5/8/15: Multi-day Severe Weather Outbreak Upcoming in the Central US

Please note that the thoughts in this post should not be used for decision making purposes. You can find “official” severe weather outlooks and watches at:

www.spc.noaa.gov

You can find “official” local forecasts, watches and warnings at:

www.weather.gov

The Storm Prediction Center (first link) has already issued a moderate risk for severe thunderstorms for Friday and also for Saturday in their day 2 and 3 outlooks respectively across parts of the Plains. This obviously garnered a significant amount of attention, after an already eventful severe weather day on Wednesday across the region, so I am going to take a look at the severe threat(s) coming up over the next couple of days and see what I think may happen.

Before jumping into the forecast models, it is very important to look over observational data regaurding the upcoming weather event to get a feel of what should happen and what ingredients should be in place. CAPE doesn’t magically develop, wind shear doesn’t just happen to appear over said CAPE for no reason, and surface lows/fronts/dry lines don’t develop because they want to. The sources of all of these ingredients can be picked out ahead of time using observational data, especially when looking at a day 1 forecast. Due to lack of a printer and some time constraints on my part I won’t be printing out raw, unanalyzed plots of the mandatory levels and doing a hand analysis right now, and will have to settle for looking over objectively analyzed data along with water vapor, observed soundings and mesoanalysis images from this evening.

current wv

Current water vapor imagery shows an upper level low pressure system slowly moving SE over California. The low appears to be approaching the base of the trough it is located in, meaning it should begin moving more east than south soon. The low is very slowly moving in water vapor loops, suggesting it probably won’t eject into the Plains until Saturday.

To the southeast of the upper low, a speed max and diffluent flow aloft is evident over the four corners region. A subtle speed max was evident over OK/TX when looping the water vapor which helped convection develop in that region Thursday afternoon and evening. As the upper level low slowly moves east over the next day, the diffluent flow aloft should also move east and emerge over the southern Plains on Friday.

current 250

250mb objective analysis valid at 0z Friday shows a similar picture to the water vapor imagery, with a clear speed max and diffluent flow aloft over the four corners region. This will likely eject over the southern Plains on Friday as the upper low and trough slowly move east.

current surfacec

Surface analysis courtesy of the WPC valid 0z Friday shows a weak lee trough/dry line across western TX. Given the bulk of the upper level ascent associated with the speed max is still generally west of this area, the weak nature of any attempt at a lee side cyclone makes sense. However, as the upper level ascent associated with the nose of the speed max and diffluent from aloft with the speed max spreads east on Friday, it is likely that a better attempt at lee side cyclonegenesis will occur on Friday over the southern plains.

The surface analysis also reveals an outflow boundary from convection that occurred on Thursday stretching from west-central TX east-northeast towards the Red River and into south-central OK…south and east of this boundary, surface winds are S-SErly with dew points ranging from the mid 60’s to the lower 70’s, with winds coming off of the Gulf of Mexico where dew points are in the low to mid 70’s. North of this boundary, winds are generally easterly and the atmosphere appears to have been overturned by convection, with much cooler temperatures and dew points.

This sharp of an outflow boundary, with a distinct temperature and moisture difference across it won’t disappear overnight and may act as a pseudo warm front on Friday that both focuses convection and perhaps limits the northern extent of instability, depending on ongoing convection Friday morning and the strength of the low level flow.

Unfortunately, 0z Friday soundings from OUN and FWD still haven’t come in or are just unavailable (it’s 4z as of this writing so I’m leaning towards the latter), which makes it difficult to ascertain the influence of Thursday’s convection on the environment north of the outflow boundary; however, soundings to the south and west can be used to assess the moisture and lapse rates that will be available for Friday:

CRP sounding

Soundings from BRO, CRP, LCH and SHV all reveal a deep layer of rich moisture (to at least 850mb)…with CRP shown above as it is the closest 0z sounding on the “warm” side of the outflow boundary and east side of the dry line to the target area for Friday. SSErly low level winds at CRP suggest this moisture rich air will be moving towards north-central TX and central and perhaps western OK.

