Extended Forecast Discussion NWS Weather Prediction Center College Park MD 328 PM EDT Thu Apr 13 2023 Valid 12Z Sun Apr 16 2023 - 12Z Thu Apr 20 2023 ...Overview with Guidance/Predictability Assessment... The medium range period will begin Sunday morning with a low pressure system likely tracking into the Great Lakes. Model consensus over the past couple of days has been trending toward a slower northeastward progression of the system across the Great Lakes on Sunday and then into southeastern Canada early next week. The 00Z CMC was the slowest piece of guidance but still within reason. The slower system progression has also slowed down the cold front and the associated precipitation forecast to move off the East Coast on Monday. Meanwhile, models have generally shown decent agreement on timing the large-scale trough going through the western U.S. early next week and then into the Plains midweek. The GFS and GEFS have been favoring a more amplified trough to reach the West Coast on Tuesday. However, both the ECMWF and CMC would dig a sharper shortwave trough across the central Rockies next Wednesday and out into the central Plains next Thursday whereas the GFS likes to lift it farther up toward the northern Plains. Meanwhile, the 00Z EC mean did not support the highly amplified trough digging into the Plains as depicted in the deterministic 00Z ECMWF. More uncertainty is seen farther upstream over the northeastern Pacific where the next system is forecast to move into the vicinity by next Thursday with poor model agreements at this point in time. Given the above considerations, the WPC forecasts begin with a blend of 40% from the 06Z GFS/GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the 00Z CMC/CMC mean Day 3 - 5, followed by a higher percentage from the ensemble means through Day 7. ...Weather/Hazards Highlights... A deepening low pressure system will help support a round of meaningful precipitation across the Midwest/Great Lakes Sunday. Cold air filtering in behind the surface low could support some springtime snow in northern portions of these areas, lingering into Monday. Meanwhile, a good chance of thunderstorms is expected ahead of the associated cold front. The central Appalachians/northern Mid-Atlantic into the Northeast may see the best chance for heavy rain in convection given the deepening closed low, instability in place, and anomalously high moisture, but the progressive nature of the storms and the dry antecedent conditions will limit the chance of excessive rainfall. Precipitation could linger across the Great Lakes to northeastern U.S. into Monday-Tuesday together with some lake enhancement. Light snow is possible across the Great Lakes to interior New York/New England as the low begins to exit the region. Precipitation in the form of lower elevation rain and higher elevation snow is likely in the Pacific Northwest on Sunday, with some expansion into the northern Rockies beginning Sunday as well and potentially lasting there through around midweek. Northern California may see increasing precipitation chances too around Monday-Tuesday. By Tuesday into midweek, precipitation could extend into the Plains as well. Some return flow moisture could allow for some rain chances across the south-central U.S. by about Tuesday and could reach the western Gulf Coast on Wednesday. It is worth noting that uncertainty significantly increases given the timing of surges of upper-level energy later into the period, leading to less confidence in the location and timing of daily precipitation chances in these areas. However, the pattern should keep precipitation chances in place across the region overall. Warmer than normal temperatures by 10-20F are likely across parts of the Great Lakes/Appalachians on Sunday before a cold front comes through Monday and cools temperatures to near or below normal through midweek. The interior West can expect temperatures 5-10F above average Sunday ahead of a possible cooling trend as upper-level energy could come in. After a relatively cool pattern in the central U.S. behind the cold front to begin the week, the ridge aloft will support temperatures warming to around 10-20F above average in the Plains for Monday and beyond, with above normal temperatures stretching toward the East Coast by Wednesday-Thursday. Kong/Tate 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 Hazards: - Heavy precipitation across portions of the Great Lakes and the Upper Mississippi Valley, Sun-Mon, Apr 16-Apr 17. - Flooding possible across portions of the Southeast, the Mid-Atlantic, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding occurring or imminent across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. - Flooding likely across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains. 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