Sunday Severe Weather Nowcast

6:40 PM:

Most of the region is still under a severe thunderstorm watch until 9pm EDT. A couple of tornadoes, winds up to 70 mph, and marble-sized hail is possible. The rest of the area is in the wind advisory until 4 AM. Severe thunderstorms are expected to continue throughout the night in those pinkish areas. Staying indoors and not traveling is advised throughout the DMV.

5:07 PM:

THERE IS A SEVERE THUNDERSTORM WARNING FOR MOST OF MOCO, AND FOR A LOT OF BALTIMORE AND CAROLL COUNTY! WIND UP TO 60 MPH AND HAIL THAT IS PENNY SIZED IS POSSIBLE! GET INDOORS, GET RESOURCES, AND STAY SAFE NOW! A WARNING MEANS DANGEROUS WEATHER IS IMMINENT, AND IS OCCURING SHORTLY OR NOW!

5:05 PM:

THERE IS A SEVERE THUNDERSTORM WATCH FOR MOST OF THE REGION EXCEPT FOR OUR WESTERN REGIONS! A COUPLE OF TORNADOES ARE POSSIBLE, HAIL UP TO SMALL MARBLE SIZE, AND WIND GUSTS UP TO 70 MPH ARE POSSIBLE!! GET INSIDE NOW AND STAY SAFE! THIS IS UNTIL 9 PM EDT SUNDAY! THE SQUALL LINE HAS MOVED INTO NW MOCO AND WILL BE MOVING SOUTHEAST! GET INDOORS AND STAY SAFE NOW! 

4:55 PM:

If you are in Northern MoCo, and not in the severe thunderstorm warning, I would still advise you to stay indoors and get ready for one. Radar indicates Northern MoCo getting directly hit by the heavy precip of this squall. Even in Southern MoCo, I would advise you to be careful. Gusty winds, hail, and possibly even a tornado are possible with this line.

4:40 PM: 

TORNADO WARNING FOR EMITTSBURG AND TANEYTOWN! GET INSIDE AND SEEK SHELTER NOW! PENNY SIZED HAIL IS POSSIBLE!

4:35 PM:

THERE IS A SEVERE THUNDERSTORM WARNING FOR CLARKSBURG MARYLAND! WIND UP TO 60 MPH AND PENNY SIZED HAIL POSSIBLE! GET INDOORS NOW! THE SQUALL LINE WILL HIT NORTHERN AND WESTERN MOCO! A WARNING ALSO MAY BE ISSUED FOR AREAS MORE SOUTH OF CLARKSBURG SO BE VARY!

4:10 PM:

This squall line may reach northwestern MoCo; however it probably wouldn't be severe, and it will be brief. Precip will likely be moderate, but will quickly dissipate if it even reaches it.

3:55 PM:

SEVERE THUNDERSTORM WARNING FOR FREDERICK, BOONSBORO, AND HAGERSTOWN! WIND UP TO 60 MPH AND PENNY SIZED HAIL IS POSSIBLE. GET INDOORS NOW!

3:45 PM:

This line needs to be watched very closely. As of now, it looks like it will miss MoCo and may touch some of Frederick county, but as it gets closer, we will know if MoCo is getting hit. This squall line has up to 60 DBZ rates and may be severe.

3:00 PM:

The National Weather Service in our region has put a timeline on when to expect severe weather. Most of MoCo and our region will see severe weather around 4-6 PM, while the southeastern part of MoCo will see severe weather around 6-8 PM. The rest of our region will see severe weather around 6-8 PM, and for our far western areas, you might see it at 2-4 PM.

1:30 PM:

The SPC has put a mesoscale discussion for most of our area, including all of MoCo. It says that damaging wind gusts will happen and a tornado or two is possible this afternoon. The SPC expects 500-1000 j/kg MLCAPE to develop with "low-level moist advection of dewpoints in the 60's F." Even with cloud cover that is increasing, destabilization is expected to occur in the afternoon from VA all the way to the PA border. High resolution guidance suggests some isolated thunderstorms may form from the rain shower activity as the pre-frontal through moves eastward. Storms that develop have the potential for damaging wind gusts and maybe even a isolated, brief tornado, because of the strong low and mid-level flow across a lot of the Mid-Atlantic. A lot of doubt remains due to weak destabilization and low confidence. Another round of tstorms may develop as the cold front moves east across central VA later this afternoon, and although conditions on the surface are cooler and drier, models show hodographs that support damaging wind/gusts with linear storms "along and ahead of the front later this afternoon". And one thing that I really wanted to spot out was "Trends will be monitored for a possible weather watch this afternoon/evening". All quotes were from the SPC and this paragraph has been trying to paraphrase the SPC's words.

