Severe Weather Threat... feb 24th-25

[David Reimer]

One particular index I'll be watching closely for this event is the 3 KM CAPE. If we can get a majority of the instability being shown into the lower level, then we'll end up with lower topped supercells (messy at that) but they will still be quite significant. As of now, the models are not getting a good handle on that with precipitation/boundary contamination. One particular area of interest may be closer to the OK/MO area if winds can back.

[Thundersnow12]

One particular index I'll be watching closely for this event is the 3 KM CAPE. If we can get a majority of the instability being shown into the lower level, then we'll end up with lower topped supercells (messy at that) but they will still be quite significant. As of now, the models are not getting a good handle on that with precipitation/boundary contamination. One particular area of interest may be closer to the OK/MO area if winds can back.

already getting impressive amounts..

[JoMo]

Not ready for severe weather yet

[Reed Stough]

Well I think the event is too far out to determine the extent if any severe weather. When comparing the 06Z and 12Z 500 mb the upper level support appears to be shearing apart, thus the trough would be weakening. Also looks like the winds are more linear so it may be more of a low topped wind event than a tornado outbreak.

Either way I am jealous - I would just love to hear some thunder in the deserts of the SW soon!

[Indystorm]

Based on parameters I am currently seeing off the 18z NAM I would think northern Arkansas seems to have the best combination of temp, CAPE, bulk shear, helicity, and dews for svr weather Thursday evening.

[Ed Lizard]

The GFS is further South (at 12Z) than the WRF, and seems to have most of the heavy storms along the warm front.

HPC discussion doesn't mention NAM per se, but discounts the GFS Southern track as being influenced by convective feedback.

[Fred Gossage]

Well I think the event is too far out to determine the extent if any severe weather. When comparing the 06Z and 12Z 500 mb the upper level support appears to be shearing apart, thus the trough would be weakening. Also looks like the winds are more linear so it may be more of a low topped wind event than a tornado outbreak.

Either way I am jealous - I would just love to hear some thunder in the deserts of the SW soon!

A lower amplitude and/or weaker shortwave would be more conducive for a cellular storm mode... because of less forcing... especially in the presence of a warm sector that is more weakly capped or not capped at all. I could spend the rest of the afternoon, evening, and night.... listing past outbreaks that have fit this rule. East of the Plains, a lot of the bigger tornado days occur with surface winds that are between SSE and SW. A lot of times, moisture return and speed shear are more important to violent tornadoes in Dixie Alley than directional shear. There is still plenty of directional shear... when combined with speed shear... to give very large helicities for storm updraft rotation. As far as veered surface winds go... it should be noted that all four F5 tornadoes in Alabama history... occurred with surface winds in the warm sector... between due south and southwest. A due southeast wind this far east can actually hinder instability, because the moisture trajectories aren't from the deeper moisture that's often present in the western Gulf of Mexico.

As far as this system itself goes... I think the severity of the event will be determined by how much convective contanimation there is in the warm sector.

[Reed Stough]

Looking at the 18Z NAM - for 2/24 and 2/25 - things just don't look that good to me.

Synoptic parameters are in place, but look a little chaotic to me. Instability seems to be lacking according to the models. BUT of course they are models and things can definitely change.

[Indystorm]

Looking at the 18Z NAM - for 2/24 and 2/25 - things just don't look that good to me.

Synoptic parameters are in place, but look a little chotic to me. Instability seems to be lacking according to the models. BUT of course they are models and things can definitely change.

Yep, instability seems to always be the one wild card in late winter/early spring systems, if it isn't lack of moisture.

[Ed Lizard]

Looking at the 18Z NAM - for 2/24 and 2/25 - things just don't look that good to me.

Synoptic parameters are in place, but look a little chotic to me. Instability seems to be lacking according to the models. BUT of course they are models and things can definitely change.

Sort of a snow weenie type response, but the upper disturbance in question is still offshore...

