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Severe thunderstorms will threaten parts of Brazil, Paraguay, and Bolivia Friday and Saturday. Damaging winds, large hail, and flash flooding are all concerns.

The Setup

A cold front will move northward across central South America on Friday. Along and ahead of the front, the environment will be supportive for some damaging thunderstorms to blossom.

The greatest severe threat Friday afternoon and evening will extend from southern Brazil northwestward through Paraguay and into Bolivia. This includes cities such as Florianopolis, Asuncion, and Santa Cruz.

Damaging winds in excess of 80 kph, large hail up to 5 cm, and flash flooding will be possible in the strongest of storms.

The severe thunderstorm threat will diminish late Friday night into early Saturday, however, the severe threat will once again grow during the day Saturday as temperatures rise ahead of the cold front.

Cities such as São Paulo up to Brasilia will be at risk for some gusty to potentially severe thunderstorms Saturday afternoon and evening.

In the wake of the cold front, a cooler more stable fall air mass will move in and end the risk for severe weather.

Rounds of rain and thunderstorms will track across the Southeast and mid-Atlantic through the rest of this week and this weekend. The soggy weather pattern will bring the risk for flooding in some areas.

How much rain will fall?

The heaviest rain is expected to set up across Virginia, Maryland, Delaware, and southern New Jersey through Sunday. In this corridor, 4-6 inches (100-150 mm) of rain is likely over a 5-day period. Outside of this region, a large area of over an inch (25 mm) of rain is expected.

It will not be raining the entire 5-day period at any one location, however, the expected rain combined with an increasingly saturated soil will bring some risks for flooding.

Why the wet pattern?

The culprits for the rain include a disturbance moving northward across the Gulf of Mexico, a second disturbance moving eastward across the Mississippi Valley, and a stationary front draped over the mid-Atlantic.

The slow movement of each of these features will keep rain chances high into this weekend.

A brief break in the rain is possible next Monday, but another disturbance will bring a risk for wet weather again next Tuesday.

 

An active upper level pattern will persist into the new week. Two areas will see a particular risk for severe storms on Monday into early Tuesday, one in the central Appalachians and Mid-Atlantic, the other in the Plains.

 

 

A cluster of storms will be ongoing Monday morning in the Midwest. As it moves across Ohio towards the central Appalachians by midday, it will encounter increasing instability thanks to daytime heating. Strong to severe storms will cross the mountains into the Mid-Atlantic later in the afternoon and early evening. Strong wind gusts to 65 mph and large hail to 1.5″ will accompany the stronger cells.

Storms will fire in the mid-afternoon hours over the Plains, especially from northwest Texas to northern Illinois. These storms will tend to move northeast with damaging wind gusts to 75 mph, hail to two inches in diameter, and perhaps an isolated tornado or two. These storms will tend to weaken slowly after midnight.