US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 448 PM EDT Mon May 17 2021 Valid Thursday May 20 2021 - Monday May 24 2021 Hazards: - Heavy precipitation across portions the Northern Rockies, Thu, May 20. - Heavy rain across portions of the Northern Plains, the Upper Midwest, and the Southern Plains, Thu, May 20. - Heavy rain across portions of the Northern Plains, Fri, May 21. - Flooding possible across portions of the Midwest and Central and Southern Plains. - Flooding occurring or imminent across portions of the Central and Southern Plains. - Flooding likely across portions of the Central and Southern Plains. - Enhanced wildfire risk across portions of the Central Great Basin and the Southwest, Thu, May 20. Detailed Summary: The main weather hazard during the medium-range forecast period (Thursday, May 20 - Monday, May 24) will be heavy rainfall across areas of the Central U.S. At the beginning of the period, the mean flow pattern will feature an upper-level low over the western U.S. and a ridge over the eastern U.S., funneling rich, gulf moisture northward across the Plains to the Canadian border. A slow moving cold front will be draped from the northern Midwest southwest through the Northern Rockies and across the Great Basin. This influx of moisture will lead to many areas of heavy rain depending on the positions of any shortwaves and frontal features given the day. For Thursday, May 20, lift ahead of a shortwave moving across Texas will encourage numerous showers and storms leading to heavy rain for eastern portions of the Southern Plains. Additionally, heavy rain is likely for portions of the northern Midwest as a wave of low pressure along the cold front moves eastward across northern Minnesota. Finally, diffluent flow aloft associated with the upper-level low will lead to heavy rain and mountain snow behind the cold front across most of Montana. Temperatures will likely stay warm enough for the precipitation to remain mainly as rain for eastern Montana, but heavy snow is forecast for the Northern Rockies to the west. The highest totals are currently forecast to remain confined to the higher mountain passes and peaks. However, it is possible that some snow will mix in with the rain across the valleys. For this reason, the western portion of this outlook area is for heavy precipitation in general. On Friday, heavy rain is likely for portions of the Northern Plains as another wave of low pressure moves to the northeast across the region and the cold front continues slowly to the southeast. Further south, a surface trough will develop in the lee of the Rockies across the Southern High Plains on Saturday in response to the approaching upper-level low. Diurnally-driven showers and storms are likely on both Saturday and Sunday but have opted not to include an outlook area at this time due to model differences on which day(s) may see more widespread storms and rain, though localized heavy rain appears possible. Beyond the outlook areas highlighted, additional areas of heavy rainfall are possible day-to-day, particularly across portions of the Central/Northern Plains and the Midwest as the ridge over the eastern U.S. begins to break down and the upper-level low over the western U.S. ejects to the northeast over the Northern Plains later in the period. Model differences on the timing and location of the heaviest precipitation preclude additional outlook areas at this time. However, it is likely given the overall wet pattern that additional areas of heavy rain may occur throughout the end of the period, and additional outlook areas may be added as confidence grows in the forecast. Elsewhere, above normal temperatures are likely from the Midwest/Great Lakes region east to the Northeast and Mid-Atlantic under the stagnant ridge of high pressure over the eastern U.S. However, the temperatures are not forecast to be particularly hazardous at this time. For Alaska, a storm system will move northeastward over the Gulf of Alaska Thursday and Friday and onshore in South-Central Alaska by Saturday. Rain is likely for Kodiak Island and southern portions of the Kenai Peninsula, Cook Inlet, and Prince William Sound on Thursday and for Yakutat and the Panhandle on Friday. However, the total rainfall at this time does not appear to be significantly anomalous to include an outlook area. Putnam