The low level moisture advection is rather weak (note only 10-20 knot winds in the low levels on this sounding), however per the surface analysis above rich moisture exists as far north and west as the dry line and outflow boundary from Thursday’s storms. This layer of moisture is deep enough, with a strong enough cap at the bottom of the elevated mixed layer (EML) that the moisture shouldn’t “mix out” with daytime heating on Friday. This suggests that moisture will be high enough to maintain LCLs that aren’t too high for tornadoes on Friday…and also suggests that low surface dew points won’t limit instability in the warm sector.

current MLCAPE

There is already a fair amount of CAPE on the CRP sounding pictured above, however mesoanalysis suggests that extensive overturning from today’s storms has significantly limited instability in northwestern TX and pretty much all of OK. Moderate buoyancy exists south of the outflow boundary and east of the dry line, however recovery farther north will be needed for the SPC day 2 moderate risk area to verify.

MAF sounding

In that respect, 0z Friday soundings from MAF, AMA, EPZ and ABQ reveal a very deep layer of steep lapse rates extending from the surface at those sounding locations to 500-600mb, with 20-30 knot WSWrly winds above 700mb at all four of these locations, suggesting the steep lapse rate air in the mid-levels will be advected east and potentially make it to the east of the dry line by Friday. Steep mid-level lapse rates are crucial for strong, sustained updrafts associated with higher end supercells, and can help buoyancy recover quickly if they advect into an area that has suffered convective overturning from the previous day’s thunderstorm activity.

Farther north, another potential focus for storms on Friday will be a stationary frontal boundary delineated on the surface analysis above. The 0z sounding from DDC reveals respectable mid-level lapse rates well north of the overturned airmass over OK, but just a skin-layer of respectable moisture. This suggests that locations well north of the OFB may see high LCLs are heating and mixing “bring down” lower dew points on Friday, which could limit CAPE and tornado potential once you get well into KS/MO.

current radar

Speaking of which, the thunderstorms from Thursday have developed upscale into a mesoscale convective system…this is moving east and I don’t think will really be in the way for anyone in TX or OK come morning. The OFB is evident on surface obs and radar loops and a roughly sketched in its 4z position on the radar image above…the OFB was perhaps slowly drifting north, and this should continue thanks to the modest southerly low level flow in the area. The dry line has retreated back to near the TX/NM boarder.

My main concern is potential nocturnal convection developing along/just north of the outflow boundary by Friday morning possibly retarding the boundaries push north and possibly influencing later storms if the elevated convection is persistent.

700mb meso

Current mesoanalysis (4z Friday) shows some mid-level warm air advection across parts of west-central TX. You would normally think “oh great, the EML is advecting east and the airmass is recovering behind the MCS” and that is true…however, this mid-level warm air advection can cause elevated thunderstorms to develop overnight into early Friday on the cold side of the OFB and potentially keep the OFB suppressed south of the Red River. Recent HRRR runs develop this elevated convection by early Friday and the very steep lapse rates observed on the 0z Friday MAF sounding support the notion of elevated convection.

This elevated convection could keep the threat farther south than what models that don’t show elevated convection would imply.

So, let’s summarize what we can imply from just the observational data and no real model input up to this point:

-Deep low level moisture slowly advecting NNWward south of an OFB left by Thursday’s storms combined with a stout EML advecting ENEward off of the southern Rockies supports moderate to strong CAPE in the warm sector east of the dry line and south of the OFB on Friday.

-Large scale ascent on the nose of an upper level speed max/jet streak should overspread the southern Plains on Friday. This will support a better lee cyclone on Friday than on Thursday, suggesting a somewhat stronger low level flow. This may help the OFB move north from its current position, potentially into or through OK. This will also support a decent intensification of the low level jet Friday evening. Modest pressure falls may also support backed low level winds to the east of the dry line. The combination of strong Wrly winds associated with the jet streak and potentially backed low level winds in response to the developing lee cyclone likely over the southern Plains should lead to enough westerly bulk shear for supercells…and enough low level shear and turning for tornadoes by evening as the LLJ ramps up as it typically does every evening in a Plains warm sector.

