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We continue to see significant issues with ProbTor that seems directly related to inflated Low Level AzShear values. This is likely due to an outflow boundary that is extremely well sampled by the KBIS radar. As this outflow boundary races ahead of convection, the actual tornado threat, which was never high to begin with, continues to drop even more.
ProbTor, on the other hand, places a tremendous amount of weight on LL AzShear, resulting in a persistently high ProbTor values.
#MarfaFront
Watching a supercell near Bismarck…
It was clear from the hail core aloft that this was severe. However, it was obvious around 2152Z that the RFD gusted out as was seen on reflectivity initially, and then later on the velocity images as the winds became less perpendicular to the beam. However, ProbTor continued to carry high probabilities, and even increased when it was clear on the velocity images there was no tornado threat. ProbTor was only keying on the outflow boundary. MRMS Rotation Tracks were also showing a swath of higher values along the outflow boundary.
-Tempest Sooner
Upper left: Orange ProbSevere with a green ring signifying escalated ProbTor
Bottom right: Low Level Rotation tracks with green ProbTor Object associated with the green ring at upper left.
Lower Left: KBIS base velocity.
Key things to notice above – a spurious looking inbound maximum to the northwest of KBIS and a corresponding uptick in Low Level Rotation tracks in response to this shear zone that likely is just an artifact of sampling near the ground. Below, we see ProbTor climbing to over 40% (red trace). Be careful with ProbTor when the storm is close to a radar.
Suggestion: Instead of 0-2km, perhaps Low Level Rotation Track should be capped at the estimated LCL AGL.
#MarfaFront
GLM Flash Extent Density product seems to have poor detection from both GOES 16/17 in the high plains. Perhaps this is related to the greater viewing angle due to latitude and being farther on the periphery. In this example, a very strong supercell was noted to have high total flash density values on ENTLN, while the GLM FED was very unimpressive. It was picking up on some flashes, but not enough to help the storm stand out from a lightning perspective. This could have negative impact on warning forecasters trying to use FED to identify stronger cells from a situational awareness perspective.
— warmbias —
All Sky Layer Cape looking funky with some discontinuities around KS? Why?
A quick look at the Data Type product (below) shows only GFS being used in these suspect areas. Major caveat emptor!
All right, time to look at storms around Bismark.
#MarfaFront
Compared KBIS observed 19z Skew-T to NUCAPS 20z Skew-T
Key points:
Lapse rate differences: Between 1-1.5C/km difference through all levels
SFC and MU CAPE was 500J/kg higher on the NUCAPS
Vertical profile is markedly different and is a much more saturated on the observed Skew-T
To further confuse matters the NUCAPS forecasted SFC CAPE values for the same location and time is 400J/kg…which is closer to the observed sounding than to the NUCAPS sounding.
—Desmond—
Looking north of the ND/MT state up in Canada. Green (Red) is Event Density from GOES-16 (GOES-17) and yellow is ENTLN cloud to ground strikes. Remarkably good agreement in position, although the data smearing is evident and parallax isn’t much of an issue (yet?) given the shallow convection.
#MarfaFront
The 12Z Bismarck sounding was quite dry and and the surface observation at Bismarck was around 32F. Low level moisture has quickly rebounded this afternoon across central and western ND, however it remains dry across eastern ND. Dewpoints have come up into the upper 40s to near 50F and temperatures have climbed into the upper 70s. The AllSkyLAP CAPE loop clearly shows that instability is increasing across portions of ND with values topping out around 900 J/KG at 1930z, though most of the retrievals are GFS based because of cloud cover lingering in the area.
I decided to look at the NUCAPS soundings across the area to see if the CAPE values were similar. What I found was in the areas of low level moisture, the NUCAPS modified and unmodified sounding had similar MLCAPE values. However, as I checked NUCAPS soundings further east into the drier air, the un-modified soundings showed MLCAPE values that were far to high (should be near zero based on other observations), but the modified soundings were pretty spot on regarding the surface observations (especially Td, T may be slightly low), and thus MLCAPE values appeared to be more reasonable.
The next six images are the comparisons between the non-modified and modified NUCAPS soundings at three different locations. The first is near the peak in instability northwest of Bismarck, the second is east-northeast of Bismarck near the gradient, and the third is farther east into the drier air.
With that said, the modified soundings seemed to do a decent job capturing the gradient in CAPE. The values appear to be a bit high compared to the AllSkyLAP CAPE (1183 vs 750 J/KG around the same time), and high also compared to the SPC Meso Analysis Page (around 500 J/KG).
Quick update…..
The Bismarck office fortunately did a 19Z sounding, which showed very weak MLCAPE, only 22 J/KG! So both the AllSkyLAP CAPE and the NUCAPS were overdone.
-Tempest Boomer
Continuing the theme of the week, today figures to be another day with few storms across the country producing some severe reports. Today we are operating across the slight risk area in North Dakota in the Bismark and Grand Forks CWAs where convection should initiate across central North Dakota this afternoon and into the evening. A third group will also be operating in the Memphis CWA looking for redevelopment behind a morning MCS along a trailing outflow boundary which could produce some marginally severe hail and wind across northeast Arkansas and into western Tennessee.
-Michael
On MXX the Hook, Strong MESO, and TVS from 1950-2008, with the cell heading N of Phenix City, the products lined up really well with AzShear and added confidence in locating TVS and projecting track. After 2008z on MXX new color scheme proved itself viable, highlighting the top end of the scales.
@2020z On MCV to S of MXX radar the forming TVS was caught about one-two VS earlier on AZ Shear than the Vel displayed any strong couplet
~ 2000z On a discrete cell ahead of the LEWP, AZ Shear had similarly stellar performance from EOX radar and high-end values lined up well with Hook, G-G shear and the tornado track
From the EOX radar the in the QLCS ~ 2016z, the AZShear did really well spotting the shear 3-4 Volume Scans prior to any exaggerated Ref/Vel signatures, this was impressive
Another score for AZShear on QLCS FC track ~ 2140, several Volume Scans prior to touchdown, the new color table >.02 and no real definitive SRM or Reflectivity signature to help out!
I’d really like to see this product in the field, ASAP!!
