Talking points for “Predicting Supercell Motion in Operations”
1) My interest in supercell motion all started with a storm near the Black Hills on 14
June 1996. We’ve been predicting supercell motion operationally since the summer of
1997. If you discover a forecast problem and you are interested in it, pursue it with
passion – you never know where it might lead you. Much of this work was completed
when I was a journey and lead forecaster. However, I had a lot of help along the way,
and am indebted to many people.
2) Supercell motion can be predicted! Severe weather and storm motion are places
where we can still truly forecast, which is why most of us entered the field of
meteorology in the first place. Models aren’t much help here (other than for the
kinematic fields).
3) Essentially two parts – background/theory and examples. The “meat” of the
presentation is in the examples. The vertical wind shear perspective of supercell
motion is key. If you get just one thing out of the presentation, get this. B2K method
comes from the paper by Bunkers, Klimowski, Zeitler, Thompson, and Weisman. This
has also been called the ID method (among other things). Note the status bar at the
bottom of the slides.
4) At least 90% of supercells are severe, so we want to be able to predict them.
Situational Awareness – Perception, Comprehension, and Projection – embodies
operational prediction of supercell motion. I.e., do this before thunderstorms develop, or
before thunderstorms split into right- and left-moving components. Question 1: What
county(counties) do you warn for in this situation, assuming the storm is just becoming
severe?
5) So what if you just went with persistence when you created your warning? Note that
the storm exhibited a “climatological” split. It also missed the county it was originally
headed for. This is why a prediction of supercell motion would be useful for situations
like this.
6) These items are based on a literature review. There are only two primary controls on
thunderstorm motion. Advection – horizontal momentum. Propagation – the three
ingredients for convection. Furthermore, this is not an exhaustive list of propagation
mechanisms. The propagation mechanisms can be inter-related. An understanding of
these propagation mechanisms will enable you to make an accurate forecast of
supercell motion.
7) Note that the storm is not an object, rather it is a process (e.g., flow in a stream
analogy, although not exactly). Determining the mean wind layer is not necessarily a
simple thing to do—no universally accepted layer (gets complicated rather quickly).
What about pressure-weighting? This mechanism affects all convection. Advection
becomes increasingly important as the mean wind speed increases. When advection is
zero, you only have propagation. The key for this slide is knowing whether you have
surface-based convection or elevated convection. In general, the mean wind depth
equals the lowest half of the thermodynamic profile. Rich Thompson, Roger Edwards,
and other SPC people have done some nice work here (22nd SLS).
8) Note: this is the most complicated slide of the presentation. The keys are (1)
propagation to the right of the shear vector and (2) the doubling of updraft strength due to
rotation. This process is germane to supercells only. USP manifests itself through
continuous propagation. This effect may be related to shear strength and
thermodynamics (studies are in progress). Especially important when advection is
weak (or opposite to mean wind), and also for “atypical” hodographs. If the shear is
exceptionally large, significant deviations from the mean wind may also occur.
9) Don’t confuse this (internal process) with propagation along boundary layer
convergence zones (external process)—discussed next. Cold pool strength is dependent
upon cloud base height, subcloud & midtropospheric relative humidity, and mid- to
upper-level storm-relative flow (shear)—how many “chilled” hydrometeors are falling
near the updraft. Especially important when the midlevel shear is weak, and/or dry air
exists below cloud base. Can produce dramatically different propagation for gust
front out ahead of storm (faster) vs. gust front as a flanking line (slower) – also see
Corfidi (2003). If the shear is confined to the low levels, the supercell may become
outflow-dominated.
10) Storm motion calculated prior to, and after, the discrete propagation was 35 knots.
Storm motion calculated which includes the discrete propagation during the middle of the
animation was 42 knots. This supercell appeared to accelerate due to gust front
propagation out ahead of the storm.
11) This process is external to the storm, but can be intertwined with gust-front
propagation. The Superior, NE, supercell of 22 June 2003 was affected by a boundary
layer convergence feature. Note that storms don’t always change direction upon
interaction with a boundary (depends upon ambient shear, angle of interception,
thermodynamics, UVV). Especially important with “high” BRN (weak shear vs. CAPE),
and with storm motion somewhat parallel to the boundary layer convergence zone (next
example)—”erratic” movement may occur.
