Archive March 18-20th, 2018 Severe Weather

[Richardjacks]

The 0z NAM run in general seems extremely off. It plummeted the surface temps for some reason and the warm front is almost down on the coast at the beginning of the day, and I can't find a meteorologically sound reasoning as to why..

That can't be right. This might be part of the reason why we are seeing cape underforecated this season and the ongoing mesonet issue; Capes are going to be plenty high...that's not an issue here. Now the dry/warm air at the 850 mb or basically, a cap...might be an issue, but plenty of potential cape above

 

[rolltide_130]

The entire run seems weird.. it randomly kills the trough after 18z, the warm front starts the day on the coast, it RAMPS the LLJ up early before it just dies towards the afternoon... I'm personally tossing it for now unless the 06z comes in different.

 

[Richardjacks]

well there is a true trend to warm the 850 T's. This is causing lower cape and creating a cap on the sounding...

 

[Bama Ravens]

Can you please provide me the link to this graphic?

This is the link to the SPC outlooks highlighted by FEMA Regions (southeast is Region 4):

http://www.spc.noaa.gov/public/fema//images/

This is the link for the SPC outlooks highlighted by state:

http://www.spc.noaa.gov/public/state//images/

Quick explanation for these: swody1 is the day one outlook, swody2 is the day two, and so on.

 

[Bama Ravens]

SPC has added an enhanced risk into the Day 2 outlook (Monday). It includes Birmingham, Huntsville, and Nashville.

The enhanced risk includes a 30% hatched area

 

[rolltide_130]

Whether or not the 0z NAM is for real or just an accident is the million dollar question..

 

[Daryl]

Looks like a rough monday on Sand Mountain!

SPC AC 180600

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
0100 AM CDT Sun Mar 18 2018

Valid 191200Z - 201200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS MIDDLE
TENNESSEE...NORTHERN ALABAMA...AND FAR NORTHWEST GEORGIA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM TENNESSEE
SOUTHEASTWARD INTO GEORGIA AND NORTHERN FLORIDA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS MUCH OF
THE SOUTHEAST...

...SUMMARY...
Tornadoes, damaging wind and hail are expected across parts of
Middle Tennessee, much of northern Alabama, and northwest Georgia. A
strong tornado or two is possible during the late afternoon. Severe
wind is also possible into northern Florida.

...Synopsis...
A compact shortwave trough will move quickly from AR Monday morning
to the TN Valley by 00Z Tuesday, with a slight deamplification. Wind
profiles will increase throughout the day, with cooling aloft
overspreading KY, TN, and northern AL/GA. At the surface, low
pressure will move from OK across TN, with a cold front extending
from Middle TN across central MS by 00Z Tuesday. Ahead of the front,
dewpoints into the lower 60s F will reach as far north as TN, with
mid 60s F over AL and GA. To the east, a warm front will lift across
northern AL and GA during the day, allowing for destabilization.
Severe thunderstorms are likely near the surface low and along the
cold front, with the primary threat area from Middle TN into
northern AL and northwest GA during the afternoon.

...Middle TN into northern AL and northwest GA...
Scattered showers and a few thunderstorms may be ongoing Monday
morning from TN into northern GA associated with warm advection near
the warm front. This activity should lift north, allowing for
heating farther upstream across western TN and MS. If early day
storms are more widespread than expected, this could have an impact
on the position of the main threat area later in the day.

A minor capping inversion will be present around 850 mb according to
forecast soundings. This will keep the warm sector free of
additional precipitation contamination. Lapse rates aloft will
steepen as the shortwave trough approaches, and wind profiles will
strengthen creating a favorable setup for tornadic supercells. The
area of strongest lift, shear and instability will exist over a
relatively limited in area, but supercells that do occur will have
significant tornado potential, along with large hail. The eastern
extent will be limited by a wedge of cooler air from eastern TN
across northern GA. Storm density will likely decrease across north
central AL, farther away from the main area of lift, but a
conditional threat of supercells capable of a tornado or two and
large hail will exist. This threat may spread across eastern AL into
western GA during the evening, as warmer air spreads east resulting
in surface based instability.

 

[champal3003]

06z Nam looks pretty ominous with PDS soundings back after the 0z run.

 

[Daryl]

Area Forecast Discussion
National Weather Service Huntsville AL
455 AM CDT Sun Mar 18 2018

.NEAR TERM...(Today)
Issued at 455 AM CDT Sun Mar 18 2018

.SHORT TERM...(Tonight through Monday Night)
Issued at 455 AM CDT Sun Mar 18 2018

Continue to be weather aware for Monday`s forecast.

A shortwave will quickly swing from the Plains on Monday morning to
the TN Valley by Monday evening while the sfc low will glide into TN,
dragging a dry line into the area by afternoon.

