2/24 Severe Potential - Southern Half of Subforum

[tornadohunter]

Although the current D4 outlook has a slight risk over Western Tenn, models seem to suggest the potential for the southern half of this subforum to see severe on Saturday. 

GFS has a 992 low over Iowa with dewpoints in Il, Ky, and In ranging from 58-63. 

 

The nam is slightly weaker with a 995 over Eastern iowa and a smaller warm area with dew points over 60. 

This one looks like worth watching. 

 

Sounding (GFS) is west southern illinois

 

[Indystorm]

It is interesting that you should mention this.  Although I have my doubts one of the WTHR Indpls mets mentioned the possibility for severe this coming Saturday.

[tornadohunter]

12Z Euro has a 982 by 12z sunday

[JPOracle]

I expect when the models start getting agreement going, it will start to look nasty over a large area of the ohio valley south of the Great Lakes

[Hoosier]

I suppose it is something to monitor.  A quick look at the GFS and NAM shows that the GFS has some workable mid level lapse rates, while the NAM is less favorable.  At this point, would say the better chances would be roughly near/south of I-70 with lower but not zero chances farther north.

[Stebo]

41 minutes ago, Hoosier said:

I suppose it is something to monitor.  A quick look at the GFS and NAM shows that the GFS has some workable mid level lapse rates, while the NAM is less favorable.  At this point, would say the better chances would be roughly near/south of I-70 with lower but not zero chances farther north.

If we didn't have potentially so much cloud cover in the warm sector, this would have a really good chance at being significant. Right now it is a wait and see though.

[CheeselandSkies]

Trough and surface system actually looking really good on the GFS right now. So good I'd like to bottle those charts and save them for May. Annoying that warm mid-level temps are an issue in February. One of those "WTF, atmosphere?" things that seem to be becoming more and more common in recent years. I really like to see cold temps upstairs with early season systems because that can go a long way toward overcoming marginal moisture at the surface (see, the last late February tornado outbreak, or March 15, 2016 in IL).

 

[hlcater]

Well at least it looks like this system is gonna have a decent severe event in Dixie, which is better than just the snow in MN. Thing with severe wx is that I find it interesting no matter where it is. If it’s not snowing here, I really don’t care about it. Snow isn’t nearly as exciting as severe for me.

[Indystorm]

New day 2 for Saturday now shows enhanced risk as far ne as Evansville with slight on north to Columbus IN and marginal up to Indpls.  Will be interesting to see how the low pressure system deepens and moves.

[snowlover2]

New day 2 shifts marginal into SW OH and slight into S IN.

 

Quote

Day 2 Convective Outlook 
   NWS Storm Prediction Center Norman OK
   1128 AM CST Fri Feb 23 2018

   Valid 241200Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
   ARKLATEX TO THE LOWER OHIO VALLEY...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTH TEXAS
   TO THE OHIO VALLEY...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
   TEXAS/LOUISIANA COASTS TO THE MIDWEST...

   ...SUMMARY...
   Scattered to numerous thunderstorms will develop across the southern
   Plains Saturday morning and shift northeastward towards the Ohio
   Valley through Saturday night. Several of these storms will likely
   be severe, with damaging winds and a few tornadoes being the primary
   threat. A few instances of large hail will be possible as well.

   ...Synopsis...
   Within a cyclonic-flow regime across the western US, a robust
   shortwave trough will eject northeast across the central Plains,
   while acquiring a negative tilt as it approaches the upper Midwest.
   In response, mid-level heights will fall over much of the Plains and
   Mississippi Valley through the day. The surface pattern will feature
   a deepening low lifting north from the Mississippi Valley towards
   the upper Great Lakes. Trailing to its south, a cold front will
   accelerate eastward towards the Ohio and Tennessee Valleys, while
   the preceding warm sector advances northward from the Mid-South to
   portions of southern Illinois, Indiana, and Ohio.

   ...Arklatex to the Ohio Valley...
   As the aforementioned shortwave trough continues northeast towards
   the upper Midwest, a strong low-level jet is forecast to organize
   across the Arklatex through the mid-day hours, before strengthening
   and translating northeast towards the Ohio Valley through the
   overnight hours. In conjunction with this evolution, the surface
   warm sector (characterized by dew points in the lower/mid 60s along
   its northern fringes) will stream northward, reaching areas from
   southeastern Missouri to southern Indiana through the period. South
   of the warm front, despite little/modest low-level heating and
   related buoyancy, favorably moist low levels should support upwards
   of 1000 J/kg of MLCAPE across parts of the Mid-South by afternoon,
   with values decreasing to 200-400 J/kg across the Ohio Valley.

