Severe WX April 22-24th, 2020 Severe Weather Threat

[Kory]

If the rumors of wanting to go high for Sunday were true, despite massive uncertainty and downtrending at issuance time, it's really alarming; hopefully the D2 will take into account the still less aggressive GFS and Euro trends

It was a screenshot from NWS chat. It was going to happen.

 

[Equus]

Yikes. Just not at all agreeing with their reasoning for that

 

[Taylor Campbell]

I’m eyeing my own backyard right now as a place to storm chase.

 

[Taylor Campbell]

Mods, can we please pin this thread to the top. It also wouldn’t be a bad thing to give me that type of access. Thank you.

 

[Tyler Penland]

Mods, can we please pin this thread to the top. It also wouldn’t be a bad thing to give me that type of access. Thank you.

Gotcha covered.

Sent from my SM-G960U using Tapatalk

 

[Taylor Campbell]

Gotcha covered.

Sent from my SM-G960U using Tapatalk

Nice! I appreciate that!

 

[NorthGaWeather]

SREF looks interesting as well. Hopefully we’ll have a better handle on things by tomorrow morning.

 

[MattW]

Why do the NAM and 3km NAM differ so much? I now the 3km is higher resolution, but the NAM has a SIGTOR of 9.6 just SW of Atlanta at one point, while the 3km doesn't really show any SIGTOR near Atlanta until later.

 

[Taylor Campbell]

This is very impressive from the 12z GFS ensembles. There’s not a single member that doesn’t show very significant severe weather parameters moving across central Alabama and most of the members show higher parameters than lower parameters.

 

[Bevo]

SPC just upgraded tomorrow's risk to enhanced.
Upgrade driven by a 10% hatched sig tornado, not a great situation for the DFW metro. Very densely populated areas within that corridor. HRRR and NAM do appear to show a strong cap in place to a point, however there's variations in the cloud cover models. Some models predict clearing skies around peak heating in the afternoon--hoping whatever happens on the mesoscale level tomorrow helps stave off the threat.

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE WEDNESDAY
AFTERNOON INTO WEDNESDAY NIGHT ACROSS PARTS OF SOUTHEASTERN
OKLAHOMA...MUCH OF NORTHEASTERN TEXAS...SOUTHWESTERN ARKANSAS AND
NORTHWESTERN LOUISIANA...

...SUMMARY...
Severe thunderstorms are expected to develop east of the Interstate
35 corridor of southern Oklahoma into north central Texas by late
Wednesday afternoon, before spreading southeastward across the
Ark-La-Tex and Piney Woods Wednesday evening, into the lower
Mississippi Valley by Thursday morning. This will be accompanied by
a risk for severe hail, a few tornadoes, and potentially damaging
wind gusts.

...Synopsis...
While mid-level ridging may begin to build across the eastern
Pacific, downstream flow across the eastern U.S. into the western
Atlantic may trend more zonal during this period. This is expected
to occur as a strong short wave trough within a distinct northern
branch of mid-latitude westerlies pivots east-northeast of the New
England coast. As an upstream perturbation rounds the crest of the
eastern Pacific ridging, and digs into the Pacific Northwest, a
fairly significant southern stream impulse is forecast to turn east
of the southern Rockies, toward to the middle/lower Mississippi
Valley Wednesday through Wednesday night.

The southern stream perturbation appears likely to maintain a
positive-tilt orientation, but models continue to suggest that it
will be accompanied by a slowly deepening surface cyclone, from the
southern Texas Panhandle vicinity early Wednesday morning into the
Mid South by late Wednesday night.

Initially cool/dry and stable boundary layer air, reinforced by a
cold front associated with the lead northern branch impulse, likely
will be entrenched as far south and west as northern Florida and the
northeastern Gulf of Mexico into the lower Mississippi Valley at 12Z
Wednesday. Some modification may already be underway across parts
of the southeastern Plains, but a more established return flow is
expected to be ongoing from across the lower Rio Grande Valley into
the vicinity of the developing cyclone. And this moisture return
may gradually broaden eastward within an evolving warm sector across
the lower Mississippi Valley by late Wednesday night.

The moisture return and associated destabilization, coincident with
large-scale forcing for ascent and strengthening deep-layer wind
fields and vertical shear, probably will support increasing severe
weather potential across the southern Plains into the lower
Mississippi Valley.

...Southern Plains...
Some uncertainties do linger, which could impact convective
evolution and associated severe weather potential. This includes
the potential for fairly strong capping associated with elevated
mixed-layer air, which may initially suppress convective
development, particularly along a sharpening dryline to the south of
the surface cyclone. This may also impact areas east of the
dryline, across much of eastern Texas, but considerable model output
appears to suggest that large-scale forcing for ascent, likely aided
by lower/mid tropospheric warm advection, may support increasing
convective development by midday across this region, before
spreading toward the lower Mississippi Valley. This activity could
be accompanied by some risk for severe hail, but probably would also
reinforce stable boundary layer conditions.

A consensus of model guidance does suggest that substantive
boundary-layer destabilization through much of the day Wednesday may
be initially confined to a fairly narrow corridor along the dryline,
roughly along/east of the Interstate 35 corridor of central/southern
Oklahoma into north central Texas by late afternoon. However,
mixed-layer CAPE of 1000-2000+ J/kg, in the presence of strong
deep-layer shear is expected to provide a conducive environment to
discrete supercell development.

