In my previous post, I diagrammed the Oxford, Kansas supercell from 19 May 2013. About 90 seconds after that shot was taken, a new area of low level rotation developed just ahead of the occlusion. This is a diagram of that moment as the inflow was forced into the updraft at that point and rapidly exposing circulation at the cloud base.
In 2010 after my first Great Plains chase, I spent a lot of time going through my photos and working to improve my understanding of storm structure. For one of those exercises, I diagrammed the Bowdle supercell. There was a lot of excellent feedback and discussion on all the structure in that beautiful, terrible, amazing storm.
Each new supercell I chase broadens my appreciation for all the ingredients and dynamics that make each one unique and awesome. I enjoy recapping each one and visualizing what was happening, so I've been keen to diagram some more recent storms.
What I especially wanted to work on were transition points in space or time where a shelf cloud is merging or morphing into a wall cloud/tail cloud and vice-versa. A lot of times it's pretty clear what's what. But because the storm and its environment don't care about taxonomy, there are plenty of cases where things ride along a diffuse spectrum. To me, it's educational to watch these things unfold and look for transitions where one thing becomes another, and what that means for the fluid movement and interaction of near-storm air masses.
So, here is a batch of a few more.
Denver Supercell - May 21, 2014
While this storm was still strengthening over Denver Metro and headed toward the airport, it had a very compact RFD shelf that I wanted to call a wall cloud at first glance. But the RFD appears to be responsible for lifting this prominent feature out ahead of the base while the true wall clouds spins away behind. Mike Olbinski's time lapse nicely demonstrates the dynamics (See the 6:24 segment of the video).
Goshen County Supercell - May 20, 2014
This was a view that to me, beautifully demonstrates the source of a weak wall cloud’s formation as the forward flank's rain foot intrudes into the updraft, drastically lowering the condensation level in that area. How much of the foreground lowering would technically be considered a wall cloud vs. RFD shelf was questionable to me.
Newkirk, Oklahoma Supercell - May 19, 2013
This is an example of a transition point where the RFD is smearing a weak wall cloud into a shelf cloud. At least that's how it seems to me. You can see that the foreground lowering is "pointing" toward the forward flank, and benefiting from lowered condensation as it rises into the base. But now it's being hurried and lifted on its way by the RFD gust front (which it "points" away from). It seems to be in a place between both 'definitions' and both dynamic conditions.
Oxford, Kansas Supercell - May 19, 2013
This storm had a shelf feature on the forward flank leading straight into the the RFD core that made me think of video of the Long Point, Illinois storm on November 17, 2013. Skip Talbot had commented that the forward flank shelf on that storm seemed to behave like a hybrid shelf cloud and inflow band.
I just finished editing and uploading video to the September 27, 2014 chase report.
It's been three years since I've had a really good storm chase in Arizona. This past Saturday, I had a chance to make up for the last couple of missed years. I wound up tracking three supercells along Highway 93 between I-40 and Wickenburg. The last of those three was difficult to observe due to bad positioning, surrounding convection, and unexpected freight trains, but the first two were spectacular amid some stunning Arizona landscapes.
The day before, I had set an initial target of Kingman, Arizona by sunrise, based on NAM and GFS placement of a closed low over Nevada. This was forecast to wrap up 30-50 kts of deep layer shear on top of a cold front over the Colorado river while rich moisture surged northward bringing mid 60 degree dew points. As is usually the case, targeting was based on a compromise between better instability further south vs. better dynamics further north. Instability and backed surface winds appeared to be maximized around daybreak and weakening by midday (1000-1500 j/kg with 100-150 m2/s2 SRH). By the night before, all models were trending downward on lapse rates, so forecast instability was dwindling. However, there was still enough to work with, and hodographs were very tantalizing by Arizona standards. This did adjust my perspective on positioning and timing. So I was able to get a bit more sleep and head out of Flagstaff a little later than planned (5AM instead of 3AM).
I stopped at Seligman about 6:00 AM. As I looked west, I was treated to a line of storms flickering with constant lightning as they trained along the cold front near Kingman. HRRR was forecasting the convection to quickly fill in eastward, along sheared out bands with some stronger embedded storms. HRRR's version of SRH, bulk shear and CAPE still looked promising and was trending toward batches of storm helicity tracks over western Yavapai county by mid to late morning.
