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Posted at 8:45 PM ET, 04/ 8/2010

PM Update: Showers and storms into night

By Ian Livingston

Main severe threat is isolated wind damage

* Trees down around town | Pollen is blowin' in the wind *
* Outside now? Radar & more: Weather Wall | CWG T-Shirts *

Our April heat is departing and it went out with a bang in some spots. Highs that made it into the mid-80s most spots added some fuel to the line of showers moving through. Though severe weather -- mainly isolated damaging wind gusts such as the 59 mph recorded at National during a storm earlier this evening -- is a possibility, incidents of such should be few.

Through Tonight: A brief shower or storm is possible early, with the bulk crossing the area between 7 p.m. and around midnight. Some storms could be strong to potentially severe. But the later arrival (as daytime heating wears off) and limited instability should keep them from being extreme. Regardless of storms, winds will gust near and past 40 mph ahead of the front. Overnight lows range from near 50 into the mid-50s.

Severe Weather Tracking Station
Warning Text: DC | MD | VA * New: Lightning Map
Radar: Latest D.C. area radar loop shows precipitation over past three hours. Powered by HAMweather. Refresh page to update. Click here to see radar bigger.
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Warnings: Severe alerts from National Weather Service. Updates w/o refresh (most browsers). Hint: Alert on map? Click matching label in key for details.

Tomorrow (Friday): Tomorrow may be somewhat of a shock to the system initially unless you have not acclimated to the heat yet. Temperatures in the 50s much of the day, along with gusty winds from the north and northwest, will make for a somewhat chilly one. Highs ultimately top out within a few degrees of 60.

See David Streit's full forecast through the weekend. And if you haven't already, join us on Facebook and follow us on Twitter.

April 90s: Not counting 2010, there were 51 occurrences of 90+ at D.C. since 1872, or about 1% of all April days. With this year included, there are 14 Aprils that have had two or more days above 90. The last time D.C. had 2 or more was in 2002 when 3 days in April topped 90. The top Aprils on record for 90+ are 1960 and 1976 with 5 each. Snow fans: The 13 winters that followed Aprils with 2 or more days of 90+ averaged 21.4" (current norm: 15.2"), with 3 of the winters being very significant (40"+).

By Ian Livingston  | April 8, 2010; 8:45 PM ET
Categories:  Forecasts  
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Next: Forecast: Welcome back, spring!


what causes the "DC Split"? I am new to the area but have always noticed this phenomenon where ever I am--growing up in central NJ and going to college in Boston--it always seems that a line or a cell would be bearing down on me and last minute will split to the north and the south. My family jokes that I am a weather "bubble", but I would love to know the atmospheric reason for this. Does it have something to do with geography and vicinity to mountains/waterways?

Thanks for the expertise--I have been wondering this for over 10 years!

Posted by: amandap2986 | April 8, 2010 4:09 PM | Report abuse

The so-called "D.C. split" is more myth than reality, although there is some truth that the northern edge of Fairfax County and southern Montgomery County (roughly, the Potomac River)seems to be a favored track for heavy thunderstorms.

What is fact, however, more than myth, is that the Appalachians often have a marked weakening efect on Midwestern squall lines as they move over and east of the mountains.....often, the heaviest storms and to strike the D.C. area form along the Blue Ridge (usually at mid-day or the early afternnoon)and move east, hitting this area between 4 and 6 PM.

There are a number of theories why the mountains weaken Midwestern squall lines as they cross, but the most likely one is that the mountains disrupt the so-called "dry line" of dry air at mid-levels which is needed to create classic severe storms and squall lines. The dry line, along with the tilted-updrafts due to vertical wind-shear vastly increases instability in the thundercloud, and the potential for large hail, strong winds, and tornadoes.....though tornadoes also need horizontal as well as vertical shear.

