The weather pattern will remain quite active across the Northern Tier as a near-stationary upper-level low paired with deep tropical moisture associated with then-Hurricane Bud team up to deliver heavy rains and some severe thunderstorms to parts of the region.
Sunday into Sunday night, we’re most concerned for flooding in parts of the Northwest, central Rocky Mountains, Northern Plains, and Upper Midwest. This is due to a rather saturated ground already in place and the additional rainfall on the way. Due to this near-stationary weather pattern, this will allow for some of the rain and thunderstorms to train over the same locations for a couple hours. These training storms will likely dominate the day from northern Minnesota and surrounding areas back through South Dakota and into the central Rockies. Near the upper-low over the Northwest, showers and some thunderstorms will also continue to spin over the same areas, thus aiding in the threat for flash flooding. Those showers are expected to persist into Sunday night while the areas that received the rainfall during the day in the Northern Plains and Upper Midwest begin to dry out thanks to the progression of a cold front. This cold front will slowly suppress the moisture to the south and east.
In terms of rainfall, some locations will receive over two inches, in particular across the Northern Plains and Upper Midwest. Otherwise, generally between half an inch and one and a half inches of rain can be expected across most areas that experiences the storms into Sunday night.
There will also be severe thunderstorms to worry about, especially Sunday evening and Sunday night. This threat will span from the Central Plains where the intense storms will be isolated whereas in the Upper Midwest scattered severe storms are forecast. These storms will threaten these regions with large hail and damaging winds and possibly even an isolated tornado or two. By the end of the day Sunday, a line of strong to severe storms is forecast to form to the south of the training rain and thunderstorms. This line also looks to train, moving in the northeast direction. They will then weaken early-Monday morning. There is some uncertainty in regards to how strong these storms will get thanks to the widespread cloud cover in the region associated with the persistent rain activity already in place. This may be the saving-grace and will prevent an outbreak of severe weather from occurring.
On Monday, rounds of rain and thunderstorms will continue across the Northwest, especially in the northern Rocky Mountains. A few showers and storms will also be around across the Northern Plains and Upper Midwest, although the risk for flooding will be much lower. This is due to the more spotty convective activity and the lighter rainfall totals. Late in the day, however, a line of strong to severe thunderstorms, some of which will contain heavy precipitation, will likely form from the Quad Cities back toward Kansas and possibly as far south and west as the Oklahoma and Texas Panhandles. Localized flash flooding will be a concern with these storms as they tap into some of the leftover moisture from the remnants of Bud. Much of these thunderstorms will then collapse and dissipate early-Tuesday morning due to the loss of instability and solar energy.
Father east, a line of scattered showers and thunderstorms ahead of a cold front will work southward across the Great Lakes, northern Mid-Atlantic, and New England Monday afternoon and evening. These storms will generally wait until the overnight hours to affect these areas thanks to the cap in place, but a few of these storms may still have the potential to turn severe once they undergo their explosive development. This cap indicates warm air aloft given how warm the air mass will be on Monday across the Northeast. Widespread record heat is even expected along the I-95 corridor.
During the midweek time period, the weather will remain active across many of these areas, especially across the Northwest and Central US.