2018 Mid-Atlantic General Severe Discussion

[mappy]

23 minutes ago, mappy said:

a body was found in the Patapsco River today

 

confirmed to be the missing National Guardsman who was helping. 

[GramaxRefugee]

3 hours ago, mattskiva said:

I would frankly find it very surprising to learn that there are states that are more regulated than Maryland, in any capacity.  Well, maybe California, but even that has to be pretty close.

 

This is true. I've been doing SWM for 35 years in several states, and I can tell you that MD is much more difficult than PA, VA, WV, NY, NC, SC, and I guess others that I don't have experience in.

However, difficult does not equal good. Bureaucratic does not equal good either.

Besides, as has been pointed out, SWM is not done for 12" of rain. Our 100 year event is 7.3 (or 8.5, depending what you use) in 24 hours.  No amount of hydrology can stop this. Hydraulics, OTOH, (the sizing of culverts and other conduits) could be improved, if there is room.

Ironically, a local stormwater group, Chesapeake Stormwater Network , http://chesapeakestormwater.net/category/publications/csn-technical-bulletins/  had their office in EC. Took almost a year for them to return after the 2016 flood (if I have that right).

[smokeybandit]

1 hour ago, mappy said:

a body was found in the Patapsco River today

 

RIP

[North Balti Zen]

I don't want to engage, really, but maybe slow one's roll with dismissing too quickly the effect that development in terms of sub-divisions and shopping centers in the hills above Ellicott City has had on what has happened in Ellicot City with the last few floods. There has been enough forest and fields lost to that development that the amount of water that now cascades into those tributaries is markedly different than where it was even 15 years ago. The previous issues that Ellicott City has had with flooding historically were from the Patapsco filling up and backing into the low end of Main Street. What happened in 2016 and over the weekend was very very different - run-off from the higher ground filling and over-whelming the streams running through the town.  That is a development issue, and that has been a point of contention for locals there as the developments have been green-lit.

[osfan24]

20 minutes ago, North Balti Zen said:

I don't want to engage, really, but maybe slow one's roll with dismissing too quickly the effect that development in terms of sub-divisions and shopping centers in the hills above Ellicott City has had on what has happened in Ellicot City with the last few floods. There has been enough forest and fields lost to that development that the amount of water that now cascades into those tributaries is markedly different than where it was even 15 years ago. The previous issues that Ellicott City has had with flooding historically were from the Patapsco filling up and backing into the low end of Main Street. What happened in 2016 and over the weekend was very very different - run-off from the higher ground filling and over-whelming the streams running through the town.  That is a development issue, and that has been a point of contention for locals there as the developments have been green-lit.

I'm sure it was somewhat of a factor or exacerbated the situation, but the bottom line is they had a very intense, long-duration event and the topography of the area literally checks all boxes in terms of what you would look for when it comes to a town and flash flooding. It is basically built in a valley surrounded on all sides by steep hills, is a narrow street lined with buildings, sits next to a major river and is also built on top of a river. It's essentially sitting in the bottom of a bucket or bowl. Fill up cups of water and dump them into a bowl from all sides and watch what happens.

The other evidence that this is almost entirely topography driven is that nearby Catonsville received even more rain than Ellicott City. While there was some flooding, it was not the catastrophic damage we saw on Main Street. Catonsville is more developed than Ellicott City, but it doesn't have buildings that sit near the Patapsco, nor is it built on top of a river.

[North Balti Zen]

Last word on this from me. Yes, Ellicott City is in a bowl. Which means it is extra precarious there to take away the forest and field cover on the hills above the town that used to be so important to control run off from heavy rains. The extra acreage (thousands and thousands of acres) that are now hard surfaces that cause and exacerbate run-off rather than help to mitigate it are a huge issue for Ellicott City. The people that live in that town have been raising the alarm on that issue for years. It's a problem for them. Not sure what can be done about it now - for better or worse, the developers were allowed to develop, and in most cases without proper mitigation as regards run-off. It could be the development above the town means that the town itself may have to be largely abandoned - not sure how any property there gets insurance after this.

[CAPE]

Obviously the topography cant be controlled, but the land development in the elevated areas all around the town absolutely exacerbates the threat- all the concrete and roads and artificial drainage to keep those areas dry(and in turn reduces the natural absorption/drainage) has forced more run off down into that town and it was already precarious by the fact it sits at the convergence of multiple stream/river valleys. Its also just really bad luck this happened yet again. I made a post a couple days before this, based on the modeled potential for slow moving/training storms, that I hoped it all would manage to avoid Ellicott City. Of course I really never thought this would happen again, just 2 years later. Really sad.

[EastCoast NPZ]

7 hours ago, Eskimo Joe said:

Agreed on the rainfall number, but the tenacity of the event is magnified by human decisions and the terrain.  10" - 12" on Delmarva is just a pond...not a raging torrent.  There are certain businesses that will probably have to be torn down this time.  

