US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 328 PM EDT Mon May 24 2021 Valid Thursday May 27 2021 - Monday May 31 2021 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Thu, May 27. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, and the Southern Plains, Sat-Mon, May 29-May 31. - Heavy rain across portions of the Southern Appalachians, the Central Appalachians, and the Ohio Valley, Fri, May 28. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Thu, May 27. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, and the Northern Rockies. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains. - Much above normal temperatures across portions of California, the Pacific Northwest, and the Northern Great Basin, Mon, May 31. - Much above normal temperatures across portions of the Southeast, the Mid-Atlantic, the Southern Appalachians, and the Tennessee Valley, Thu-Fri, May 27-May 28. - Much below normal temperatures across portions of the Upper Mississippi Valley and the Northern Plains, Thu, May 27. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Fri-Sun, May 28-May 30. Detailed Summary: This upcoming Day 3-7 period, which includes the Memorial Day weekend, will feature a fairly active weather pattern, mostly focused around a wavy surface front stretching across the central and eastern CONUS. On Thursday, an area of low pressure along the front combined with flow of deep Gulf moisture, will bring widespread moderate to heavy rainfall to the upper Midwest, mid-Mississippi Valley, and south-central Plains. Amounts will mostly fall into the 1-2 inch range, but isolated heavier amounts are likely, especially along the trailing cold front. In addition to heavy rainfall, there is a threat for severe weather from Missouri southwestward through eastern Kansas and much of central and eastern Oklahoma. On Friday, this low will advance into the Ohio Valley and weaken, with a secondary low possibly developing off the mid-Atlantic coast Friday night-Saturday. Associated rainfall will push into this region, but likely will be less significant than over the central U.S. the previous day as the feed of Gulf moisture lessens. Also very concerning will be the heavy rainfall potential over the central states during the long holiday weekend. A series of short wave troughs, one moving across the northern Plains, along with less well-defined impulses lifting out of a closed low forming in southwestern states will support multiple periods of organized shower/thundershower activity. Helping to focus the rainfall are the above mentioned wavy front along with a second boundary associated with the trough crossing the northern Plains. While there is uncertainty with exact details of timing and location, the general consensus supports the heaviest precipitation over Kansas, Nebraska, and possibly southward into western Texas and eastern New Mexico. With a nearly continuous feed of Gulf moisture, there are indications that some parts of this region will see repeat periods of shower/thundershower activity, which could lead to significant rainfall totals and the possibility of flash flooding. Farther east and south of the front, hot weather will persist across the Carolinas, Georgia, Mississippi and northern sections of Florida Thursday-Friday. High temperatures are expected to top out in the mid-upper 90s both days (about 10-12 degrees above normal and close to record levels), then settle back closer to normal as a modestly cooler air mass builds in behind the developing low along the mid-Atlantic coast. On the other side of the CONUS, upper-level ridging will strengthen over the Pacific Northwest, resulting in above to much above normal high temperatures Sunday and Monday. Finally, there's still some cool weather over the CONUS...namely over the northern High Plains and upper Midwest on Thursday due to a combination of a Canadian air mass and increasing clouds/precipitation associated with a short wave trough. The core of the coolest air will center over North Dakota, where high temperatures will range from 15-25 degrees below normal, with lows near or below freezing Thursday into Friday morning. Over Alaska, a deepening upper-level trough south of the Aleutians and downstream upper ridging will result late this week in broad southerly onshore flow from the south-central mainland into the northern Panhandle. A series of short waves and associated frontal systems will be able to tap deep moisture from the tropical Pacific to support unsettled weather through much of the period. There's some uncertainty with details, but at this time, the heaviest precipitation Friday will be across the northern Panhandle, then shift somewhat westward along the southern mainland over the weekend. Klein