Posted by: MMCarhelp | April 8, 2010 4:37 PM | Report abuse

I'm not sure it's a total myth, but you can probably claim most places see splits in convection from time to time as convection is not often completely widespread by it's very nature. Leeward sides of mountains generally have issues with precipitation, partly because upslope flow tends to "rain out" systems and then on the other side you have compressional drying. The dry line issue may be part of it, but the lee trough often acts like a dryline and is usually where we get big storms on rather than the front itself.

In this case it seemed a possibility because the parent surface low is quite far to the north and there is limited low-level moisture. So, in a situation like that you can get storms forming south of the low and then again well south in the higher moisture area ... leaving the area in between somewhat drier.

Posted by: Ian-CapitalWeatherGang | April 8, 2010 4:54 PM | Report abuse

Let's hope those storms come later rather than sooner. I had my umbrella and raincoat at the ready this morning (because of course I had read CWG and knew what to expect!), but of course managed to leave the house without them in hand. School ends tonight around 8:30-9:00 and I am parked a football field away....

Posted by: Snowlover2 | April 8, 2010 4:59 PM | Report abuse

Getting some light rain now across the Blue Ridge. Temp=71.2F; Pressure 29.6 in

Posted by: spgass1 | April 8, 2010 5:12 PM | Report abuse

Centreville VA is very cloudy, very warm, and very windy.

Posted by: FIREDRAGON47 | April 8, 2010 6:09 PM | Report abuse

First batch went north but stuff near Charlottesville looks to make it in over the next hour or so.

Posted by: Ian-CapitalWeatherGang | April 8, 2010 6:43 PM | Report abuse

Aiyyo look at the radar we're about to get clobbered

Posted by: fleeciewool | April 8, 2010 6:56 PM | Report abuse

We're experiencing a thunderstorm with very heavy rain and gusty winds at the moment.

Posted by: david_in_stafford | April 8, 2010 7:23 PM | Report abuse

Centreville VA Heavy rain with one good lightning flash/thunder grumble.

Posted by: FIREDRAGON47 | April 8, 2010 7:37 PM | Report abuse

Storm coming into DC just warned.

Posted by: Ian-CapitalWeatherGang | April 8, 2010 7:49 PM | Report abuse


I believe it was you who went to see the Bluebells at Bull Run. How were they? I am planning to visit Turkey Run this weekend along the Potomac and absorb the beauty of the ephemeral flowering plants. Hopefully I am right on time. We know how fleeting the window can be.

Posted by: pjdunn1 | April 8, 2010 7:50 PM | Report abuse

I hope this storm makes its way right up 95 to me!

Posted by: Brian-CapitalWeatherGang | April 8, 2010 7:56 PM | Report abuse

Thunder here in Silver Spring.

Posted by: Murre | April 8, 2010 7:56 PM | Report abuse

And light to moderate rain.

Posted by: Murre | April 8, 2010 7:59 PM | Report abuse

PJDunn: They were pretty nice... I imagine they'll still be looking good this weekend... a couple photos from last Sunday... hope you have a good trip... sounds like the weather will be great

Today I noticed the bloodroot are past peak, but rue anemone are coming out... too breezy for photos today... the trillium are normally spectacular in early May

Temp dropped to 58.3F so far tonight...

Posted by: spgass1 | April 8, 2010 8:04 PM | Report abuse

DCA has verified the warning, gust to 59 mph.

Posted by: Ian-CapitalWeatherGang | April 8, 2010 8:05 PM | Report abuse

Tornado Watch now extending northeast from the NC/SC border to *roughly* Louisa, Virginia.

Posted by: Murre | April 8, 2010 8:07 PM | Report abuse

While the rain pours down outside, Ian's statistical evidence that hot Aprils are sometimes followed by winters with 40"-plus snowfalls is balm to my allergy-plagued sinuses. Just wish I could hop a plan to Michigan's U.P., where they are getting lake effect snow this evening.

That 1976 mid-April heat wave was truly awful and as noted in an earlier thread, the winter that followed was something else.