Once I reached Highway 93 a little before 8AM, a weak line of storms was passing overhead while a stronger line was approaching from the west. That stronger line featured a healthy cell just to my southwest with a persistent velocity couplet. It looked like it would cross Hwy 93 a couple miles south of I-40 and I had some difficulty deciding on which highway to intercept it (no decent side-road options in this area, just the two main perpendicular highways). I would have preferred the east-west option of I-40 for more fine-tuned positioning and safety margin on storms that are moving mainly northward. But I finally decided on my north-south Hwy 93 option since it would give more opportunities to pull off, start and stop instead of being trapped on the interstate with fewer offramps. As the updraft for the embedded cell moved by my position, it wasn't much to look at, and just seemed to be putting a bit of a kink in the line.
Because of the added safety cautions being on the north-south road, and new convection fluctuating to the south of my target cell, I decided to stay a lot further south of it than I wanted. About the time it neared the I-40/Hwy 89 junction, it didn't look as strong on radar, but the trailing gust front finally pushed hard and it expanded outward in spectacular fashion. As a kink in the shelf took shape, a rugged, conical lowering materialized behind the surging gust front before getting smeared out.
After that amazing encounter, I headed further south to get in front of some more developing convection. Along the way, I caught brief views of stronger updraft bases between precipitation cores, and irresistible stormy views in the desert terrain.
By this time, HRRR was trending toward better options further east into central Yavapai county. So I headed east to Congress when a strong cell with a developing couplet started moving in from the southwest, so I headed back south toward Wickenburg on Highway 89. As I cleared heavy rain on some cells popping up overhead, the base of the Wickenburg storm came into view. It was sporting a lowering and a huge tuft of scud was rising up from the ground to meet it. As that merged with the rain free base, the storm had the embryo of a wall cloud.
While photographing it, I noticed that the storm appeared to be deviating a bit to the right. So I jumped back in the car and headed further south to avoid getting cut off. During that short drive, the wall could got sturdier as it made attempts at a tail cloud along the interface of a the forward flank. It reminded me a lot of another beautiful supercell I intercepted in Wyoming back in May.
I was so busy trying to coordinate video and photography, and keeping track of overall storm motion that I couldn't relax and see if any slow rotation was present. At this point, if there was any rotation in the wall cloud, it wasn't fast enough to make me think it was about to plant something out in the mesquite and palo verdes. However what was concerning is that the storm was turning even further to the right and I had to hustle out of the way. As I raced south, I started getting shoved around by RFD that was starting to fill in with rain. Once I was clear, I got view of the back of the RFD core and right edge of the wall cloud as it moved away.
I still wasn't confident if I was seeing low level rotation, but I did call it in to the Phoenix NWS office as exhibiting supercell structure. Looking at the video and time lapse sequences later, it definitely was twirling behind the RFD. The view of the entire cell moving away over the desert was incredible. The dark base and flanking line contrasted against the bright white RFD core and anvil against a deep blue sky. It was spectacular.
After savoring the experience, I had a choice between more cells moving into Yavapai County, or a building squall line moving into Phoenix. I was feeling a little hooked on the supercell storm mode, so I didn't head into Phoenix for the amazing show that storm put on. Instead, I repositioned west of Congress for the next line of storms as another cell with a velocity couplet was moving in. Unfortunately, it was surrounded by more developing convection and I only had brief views of structure between morphing rain cores. I was about to make a hopefully decent intercept, but was stopped short as a freight train took its sweet time blocking my best approach.
Once that storm was gone, I decided to take the scenic route back toward I-17 and took Highway 89 to Prescott. The long drive up the mountain switchbacks took me into the dark of the foggy cloud bases where mounds of small hail still survived in places against an onslaught of heavy rain. As I descended the east side of the mountain range I came across several sections of road that had recently seen flooding and had to stop at one point to pull an 8-inch diameter log out of the road while essentially taking a full on shower.
As I made my way into Prescott, DPS was roaming around pulling debris out of the streets. Further in, I passed a storm drain that was fountaining water, and the town was a waterlogged, debris strewn mess. The worst had passed though, and there were no 'turn around don't drown' scenarios to deal with.
As I made my way up I-17 back to Flagstaff, I skirted the west edge of the weakening line of storms and noticed an eddy on radar moving over east Flagstaff. Looks like this might have been a bookend vortex on the line. I wish I could have seen what that looked like, but that will have to wait for another time.
