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2022 Mid-Atlantic Severe Wx Thread (General Discussion Etc)


Kmlwx
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So Wednesday evening into Wednesday night looks interesting per afternoon LWX AFD 

The cold front approaching from the west is forecast to pass through
our region sometime late Tuesday evening and into Wednesday. There
remains some uncertainty on the timing of the frontal passage and
how far south of our region it will sink on Wednesday. Based on
current guidance, the front should pass fully through the region
during Wednesday morning. The GFS and Euro models have the front
stalling just south of our region on Wednesday while the NAM moves
the front further southward along the NC/VA border. This difference
in location will be a big factor for precipitation during the day on
Wednesday. The more southern solution for the NAM, means a strong
westerly flow is able to form which will keep most of our region dry
during the day while the GFS/Euro solutions would mean our region
could see continued chances for showers. Went middle ground with
Pops and kept mainly chance/low end likely pops for Wednesday.

The big question for Wednesday evening will be how far north the
front is able to move as it returns northward. As the front lifts
northward, a surface low is forecast to form along the boundary as
an upper level shortwave passes overhead. The GFS and Euro are more
progressive with the northward propagation of the front while the
NAM keeps the boundary overhead of our region Wednesday evening. It
seems the differing solutions will mean the difference between an
increased SVR threat and increased flood threat. The GFS and Euro
would mean a bigger SVR threat due to increased CAPE and shear along
the boundary.  The NAM solution on the other hand would mean a
bigger flood threat. Both the 6z and 12Z NAM are highlighting areas
south of I-66 for the potential for rain amounts of 2 to 5 inches in
a 12 hour period. We should have a better idea of the overall threat
for SVR and flood once Wednesday moves into the range of the high-
Res guidance.
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Snippet from Mount Holly AFD this morning-

Things get potentially more interesting for Wednesday night. A new area of low pressure looks to take shape over the midwestern states along the lingering frontal boundary. This will then deepen as it moves northeast towards eastern PA through Wednesday night. There are still model differences regarding the strength and track of this system but the general trend appears to be towards a stronger system . Both the GEM Reg and especially the GFS depict a robust shortwave driving this system as it eventually takes on a neutral to negative tilt by early Thursday. The system will be accompanied by widespread showers and storms by late Wednesday evening through the overnight and this could actually come through in multiple rounds. And in fact we have concerns for both the potential for severe weather and heavy rainfall with an associated flood/flash flood risk. This is because strengthening low moving right near the area could result in deep layer shear increasing to 40 to 50 knots. In addition both the GFS and GEM depict a strong LLJ at 850 mb advecting in PWATs of around 2 inches with this LLJ also helping to aid in increased low level shear. It is still too early to be confident in exactly how this will evolve as, along with the uncertainty regarding the strength of the system, a potentially limiting factor will be the timing during the overnight during the diurnal min of instability. Even so though, the strong low level warm and moist advection could still result in at least several hundred j/kg of CAPE which may be enough given the strong dynamics and shear.

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A couple of quick thoughts about Wednesday night:     We're definitely going to have an interesting convective overnight, as the strong wave lowers heights across the region and increases the shear.      If this were occurring during the day, we'd likely have a good shot at some severe, but the after dark arrival of the main forcing reduces that potential.     Right now, it looks like the best shear will arrive after the main round of convection, but if we can overlap storms with the improved shear, a few damaging wind gusts or an isolated tornado probably can't be ruled out.    For flash flooding potential, the rapidly increasing PW values certainly make it a possibility - my initial thought is that overall dry ground and quick movement of the system will likely limit the flash flooding potential, but it's certainly on the table, especially if there is some lead convection during the late afternoon hours before the main show later at night.

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11 hours ago, high risk said:

A couple of quick thoughts about Wednesday night:     We're definitely going to have an interesting convective overnight, as the strong wave lowers heights across the region and increases the shear.      If this were occurring during the day, we'd likely have a good shot at some severe, but the after dark arrival of the main forcing reduces that potential.     Right now, it looks like the best shear will arrive after the main round of convection, but if we can overlap storms with the improved shear, a few damaging wind gusts or an isolated tornado probably can't be ruled out.    For flash flooding potential, the rapidly increasing PW values certainly make it a possibility - my initial thought is that overall dry ground and quick movement of the system will likely limit the flash flooding potential, but it's certainly on the table, especially if there is some lead convection during the late afternoon hours before the main show later at night.

       Timing looks a bit different on the evening guidance.     Looks like the main batch of convection will occur more in the 7pm - midnight period.     We'll have a bit more instability to work with, but wind profiles look very meh.   So widespread severe seems unlikely, but perhaps the flash flooding potential is a bit higher.    The bigger question is what happens ahead of the cold front in the early morning hours.    Wind profiles will strengthen significantly, and while instability won't be impressive, there might be just enough to favorably interact with the shear.    This is probably our better chance for SVR, although the CAMs don't have a lot of storms for that time - the better chance seems to be in northern MD.

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  The late afternoon / evening seemed to always have limited potential here.    Shear is weak, and CAPE is modest;   a small amount of downdraft CAPE did admittedly keep the door open for a few wind gusts (and it's probably still open), but it wasn't a strong likelihood.

    I'm still watching the period much later tonight along the cold front.    The HRRR wants to sweep a nice line through northern and perhaps north-central MD.   CAPE will be limited due to time of day, but the shear will be much improved.      The HRRR is definitely more aggressive than other CAMs with this feature, so we'll see how it plays out.

 

hrrr_ref_frzn_neus_fh12-12.thumb.gif.644be2ee9c5f4baed8edf8c9ec826724.gif 

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2 minutes ago, Amped said:

Just got a Warning in Howard county for a Tornado.  There is a couplet though it doesn't appear to be too strong.

Might (probably) warned off BWI terminal. Just saw an LSR come out of a tree on a house in that area.

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I still don't understand how we ended up with a few modest supercells in the area.     The shear on the 00Z IAD sounding is nothing of note.    And the Howard County rotation suddenly emerged at the back edge of the complex that brought torrential rain to that corridor.     That cell was definitely not ingesting high theta-e air.

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