A consolidating frontal boundary currently stretching from the Northeast southwest through the Ohio and Tennessee Valleys and into the Southern Plains that has led to days of active weather will continue to be the focus for storm development for the next couple of days.
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A couple days of widespread, very heavy rainfall and the risk for flash flooding is in store for portions of the Northeast.
Upper level energy rotating around the base of a deep upper level low over south central Canada as well as the slow moving front approaching from the west will help to trigger numerous showers and storms.
A steady stream of anomalously high moisture continues to flow into the region and will help to fuel very heavy downpours with efficient, high rain rates.
The mean southerly flow east of the front will also lead to the tendency for storms to repeatedly move over the same locations, leading to the potential for several inches of rain each day.
A Moderate Risk of Excessive Rainfall (level 3/4) is in effect Sunday from interior New England south southwest into the Mid Atlantic where the highest probabilities for significant heavy rainfall overlap wet antecedent conditions, leading to the risk for scattered to widespread instances of flash flooding.
A bit more unstable atmosphere residing over the Mid Atlantic and Carolinas may also lead to some more organized storms capable of producing gusty winds, with a Slight Risk of severe weather (level 2/5) in place from the Storm Prediction Center.
A Moderate Risk of Excessive Rainfall is also in effect Monday for interior New England as conditions remained primed for heavy rainfall as an organizing area of low pressure along the coast helps to drive onshore flow along the higher terrain of the region.
The presence of widespread storms as well as the frontal boundary moving through will lead to cooler temperatures broadly across the Northeast, with 70s for the Upper Ohio Valley, Lower Great Lakes, and portions of New England and 80s elsewhere.
Storm chances will be much lower Sunday afternoon for the Central/Southern Plains after remaining storms overnight Saturday clear out Sunday morning, with storm chances remaining ahead of the front in the Lower Mississippi Valley and Southeast.
A Slight Risk of Excessive Rainfall (level 2/4) stretches from the Southern Plains southeast into the Lower Mississippi Valley following the likely track of organized convection/redevelopment persisting through the day with locally heavy rainfall possible.
A few of the more robust, organized storms redeveloping along the frontal boundary may also produce some gusty winds, with a Slight Risk of severe weather in effect.
Following the departure of storms early Sunday, atmospheric conditions will rebound a bit more Monday ahead of another cold front pushing southward into the Plains/Midwest, with increasing precipiation chances for portions of the Upper Midwest and Central/Southern Plains as well as the Northern High Plains and Rockies.
A Slight Risk of severe weather is in place over portions of the Central/Southern Plains where storms may produce gusty winds and large hail.
High temperatures overall will remain well below average Sunday across portions of the Central/Southern Plains under the influence of the southern frontal boundary and clouds/precipitation, hovering in the upper 70s to low 80s.
Conditions will return closer to average Monday with mid to upper 80s expected.
Locations from the Northern Plains east into the Upper Midwest and Great Lakes region will see a warm up ahead of the incoming cold front, with highs in the mid 80s to low 90s.
Temperatures will drop into the 70s for locations behind the front in the Northern Plains/Upper Midwest Tuesday.
Meanwhile, to the south and west of the frontal boundary, conditions will remain hot from Florida to Texas and the Southwest as well as the interior Pacific Northwest.
Highs in the mid 90s continue to be close to record tying/breaking high temperatures for the Florida Peninsula, and a Heat Advisory is in effect Sunday for South Florida where these hot temperatures combined with high humidity will lead to heat indices in the 105 110 degree range.
Hot temperatures over portions of west Texas and the Southwest have begun to broaden in coverage as an upper level high builds in aloft with heat related warnings and advisories stretching from east Texas west through the Desert Southwest.
High temperatures in the upper 90s to 100 for east Texas combined with high humidity will lead to heat indices in the 105 110 degree range over the next couple of days.
To the west, humidity wont be as high, but temperatures will be in the 100s, with some 110s in the Desert Southwest.
Mean upper level ridging northward through the Great Basin and into the Pacific Northwest will lead to one more day of much above average temperatures for the interior Pacific Northwest.
Highs will be in the upper 90s for many locations before an upper level wave approaching Monday helps to drop temperatures down into the 80s and much closer to average.
Temperatures will generally be above average across portions of the Rockies and Great Basin, with highs into the 90s.
The noted upper level wave over the Pacific Northwest as well as a second upper wave off the coast of California will keep temperatures much cooler along the West Coast, with highs in the 60s for the immediate coast and 70s to low 80s for inland locations. ■