Agree that the tenacity is probably somewhat magnified, but the root-cause is still topographical and meteorological.  That much rain, and more, could never produce a raging torrent on pancake-flat ground like what is found on Delmarva.  EC has hills upstream that funnel down to a major river, upon which they've built a town.  Obviously, if there was no development, then there'd be nothing to be impacted, but there'd by a raging torrent headed downstream regardless.

[GramaxRefugee]

3 hours ago, North Balti Zen said:

... and in most cases without proper mitigation as regards run-off....

Don't know what year you have in mind, but HoCo has required 2,10,100 year attenuation since the 1970s. I can think of some spots without SWM (1960s) but not much. And, if it is redeveloped, even the existing impervious is required to be managed at 75%. 

You can't even build a single lot house without SWM nowdays.

Can't explain in a combox, but just trying to help.

Clik on that CSN link I posted. They're sort of a fun group to read.  

[87storms]

5 hours ago, osfan24 said:

I'm sure it was somewhat of a factor or exacerbated the situation, but the bottom line is they had a very intense, long-duration event and the topography of the area literally checks all boxes in terms of what you would look for when it comes to a town and flash flooding. It is basically built in a valley surrounded on all sides by steep hills, is a narrow street lined with buildings, sits next to a major river and is also built on top of a river. It's essentially sitting in the bottom of a bucket or bowl. Fill up cups of water and dump them into a bowl from all sides and watch what happens.

The other evidence that this is almost entirely topography driven is that nearby Catonsville received even more rain than Ellicott City. While there was some flooding, it was not the catastrophic damage we saw on Main Street. Catonsville is more developed than Ellicott City, but it doesn't have buildings that sit near the Patapsco, nor is it built on top of a river.

flooding and white water rapids flowing down a main street are pretty different.  i'm not gonna say i'm right...not enough knowledge in this area...but i gotta lean towards development off the bat as being a significant issue here.

[yoda]

Channel 5 running lead story on this issue

[H2O]

Development is the main culprit with EC.  When you look at other historical floods for them they are mostly river based and not from water flushing down the hills and streets.  Very little is required of developers for making sure that the down stream infrastructure is adequate for the size of neighborhoods they want to build.  So you have land changing from permeable to needing SWM.  The thing about that is the velocities of the run off has changed drastically.  A new development will get that water off its area and into the natural drainage a hell of a lot faster than if it were trees and soil.  Once you get a ton of water moving at a fast speed it creates its own path.

Oddly enough, a storm drain 85% full carries more water than a pipe 100% full. 

[smokeybandit]

The gofundme for Hermond is approaching its 25k goal in less than 24 hours

[EastCoast NPZ]

1 hour ago, H2O said:

Development is the main culprit with EC.  When you look at other historical floods for them they are mostly river based and not from water flushing down the hills and streets.  Very little is required of developers for making sure that the down stream infrastructure is adequate for the size of neighborhoods they want to build.  So you have land changing from permeable to needing SWM.  The thing about that is the velocities of the run off has changed drastically.  A new development will get that water off its area and into the natural drainage a hell of a lot faster than if it were trees and soil.  Once you get a ton of water moving at a fast speed it creates its own path.

Oddly enough, a storm drain 85% full carries more water than a pipe 100% full. 

Odds are that those past floods didn't feature a foot of rain in 3 hours.  Big difference in mechanisms for a general river flood vs. A flash flood event.

[Eskimo Joe]

Slowly getting interested in the flooding and conditional severe potential for the Thursday - Saturday timeframe.  SE flow + lift + remnant tropical system = intriguing.

[yoda]

24 minutes ago, Eskimo Joe said:

Slowly getting interested in the flooding and conditional severe potential for the Thursday - Saturday timeframe.  SE flow + lift + remnant tropical system = intriguing.

Meh on the conditional severe... don't see much risk for anything storm-wise then heavy rain and lightning

[George BM]

Eskimo Joe intrigued while Yoda is 'meh'?! 

What universe am I in and how do I get back to the correct one?

[Kmlwx]

EJ has been much more bullish on events this year so far. 

[yoda]

12z NAM soundings suggest poor lapse rates and marginal shear... I guess we could see an isolated wind damage threat though water-loading as LWX mentioned in their morning AFD... but I don't see any other risk for severe at all during the time period EJ mentioned

[osfan24]

1 hour ago, Eskimo Joe said:

Slowly getting interested in the flooding and conditional severe potential for the Thursday - Saturday timeframe.  SE flow + lift + remnant tropical system = intriguing.

Yeah the NAM and GFS are both hinting at that, especially Saturday, though obviously out of NAMs range.

[Eskimo Joe]

I'm not "in", just intrigued.  I agree with @yoda that severe is incredibly conditional, but whenever we get these kinds of setups you can get sneaky wet microbursts or even a lone spinny.