[Fred Gossage]

Sort of a snow weenie type response, but the upper disturbance in question is still offshore...

Not a weenie response at all. Lower amplitude troughs like this, while are more conducive for supercell events because of the geometry and strength of forcing, are also more fickle when it comes to exact details. They are much more dependent on exact placement of individual vorts... and this throws models for a loop until the upper-level energy comes onshore and is sampled by the RAOB network. I know you fully know this, or you wouldn't have even mentioned it... but it's an excellent point to make about the system at this stage in the game.... for the other readers following along.

[Indystorm]

What you say is true about the onshore data, Fred, and that has been my usual position until met Baroclinic Instability on this forum reminded us all only half in jest that west coast forecasters don't have the luxury of the input from onshore samplings before they make forecasts for their area. His point was that we can gain some info even from the data sparse Pacific that can be useful to at least highlight a synoptic pattern in general. Of course I've read enough to know that with svr wx outbreaks you need to look at so many day of mesoscale features that it really is a challenge in forecasting.

[Fred Gossage]

What you say is true about the onshore data, Fred, and that has been my usual position until met Baroclinic Instability on this forum reminded us all only half in jest that west coast forecasters don't have the luxury of the input from onshore samplings before they make forecasts for their area. His point was that we can gain some info even from the data sparse Pacific that can be useful to at least highlight a synoptic pattern in general.

That's very true... but when we're talking about trying to get specific details out of a situation like this... it's always better to wait.

[thewxmann]

A few notes...

1) The energy responsible for this threat is still out in the Pacific Ocean. It will makae landfall in Baja and eject northeastward starting on Wednesday. It doesn't even emerge into the CONUS until early Thursday, the day of the event. Translation: this thing won't even be in the ROAB network until the day of the event. Expect models to handle trough orientation, timing, and geometry poorly up until the morning of the event. Recent events such as Lone Grove (when the biggest threat was supposedly in E OK and W AR), Super Tuesday (weak s/w providing focus for pre-frontal supercellular development, poor trough handling after 0Z 6 Feb IIRC), etc. should remind us that even the most minute details make a big difference in severe weather setups.

2) The lack of a cap poses a concern. There is a non-negligible possibility that storms fire too early. It could be a very convectively messy day. However, given some insolation and clearing.... (read next bullet)

3) ...Instability should not be a problem (with caveat mentioned above). The sounding Ed Mahmoud posted shows conditionally unstable lapse rates even with a very moist vertical profile. Assuming model-progged H5 temps around -15C, I think lapse rates will be steep enough to yield ~1000 J/kg of SBCAPE if not more come the day of the event. So often we see at this juncture model soundings that show moist-adiabatic lapse rates, and even in those cases we somehow manage 500-1000 J/kg.

[thewxmann]

What you say is true about the onshore data, Fred, and that has been my usual position until met Baroclinic Instability on this forum reminded us all only half in jest that west coast forecasters don't have the luxury of the input from onshore samplings before they make forecasts for their area. His point was that we can gain some info even from the data sparse Pacific that can be useful to at least highlight a synoptic pattern in general. Of course I've read enough to know that with svr wx outbreaks you need to look at so many day of mesoscale features that it really is a challenge in forecasting.

Yes, west coast forecasters do not have that luxury, and as a result the weather forecasts here can be way off.

For example: for the last system, the main energy was forecasted to go way south of us (Bay Area) on Friday and Saturday. We were to get a tenth of an inch of rain at best on both days, areas north of us were to get nothing. Instead the "pivot point"/low went right over us... the combined Fri-Sat rainfall totaled 2" in some areas and the dynamics were strong enough to bring snow to the hills above Berkeley (about 1500'). Nobody saw that coming.

Meanwhile, for this piece of energy offshore of us right now (that will be responsible for the severe weather event later this week), ECM and GFS swapped solutions about 50 times, sometimes bringing the system onshore, sometimes keeping it well offshore. Not until yesterday did they stabilize on the just-offshore solution.