After the warm front lifts through in the morning, southerly winds
will be in place, returning moisture. Model soundings show a cap
~850mb to help suppress convection ahead of the dry line despite
elevated instability being present. If convection from the warm front
lingers, or if there is any late morning convection, this would have
some effect on the available instability and moisture for the
afternoon/evening storms.

As the shortwave approaches, midlevel temps cool, wind profiles
strengthen and the 850mb cap erodes. The developing low-level jet
will help speed moisture recovery with dewpoints reaching the lower
60s. Not to mention it will provide deep layer shear of 60-70kts and
~30kts of 0-1km shear. Mixed-layer CAPE values should easily exceed
500 J/kg with a few short-range models suggesting higher values. This
combination will likely yield severe storms between 18-00Z Monday.
Hazards include all modes, especially large hail. Uncertainty still
exists for the tornado threat, as a wedge ridge along the
Appalachians could create backed surface winds which would enhance
the low-level shear. There is also still some uncertainty about where
storms will initiate, since any delay could push the threat area
further to the east. In short, this resembles almost more of a
Plains-type severe pattern than a typical Southeast pattern. Will
look for storms to develop across NE Miss and NW Alabama around
midday and push east into Georgia by late evening.

 

[Daryl]

Area Forecast Discussion
National Weather Service Birmingham AL
438 AM CDT Sun Mar 18 2018

.SHORT TERM...
Today and Tonight.

.LONG TERM...
Monday through Saturday.

Confidence that the area will see significant severe
thunderstorms continues to increase for Monday, as models are
coming into better agreement on the synoptic pattern. As usual,
there are some mesoscale details that will determine the exact
timing and type of threat. Models are coming into better agreement
with the timing and placement of a compact upper low that will
eject out of the western CONUS trough into northeast New Mexico
this afternoon. It will deamplify slightly as it moves eastward to
the TN/KY border region as it approaches confluent flow
associated with the eastern Canadian trough, but still remain a
potent system with strong height falls and a neutral tilt. A
strong WSW mid and upper level jet max (~90kt at 250 mb, ~70 kts
at 500 mb, ~65 kts at 700mb) will be located to its south, which
will move over Central Alabama. A ~35-40kt southwesterly LLJ at
850mb will accompany a 998mb surface low moving across Tennessee.
This surface low will be close to steady-state in strength during
the afternoon, before effectively splitting into two pieces as it
encounters the southern Appalachians Monday night. A trailing
pre-frontal trough/dry line feature will move into West Alabama
from Mississippi Monday afternoon.

Starting off Monday morning, not expecting to see a whole lot of
activity on the radar due to dry air and capping aloft associated
with an elevated mixed layer and weak shortwave ridging aloft.
Showers and storms associated with Sunday night`s shortwave will
likely be mainly north and east of the forecast area. But with
low-level isentropic lift there could be some light showers, and
perhaps some isolated leftover thunderstorms from Sunday night.
There will probably be some low stratus as well. One possible
wrench in the forecast is a secondary LLJ in the NAM and some of
the global models that these models show causing an increase in
moisture and isentropic lift across southeast Alabama and the
Florida panhandle during the late morning with associated
precipitation. This could affect destabilization in the afternoon,
or if accompanied by thunderstorms could result in an isolated
severe threat developing earlier than expected in the far
southeast counties. However, the CAMs are much further east with
this activity, closer to the Florida Big Bend and South Georgia.
These may have a better handle on the situation if they are
handling Sunday night`s preceding convection better than the
global models. Also, the models that show this feature all still
indicate very strong instability developing by Monday afternoon
across the entire area, so it may not matter much.

Most of the day on Monday may be a nice warm sunny spring day, but
folks should not let their guard down. This daytime heating will
allow CAPE values of 2000 to 3000 J/kg to develop due to
temperatures in the upper 70s to low 80s. The 00Z NAM did show an
unlikely situation where low stratus clouds never mix out all and
surface temperatures remaining much cooler, preventing convective
initiation. But this seemed extremely unlikely given the strong
southwesterly flow and dry air aloft. The 6Z NAM has already
trended back in line with strong daytime heating, but is slower
with the storms than the guidance consensus as is its typical bias.
The WRF-ARW, NSSL WRF, and WRF-NMM were in good agreement in
explosive thunderstorm development along the pre-frontal trough/
dry line as upper- level forcing arrives, between about 4PM and
7PM roughly along/northwest of the I-59 corridor. These storms
will probably initiate right over Central Alabama rather than
moving in from Mississippi. The convective mode will be a broken
line of semi- discrete supercells, due to westerly 55-60kt 0-6 km
bulk shear vectors orthogonal to the initiating boundary. Dry air
aloft/capping will likely inhibit storms from developing ahead of
this line, except perhaps in the far northeast counties, so
confidence in timing out this line is increasing. The environment
will be very favorable for large to very large hail to develop due
to the rotating updrafts and fat CAPE profiles more typical of
the Plains than the Southeast, with significant hail >2" diameter
appearing likely with some of the storms. Damaging winds will also
occur with the hail-enhanced downdrafts due to high DCAPE.

