A blocking pattern over the eastern US will allow for rounds of heavy rain and thunderstorms as a persistent flow of tropical moisture from the Gulf of Mexico and Caribbean Sea track up the East Coast. The combination of a strong high pressure, which spins clockwise, off the Eastern Seaboard and a gradually weakening trough of low pressure, which rotates counter-clockwise, along with a stationary frontal boundary will all work together to bring the risk for wet weather and flooding to the East Coast. By the end of this week, some towns will measure a rainfall total in excess of half a foot.
We’ve already seen flooding take place in parts of the Mid-Atlantic on Saturday where up to 7 inches of rain was measured in the state of Virginia. Additional flash flooding will be possible into Sunday night, especially from the Tidewater of Virginia up through southeastern Pennsylvania. This will put cities like Washington, DC, Baltimore, and Wilmington, Delaware at risk for ponding or even impassable roadway conditions.
For the remainder of the afternoon Sunday, there will be scattered rounds of showers and thunderstorms that affect the Ohio and Tennessee River Valleys, as well as across the east coast, including all of the Northeast, Mid-Atlantic, and Southeast. If you take a look at the radar loop, you’ll be able to visualize this convection rotating in a counter-clockwise direction as moisture revolve around the idle upper-level low centered over the eastern Ohio River Valley. On the extreme-southern periphery of this storm system will be the risk for severe weather as a strong cluster of thunderstorms affects northern Florida through the first half of tonight.
Later tonight much of the convection will shut off under the upper-level low due to the lack of solar energy once the sun sets. It will be a different story from the Mid-Atlantic coast up through New England, however. It’s this region that will be under the axis of the greatest precipitable water, or atmospheric moisture. Therefore, numerous batches of moderate to heavy rain and thunderstorms will be streaming in from the south Sunday night. Some of these storms will be training, so flash flooding will be a concern, especially in the Mid-Atlantic. Also waterspout action may be possible along coastal sections of the Northeast.
On Monday, we’ll be monitoring the Bermuda High offshore as it expands to the west. This westward expansion will slowly push that axis of the most rich tropical moisture to the west, putting much of the Mid-Atlantic in play for heavy rain. This constant feed of moisture from the tropics will keep the persistent rounds of rain and thunderstorms around in this part of the country. Some areas, especially near the coast and across the northern Mid-Atlantic, may even experience a washout of a day. A few isolated severe thunderstorms cannot be ruled out. Now while it will be a warm and humid, tropical-like day in New England as well, that significant precipitable water we had before will be lacking, thus keeping most of the region dry. A few spotty showers will still be possible inland, however.
The southeastern US will also be at risk for heavy thunderstorms, some of which will bring localized flooding during the day Monday, especially in the Carolinas. Typical of the summertime, these storms will develop midday, starting off isolated but becoming widespread by the end of the day. Farther south, a cold front that will become stalled out over central Florida will keep the rain and thunderstorms around across much of the peninsula. A few isolated storms will also be possible.
The high pressure will work a bit farther to the west on Tuesday, bringing in lower atmospheric moisture into eastern parts of the northern Mid-Atlantic, including eastern New York. So while New England and eastern New York will generally be dry, it will be more of a washout just to the west with widespread heavy rain and thunderstorms spanning from western New York down through the Carolina coast. This is thanks in part to the prevalent precipitable water values greater than 2 inches, which is indicative of an extremely-moist air mass aloft. This consistent firehose of moisture will keep the flash flood threat in place as some towns likely receive over 3 inches of rain in just a 24-hour time period.
As we get into Wednesday, the attention will shift toward the Upper Midwest and northern Plains. What’s happening there will have significant implications on the weather pattern downstream. A new trough of low pressure will begin to tilt into the region, which will cause the stream of tropical moisture to shift back to the east. This comes as the Bermuda High weakens, allowing for this fluctuation to take place.
The axis of tropical moisture will now be located just off the East Coast and up through the Hudson River Valley on Wednesday. This will allow western portions of the Mid-Atlantic to dry out, while the areas closer to the coast and western New England deal with numerous rounds of heavy rain and storms.
Thursday, the eastward progression of the moisture will continue to take place as a cold front begins to dry out the atmosphere across the Mid-Atlantic and Southeast. That will allow for mostly dry weather besides a spotty shower or two. Skies will also be much clearer with mostly sunny conditions expected. Another big different on Thursday aside from the fact that it will be dry outside is the drier air. Dew points will take a plunge from the swampy 70s to the more pleasant low 60s and mid 50s away from the coast.
Unfortunately, not everyone will experience this relief just yet. We’re talking about you, New England. A final round of heavy thunderstorms moves through ahead of the cold front, tracking from the western half in the morning to the eastern half in the afternoon. Behind that will be clearer skies and noticeably drier air.
Finally on Friday, a secondary cold front will begin to approach. Dew points may actually rise ahead of it by a few degrees as some moisture streams back in. It will definitely be more limited however, so instead of widespread convection associated with this next frontal boundary, it will be more scattered or even isolated across interior sections of the Mid-Atlantic.
That front will push through coastal areas Friday night, allowing for a pretty nice weekend with mostly dry conditions and drier air. Temperatures will be seasonably cool thanks to the dip in the jet stream overhead. We wish we could tell you it will be all sunshine next week, but for now it looks like a new round of rain will move in next Monday into Tuesday, but fortunately will be brief unlike this prolonged stretch of stormy weather.