[SluggerWx]

Without a major trough axis to force things linear, with sfc pressures that low, with that much shear, instability, and moisture likely to be in place...it's too far out to say for certain how bad this is going to end up being, but the potential ceiling for this is certainly for it to end up being a blockbuster tornado outbreak. Of course, those take too many perfect mesoscale variables to occur, but this setup could be very very interesting. In fact, it reminds me of the setup for the Tri-State tornado, with the path of that sfc low, with the area under threat (remember there were violent tornadoes in the south later that afternoon), and with the movement of the tornadoes, it can be inferred that at least the direction of the flow aloft in the outbreak area was similar.

It seems like we always get a 'big' event in Feb or March in the southeast, nearly every year.

[baroclinic_instability]

Not a weenie response at all. Lower amplitude troughs like this, while are more conducive for supercell events because of the geometry and strength of forcing, are also more fickle when it comes to exact details. They are much more dependent on exact placement of individual vorts... and this throws models for a loop until the upper-level energy comes onshore and is sampled by the RAOB network. I know you fully know this, or you wouldn't have even mentioned it... but it's an excellent point to make about the system at this stage in the game.... for the other readers following along.

What you say is true about the onshore data, Fred, and that has been my usual position until met Baroclinic Instability on this forum reminded us all only half in jest that west coast forecasters don't have the luxury of the input from onshore samplings before they make forecasts for their area. His point was that we can gain some info even from the data sparse Pacific that can be useful to at least highlight a synoptic pattern in general. Of course I've read enough to know that with svr wx outbreaks you need to look at so many day of mesoscale features that it really is a challenge in forecasting.

Agreed with Indy. Upper air RAOB data certainly helps--but as a counter to your argument Fred--the cutoff low is not progged to enter the mainland until Thursday 0Z and most of it is in MX with the system fully in the US network by 12Z Thursday. The event itself happens on Thursday night into early Friday--so forecasts and details need to be sorted to some degree with the available data and some degree of uncertainty needs to be relayed regarding meso/synoptic details. Waiting is not really an option except for the morning and noon updates for SPC.

[baroclinic_instability]

A few notes...

1) The energy responsible for this threat is still out in the Pacific Ocean. It will makae landfall in Baja and eject northeastward starting on Wednesday. It doesn't even emerge into the CONUS until early Thursday, the day of the event. Translation: this thing won't even be in the ROAB network until the day of the event. Expect models to handle trough orientation, timing, and geometry poorly up until the morning of the event. Recent events such as Lone Grove (when the biggest threat was supposedly in E OK and W AR), Super Tuesday (weak s/w providing focus for pre-frontal supercellular development, poor trough handling after 0Z 6 Feb IIRC), etc. should remind us that even the most minute details make a big difference in severe weather setups.

2) The lack of a cap poses a concern. There is a non-negligible possibility that storms fire too early. It could be a very convectively messy day. However, given some insolation and clearing.... (read next bullet)

3) ...Instability should not be a problem (with caveat mentioned above). The sounding Ed Mahmoud posted shows conditionally unstable lapse rates even with a very moist vertical profile. Assuming model-progged H5 temps around -15C, I think lapse rates will be steep enough to yield ~1000 J/kg of SBCAPE if not more come the day of the event. So often we see at this juncture model soundings that show moist-adiabatic lapse rates, and even in those cases we somehow manage 500-1000 J/kg.

Just saw your response. I agree--general highlights can be garnered and must be made without waiting for sampling.

[ksstormhunter]

Will be interested to see, I am picking up on Convection for areas such as Dallas, Tulsa, I have pushed out my Convection tables on the SmartModel to test it out. Love to get some feedback and see, right now just picking up on general TSTMS.

[Chicago Storm]

DAY 3 CONVECTIVE OUTLOOK

NWS STORM PREDICTION CENTER NORMAN OK

0224 AM CST TUE FEB 22 2011

VALID 241200Z - 251200Z

...THERE IS A SLGT RISK OF SVR TSTMS FROM ERN OK/ARKLATEX EWD ACROSS

AR...SRN MO...NRN MS/AL AND INTO THE TN AND OH VALLEYS...

