A broken line to the northwest and west may still pose a storm risk, as noted in the update below. The timing for that would probably be in the 7-9 p.m. period locally. It’s not impossible these storms could be strong to severe, especially northern Md. and north. It does seem less likely this batch would try to produce a tornado nearby given that the center of Fred continues to pull away.
This will be the last update unless necessary. PM Update will be posted soon.
4:10 p.m. — Quieting down. For now?
Showers and storms that passed this afternoon are exiting the area and heading into the bay. It’ll still take a while longer to clear them in southern Md., and near the shore.
Another broken line is trying to maintain itself west of Hagerstown to Winchester and it is headed east. Our best guess is that northern Maryland has the highest odds to see more storms from this batch, although they could extend south into the D.C. area. If so, this would be closer to sunset.
Prior to that, it’s not impossible we see a pop up storm but it doesn’t seem terribly likely in any one spot. We’d imagine the tornado watch will start to be canceled in the time ahead, from west to east, although the risk for a quick spin up is still palpable across northern Maryland and into Pennsylvania.
3:05 p.m. — More showers and storms blooming near and east of I95
The broken band of showers and storms that is slowly crawling through the area has sprouted a number of new storms lately. They are mainly north of D.C., and another big batch of stronger storms is moving through southern Maryland. This activity should stay east of the city, overall. A risk of quick spin ups remains, mainly near and east of I95.
On another note: Humidity is off the charts right now. Tropical air mass is tropical.
Scroll to the bottom for earlier updates
Original post below
As the remnants of Fred steam north-northeast through the Appalachians, a severe weather threat develops locally Wednesday afternoon and perhaps lasts into the evening.
On Wednesday morning, a developing line of broken showers and storms extends from west of Richmond through Charlottesville and up into Western Maryland. The line should move through our area between about 11 a.m. (west) and 5 p.m. (east). While the line itself is moving east-northeast, individual storms in it are moving more toward the north. It’s also possible another broken line will pass Wednesday evening, although it seems odds for that are best north of the city.
Given considerable vertical wind shear, or turning winds with height, the potential for brief tornadoes is there with any storms that become intense. There’s also a risk of some flooding, damaging wind gusts and dangerous lightning.
Timing for main line(s): At least one broken line of showers and storms is likely to crawl across the area. The first is focused from midday into afternoon. Another is possible during the evening, although this line is less certain to develop locally.
Storm motion and duration: Broken lines moving southwest to northeast, with individual storms moving north.
Main threats: Heavy rain, lightning, brief tornadoes, localized damaging wind gusts.
Small chance: Flooding.
Rainfall potential: Highly variable. Up to an inch or two in the heaviest storms.
All clear: By late evening.
Confidence: Medium. Coverage of storms is uncertain, but scattered activity is likely.
Discussion
With the passage of post-tropical remnants of Fred Wednesday, the weather hazards for the D.C. region are focused on two areas: isolated to scattered tornadoes and torrential, tropical-like downpours leading to flash flooding.
As of early Wednesday, the circulation of former Tropical Storm Fred was located along the West Virginia-Ohio border, tracking to the north-northeast. You can see the projected location of the surface disturbance in the forecast chart for early Wednesday afternoon (below).
Fred is undergoing a process called “extratropical transition,” whereby the purely tropical system has been transitioning to a hybrid storm, taking on characteristics of an ordinary extratropical cyclone. This includes the development of a cold front.
Fred still packs a wallop of spin in the upper atmosphere, as shown in the figure below. “X” marks the spot in the center of spin. It remains quite a dynamic and potent disturbance.
Don’t let the westward location of Fred’s low-level center lull you into thinking we’ve largely missed its impacts. In fact, the more dangerous breeding ground for inland tornadoes lies to the east of the center of circulation, often removed from the heaviest rain, in the so-called eastern semicircle. There, intense wind shear and concentrated, low-level spin often combine to produce rotating storm cells.
Our concern Wednesday afternoon will be the development of one or more spiral rain bands, along and east of the Appalachians, which may contain rotating supercells. These storms will feed off an unstable atmosphere, due to a combination of daytime heating and impressive low-level moisture feeding into Fred from the south.
In fact, this plume of tropical moisture is readily visible in an analysis of “total precipitable water.” You can see it arcing into the core of Fred east of the Appalachians (dark green shades), and ahead of the cold front, in the figure below.
The unusually high humidity levels, in fact, extend through a deep layer. With all this moisture to work with, converging winds in any spiral band will have the potential to produce locally torrential downpours, with possible repeat passage of storm cells over the same locations. Hence the reason for flash flood watches for the western half of the D.C. metro area.
The Storm Prediction Center is closely monitoring our region for a severe weather watch, which may be in the form of a tornado watch or severe thunderstorm watch. The risk for tornadoes stands at 5 percent probability within 25 miles of any point across the area. Formative parameters for tornadoes are maximized right over the D.C. region — note the bull's eye in the figure below!
Any tornadoes are likely to be on the weaker, short-lived side — as the majority of inland, tropical cyclone-spawned tornadoes tend to be.
In 2004, a tremendous “swarm” of tornadoes was unleashed across the Mid-Atlantic as the post-tropical remnants of Hurricane Ivan passed to the west, leading to Virginia’s biggest tornado day on record. It’s doubtful that we will experience something of that magnitude Wednesday afternoon, but it’s nonetheless important to remain vigilant for a handful of these “spinners” to erupt.
Earlier updates
1:25 p.m. — Showers and storms focusing on the immediate area and east
A broken band of downpours and storms continues to slowly push through the area. It is focusing on the Interstate 95 corridor and to the east.
So far, the main story locally has been heavy rain and some lightning. Downpours are quickly lowering visibility and putting down rain rates of 1 to 2 inches per hour, although they don’t tend to last that long in any one location. The environment for tornado development remains quite impressive, despite the lack of tornado activity so far. There may be an uptick in the tornado threat as this band heads into more unstable air near and east of Interstate 95.
Although the tornado threat is elevated, typical twisters in this kind of environment are quick and relatively weak, although they can cause damage.
By about 4 p.m. this activity should be mainly to our east.
11:40 a.m. — Tornado watch as storms begin to form
A tornado watch has been issued by the National Weather Service. It runs until 8 p.m.
Showers and storms are increasing in coverage over western and southern parts of the area. This activity will slowly move east over the next few hours as severe weather potential, including tornadoes, grows.
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Updates: Tornado watch until 8 p.m. as remnants of Fred pass the D.C. region - The Washington Post
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