   Countering these lower values of buoyancy, a strong kinematic
   profile will evolve across much of the region, especially from
   northern Arkansas to the lower Ohio Valley. Within this region,
   925-850mb flow around 60-70 kt will contribute to sizable values of
   storm-relative helicity through the evening hours. In turn, as a
   narrow band of convection organizes from the Arklatex to the Ozarks
   through the day, shear profiles should encourage several bowing/LEWP
   structures, with embedded supercells possible. Furthermore, forecast
   soundings and high-res guidance depict a considerable component of
   low-level shear perpendicular to several bowing segments, enhancing
   the potential for tornadoes -- a few of which could be strong --
   during the afternoon and evening hours. These cells will then race
   towards the Ohio and Tennessee Valleys during the evening and
   overnight. Moist adiabatic low-level profiles, while not conducive
   for high values of buoyancy, may still prove favorable for a few
   swaths of damaging winds (with an attendant line-embedded tornado
   threat), as any low-level rotating elements will enhance upward
   vertical motion and convective intensity.

   Outside of the main band of convection, although forcing for ascent
   will not be particularly strong earlier in the day, an isolated
   discrete supercell or two may form across the Mid-South within
   warm/moist low-level confluence Saturday afternoon. Favorable
   storm-relative helicity and effective shear would support a
   conditional damaging wind and tornado threat during this time frame
   as well.

   ..Picca.. 02/23/2018

 

[Indystorm]

Right now I want to focus on the PAH area for Saturday evening with good combination of CAPE, shear, and helicity values.  We'll see how things transpire.

 

The bottom line is that we should eek out enough surface-based
instability to support thunderstorm development along the cold
front and possibly the warm front if it is unable to lift north of
the area. Given the amount of forcing, it will most likely take
the form of a loose QLCS and with 0-1km and 0-3km helicity values
300-400m2/s2, some embedded supercell structures will certainly be
possible. Regardless of the mode of convection, the 0-1km shear
will support a tornado threat, especially near the warm front and
triple point. In addition, a widespread damaging wind threat will
exist.

[Hoosier]

Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1146 PM CST Fri Feb 23 2018

   Valid 241200Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
   THROUGH THIS EVENING ACROSS MUCH OF SOUTHERN AND EASTERN
   ARKANSAS...NORTHERN LOUISIANA...NORTHWESTERN MISSISSIPPI....A
   PORTION OF NORTHWESTERN ALABAMA...WESTERN TENNESSEE...SOUTHEASTERN
   MISSOURI...SOUTHERN ILLINOIS...WESTERN KENTUCKY AND A PORTION OF
   SOUTHERN INDIANA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
   AREAS OF THE SOUTHEASTERN LOWER PLAINS...THE LOWER MISSISSIPPI AND
   LOWER OHIO VALLEYS...

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
   SLIGHT RISK...AS FAR NORTH AND EAST AS THE SOUTHERN GREAT
   LAKES...UPPER OHIO VALLEY AND WESTERN SLOPES OF THE APPALACHIANS BY
   DAYBREAK SUNDAY...

   ...SUMMARY...
   Severe thunderstorms are expected to develop today through tonight
   across the Ark-La-Tex region, east northeastward through the lower
   Mississippi and Ohio Valleys.  These will be accompanied by
   potential for damaging wind gusts and a few tornadoes, a couple of
   which could be strong.

   ...Discussion...
   Subtropical ridging centered near the south Atlantic coast continues
   to weaken, but appears likely to maintain a prominent influence
   across much of the Southeast through this period.  As it does, a
   vigorous and progressive short wave trough, now beginning to pivot
   northeast of the Four Corners region, is forecast to take on a
   negative tilt and accelerate northeastward through the Upper Midwest
   by 12Z Sunday, as another significant upstream short wave trough
   digs across the Great Basin.  At the same time, models indicate that
   a strong subtropical jet streak will nose across the Texas Big Bend
   region, toward the Ozark Plateau and lower Ohio Valleys.

