US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 419 PM EDT Wed Apr 21 2021 Valid Saturday April 24 2021 - Wednesday April 28 2021 Hazards: - Heavy precipitation across portions of northern and central California, Sun, Apr 25. - Heavy rain across portions of the Southern Plains, the Lower and Middle Mississippi Valley, the Great Lakes, the Ohio Valley, the Tennessee Valley, the Southern Appalachians, and the Southeast, Tue-Wed, Apr 27-Apr 28. - Heavy rain across portions of the Middle and Upper Mississippi Valley, and the Great Lakes, Wed, Apr 28. - Heavy rain across portions of the Tennessee Valley, the Southeast, the Southern Appalachians, and the Mid-Atlantic, Sat, Apr 24. - Heavy rain across portions of Maine, Sun, Apr 25. - Severe weather across portions of the Central and Southern Plains, Tue, Apr 27. - Severe weather across portions of the Lower Mississippi Valley, the Tennessee Valley, the Southern Appalachians, and the Southeast, Sat, Apr 24. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Northeast, Sun-Mon, Apr 25-Apr 26. - Much above normal temperatures across portions of northern Mainland Alaska, Sat-Sun, Apr 24-Apr 25. Detailed Summary: Two storm systems will bring areas of significant weather to portions of the lower 48 during the period. In the East, low pressure tracking over or near the Tennessee Valley on Saturday will continue through the Mid-Atlantic and develop into a nor'easter that may linger close to New England and the Canadian Maritimes into early next week. This system is likely to bring a period of heavy rain and/or severe weather to southern areas and then may produce potentially heavy rainfall and strong winds over parts of the Northeast. Meanwhile an eastern Pacific system pushing into the West during the weekend and continuing eastward next week will spread a broad area of precipitation from the West Coast through the east-central U.S. The best focus for highest precipitation totals over the West will be over the Sierra Nevada, while areas from the eastern half of the Plains to west of the Appalachians will see the potential for heavy rainfall and/or severe weather. On Saturday there is reasonable consensus for the severe potential over the South and heavy rainfall over the Southeast into southern Mid-Atlantic as depicted on the chart in association with the system affecting the East. It is possible that some bands of locally heavy rain may fall to the north of these areas. By Sunday confidence decreases in the precise coverage and magnitude of rainfall over New England depending on exactly how the nor'easter evolves. The most common signal is for at least southeastern Maine to see a period of heavy rainfall with this system. Expect the strongest winds to be confined near the coastal areas of New England from late Sunday or Sunday night through Monday. The system moving in from the eastern Pacific will initially spread moisture through the West this weekend into early next week. The best confidence in precipitation (rain and higher elevation snow) reaching heavy thresholds is over and near the Sierra Nevada around Sunday so the map focuses on this area. It is possible that rain and mountain snow may approach heavy criteria on a localized basis over parts of the Pacific Northwest and in favored terrain over the Interior West, Southwest, and Rockies. There may also be a period of breezy to windy conditions from portions of the Interior West into the High Plains but likely remaining below strength/duration criteria. After Monday the Pacific/western U.S. system will reach far enough east to produce increasing potential for heavy rainfall that should extend from parts of the Plains and Mississippi Valley on Tuesday into the east-central U.S. by Wednesday. Guidance is generally suggesting two areas of heavy rain--one from southeastern portions of the Plains eastward/northeastward corresponding to the most favorable combination of moisture and instability, and a second area somewhere between the central or northeastern Plains into the Upper Mississippi and northwestern Great Lakes in association with surface low pressure and leading front as well as an upper low that could form. The northern area has somewhat lower confidence in the details. Today's chart depicts the most common signal for the Upper Mississippi Valley on Wednesday but with the understanding that some potential for heavy rain will exist over a broader area that day and possibly a bit to the west already on Tuesday. Some areas across the southern tier will be sensitive to additional rainfall after the heavy rainfall event late this week. Expect an increase of moisture into the Northwest by Tuesday or Wednesday with the approach of a frontal system. Over the past day guidance has been offering poor agreement for when this moisture will arrive and how much precipitation will fall. It will be possible for some locations to see heavy amounts but uncertainty is too great to depict any area at this time. An upper ridge over Mainland Alaska to start the period early Saturday will ultimately give way to flow around a Canadian upper low next week. Some above-freezing high temperatures should persist over northern locations into the weekend after peaking late this week but this area of relative warmth will see a reduction in coverage from the north. Systems that may affect the Aleutians and Alaska Peninsula during the weekend as well as later in the period next week should have precipitation and winds that remain below hazardous criteria. Rausch