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Archive March 18-20th, 2018 Severe Weather

It is now sunny here in McMinnville Tennessee and is well up into the 60s
 
New SPC Day 1 Convective Outlook

fema04_swody1.png


AL_swody1.png


...SUMMARY...
Tornadoes (some strong), very large hail, and damaging winds are
expected from eastern parts of the Mid-South across the Tennessee
Valley region toward the southwestern Appalachians.
Click to expand...

In the main threat area, scattered thunderstorms -- including
supercells -- should develop mid/late this afternoon across parts of
western/middle TN, northeastern MS and northern AL. Tornadoes,
large/damaging hail, and severe wind gusts all are possible with
this activity. The most favorable combination of ingredients still
appears to be over the enhanced-risk area. Enough uncertainties
remain to preclude more tightly focused/higher probabilities at this
time, though an upgrade of part of this area may be needed as
mesoscale diagnostic tendencies and 12Z guidance inform update
decisions.

An ongoing/weakening MCS moving southeastward into more stable air
over northern FL should dissipate through the remainder of the
morning, but has produced an outflow boundary south of the
aforementioned warm front, from the northern peninsula northwestward
over south-central AL. This boundary will erode gradually from west
to east and retreat northward today into this evening, perhaps
merging with the synoptic front. The boundary and frontal zone will
be important sources of vorticity, even as they retreat
northward/northeastward.

An arc of enhanced UVV now located near the surface occluded front
is expected to shift eastward today and meet the returning warm
sector across western/middle TN and northern MS mid-late afternoon.
As this occurs, thunderstorms will develop in an air mass
destabilizing from both diabatic surface heating and warm advection,
with surface dew points 60s F and strong deep shear. Substantially
southerly surface flow component will be maintained amidst pressure
falls ahead of the low, and height falls aloft, contributing to
enlarged hodographs and effective SRH 200-400 J/kg in the warm
sector and potentially exceeding 400 J/kg in the warm-frontal zone.
With MLCAPE 2500-3000 J/kg in the eastern MS/western AL part of the
pre-cold-frontal air mass to 500-1000 J/kg in the warm-frontal zone
over TN, sufficient instability will exist to support sustained
supercells, some producing tornadoes. Supercells may be discrete
and/or at least loosely embedded in a southwest-northeast primary
convective band. Also in that environment, beneath steep
low/middle-level lapse rates (associated with eastward transport of
a residual EML), and sufficient shear for optimal updraft tilt and
precipitation size sorting around mesocyclones, large hail is likely
as well -- including significant/damaging stones above 2 inches in
diameter.
Click to expand...
 
day1probotlk_1630_torn.gif


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CDT Mon Mar 19 2018

...TN Valley and Southeast...
Have upgraded to Moderate risk for the increasing likelihood of
several tornadic supercells between 21-03Z, centered on northern AL
and portions of adjacent states. Expansions to Enhanced and Slight
risks have also occurred across parts of GA and SC for this evening
and tonight. Lowest confidence remains on northern extent of the
strong tornado risk where the degree of surface-based
destabilization is quite uncertain.

An elevated storm cluster is ongoing across far northern MS to the
Mid-MS Valley, still removed from appreciable surface-based
instability that is present near/south of the warm front in
north-central MS/AL. See MCD 144 for additional short-term
information. Guidance is highly varied in the degree of warm front
movement today, which may be in part explained by differences in the
handling of the ongoing convection. The NAM robustly destabilizes
into middle TN but has a distinct absence of convective precip where
convection is ongoing. While the RAP has substantially less buoyancy
in TN with SBCAPE >500 J/kg remaining largely south of the state
border. While low/deep-layer shear and hodograph size will become
increasingly favorable for rotating storms, confidence in the degree
of northward destabilization in the late afternoon across TN is low.
However, farther south in parts of MS/AL/GA, confidence is greater
in moderately large buoyancy developing with MLCAPE of 1500-2500
J/kg becoming common south of the warm front. The northern extent of
this instability plume with enlarged low-level hodographs should
render the potential of several tornadic supercells during the late
afternoon and early evening, a couple of which may be long track.
Morning CAMs suggest the southern extent of probable supercell
development will be across north-central AL, with convection
becoming increasingly sparse to the south/southwest.
 