   In response to these developments, fairly strong surface
   cyclogenesis still seems probable across the central U.S. today
   through tonight.  There has been variability among the model output
   concerning the evolution, likely at least in part due to the slow
   erosion of a lingering shallow wedge of cold surface-based air over
   the southern Plains.  While the onset of more rapid deepening of the
   surface low  remains in question, it now seems most probable that
   surface frontal wave development will take shape over northeastern
   Oklahoma into the Missouri Ozarks by midday today, before deepening
   more rapidly while migrating northeastward, and eventually occluding
   over the Upper Midwest by early Sunday.

   Strengthening wind fields (to 50-70 kt in the 850-500 mb layer) and
   shear within a large portion of the developing warm sector will
   contribute to increasing severe weather potential ahead of a
   southeastward advancing cold front trailing from the surface
   cyclone, as southerly return flow off the northwestern Gulf of
   Mexico supports an influx of high moisture content air.  However,
   less than optimal thermodynamic profiles due to potential widespread
   convective development and cloud cover may temper this risk
   somewhat.  Regardless, severe thunderstorm development still seems
   probable across portions of the southeastern Plains into the Ozark
   Plateau by midday, before increasing across the lower Mississippi
   into lower Ohio Valleys late this afternoon and evening.

   ...Southern Plains into Ohio Valley...
   The rather broad western/northwestern gradient of severe
   probabilities is generally reflective of lingering uncertainty
   concerning the specific track of the surface cyclone and timing of
   its most rapid deepening.  The broad eastern/southeastern gradient
   generally reflects uncertainties associated with late night timing
   and the likelihood of progressively weaker warm sector instability.

   Severe weather potential still seems likely to become maximized
   across parts of southern and eastern Arkansas into adjacent portions
   of the lower Ohio Valley late this afternoon and evening, when/where
   it appears that forcing for ascent and strong shear profiles may
   best coincide with at least modest boundary layer instability (CAPE
   on the order of 500-1000 J/kg).  Substantive intensification of a
   pre-frontal squall line is expected during this period, and
   sustained discrete supercell development appears possible just ahead
   of the line.  In the presence of a moist boundary layer with surface
   dew points in the mid 60s F, rather large and clockwise curved
   low-level hodographs will support the risk for a few tornadoes.  One
   or two of these could potentially be long-lived/tracked and strong. 
   Otherwise, damage wind gusts are expected to become the primary
   threat with the evolving line, which will spread across and east of
   the lower Mississippi Valley late this evening through the overnight
   hours.

   It still appears that the strong/severe thunderstorm development
   could commence near or shortly after day break today across parts of
   central and northwest Texas.  Aided by forcing for ascent associated
   with low-level warm advection, beneath increasingly divergent upper
   flow associated with coupling subtropical and polar jets, a sizable
   cluster of storms may evolve across the Red River Valley into parts
   of the Ozark Plateau through the midday/early afternoon hours.  This
   may mostly be rooted above lingering cold/stable surface-based air,
   with severe hail the primary hazard.  However, at some point this
   may become increasingly rooted within a destabilizing boundary layer
   across the Ark-La-Tex into the lower Mississippi Valley.  As alluded
   to earlier, this seems most probable across southern/eastern
   Arkansas and adjacent northern Louisiana late this afternoon. 
   However, it is possible that a severe wind/tornado threat could
   commence earlier and to the west.

   ..Kerr/Leitman.. 02/24/2018

 

 

 

[Hoosier]

First model I looked at was the 00z NAM, and my thought was how can SPC have the 2% tor and 5% wind areas so far north.  But upon examination of the NAM, it looks unrealistic with a shallow cool layer roughly north of I-70.  Threat certainly looks low end with northward extent, but I'm pretty sure the NAM temps are too cool.

[Indystorm]

Unless that low pressure deepens significantly the svr weather threat will be from EVV and PAH on south of the Ohio later today.  Effective WF right now is by Union City TN and the TN/KY border.  However, Edmond OK had 37 degrees with heavy rain, lightning and thunder this morning so the kinematics are there.  Heavy cloud cover will also be an issue unless we get some breaks.

[JPOracle]

Area to watch evening and overnight will be the OH River up as maybe localized severe threat to the SW PA, i expect it to be marginal but a few wind reports or a small tornado could be possible in OH,IN,PA overnight but highly conditional for sure

[Indystorm]

 Mesoscale Discussion 0091
   NWS Storm Prediction Center Norman OK
   0258 PM CST Sat Feb 24 2018

   Areas affected...Northeast AR to Western KY/TN

   Concerning...Severe potential...Tornado Watch likely 

   Valid 242058Z - 242200Z

   Probability of Watch Issuance...80 percent

   SUMMARY...Tornado threat will increase across northeast AR into
   western KY/TN into the early evening hours. Tornado watch will
   likely be issued soon.