Concerning the tornado potential, the 850mb-925mb flow will be a
bit veered to the southwest and not extremely strong. This veered
flow will probably mix down to the surface; the NAM is more backed
but does not seem to be mixing the boundary layer properly as
mentioned related to the spurious looking low stratus mentioned
above. The surface low will also not be deepening, so isallobaric
flow will not be a factor. So near-surface streamwise vorticity
does not appear to be enhanced, with the critical angles between
the 0-0.5km shear vector and 0-0.5km storm-relative flow being
mainly well below the optimal 90 degree angle. This could be a
limiting factor for the strong (EF2+) tornado potential. Still,
the presence of strong rotating updrafts in a high CAPE
environment with around 200m2/s2 0-1km SRH can compensate somewhat
for the less than favorable wind directions. Therefore, think
there is a threat for a couple to maybe a few tornadoes, even if
they may be EF0/EF1 especially the further north you go closer to
the surface low, and especially right around/just after sunset
when the nocturnal LLJ begins to strengthen. One potential area
of concern for strong tornadoes, however, is the far northeast
counties where flow could remain a bit more backed, near a
lingering wedge front over north Georgia. Confidence remains too
low to mention the potential for a strong/significant tornado in
the HWO/graphics, but we will continue to monitor closely as we
get closer. Either way, these will be dangerous storms that
everyone needs to pay attention to. .

 

[mbrewer]

Area Forecast Discussion
National Weather Service Birmingham AL
438 AM CDT Sun Mar 18 2018

.SHORT TERM...
Today and Tonight.

.LONG TERM...
Monday through Saturday.

Confidence that the area will see significant severe
thunderstorms continues to increase for Monday, as models are
coming into better agreement on the synoptic pattern. As usual,
there are some mesoscale details that will determine the exact
timing and type of threat. Models are coming into better agreement
with the timing and placement of a compact upper low that will
eject out of the western CONUS trough into northeast New Mexico
this afternoon. It will deamplify slightly as it moves eastward to
the TN/KY border region as it approaches confluent flow
associated with the eastern Canadian trough, but still remain a
potent system with strong height falls and a neutral tilt. A
strong WSW mid and upper level jet max (~90kt at 250 mb, ~70 kts
at 500 mb, ~65 kts at 700mb) will be located to its south, which
will move over Central Alabama. A ~35-40kt southwesterly LLJ at
850mb will accompany a 998mb surface low moving across Tennessee.
This surface low will be close to steady-state in strength during
the afternoon, before effectively splitting into two pieces as it
encounters the southern Appalachians Monday night. A trailing
pre-frontal trough/dry line feature will move into West Alabama
from Mississippi Monday afternoon.

Starting off Monday morning, not expecting to see a whole lot of
activity on the radar due to dry air and capping aloft associated
with an elevated mixed layer and weak shortwave ridging aloft.
Showers and storms associated with Sunday night`s shortwave will
likely be mainly north and east of the forecast area. But with
low-level isentropic lift there could be some light showers, and
perhaps some isolated leftover thunderstorms from Sunday night.
There will probably be some low stratus as well. One possible
wrench in the forecast is a secondary LLJ in the NAM and some of
the global models that these models show causing an increase in
moisture and isentropic lift across southeast Alabama and the
Florida panhandle during the late morning with associated
precipitation. This could affect destabilization in the afternoon,
or if accompanied by thunderstorms could result in an isolated
severe threat developing earlier than expected in the far
southeast counties. However, the CAMs are much further east with
this activity, closer to the Florida Big Bend and South Georgia.
These may have a better handle on the situation if they are
handling Sunday night`s preceding convection better than the
global models. Also, the models that show this feature all still
indicate very strong instability developing by Monday afternoon
across the entire area, so it may not matter much.

