatures in their wake from the Midwest to the Northeast. Temperatures remain spring-like through the remainder of the medium range period. A low pressure system in northeastern Mexico and inverted troughing into Texas with southeasterly flow into the state will support showers and thunderstorms over across the region on Sunday. Some heavy rain rates developing inland and propagating eastward into the Gulf supported by a bit of weak upper-level energy could produce isolated instances of flash flooding over segments of southern Texas. The 00z GFS and 12z EC diverge with respect to the axis of heaviest precipitation with the GFS signaling a northern track and the EC suggesting a more southerly solution. The day 4 marginal risk area for southern Texas was drawn with that in mind. Temperatures are likely to drop well below normal (15-20 degree anomalies) between Sunday and Monday over much of Texas due to high pressure over the Great Plains causing cool air to advect into the region. The next system to enter the West will bring rain showers to the region and snowfall to the highest elevations of the Cascades and Northern/Central Rockies between Sunday and Tuesday. Showers and thunderstorms are then forecast to develop across the Great Plains and Mississippi Valley on Tuesday as the system reorganizes over the Central Plains. An ascendant warm front will become the focus for rainfall across the Lower Mississippi Valley and Southeast next Tuesday before shifting into the Central/Southern Appalachians and Mid-Atlantic by next Wednesday into Thursday. Kebede 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 - Heavy rain across portions of the Southern Plains, Sun, Apr 23. - Heavy rain across portions of the Central Plains, the Mississippi Valley, and the Southern Plains, Mon-Wed, Apr 24-Apr 26. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern Plains, Tue, Apr 25. - Heavy rain across portions of the Southeast, the Lower Mississippi Valley, the Southern Appalachians, and the Tennessee Valley, Wed, Apr 26. - Heavy rain across portions of the Northeast, Sun-Mon, Apr 23-Apr 24. - Severe weather across portions of the Southern Plains, Tue, Apr 25. - Flooding possible across portions of the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Lower Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Much below normal temperatures across portions of the Plains and Mississippi Valley, Sun, Apr 23. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Sun-Tue, Apr 23-Apr 25. - Much below normal temperatures across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Appalachians, the Great Lakes, and the Ohio Valley, Sun-Mon, Apr 23-Apr 24. 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