A near-stationary weather pattern will set up this week as a ridge of high pressure sits over the Eastern US, thanks to a strong Bermuda High offshore. This will allow for warm, summer-like temperatures. Unfortunately, an upper-level low near the eastern Gulf Coast will hang around through at least the start of the week while fast-moving disturbances move through the Northern Tier. All of these components will come together to bring days of rain and thunderstorms to much of the Eastern US, especially the Southeast and East Coast.
There is also the risk for heavy rain this week due to highly-anamolous moisture flowing northward from the Caribbean Sea. The loop below from the European model highlights the forecast precipitable water values over the Eastern US through the next ten days. The brighter colors signify higher moisture content in the atmosphere, and thus the greater chance for heavier rainfall rates because the atmosphere is able to hold more water. Notice the constant stream of this moisture flowing up the East Coast of the country through at least the weekend.
Not only will this moisture be above normal levels for this time of the year, but it will break many precipitable water records along the East Coast. For example in Miami, the record predictable water value on Sunday, May 13th is 1.87 inches, but the model guidance is suggesting that values may reach or even exceed 2.5 inches. This represents a very saturated, moist air mass, and therefore the risk for multiple inches of rain. Later on in the week, these higher levels of moisture may stream into the Northeast and New England. These values will peak during the weekend. The record predicable water value on Friday, May 18th in New York City is 1.55 inches; values this year may accomplish 2 inches.
On Sunday, rain and thunderstorms will be confined to South Florida. It’s not until Monday when that shield of rain will work farther to the north, getting into the remainder of the Florida Peninsula. This rain will then expand into the Carolinas and Georgia on Tuesday while continuing in most of Florida. Some of this moisture will also work into the Northeast, which coincides with one of the upper-level disturbances passing through, so heavier downpours will be possible within the rounds of rain and thunderstorms from the Ohio Valley through the Mid-Atlantic. A few thunderstorms will also develop due to the hot temperatures in the remainder of the South. By Wednesday, showers and thunderstorms will become a bit more prevalent in the Southern Plains. Widespread rain and thunderstorms will be the story to the east, across the Southeast up through southern New England. High pressure may then keep the moisture to the south, thus keeping most of the Northeast dry on Thursday while storms persist in the Southeast, southern Mid-Atlantic, and Ohio Valley. Additional rounds of showers and storms will continue through the weekend. A new storm system will also develop over the Plains and track eastward while running into the higher atmospheric moisture. That may bring heavier rains to portions of the Midwest, Great Lakes, and Northeast regions. A cold front will then sweep the moisture offshore early-week, making for a nicer and definitely drier period of weather.
This persistent tropical flow will dump a lot of rain. It’s been awhile since we had a rainfall forecast showing a widespread two to three inches of rain. There is even a large area of Florida that is expected to receive at least three to five inches of rainfall. Localized areas in the state may receive up to ten inches, so there is definitely the risk for flooding not only in Florida but across much of the East.