A quasi-stationary boundary draped across the Southeast that was the trigger for widespread flash flooding and severe weather over the weekend will be the focus for these threats again Monday.
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Storms will likely be ongoing Monday morning near the boundary across portions of central Alabama and Georgia.
Anomalously high moisture continuing to stream northward from the Gulf will promote heavy rain rates.
The quasi-stationary nature of the boundary will also lead to repeated, overlapping storm development and motions increasing the potential for locations to see multiple rounds of heavy rain and higher rainfall totals.
A Slight Risk of Excessive Rainfall has been introduced over this region as these storms continue over locations that saw heavy rainfall Sunday, increasing the risk for additional scattered instances of flash flooding.
In addition, some storms may become severe as a moderate to strongly buoyant airmass remains in place to the south of the boundary with continued strong flow aloft.
The Storm Prediction Center has issued a Slight Risk for severe weather (level 2/5) over a similar region as the flash flood risk from southern Georgia into the South Carolina Low Country for the threat of hail, strong winds, and a tornado or two.
By Tuesday, an upper-level shortwave will help to push a better organized frontal system south towards the Gulf, helping to clear out the region while keeping storm motions a bit more progressive and expected rain totals lower compared to the prior days.
However, anomalously high moisture will still be present keeping the threat of higher rain rates and localized heavy rain possible.
This led to the retention of a Marginal Risk of Excessive Rainfall (level 1/4) from the Central Gulf Coast east into southern Georgia and northern Florida.
High temperatures will remain above average across the South Monday with upper 70s to low 80s forecast.
Temperatures will be even warmer in Florida with near record-tying/breaking highs upwards of 90.
Highs will cool into the mid-60s to low 70s for most of the region Tuesday as the front finally pushes southward into the Gulf.
The frontal system pushing through the South will also move eastward through the Mid-Atlantic Monday as another cold front over Canada slides southward into the Interior Northeast.
This will increase precipitation chances regionally for the Northeast, with light to moderate showers for the Mid-Atlantic and southern New England and a wintry mix into the Interior Northeast, though little to no accumulations of sleet/snow are expected.
Highs will generally be seasonable for the Interior Northeast/New England with 40s and 50s forecast.
Energetic flow aloft will help to encourage additional waves of low pressure along the Canadian front trailing through the Great Lakes and into the Plains, increasing precipitation chances for these regions as well.
A few inches of accumulating snow are forecast for portions of the Central High Plains Monday and for the Upper Great Lakes Tuesday.
Highs will be a bit below average over the Midwest as temperatures generally remain in the 40s.
Very chilly, much below average temperatures are forecast for the Plains Monday with highs in the 20s and 30s.
Highs will warm up into the 40s and 50s for the Central Plains Tuesday.
A powerful Pacific storm system will approach the West Coast Monday, with high Pacific moisture quickly overspreading southern portions of the Pacific Northwest and California and bringing yet another round of heavy coastal/lower elevation rain and mountain snow.
The heaviest rainfall is expected Tuesday along coastal Central California.
There is a Slight Risk of Excessive Rainfall along the terrain of the coastal mountains as warmer air moving in will help to raise snow levels and increase the threat of runoff into downstream rivers, as the entire region remains sensitive to the risk of flooding given very wet antecedent conditions.
Very heavy snow is forecast for higher elevations of the northern Coastal Ranges and Sierra Monday-Tuesday, with totals of several feet possible.
In addition to the precipitation, gusty winds are also likely Monday along coastal California and the Sacramento Valley.
Moisture will spread further inland into the Pacific Northwest and Great Basin Tuesday with a mix of rain and snow for lower elevation valley locations and accumulating snow for the Cascades, Blue Mountains of Oregon, and Sawtooth Range of Idaho.
High temperatures will continue to remain much below average for most of the West Monday, with highs in the 30s and 40s for the Great Basin and Northern/Central Rockies; 50s for the Pacific Northwest, northern/central California, and the Southern Rockies; and 60s to low 70s for the Desert Southwest.
Highs will warm up a bit regionally Tuesday as upper-level ridging briefly builds in following the departure of a trough to the East, though temperatures over northern/central California will remain unseasonably cool. ■