..ERN OK/ARKLATEX EWD INTO THE TN AND OH VALLEY

A COMPLEX...AND RAPIDLY CHANGING SCENARIO WILL UNFOLD ON THU. WHILE

SOME SIGNIFICANT MODEL DIFFERENCES STILL EXIST...A SIGNIFICANT

SEVERE THREAT APPEARS LIKELY REGARDLESS OF FINER SCALE DETAILS WHICH

WILL BECOME CLEARER IN LATER OUTLOOKS.

STRONG TO SEVERE STORMS...WITH MAINLY A HAIL THREAT...MAY BE ONGOING

ACROSS PORTIONS OF OK THU MORNING. THESE STORMS WILL LIKELY BE

ELEVATED. ADDITIONAL STORMS WILL BE ONGOING FROM SERN KS ACROSS MO

AND INTO NRN AR AND THE LOWER OH RIVER VALLEY ALONG AND N OF A WARM

FRONT...BUT THESE ARE NOT EXPECTED TO BE SEVERE.

THE PRIMARY SEVERE THREAT SHOULD DEVELOP DURING THE AFTERNOON ALONG

THE OK/AR BORDER FROM THE ARKLATEX INTO SWRN MO AS COOLING ALOFT

OVERSPREADS A TIGHTENING DRYLINE SURGE BENEATH SUBSTANTIAL DCVA

ALOFT. WHILE PROGGED INSTABILITY FIELDS ARE NOT STRONG...THEY WILL

BE SUFFICIENT GIVEN A MOIST BOUNDARY LAYER AND VERY STRONG FORCING

FOR ASCENT. SHEAR PROFILES WILL FAVOR SUPERCELLS AND BOWING

STRUCTURES. ENVIRONMENTAL SHEAR WILL BE MORE THAN SUFFICIENT FOR

TORNADOES...AS WELL AS DAMAGING WIND GUSTS. ALTHOUGH THE LOCATION IS

NOT PRECISELY KNOWN AT THIS TIME...AN ENHANCED TORNADO THREAT MAY

EXIST ALONG THE JUST S OF THE SURFACE LOW TRACK EWD ALONG THE WARM

FRONT...WHERE LOW LEVEL SHEAR WILL BE MAXIMIZED.

A SEVERE THREAT IS LIKELY TO PERSIST THROUGH THE LATE EVENING/EARLY

MORNING HOURS AS FAR E AS SRN INDIANA AND CNTRL KY AS THE LOW

CONTINUES TO DEEPEN.

[baroclinic_instability]

Going with 30 percents already. Seems they are going for more rapidly deepening solutions such as the Euro/UK based off their wording. I agree with it.

[MoeWx]

Going with 30 percents already. Seems they are going for more rapidly deepening solutions such as the Euro/UK based off their wording. I agree with it.

So do I. Given the last 2 storms we ended up some 10°F warmer than previous model projection. If that happens with this set up folks siding with GFS right now will be in for a rude surprise :/ That said I think GFS is just being GFS right now. with the cold/progressive solution. At least for So IL, S IN and W/ Central KY Though the OP and GEFS both show similar limited northward LLJ.. Will be fun to watch over the next few days.

[on_wx]

They're here

[janetjanet998]

12z NAM not as strong but may be more unstable

[David Reimer]

12z NAM not as strong but may be more unstable

It looks like its slowed the progression of the surface low down dramatically, keeping it located northwest of D/FW for most of the day. Its a shame that the low level flow veers out, as this would have been a classic dryline setup here in North Texas. It definitely did come in with more instability for some areas, especially northeast of the surface low.

[Reed Stough]

Still watching...

Models look a little more interesting this morning....slightly more instability and better dry-line setup.