[Eskimo Joe]

WPC puts us in the slight risk for flash flooding Thurs & Fri

[Mrs.J]

5 hours ago, H2O said:

Development is the main culprit with EC.  When you look at other historical floods for them they are mostly river based and not from water flushing down the hills and streets.  Very little is required of developers for making sure that the down stream infrastructure is adequate for the size of neighborhoods they want to build.  So you have land changing from permeable to needing SWM.  The thing about that is the velocities of the run off has changed drastically.  A new development will get that water off its area and into the natural drainage a hell of a lot faster than if it were trees and soil.  Once you get a ton of water moving at a fast speed it creates its own path.

Oddly enough, a storm drain 85% full carries more water than a pipe 100% full. 

So somewhere I heard that the mandate that required developers to put in SWM ponds in new neighborhoods is no longer in effect? That is was done away with 10 years ago?

My neighborhood started building in 2007 so just before the time frame doing away with said requirement. We have 5 SWM ponds on my side of the neighborhood alone. The East Side of our community, my community is split by Ballenger Creek Pike, has 4 and the new neighborhood behind that which only started building in 2016 has put in 3. They are amazingly effective. We are bordered by Ballenger Creek which has been prone to flooding. The 8+ inches we had on that deluge back on May 15th all the ponds were full but by morning because of the drainage they put into the bottoms all the water was gone and Ballenger Creek was only flooded from what was coming from upstream. 

I guess what I am asking is it true that the requirement to build SWM ponds was done away with? Because if so that is just as stupid as taking away the fee imposed on builders per dwellings for money towards schools. 

[yoda]

SPC moves MRGL into I-81 corridor and west of LWX CWA in Day 2 1730 OTLK... mentions chance for some locally damaging winds due to ample CAPE but weak shear/flow aloft

[Eskimo Joe]

Someone's gonna jackpot in the rainfall category with a setup like this.

Quote

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
Concern for heavy rain and flooding this weekend continues to grow. 

Blocking over the northwest Atlantic will help to create a cutoff 
upper low on Saturday, which will slide south across our area 
through the day. A surface low will begin to strengthen, in 
association with the aforementioned upper low, and move from 
northwest to southeast across the region along a stalled front 
draped across the region. This will be the driver for showers and 
thunderstorms throughout the day on Saturday. Precipitable water 
values will remain abnormally high throughout the day on Saturday, 
with values reaching 1.75-2 inches. This, coupled with weak steering 
flow and deep warm cloud layers, will provide a heightened risk for 
slow-moving convection across the area. With the ground already 
saturated in many areas due to recent heavy rains, the flood threat 
is even higher. 

The potential exists for an area of moderate to heavy stratiform 
rain to develop Saturday night into early Sunday morning as the 
surface low strengthens and begins to drift offshore. However, due 
to uncertainty in the exact track at this time, it is uncertain if 
or where this might set up. Showers will then linger into Sunday as 
well, keeping temperatures much cooler, with highs only in the low 
to middle 70's. 

Model consensus suggest that the low pressure system/front and 
attendant heavy rainfall risk should exit to the south and east by 
early next week as high pressure noses in from southeastern Canada 
and the eastern Great Lakes. Temperatures should return to average 
by mid-week. 

 

[yoda]

Probably going to see FFW's for most of the LWX CWA issued tomorrow morning IMO

[Eskimo Joe]

16 minutes ago, yoda said:

Probably going to see FFW's for most of the LWX CWA issued tomorrow morning IMO

Agreed.

[CAPE]

1 hour ago, Eskimo Joe said:

Someone's gonna jackpot in the rainfall category with a setup like this.

 

Mount Holly highlighting the threat in their afternoon AFD as well-

The forecast gets more complex Friday night into Saturday. Upper
level trough will dive south and east from the Great Lakes and
looks to evolve into slow moving closed upper level low near or
just south of the region. Meanwhile at the surface, a cold
front will move down from the north and stall somewhere in the
central to southern part of the CWA by late Saturday with an
associated low developing along it. This is a concerning set up
as these factors will likely lead to continuing showers and
embedded storms affecting much of the region. In fact
precipitation will likely be more widespread by this time with
instability decreasing north of the front over northern portions
of the forecast area. PWATs look to remain in the neighborhood
of 2 inches so this will unfortunately mean a continuing flash
flood threat with the threat of river flooding also a
possibility by the middle to latter part of the weekend given
the duration and more widespread nature of the precip by this
time. Will highlight this in the HWO. This is still a few days
away and forecast models not in perfect agreement on the details
but all are indicating this increasing potential of widespread
moderate to heavy rain over the weekend.

[Eskimo Joe]

I'm liking this setup.  GFS is usually bone dry...if it gets wet watch out.

[yoda]

16 minutes ago, losetoa6 said:

Gfs really closes off at 500 over us Saturday and look at 700 mb moisture core ..it's right over us .Interesting like others have said. Someone could get crushed .

. 

I dont think I have ever seen that before