12) Note there is a large-scale boundary from NE to SW, just to the west of the line of
storms. Storms moving parallel to large-scale boundary/convergence axis, with boundary
layer propagation noted in several locations along the line, especially on the southern
extent. Storm motion calculations were quite different, depending upon whether
continuous or discrete motion was determined (e.g., 35 knots vs. 20 knots). This has a
big effect on your SRH calculations as well. In addition, sometimes storms accelerate
when moving down boundaries.
13) This is a “pure” example. Not much in the literature. May promote a positive
interaction between convective cells, but may also affect each other negatively.
Especially important with “high” BRN and low CIN (large CAPE vs. shear), differential
cell motions, or “linear” forcing. Can be difficult to separate from propagation due to
gust fronts and boundary layer convergence.
14) The first storm was initially “anchored” to the east side of the Black Hills due to a
terrain-induced convergence zone (discussed next). After the second storm merged with
the first one, they accelerated eastward. The merger likely strengthened the cold pool in
this case.
15) Can be very important in the West, but also near other smaller topographic features.
Can be similar to boundary layer convergence, but forcing mechanisms are distinctly
different. Examples in Kuo and Orville (1973), Banta and Shaaf (1987), Banta (1990),
and Akaeda et al. (1995). Most important for weak mean wind.
16) Note how the supercell keeps going through a regenerative process whereby new
cells initiate to its west (i.e., 16+ first echoes)—we’re only seeing the end of event.
Nearby storms not affected by orography can have significantly different motion.
Eventually merger leads to acceleration. Summarize the two controls – advection and
propagation. Question 2: list the five propagation mechanisms just discussed.
17) 260 hodographs used for study. Only the mean wind and updraft-shear
propagation are assumed to be important (others couldn’t be done with hodograph). See
paper for details on other methods (in references). Note caveat about BUFKIT (discussed
later). Also note Rasmussen and Blanchard (1998) is quite similar to B2K, but without
explicit prediction of the mean wind.
18) Use this as a guide, with a goal of just “eyeballing” supercell motion when you
become proficient. It is also very useful to overlay the forecast supercell motion with
radar data, as will be shown in later examples. [Take time to go through this.] (1)
Explain how the shear vector (hodograph-relative) is different from the mean wind
vector (ground-relative). (2) Note the difference between “ground-relative” motion and
“shear-relative” motion. Shear-relative motion will be the general convection throughout
the rest of the presentation.
19) Hodograph convention: blue – 0-4km, green 4-8km, open circles every 1 km.
Participation: draw the BL to 6km shear vector; draw the propagation vectors for the
RM/LM supercells; and draw the ground-relative storm motions.
20) Hodograph convention: blue – 0-4km, green 4-8km, open circles every 1 km.
Participation: draw the BL to 6km shear vector; draw the propagation vectors for the
RM/LM supercells; and draw the ground-relative storm motions. Note: the mean wind
is toward the east, but the RM supercell motion is forecast toward the southwest.
21) Recent studies have generally supported the results of B2K. However, a deeper
mean wind, on average, may be more appropriate (especially in warmer and more
“southern” locations). We’ll be looking at several examples to show how the deviation
can vary from the 7.5 m s-1 “constant”.
22) This works in South Africa, Australia, Japan, etc. (i.e., it is universally applicable).
My goal is to make you understand supercell motion well enough such that you won’t be
surprised when you see “strange” motions in operations.
23) As with any algorithm you use, it is important to understand the weaknesses. The
motion of some HP supercells and supercells right along strongly convergent boundaries
in weak shear are the toughest to predict. This is why it is important to understand the
other propagation mechanisms – and this is where forecasting really comes into play.
24) Vertical wind shear is Galilean invariant; the mean wind is not. This is really a
simple concept—if the results remain the same after a coordinate transformation, then the
method is Galilean invariant.
25) Hodograph convention: blue – 0-4km, green 4-8km, open circles every 1km. Note
that VRM is always to the south, and a little east, of VMW on these three corresponding
slides. However, V20R85 and V30R75 change with each of these three slides.
