A slowly weakening upper low will remain a focus for moderate to heavy rainfall across the Southeastern U.S...
Another upper cyclone moving through California should spawn
diurnally-enhanced showers across much of the Southwestern states...
The month of May continues to feature a very blocked upper-level pattern with a pair of closed lows dominating the headlines. This stagnant mid-latitude setup has allowed the prevailing westerlies to migrate well to the north across the international border. This should consequently confine the modified Arctic air to Canada for most of the period before the boundary sinks southward into Western Montana by Tuesday evening.
As the upper low currently crossing Northern Alabama continues its slow push to the east, a steady plume of Atlantic moisture will work its way toward the Eastern U.S. The overall intensity of this system is forecast to weaken as the low center begins to fill and move toward the east-northeast.
However, still many factors suggest the heavy rainfall threat should continue given the adequate moisture in place combined with the upslope flow against the Southern/Central Appalachians. Additionally, storm motion is expected to remain slow which will focus heavier elements over certain areas longer.
As a result, the Weather Prediction Center has kept a low-end threat for excessive rainfall over the Southern Appalachians/Upper Tennessee Valley through Monday evening.
Looking back toward the Southwestern U.S., another closed low will meander across Northern California through Monday morning. Thereafter, a slow movement toward the east is expected with a gradual loss in intensity while nearing the Desert Southwest by Tuesday afternoon.
The overall moisture content will at first be rather limited which should maintain lower rain/snow amounts. The primary driver for precipitation should be from the upper low itself which will enhance the vertical motion within the air column. Additional contributions will come from upslope flow against much of the elevated topography across California and into the Central Great Basin.
As the system makes it way eastward, moisture will be on the increase from the Western Gulf which is forecast to allow more
widespread shower activity to develop by midday Monday onward. This should impact the Four Corners region up into Central Rockies with a separate area of light showers forming along a low-level convergence zone by Tuesday morning across the Great Plains.