What an amazing chase. No tornadoes, but three Arizona supercells in one chase day is so much better than I could have hoped for. It has me seriously itching for springtime in the Plains—only 6 or 7 months to go!
Today, a strong multicell hammered east Flagstaff, dropping up to 2 inches of precipitation including mounds of hail during a 45 minute time frame. Some meager vorticity got stretched into the blue-green updrafts at times and made me wish I had a better vantage.
Still, the view from the middle of town was pretty good as a clearing eventually showed a nice rain free base west of Mt. Elden.
I noticed some laminar striation under the base, but thought it was just an indication of strong inflow. After checking the photos, I noticed that structure may have actually been a shear funnel that lasted a little more than a minute from 4:42 to 4:43 PM.
I wasn't fully prepared, and decided to shoot a 96 frame time lapse when all I had was a monopod. After Effects stabilization managed to rescue the time lapse pretty nicely.
So much for Virtual Chases. I'm planning to head out to the plains for a few days to try chasing some marginal setups. Not sure if I'll be able to post a forecast ahead of each day, but I'll start with my first opportunity this Sunday.
Target: Northeast Wyoming
With all the frustrations of a flimsy severe weather pattern setting in for the latter half of May, it still looks like there will be isolated/conditional opportunities in the northern plains and elsewhere as a trough moves onshore this weekend and starts cutting off in the southwest early in the week.
For Sunday afternoon/evening, per guidance from NAM, I'm looking at a target in northeastern Wyoming as 40-50 kts of southwest H5 flow moves in over an area of low to mid 50 dew points advects around a consolidating surface low in that target area. A narrow, meridional zone of 1500-2000 j/kg CAPE will develop along the eastern Wyoming border with the CAP weak or open by mid to late afternoon in the northern portion of this ribbon. Today's 12Z NAM forecasts a focal point for convection in this area by 00Z with that area potentially ingesting 100-150 m2/s2 0-1km SRH. Mixed layer LCLs are in the 1500 meter range and heighten quickly the further south you go.
If this plays out well, I might hope to see an isolated, high-based supercell drifting slowly into South Dakota. I have no expectations of anything tornadic with this setup. And if today's 06Z GFS has its way, then I can expect a bust with maybe some elevated showers in South Dakota (GFS keeps surface moisture about a hundred miles further east under stronger capping). 00Z ECMWF agrees better with NAM on placement of moisture and location of precipitation, so I hope that agreement favors my focus on NAM.
This will be my first 2014 chase day during a week of very iffy conditions. I'm definitely concerned that I'm setting myself up for a series of busts, but also excited to take on the challenge.
Chase Target: Zanesville, Ohio
I'm relying on guidance from NAM for this forecast and have run out of time to compare with GFS.
A surface low will be consolidating over western kentucky as afternoon progresses. A warm front will be established northeast of the low through southern Indiana, and northern Ohio. A cold front will nudge into western Kentucky/Tennessee and eastern Mississippi. Mid 60 dew points will advect into the warm sector. CAPE values of 1500 j/kg reside from central Kentucky northeastward across southeastern half of Ohio and western Pennsylvania while a pocket of 2000+ j/kg resides over southern Ohio during the afternoon.
Mid level jet placing 40-50 kts of bulk shear over boundaries with enough extending over the warm side to affect convection. Strong 0-1 km SRH resides mainly north of the boundaries, but stronger pockets develop over western Kentucky during early afternoon, and 100+ m2/s2 over southeastern Ohio toward 00Z. Convection is forecast to be ongoing and messy throughout the day, so it's difficult to for me to anticipate how that plays out in this area. My target is in southeast Ohio where 0-1km EHI rises over 1.5 by 00Z and the lid strength index points to wide openings for surface based storms in the area.
Nearest Tornado Report: 94 Miles West-southwest
Nearest Severe Report: 3 Miles Southeast [Hail: 1.5 inch]
Chase Target: Ashland, Nebraska
A trough over the rockies places 50kt H5 flow from the Texas/Oklahoma Panhandles through Kansas, Nebraska and Iowa by midday. The trough tightens over the afternoon, weakening flow across Iowa to just the extreme southwest part of the state.