-The potential for elevated convection north of the OFB developing by Friday morning may retard the boundaries progress northward. Ongoing convection may cause the storm mode to be messy. Otherwise, westerly shear vectors across a north-south oriented dry line combined with a cap due to the EML suggest storms would initially be discrete supercell…with some upscale growth into an MCS or two during the evening possible due to respectable large scale ascent and also an intensifying nocturnal LLJ. Given the potential for elevated convection developing early, there is some doubt on discrete supercells occurring.

-Well north of the remnant OFB, along the stationary front (that should turn warm on Friday in response to a deepening lee cyclone and associated low level wind response) in KS and MO decent lapse rates but a lack of moisture may allow some severe threat to play out…however the tornado threat may be lower due to somewhat weaker buoyancy and higher LCL heights due to lower surface dew points.

-The dry line is currently well west along the TX/NM boarder…thanks to ascent shifting east I’d expect the dry line to shift east on Friday as well. However, it is starting fairly far west, so a severe threat could exist back into far western TX/OK (the Panhandles).

With all of the above taken into consideration, before I look at the models, here is a rough sketch of where I think the best tornado threat may occur on Friday:

severe 1

This I believe is fair as it takes into account the possibility of the OFB lifting well north given the strengthening Srly low level flow…but also gives weight to potential elevated convection in the morning keeping it farther south. This also includes some risk up to the stationary/warm front n SE CO/KS/SW MO. If I had to assign SPC style probabilities, the green would be 2%, yellow 5% and red 10%. Although the environment looks good, uncertainties regarding early convection make it harder to go any higher than the 10% odds within 25 miles of a point of seeing a tornado.

Let’s look at some models:

sig tor ingred

Starting off with the SREF “significant tornado ingredients” parameter valid at 21z Friday (it’s a joint probability among the ensemble members of having a “low end” significant tornado environment with convective precip in place), you can see that a good number of members have ingredients in place over western OK and parts of NW TX. By this point the SREF mean doesn’t develop a true lee cyclone (perhaps an attempt over SE CO), but there is a clear dry line extending from SE CO through the TX panhandle south into south-central TX…a clear stationary front across central KS into northern MO…and perhaps a remnant OFB of some sorts over central OK (note the chance in wind speed in that area).

The wind field makes sense given the observational data discussed above and inferences made from that data…however I am concerned that the OFB may stay a bit farther south than this for reasons discussed above.

sig tor ingred 2

By 0z, the SREF mean shows surface winds backing some across the warm sector in OK and TX, resulting in stronger low level shear and a slight increase in probabilities across the board. This is reasonable as the low level jet typically intensifies in the evening and surface winds also seem to back some in the evening…especially if pressures are falling.

SREF sig tor 3

When looking at the probability of the significant tornado parameter being greater than 3, according to the SREF valid at 0z Saturday, you can see high probabilities near the area I outlined above, with better than 50/50 probabilities extending down the dry line in TX. Given the potential for strong CAPE, strong bulk shear and improving low level shear during the evening, this sort of parameter space isn’t unreasonable in my opinion.

NAM 21 250

When looking at the 0z Friday NAM; at 21z, as expected based on the observational data looked over above, upper level diffluence/large scale ascent is evident over OK/north-central TX as a speed max ejects out over the Plains. This helps favor a weakening of the cap associated with the EML and storm initiation. My one concern is that this large scale ascent spreads over the dry line over western TX before 18z, which could combine with potential ongoing elevated convection to cause a messy situation.

NAM s

A forecast sounding from the NAM valid at 21z along the Red River in far SW OK shows strong buoyancy, respectably low LCL heights, good low level turning and sufficient bulk shear for supercells, and no cap. This sounding would suggest that storms would fire and that any discrete storm would be a supercell capable of very large hail and a tornado.

I am skeptical about the lack of a cap based on the strong EML advecting east…however, the backed but fairly weak low level winds near the remnant outflow boundary, strong CAPE and strong bulk wind shear make sense based on the observational data discussed above.

NAM STP 24

By 0z, the NAM has a nice bullseye of fairly high significant tornado parameter values along the OFB across central OK. Given the likely ramp up of the low level jet Friday evening and expected good deep layer shear and good low level turning along the outflow boundary along with strong instability, this makes sense.