Aided strengthening wind fields to 40-50+ kt in the 850-700 mb
layer, contributing to sizable clockwise curved low-level
hodographs, the supercells may be accompanied by increasing risk for
a few tornadoes, some strong, while spreading eastward into early
evening.

Thereafter, continued gradual low-level moisture advection and
boundary layer destabilization ahead of the convective activity,
coupled with mid-level cooling, may support the evolution of an
upscale growing convective system with a transition to increasing
potential for damaging wind gust, along with a continuing risk for
tornadoes. This is expected to overspread the Ark-La-Tex region and
Piney Woods of Texas through late Wednesday evening, before
progressing eastward into the lower Missisippi Valley overnight.

..Kerr.. 04/21/2020

I am particularly curious--most of the local forecasters say that if we get showers in the morning, it will help reduce the afternoon/PM threat. However, I've heard outflow boundaries left over from previous storms and daytime heating can intensify the tornado threat. How do morning/early afternoon showers work to limit the severe potential?

 

[Equus]

Well then. Could be a pretty sizable event both days...

 

[Bevo]

Well then. Could be a pretty sizable event both days...

Yeah, not even the October 20th event in the DFW Metro had hatched sigtor chances IIRC. The short term models aren't showing a ton of discrete cell development at the moment, save for a small window of 4-6 PM where there's a couple cells migrating across the west and south part of the Metro. I'm a little confused (but glad?) that it's not depicting development firing aggressively along the dryline in the Wednesday 18z-00z window. Why would this be? Is it because of the strong CIN? Some of the parameters on the soundings are INSANE and it's showing almost 5,000 MCAPE during that time.

 

[Jpgood20]

Just watched Brett Adair’s recent video on Facebook. He definitely shows some concern for this particular system as it moves into MS and AL. Pretty cool guy

 

[andyhb]

With the northern target tomorrow well documented near the dryline, some attention should be paid to the southern target in E TX and W LA, where several CAMs have a long-lived supercell passing through a pretty high end parameter space tomorrow afternoon into evening.

 

[Bevo]

With the northern target tomorrow well documented near the dryline, some attention should be paid to the southern target in E TX and W LA, where several CAMs have a long-lived supercell passing through a pretty high end parameter space tomorrow afternoon into evening.

This guy? Dryline comes to a halt, even "pinches" in some models after dark...

 

[Kory]

Whew those are some nasty lapse rates that slide in behind the morning round on the NAM. If convection fires and remains cellular, I could see a mega hail threat.

 

[Jack16]

Hazardous Weather Outlook
National Weather Service Peachtree City GA
1151 AM EDT Tue Apr 21 2020

DAYS TWO THROUGH SEVEN...Wednesday through Monday...

Strong to severe thunderstorms will be possible across north and
central Georgia on Thursday. Multiple rounds of severe weather
could occur from early Thursday morning through Thursday night.
There will be a potential for a few strong tornadoes, damaging
wind gusts, large hail, and flash flooding. There is some
uncertainty of timing and placement of these threats with further
details expected to become clearer in the days ahead.

Thunderstorms will also be possible again on Saturday, primarily
across central and eastern Georgia.

 

[Jack16]

Hydrologic Outlook
National Weather Service Peachtree City GA
939 AM EDT Tue Apr 21 2020


.IMPACTS...
With the expected rainfall amounts, isolated flash flooding is
possible, particularly along the the I-20 and I-85 corridors
where the rain is expected to be the heaviest. Periods of heavy
rain can overwhelm or clog storm drains and ditches with debris.
Take time ahead of the rain to clear leaves and debris from
drainage systems.

Minor flooding of some of the larger creeks or rivers is possible
with the forecast storm total rainfall, especially as some of
these channels are already high from the weekend rain. A few
rivers may experience a period of moderate flooding as well.
Quickly accumulating rainfall can also produce widespread flooding
of smaller, fast-responding creeks. Again, at this time, the
highest threat for flooding remains over middle Georgia.


.ACTIONS...
Stay alert to changing forecasts. A Flash Flood or Flood Watch
may be issued for parts of the area. Know what county you are in
and the names of rivers and creeks in your area. Do not be caught
off guard. Know what to do if a Flash Flood Warning is issued,
particularly if you live or are visiting an area near a creek or
river.

 

[Bama Ravens]

They've been on the money lately and I'm good friends with one of their convective forecasters. He's not sold on the threat and is thinking the NAM is out to lunch with its solution. Could change but BMX is skeptical and they're traditionally money with these types of events.

Oh, I trust the BMX guys, and they seem to be more accurate in their outlooks than SPC. I didn't mean for my comment to come across as a slight at them, but rather just to mention that, for the general public, the two maps are confusing when they look different.

 

[Taylor Campbell]

Oh, I trust the BMX guys, and they seem to be more accurate in their outlooks than SPC. I didn't mean for my comment to come across as a slight at them, but rather just to mention that, for the general public, the two maps are confusing when they look different.

Fortunately, I have the ability to tell which one currently stands to be more accurate and it is not BMX with that forecast.