Most of the day on Monday may be a nice warm sunny spring day, but
folks should not let their guard down. This daytime heating will
allow CAPE values of 2000 to 3000 J/kg to develop due to
temperatures in the upper 70s to low 80s. The 00Z NAM did show an
unlikely situation where low stratus clouds never mix out all and
surface temperatures remaining much cooler, preventing convective
initiation. But this seemed extremely unlikely given the strong
southwesterly flow and dry air aloft. The 6Z NAM has already
trended back in line with strong daytime heating, but is slower
with the storms than the guidance consensus as is its typical bias.
The WRF-ARW, NSSL WRF, and WRF-NMM were in good agreement in
explosive thunderstorm development along the pre-frontal trough/
dry line as upper- level forcing arrives, between about 4PM and
7PM roughly along/northwest of the I-59 corridor. These storms
will probably initiate right over Central Alabama rather than
moving in from Mississippi. The convective mode will be a broken
line of semi- discrete supercells, due to westerly 55-60kt 0-6 km
bulk shear vectors orthogonal to the initiating boundary. Dry air
aloft/capping will likely inhibit storms from developing ahead of
this line, except perhaps in the far northeast counties, so
confidence in timing out this line is increasing. The environment
will be very favorable for large to very large hail to develop due
to the rotating updrafts and fat CAPE profiles more typical of
the Plains than the Southeast, with significant hail >2" diameter
appearing likely with some of the storms. Damaging winds will also
occur with the hail-enhanced downdrafts due to high DCAPE.

Concerning the tornado potential, the 850mb-925mb flow will be a
bit veered to the southwest and not extremely strong. This veered
flow will probably mix down to the surface; the NAM is more backed
but does not seem to be mixing the boundary layer properly as
mentioned related to the spurious looking low stratus mentioned
above. The surface low will also not be deepening, so isallobaric
flow will not be a factor. So near-surface streamwise vorticity
does not appear to be enhanced, with the critical angles between
the 0-0.5km shear vector and 0-0.5km storm-relative flow being
mainly well below the optimal 90 degree angle. This could be a
limiting factor for the strong (EF2+) tornado potential. Still,
the presence of strong rotating updrafts in a high CAPE
environment with around 200m2/s2 0-1km SRH can compensate somewhat
for the less than favorable wind directions. Therefore, think
there is a threat for a couple to maybe a few tornadoes, even if
they may be EF0/EF1 especially the further north you go closer to
the surface low, and especially right around/just after sunset
when the nocturnal LLJ begins to strengthen. One potential area
of concern for strong tornadoes, however, is the far northeast
counties where flow could remain a bit more backed, near a
lingering wedge front over north Georgia. Confidence remains too
low to mention the potential for a strong/significant tornado in
the HWO/graphics, but we will continue to monitor closely as we
get closer. Either way, these will be dangerous storms that
everyone needs to pay attention to. .

Interesting that the BMX forecaster mentions the critical angle not being near 90 degrees, and it supporting the notion that there may not be as many stronger tornadoes. I understand what they are getting at, but I've read at least one study that mentions that the 90 degrees CA is more of a Plains thing, and climatology has shown that the South can get by with much less and still produce significant tornadoes.

Sent from my SM-J727V using Tapatalk

 

[rolltide_130]

Interesting that the BMX forecaster mentions the critical angle not being near 90 degrees, and it supporting the notion that there may not be as many stronger tornadoes. I understand what they are getting at, but I've read at least one study that mentions that the 90 degrees CA is more of a Plains thing, and climatology has shown that the South can get by with much less and still produce significant tornadoes.

Sent from my SM-J727V using Tapatalk

Yeah critical angles don't mean as much here. However, down across central AL, this will be unusual in that the shear will be more directionally driven and less speed driven so I could see how that may not be optimal.

Across North AL, the critical angles will probably mean less..

 

[Richardjacks]

Good grief on the 12z nam soundings. PDS tor for BHM but no SARS quality matches...some insane numbers though.

 

[chaser]

Good grief on the 12z nam soundings. PDS tor for BHM but no SARS quality matches...some insane numbers though.

I think someone will get a big tornado..

Sent from my H1611 using Tapatalk

 

[Kory]

Mesoscale, mesoscale, mesoscale....

We’re getting models trying to resolve boundaries and what not.

 

[Xenesthis]

Good grief on the 12z nam soundings. PDS tor for BHM but no SARS quality matches...some insane numbers though.

This Seems a bit overdone don’t you think? I mean it’s been along time since I’ve seen numbers like that. Just seems like the model might be overdoing it a bit

 

[Richardjacks]

12 NAM suggest Bham could be in the crosshairs...STP has trended up last 2 runs....it has clouds clearing by mid afternoon, which seems a little slow to me.

 

[warneagle]

Should the lack of quality matches raise eyebrows about the potential validity of the forecast sounding or is it not necessarily significant?

 

[Xenesthis]

12 NAM suggest Bham could be in the crosshairs...STP has trended up last 2 runs....it has clouds clearing by mid afternoon, which seems a little slow to me.

I also find it interesting that the significant tornado perameter Doesn’t really extended to Tennessee. However the SPC has the enhanced risk well into Tennessee. If this modeling stays the same they will have to adjust that southward.

 

[Kory]

Should the lack of quality matches raise eyebrows about the potential validity of the forecast sounding or is it not necessarily significant?

Fred and I have been chatting. He brought up 4/10/09 as an analog. Looks like a good match to me. Maybe adjust that a touch westward.