12:45 PM:

The SPC has updated their maps! Let's see what it shows.

The SPC has reduced the slight risk range to include almost none of Frederick county, and has also put a part of Northern Loudoun County in the "Marginal" area. All of MoCo and points south are in the slight risk, but our northwestern regions are in the marginal risk, and some of western Maryland is even in the "non-severe" thunderstorms risk! They have moved the marginal further north, into New York and even some of southern New England!

The SPC has most of our region in the 2% risk for any location to see a tornado 25 miles away from a certain point, while the southern parts of our region are in the 5% risk for any location to see a tornado 25 miles away from a certain point. Only our far northwestern regions are in the less than 2% to see a tornado.

Most of our region are in the less than 5% chance to see hail 25 miles away from a certain point, but the southern part of our region is still in the 5% chance to see hail 25 miles away from a certain point. Hail isn't the threat here.

This is why most of our region is in the slight risk; due to the damaging winds. We have a 15% chance to see 65+ mph winds 25 miles away from a certain point, and our northern regions are in the 5% chance to see 65+ mph winds 25 miles away from a certain point. The mountains are in the less than 5% risk to see damaging winds!

FROM the SPC: 

"...Mid-Atlantic into southern NY this afternoon/evening... As previously mentioned, the degree of destabilization is more in question with northward extent across VA/MD/DE toward eastern PA and NJ. Surface temperatures in the 65-70 F range will be needed to much in the way of SBCAPE and a damaging-wind threat with a forced band of convection along the cold front. Will maintain the character of the previous outlook, but the northward extent of the severe threat will depend on some cloud breaks this afternoon. The Marginal area has been expanding some into southern NY to reflect the potential for just enough low-level warming/moistening to support minimal SBCAPE and the low chance for strong/damaging gusts with a shallow convective band along the cold front."

They think that the cloud breaks in the afternoon will determine if the northern regions get severe weather, which is what I think too. They have extended the marginal risk into southern NY for the potential to support a small amount of instability and the chance for some strong/damaging wind gusts.

12:30 PM:

The second course of precip is slowly starting to fill in on the LWX radar, and will be moving into the are soon, in the next 1-2 hours. Storms will be starting to form, some severe, and 2-6 PM will be the prime time for severe storms to occur. Tornadoes are possible too.

12:15 PM:

The squall line is seeming to be missing Northern MoCo, or at least the heavy precip part of it. Even still, Southern MoCo definitely got hit.

12:00 PM:

That isolated squall line has reached the far SW part of our county! It will be moving northeast, and most everyone should see some moderate - heavy rain before it leaves quickly. There probably won't be any thunder/lightning and it will not be severe.

11:50 AM:

That line that I circled needs to be closely watched. Theres already many squall lines forming, one in western PA and one thats coming in our way. That is the front/boundary with storms forming on it, and some of them could be severe. The second course of precip is slowly moving northeast, but isnt close enough to be picked up by the LWX radar.

11:35 AM:

An isolated squall line is moving northeast, and has MoCo right in its path. This squall line contains pretty heavy precip, with 30-40 DBZ rates. It will come in fast and come out fast. Some thunder/lightning may be possible with this line. Probably not severe though.

10:30 AM:

The SPC has mentioned downgrading some of their risks for the entire region, and they have removed the signature/hatched risk for the entire area effected by severe weather. I would like to point out a mistake that I made in my previous post; I referred the wind risk by MPH, when it should have been knots. The hatched area means a 10% or more risk of wind speeds 65 knots or more(75mph+). Here are the latest SPC maps:

Slight risk for most of the region, with a marginal risk for the northern parts of our region.

A 5% risk of a tornado from any location 25 miles away from a certain point in the brown area, and for our northern area, its a 2% chance of a tornado from any location 25 miles away from a certain point in the green region.

There is a 5% chance in the green area for 1" or more in diameter hail from any location away from a certain point, and less than 5% elsewhere.

The main threat here is damaging winds, but the hatched area for more severe winds has been removed. You still need to be taking this seriously, as these are still chances for 65+ mph winds. Most of our area is in the 15% chance of 65 or more mph winds 25 miles away from a certain point in the yellow range, with our northern regions being in the 5% chance, and just south of us in the 30% chance range. The greatest threat is in the red.