An upper-level cut-off low that has been residing off the coast of southern California will begin to move eastward over the Southwest the next couple of days.
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Meanwhile, a cold front will move southward over the Plains and Southern Rockies late Saturday and throughout the day Sunday.
These features will help to trigger and support widespread showers and thunderstorms across the region throughout the weekend.
Anomalously high moisture, the presence of modest CAPE, and the tendency for storms to repeatedly move over the same areas have prompted a Slight Risk of Excessive Rainfall Saturday across the Sonoran Desert from California into portions of southern Arizona with scattered instances of flash flooding anticipated.
The threat transitions eastward with the upper-level low on Sunday to southern New Mexico and southwestern Texas with another Slight Risk of Excessive Rainfall in effect.
Multiple rounds of storms along the front are forecast to result in areal average rainfall totals between 1-2", with locally higher amounts of 2-4" possible.
While the flood threat will not be as high outside of the more sensitive terrain of the Southwest and Southern Rockies, the widespread storms over the Southern Plains will likely result in rain totals between 1-2", locally higher, here as well.
Over the east, a deep, amplified upper-level trough will continue to help dominate the weather story as a series of shortwaves rotate around the trough and multiple surface fronts progress across the eastern/central U.S.
Some lingering showers will continue to move northward over Maine and progress eastward through this evening ahead of a slow moving cold front.
A flood watch remains in effect through Saturday morning as a couple inches of additional rainfall on top of already wet soils could lead to minor flooding.
The focus then turns further west as progressively colder air is advected southward over the Midwest and Plains.
Highs in the mid-40s to mid-50s will spread southward over the Great Lakes Region and broader Midwest behind a cold front on Saturday.
Temperatures will cool significantly over the Northern Plains by Sunday as well as a second cold front moves southward, with highs generally in the mid-40s to low 50s.
In addition, the series of shortwaves rotating around the upper-level low will lead to several rounds of precipitation through the weekend across the Great Lakes Region.
Generally light to moderate showers are expected during the day with some wet snow mixing in overnight, though little to no accumulation is expected.
An amplified upper-level ridge will generally remain in place through the weekend over the Pacific Northwest continuing the warmer than average weather pattern for the region.
Highs on Saturday and Sunday will be in the upper 70s to low 80s for the I-5 urban corridor, with low 90s possible for the Rogue Valley in Oregon.
Many of these highs will be close to either tying or breaking daily records.
While not record-breaking, highs will still be 10-20 degrees above normal for the rest of the interior Northwest, Northern Rockies, and Great Basin, with mid- to upper 70s forecast. ■
A clipper system will move quickly across the northern Plains into the Midwest Friday and the Northeast by Saturday, bringing a wintry mix of rain and snow showers ahead of a sweeping cold front.