Posted by: JerryFloyd1 | April 8, 2010 8:09 PM | Report abuse

Two tornado warnings (one now expired) southwest of Farmville, in storms which look like they are heading towards Richmond and Fredericksburg.

Posted by: Murre | April 8, 2010 8:12 PM | Report abuse

Sounds like some trees down in the city... storm bowed out as it was moving in.

Posted by: Ian-CapitalWeatherGang | April 8, 2010 8:12 PM | Report abuse

In Reston, VA, we are very often favored by this "split", and I see it ALL the time. More than that even, Ashburn, VA, Leesburg, VA and MoCo MD will often receive heavy cells, full of large hail and such, yet we are typically only skirted by these events. Most storm systems here move in a ENE path, and that at least explains the Mid-Loudoun Co. VA/MoCo MD being slammed aspect, whereas we are often spared.

I am grateful for this "anomaly" as well, because there were a few years in a row about a decade ago, where we actually received multiple close-calls with tornadoes and the like, and even though I am a weather fanatic (Including severe weather), I don't want my house being wiped out.

One notable aspect of a possibility behind the "split", is the fact that Reston, VA exists upon a ridge (Between 400-500' + above Sea Level), whereas West of here (ie, IAD, Ashburn, VA, etc.), they are within a valley (Which averages around 200' above Sea Level). To the West of that, they are yet again bordered by ridges, foothills, and mountains. Seeing how a system (Including Tornadoes) often takes the path of least resistance (Other factors aside of course), the shaping of this valley directly coincides with the typical ENE storm tracking, and subsequently the "split" as I often observe it.

Btw, we just experienced our storm here, and despite the SVR Storm Warning, we managed to keep to the fundamentals (Some lightning, thunder, and a considerable downpour). The precipitation rate has stayed pretty consistent at right around .25" an hour (For the past 20 minutes).

What a bummer though, I kept the cover off my car so that the rain would wash away the pollen, and what am I left with? Flower petals from the Pear Tree, and all kinds of garbage. Good thing I checked my rain funnel before this storm as well, because despite the debris trap I have in place, it was still clogged by the small pieces of flowers from the trees. Hopefully the pollen will be down after this though, and hello cooler weather!

Posted by: TheAnalyst | April 8, 2010 8:18 PM | Report abuse

Wow, a 10 Degree Fahrenheit drop in less than an hour!!! Sweet. Some snow is also being forecasted for the Virginia Blue Ridge through Firday, and WV as well.

Posted by: TheAnalyst | April 8, 2010 9:06 PM | Report abuse

I'm checking in here for some rainfall totals . . . . but no love!

Posted by: ah___ | April 8, 2010 11:27 PM | Report abuse

The Analyist, what you are describing is not a "split", but pretty much what I described in my previous post.....the tendency, on a NW-SE course, for the heaviest storms to follow a Northern Fairfax-Southern Montgomery path; more or less along the river. This is dictated more by the movement of the whole cell, not necessarily a split.

Also, tornado paths are not dictated by ground level, such as valleys of ridges. Tornado paths are dictated by the parent storm's general directon of movement and where the rising air in a thunderstorm reaches its maximum tendency to spin and form wall clouds and funnels.

Posted by: MMCarhelp | April 8, 2010 11:51 PM | Report abuse

I saw there is a freeze watch for portions of WV and SW VA, but I'm skeptical boundary layer air will be cold enough for snow... Bulk of moisture appears to already be east of the Blue Ridge...

Current conditions: 50.5F and drizzle

Posted by: spgass1 | April 8, 2010 11:59 PM | Report abuse

No lightning or thunder in Woodbridge, just plain old rain. Light to moderate to steady nothing unusual, pretty decent temp. drop no where near any kind of record. Gusty in the afternoon then it died.

Posted by: zcxnissan | April 9, 2010 12:06 AM | Report abuse

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