Last edited:
Daryl said:
day1probotlk_1630_torn.gif


Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CDT Mon Mar 19 2018

Valid 191630Z - 201200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS NORTHERN
AL...NORTHWEST GA...FAR SOUTHERN MIDDLE TN...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS MIDDLE TN TO
CENTRAL AL AND NORTHERN/CENTRAL GA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SOUTHEAST
STATES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SOUTHEAST
STATES...

...SUMMARY...
Strong tornadoes, very large hail, and damaging winds are expected
across parts of the Tennessee Valley and Southeast during the late
afternoon and evening.

...TN Valley and Southeast...
Have upgraded to Moderate risk for the increasing likelihood of
several tornadic supercells between 21-03Z, centered on northern AL
and portions of adjacent states. Expansions to Enhanced and Slight
risks have also occurred across parts of GA and SC for this evening
and tonight. Lowest confidence remains on northern extent of the
strong tornado risk where the degree of surface-based
destabilization is quite uncertain.

An elevated storm cluster is ongoing across far northern MS to the
Mid-MS Valley, still removed from appreciable surface-based
instability that is present near/south of the warm front in
north-central MS/AL. See MCD 144 for additional short-term
information. Guidance is highly varied in the degree of warm front
movement today, which may be in part explained by differences in the
handling of the ongoing convection. The NAM robustly destabilizes
into middle TN but has a distinct absence of convective precip where
convection is ongoing. While the RAP has substantially less buoyancy
in TN with SBCAPE >500 J/kg remaining largely south of the state
border. While low/deep-layer shear and hodograph size will become
increasingly favorable for rotating storms, confidence in the degree
of northward destabilization in the late afternoon across TN is low.
However, farther south in parts of MS/AL/GA, confidence is greater
in moderately large buoyancy developing with MLCAPE of 1500-2500
J/kg becoming common south of the warm front. The northern extent of
this instability plume with enlarged low-level hodographs should
render the potential of several tornadic supercells during the late
afternoon and early evening, a couple of which may be long track.
Morning CAMs suggest the southern extent of probable supercell
development will be across north-central AL, with convection
becoming increasingly sparse to the south/southwest.

During the evening, convection should become increasingly
clustered/semi-discrete, but a risk for a few strong tornadoes
should continue across parts of northern/central GA near the warm
front. Damaging wind/tornado risk should persist but become
increasingly localized across parts of SC tonight.

A separate area of scattered storm clusters should emanate northeast
from ongoing convection along the FL Gulf Coast. This activity
should be aided by a minor mid-level impulse evident over LA and
modest warm advection atop a modifying outflow boundary in the wake
of an early morning MCS that traversed north FL. A few embedded
supercells capable of all hazards are possible with this activity.
Click to expand...

dont like that
 
Y'all can edit out most of the verbiage of a post when your quoting someone, that way when someone posts a wordy forecast from NWS or other sources, you can cut down on us having to scroll so much!!


Sent from my iPhone using TalkWeather
 
I think the moderate is placed very well and the SPC seems to have a good handle on the forecast. Expect a few adjustments to size or location of moderate in the afternoon update depending upon the progression of the warm front as well as any refinements to the extent of shear conducive to supercell formation and/or twisting of winds at height.

For now I think they've got it pegged. Could move a bit north/south or be expanded either/or/both in afternoon update, but I wouldn't expect any large shifts. I do think Tuscaloosa and Bham will end up getting in on the action, but they may just stick with ENH S of I-20 I'm the Western sections although BMX disco yesterday pointed out how better instability can overcome more unidirectional wind fields. We'll see.
 
It is now 68° in McMinnville Tennessee With very thin clouds and partial sunshine. I do think you will see that Moderate needs to be expanded at least up to the I-40
 
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