A surface low over western Kansas drapes a sharp dryline southward across far east border of the Texas Panhandle and southward from there. At midday, a cold front lines up from the central Kansas/Nebraska border through southwest Iowa and then a warm front up to northeast Iowa. By 00Z, the boundaries have advanced across northeast Nebraska and northern Iowa. East and south of the dryline/cold front/warm front, CAPE values reach or exceed 3000 j/kg by 21Z. LCLs will be rising throughout the warm sector throughout the day, but the southeast Nebraska and southern Iowa have best opportunity to remain at or below 1000 m through 00Z. After that, they lower to 500-750 m across Iowa.
0-1km SRH gets over 100 m2/s2 on the warm side of the boundary, and spiking over 300 m2/s2 immediately on the other side. Hodographs along the warm front have huge, open, clockwise hodographs. Pretty scary actually. Southeast Nebraska at 21Z shows 0-1km EHI over 3 while Iowa waits until 00Z to reach that level. NAM convective forecast puts discrete cells and then ongoing convection in southeast Nebraska as well.
Nearest Tornado Report: 10.9 Miles Northwest
Nearest Severe Report: 9 Miles West [Wind: 60 MPH]
A monster supercell erupted in south-central Nebraska and tracked east-northeast along the warm front north of Lincoln, and then north of my target of Ashland before moving into Omaha. It was a long track tornado producer, with the closest tornado report being 10.9 miles northwest of Ashland. A scattering of other tornado reports came from far southeast and far northeast Iowa/far southeast South Dakota, as well as a few from a dryline storm in central Kansas. Hoping no one was hurt by these.
Chase Target: Odessa, Missouri
At 21Z a surface trough will stretch from North Dakota to northeast Kansas, migrating slowly southeast. A cold front will drape from the Nebraska/Iowa border, across east Kansas and into northwest Oklahoma where it transitions to a dryline southward across west Texas. Mid to possibly upper 60 dewpoints will be pushing northward into northern Missouri on 15-20 kt surface winds. As convection fires from 18Z-21Z, MLCAPE values could reach 3000 j/kg east of the front, with NAM aggressively plotting 4000 j/kg in west-central Missouri.
Mid/upper level support takes the form of only slightly amplified flow with pockets of 40 kts at 500 mb across the central plains and into the midwest. The warm sector appears to get its best support across the northeastern three-fourths of Missouri with 50 knots of bulk shear over northwest and north central parts of the state. 0-1km SRH reaches 150 m2/s2 over the northwest quadrant of Missouri as well by 00Z.
NAM forecasts convection firing in this better zone by 21Z with open, clockwise forecast hodographs showing up in both NAM and GFS. Lid Strength Index opens a brief opportunity for surface based storms in the 21Z time frame in this northwest Missouri as well, so placing my target at Odessa, east of Kansas City.
Nearest Tornado Report: 16 Miles North-northeast
Nearest Severe Report: Same
Numerous severe storms erupted along the cold front and dryline around 20Z with a few supercells turning right and strengthening. The one that passed just north of Odessa generated a few tornado reports—the nearest 16 miles to the north-northeast.
Chase Target: Fredericktown, Missouri
I didn't have a lot of time to prepare this. I targeted a location east of the cold front in southeast Missouri where the H5 speed max would provide bulk shear in the 40-50kt range between 21Z-03Z. NAM forecasts dew points in the mid 60s in southwest Missouri with CAPE reaching 2000 j/kg. 0-1 km SRH is not good, reaching maybe into 100 m2/s2 after 00Z, so I don't see more than a very odd chance at tornado potential. NAM forecasts better parameters and reflectivity further north than GFS, so I tried to split the difference between them.
Nearest Tornado Report: None Reported
Nearest Severe Report: 16 Miles North-northwest [Hail: 1 inch] || 25 Miles Northeast [Wind: Roof off house]
Insolation had an early start in my target area, which helped several severe warned and two tornado warned storms pass near Fredericktown. A wind report 35 miles south had this to say:
3 S GREENVILLE Wind Report
County, State: WAYNE, MO (marker location is approximate)
Lat.: 37.08, Lon.: -90.45
Time: 2014-05-09 20:00
UTC Wind Speed: Unknown
TREE LIMBS DOWN AND DAMAGE TO CAMPER AWNINGS AT THE OLD GREENVILLE CAMPGROUND. UNCONFIRMED REPORT OF FUNNEL CLOUD ASSOCIATED WITH DAMAGE. DAMAGE PATH ESTIMATED AROUND O (PAH)