GFS s

A sounding off of the 0z GFS valid at 21z Friday for the same location as the NAM sounding is fairly similar, however the low level winds aren’t as backed and there’s a little less CAPE. The GFS shows some hint of a cap beneath the EML which conceptually makes sense. This sounding probably wouldn’t stop storms from firing, but would require some source of mesoscale lift to get it done. With a dry line and remnant OFB likely in place, we should have that source of lift.

GFS 21 sfc

The 0z GFS appears to lift the OFB north quicker than the NAM and SREF…by this point, it appears to be near the KS/OK boarder. I’m skeptical of this due to the potential for elevated convection to fire early and keep the OFB from moving north quicker.

At the end of the day, I don’t see anything on the models to make me change my rough estimate map posted above. I believe that capping associated with the EML will make surface based storms most likely to fire near the OFB/dry line intersection early to midafternoon…however, early day elevated convection may keep the OFB farther south. This places parts of the eastern TX Panhandle and locations near the Red River in NW TX and SW OK in the line of fire. Shear may be locally enhanced by the OFB locally increasing the tornado threat. Instability and bulk shear will support supercells capable of very large hail in addition to tornadoes. If elevated convection is persistent it may make it harder to see discrete supercells here and shrink the “best” tornado risk area even more or even eliminate it. We’ll know more in the morning Friday.

Farther south along the dry line, large scale ascent may be somewhat weaker due to being in a less favorable quadrant of the jet streak/speed max ejecting into the Plains on Friday, making initiation more questionable. However, should storms fire, steep mid-level lapse rates, good low level moisture and sufficient bulk shear suggest that supercells capable of producing very large hail and perhaps a couple of tornadoes are possible. This threat is more conditional however due to questions over initiation.

Farther north, from central OK into SE CO, southern/central KS and western MO, respectable mid-level lapse rates and more questionable low level moisture suggest weaker buoyancy. However, large scale ascent due to divergent flow aloft and the presence of the stationary/warm front may cause some convection to fire. Sufficient bulk shear may support supercells capable of producing hail and perhaps a tornado or two.

Quick look at Saturday:

GFS 48 250

A stronger jet streak/speed max will eject out into the southern and central Plains on Saturday, with outstanding upper level divergence/large scale ascent over the Plains from NE south through KS, OK and into TX.

Rich low level moisture (dew points well in the 60’s in the entire warm sector) and continued advection east of an EML should again support moderate buoyancy on Saturday in the Plains…and stronger upper level diffluence/large scale ascent should promote strong lee cyclonegenesis potentially over eastern CO that results in stronger low level winds and more favorable low level shear for supercells and tornadoes.

GFS OUN

This GFS forecast sounding for OUN valid 0z Sunday shows moderate MLCAPE and very favorable wind profiles for supercells and tornadoes…note the turning of the low level winds and gradual increase with height in the winds.

GFS KS

Farther north in central KS, the low level shear still looks good on forecast soundings however the GFS shows a veer-back-veer wind profile with height, which suggests potentially messy storm modes, which could limit the tornado threat somewhat farther north. Something to consider as the GFS has been consistently showing this. The 0z NAM soundings from the same area also show a hint at VBV winds with height.

SREF sig tor ingr

The SREF significant tornado ingredients probability for 0z Sunday shows that many members are indicating favorable conditions for tornadoes across the Plains…with a stronger lee cyclone over eastern CO, a dry line extending south across western KS/OK/TX, and a warm front extending east across NE/MO.

SREF sig tor 3 sat

The SREF shows respectably high probabilities of significant tornado parameters higher than 3 for Saturday evening as well, meaning many members are coming in with a high significant tornado parameter. Consistency among models can give a forecaster confidence in a call.

Just off of this quick look at Saturday, there will likely be two plays:

  • Near the triple point/along the warm front somewhere in western/northern KS. VBV wind profiles have me somewhat concerned about a messier storm mode here, however low level shear and reasonably good buoyancy otherwise are favorable for tornadoes.
  • Along the dry line in OK into TX. Low level shear will likely be somewhat weaker here…however more favorable wind fields aloft may mean a cleaner and more discrete storm mode. Buoyancy may also be a bit stronger down here.

Ongoing convection may also play a role on Saturday, however it’s a bit early to try to take a stab at that.

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