   DISCUSSION...Surface low continues to track east-northeast across
   northern AR along a slowly advancing warm front that currently is
   draped from western KY into northern AR north of BVX. 20z sounding
   from LZK exhibits a very moist and adequately buoyant sounding for
   robust deep convection, with very strong shear, supportive of
   supercells and organized line segments. This environment is
   gradually spreading northeast and supercell/tornado threat should
   increase toward the confluence of the MS/OH River region over the
   next several hours. Tornado watch appears warranted for this
   evolution and will likely be issued soon.

   ..Darrow/Hart.. 02/24/2018

[Indystorm]

[Indystorm]

[monsoonman1]

Pretty sure there's an unwarned tornado in KY

Sent from my SM-G920V using Tapatalk

[yoda]

 

[Chicago Storm]

Another lackluster performance by the SPC for yesterday/last night.

[hlcater]

2 hours ago, Chicago Storm said:

Another lackluster performance by the SPC for yesterday/last night.

Any justification for that? Because I disagree. I think it’s gonna verify in the end. If you look at the reports, the wind verified for sure, and I have a feeling the 10% will verify as well, especially over KY and TN where there were multiple confirmed tornadoes. Whether the hatch verifies remains to be seen. I may be a bit bullish here, but I personally wouldn’t be surprised to see 15-20 tornadoes confirmed in the end.

[CheeselandSkies]

They ended up being too far west with the 10 hatch vs. where the strongest tornadoes occurred, but the overall gist of the forecast (a QLCS with wind and a few embedded tornadoes, with a supercell or two out front producing a couple of potentially strong tornadoes) seemed pretty spot on.

I believe there was one storm-related fatality (not sure if it was confirmed as tornado-related yet or not) in AR within the 10 hatch, and the KY/TN storms were within the 5% area. However, the Adairville area killer occurred just east of the watch that was in effect at the time, and (more importantly) before the warning was issued.

The QLCS wasn't as prolific across AR as I thought it might be, I think due to limited destabilization, but that was a known caveat going in.

[Chicago Storm]

18 minutes ago, NWLinnCountyIA said:

Any justification for that? Because I disagree. I think it’s gonna verify in the end. If you look at the reports, the wind verified for sure, and I have a feeling the 10% will verify as well, especially over KY and TN where there were multiple confirmed tornadoes. Whether the hatch verifies remains to be seen. I may be a bit bullish here, but I personally wouldn’t be surprised to see 15-20 tornadoes confirmed in the end.

Not even close. It was just a terrible set of outlooks, from DY2 right on into DY1. Part of the problem was Broyles going gung-ho with his usual heavily NAM biased forecast on the original SWODY2, and then following forecasters sticking with a sig forecast. 10% hatched was never in the cards, and orientation/placement was comical. The original DY1 had 2% tor up to GYY and DTW, which is so lol worthy it's almost not funny.

[hlcater]

52 minutes ago, Chicago Storm said:

Not even close. It was just a terrible set of outlooks, from DY2 right on into DY1. Part of the problem was Broyles going gung-ho with his usual heavily NAM biased forecast on the original SWODY2, and then following forecasters sticking with a sig forecast. 10% hatched was never in the cards, and orientation/placement was comical. The original DY1 had 2% tor up to GYY and DTW, which is so lol worthy it's almost not funny.

I still think the 30% wind placement was acceptable, I'm not saying the forecast was perfect, but it wasn't bad.  If you were expecting an outbreak of supercells(to be fair broyles was on his D2 forecast, but broyles is trash) all the forecasters after seemed to lean more towards a QLCS over supercells, which is exactly what happened. It doesn't take supercells to verify a 10%, and I think the final tally will for sure verify the NE half of the 10%. That being said, I agree the placement of the 10% over SW AR was off, and in my original post I tried to somewhat allude to that. I never saw the original D1 outlook, so the DTW 2% was news to me, and that was a pretty poor call, but even then it was only a 2%. 

We are already up to 8 confirmed tornadoes.

[CheeselandSkies]

Yeah, we have to keep in mind that although it is a relatively high probability compared to any given day, it doesn't take all that many tornadoes to verify a 10% probability of one within 25 miles of any point in the zone.