If anything can get going it looks like from about 150 NM South of TUL to LIT to about 20 NM N of MEM, may be the prime spot - IF the models are correct.

[Thundersnow12]

would be talking a bigger threat if these winds were more backed..

[David Reimer]

would be talking a bigger threat if these winds were more backed..

I'd be very cautious right now in attempting to determine the amount of backing. The models have really taken a shift since the 0Z runs showing a rather elongated low for most of the day before it begins to eject and deepen overnight Thursday into Friday Morning. If the low was to deepen quicker then expected or the models (which run on a 20 to 40KM grid) not being able to define the exact position of the low pressure. In addition, if we get any mesoscale features (such as a boundary from morning convection) or a mesolow backing winds from the surface up to 925 MB, could significantly increase the chances of supercells here in North Texas. That is a incredible amount of 3 KM CAPE and reminds me of March 8, 2010. If we get a boundary in the area to locally back the winds and a cell can ride on that, then we'll get one hell of a tornadic supercell. While the models right now don't necessarily show a prime setup for tornadoes, especially here closer to the dryline in North Texas/Oklahoma, I'm not totally sold that the winds won't back. Obviously this feeling comes from past experience instead of what the models are showing, but the NAM is having issues pinpointing down a specific area of lowest pressures that would eventually decide the area where winds back. I think SPC and some local offices share that concern, as if they were to follow the models from last night they would have backed off the wording, but they didn't.

My premonition aside, my 'target area' right now would be closer to the triple point here in North Texas, eventually heading eastward towards the AR/LA/TX junction. If we get cells firing out east and instability is closer to 1,000 J/kg with that LLJ cranking like it is, then we'll end up having tornadoes. With PWAT values over 1.5 inches, they'll be rain-wrapped and difficult to see until you get hit by airborne debris.

[Ed Lizard]

^ What is that? I am not familiar at all with it.

Some kind of analog derived risk assessment?

I can find it on Google, now to figure out what it means...

http://www.spc.noaa....xper/mref_mars/

Edit part 2

Learns omething new every day...

http://www.spc.noaa.gov/misc/carbin/Help.html

[blackjack123]

A few notes...

1) The energy responsible for this threat is still out in the Pacific Ocean. It will makae landfall in Baja and eject northeastward starting on Wednesday. It doesn't even emerge into the CONUS until early Thursday, the day of the event. Translation: this thing won't even be in the ROAB network until the day of the event. Expect models to handle trough orientation, timing, and geometry poorly up until the morning of the event. Recent events such as Lone Grove (when the biggest threat was supposedly in E OK and W AR), Super Tuesday (weak s/w providing focus for pre-frontal supercellular development, poor trough handling after 0Z 6 Feb IIRC), etc. should remind us that even the most minute details make a big difference in severe weather setups.

2) The lack of a cap poses a concern. There is a non-negligible possibility that storms fire too early. It could be a very convectively messy day. However, given some insolation and clearing.... (read next bullet)

3) ...Instability should not be a problem (with caveat mentioned above). The sounding Ed Mahmoud posted shows conditionally unstable lapse rates even with a very moist vertical profile. Assuming model-progged H5 temps around -15C, I think lapse rates will be steep enough to yield ~1000 J/kg of SBCAPE if not more come the day of the event. So often we see at this juncture model soundings that show moist-adiabatic lapse rates, and even in those cases we somehow manage 500-1000 J/kg.

Yeah no doubt the Super Tuesday Outbreak was much bigger than the outbreak the day that EF4 hit Lone Grove. Although I thought the Lone Grove tornado may have been possibly stronger before it hit the town. I could be wrong but I thought they were indicating like 180-200kts of gate to gate shear which is possibly EF5 intensity and a very explosive environment over a small area(78temp, 64dew, and 2000-3000J/kg). The Lone Grove had a smaller explosive environment over parts of central south Oklahoma vs most of Dixie Alley. Yeah a few ingreidents could make a big difference.