I have to wonder why they don't address Mr. Broyes' longstanding and well-known bullish bias on Day-2 and beyond outlooks. Maybe put him on watch/MD duty.

[Stebo]

The wind was fine, the tor will end up being close to verification just not in the right area, it should have been more wsw-ene oriented zone. I don't think they did terrible and the forecast was refined through the day. 

[Geoboy645]

1 hour ago, CheeselandSkies said:

Yeah, we have to keep in mind that although it is a relatively high probability compared to any given day, it doesn't take all that many tornadoes to verify a 10% probability of one within 25 miles of any point in the zone.

I have to wonder why they don't address Mr. Broyes' longstanding and well-known bullish bias on Day-2 and beyond outlooks. Maybe put him on watch/MD duty.

But then where are we going to get the broyles memes?

[hlcater]

1 hour ago, CheeselandSkies said:

Yeah, we have to keep in mind that although it is a relatively high probability compared to any given day, it doesn't take all that many tornadoes to verify a 10% probability of one within 25 miles of any point in the zone.

I have to wonder why they don't address Mr. Broyes' longstanding and well-known bullish bias on Day-2 and beyond outlooks. Maybe put him on watch/MD duty.

June 13th Broyles MD for Eastern AL and Georgia.  (I had some time on my hands, and was bored. I still don't know what drove me to do this.)

Quote

Mesoscale Discussion 0688
   NWS Storm Prediction Center Norman OK
   0217 PM EDT Wed Jun 13 2018

   Areas affected...northereastern Alabama and northwestern Georgia

   Concerning...Severe potential...Tornado watch needed soon

   Valid 131817Z - 132015Z

   Probability of Watch Issuance...95 percent

   SUMMARY...As diurnal heating progresses and convT is reached,
numerous thunderstorms are expected to develop. In the prescence 
of increasing shear and continued destabilization, these storms 
should quickly become severe and would be capable of all severe hazards.
A WW will be coordinated soon.

   DISCUSSION...Current observations across the discussion area suggest that
convective temperature is close to being reached in the prescence of strong
diurnal heating leading to temps in the upper 80s along with a general 10kt SE
wind contributing to dews in the mid 70s. This combination, combined with 
impressively steep lapse rates of 6.0 c/km should yield SBCAPE values in the 
vicinity of 3000 j/kg.

This instability in combination with impressive deep layer shear at around 
15 knots should provide sufficient updraft organization for discrete supercells
capable of all severe hazards. Current RAP and NAM soundings seem to suggest 
effective SRH values around 25-50 m2s2 and impressive 0-3km hodographs. Given 
this, it seems as if several strong tornadoes will be possible and a PDS WW is 
under consideration. Aside from the tornado threat, storms should be capable of 
very large hail, perhaps in excess of 2 inches in diameter, especially early in 
the convective evolution.

By 23z, thunderstorms will have organized themselves into one or more bowing 
segments capable of damaging straight-line winds, of which a few events may be 
sig severe, small hail,and perhaps a tornado or two. These thunderstorm complexes 
are expected to propagate ESEward with time. By 04z, nocturnal cooling should 
begin to inhibit, or at least elevate remaining convection, and all convection 
should be dissipated by 07z, as instability continues to wane.

   ..Broyles.. 06/13/2018

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...FFC...BMX...HTX...

   LAT...LON   34109924 35029876 35419803 35339696 34979637 33789611
               32919656 32699808 32919902 33439936 34109924 

 

[Quincy]

I don't think it was that bad of a forecast by SPC either. They kept the option open for a conditional significant tornado threat (isolated supercell) farther southwest into the Arklatex region, while such activity ended up occurring farther northeast with the initial discrete cells. SPC has had far worse busts than this. 

This is a case of shear being more of a factor than instability. As we often see with cool season severe events, you don't need much instability when shear is significant enough. 

I'd also imagine that hodographs were larger with eastward extent, perhaps being slightly overlooked by the area's less impressive instability. (KY/TN)

It is interesting that there was an overall lack of severe reports over southern AR/LA/TX. Three factors come to mind, including lack of prefrontal discrete cells, kinks in the mid-level wind fields and most of the action coming from a squall line. This squall line did not pack much of a punch from the I-30 vicinity and points southeast due to nocturnal cooling and waning instability after dark. 

The images above are time sensitive and may not display correctly in a few days.