26) The hodograph has been shifted 10 m s-1 to the left of the previous position,
otherwise the shape is exactly the same. Note that VRM is always to the south, and a little
east, of VMW on these three corresponding slides. However, V20R85 and V30R75 change
with each of these three slides.
27) The hodograph has been shifted 10 m s-1 to the left of the previous position (20 m s-1
to the left of the original position), otherwise the shape is exactly the same. Note that
VRM is always to the south, and a little east, of VMW on these three corresponding slides.
However, V20R85 and V30R75 change with each of these three slides.
28) What does “to the right of the mean wind” mean in this case? Can you see why
forecasting supercell motion to the right of the mean wind can fail from this and the three
previous examples?
29) The B2K method can be very useful for highlighting the potential for slowmoving supercells. Can have slow-moving supercells with relatively strong shear (i.e.,
06-km bulk shear ≥ 25 m s-1). The other propagation components of storm motion (i.e.,
gust front, boundary layer convergence, storm mergers, and orography) may also help to
effectively “anchor” supercells.
30) We are about 1/2 of the way through the presentation. Up to this point we have
discussed background materials for understanding storm motion and the B2K method.
Next we will apply what we learned to real-world situations. Question 4: What five
thunderstorm propagation mechanisms were discussed earlier in this presentation?
Question 5: What two components does the B2K method use for predicting supercell
motion?
31) The RUC40 is the model frequently used at Rapid City to overlay the mean wind
and supercell motion on radar images. These fields can be accessed via the Volume
Browser (i.e., these are baseline fields). Need to be on “State” or “Regional” scale to
overlay RUC40 data.
32) This example epitomizes what I’ve been talking about regarding SA and prediction
supercell motion. Here is an actual event a forecaster at Rapid City experienced. Note:
don’t focus on what part of the country these examples come from; that is not important.
The results should be “universally” applicable.
33) This is the MRR hodograph for midway through the event. Note the strong 0-5km
shear – double the values observed for “low-end” supercells (~60 knots). [Demonstrate
the B2K method, and emphasize the difference between the updraft-shear propagation
and ground-relative storm motion.]
34) Note the legend in the lower left-hand corner of the display. Cyan vectors represent
the 0-6km mean wind (knots) from the RUC40. We saw the strong shear, so how is this
storm going to evolve? Question 6: How would you draw up your “WarnGen” box—
assuming it has just become a supercell?
35) Note the radar is to the northwest of the supercell. Since the storm is becoming a
supercell, we know the motion must change. Does it make sense to go with persistence
in the warning? Moreover, it may be prudent to wait for at least two volume scans (at
two different elevations) before buying into the idea that the storm is a supercell – which
it clearly already is in this case. In addition, WarnGen “boxology” comes in to play here,
because if you use the predicted supercell motion to draw your polygon, then the
WarnGen “dot” should not be placed on the past position of the storm. Try practicing on
the WES.
36) Note the legend in the lower left-hand corner of the display. Using the Volume
Browser and the RUC40: the green (tan) vectors represent right-moving (left-moving)
supercell motion, and the blue vectors represent the 0-6km mean wind. The distancespeed tool has been used to track observed storm motion. Speed in knots. All slides
hereafter will have this convention. Note: the supercell took a “hard” right turn toward
the south. The RUC did reasonably well, and even “caught on” to the strong rightward
deviation toward the end.
37) The storm traveled along a N-S instability axis, which I believe is due to the storm
motion dictated by the vertical wind shear profile. I don’t really see any discrete
propagation here.
38) This was a significant storm – ten large hail reports (and one 2.75”).
39) Strong 0-5km shear – double the values observed for “low-end” supercells (~60
knots). The supercell deviated a little farther away (than forecast) to the right of the shear
vector. [Demonstrate the B2K method.]
40) The forecaster’s SA was enhanced in this case! This was in the “middle of
nowhere”, but imagine if you had to make the same warning decision and the initial
storm was upstream of a major city (e.g., Dallas, Nashville, Kansas City, etc.).
41) This example shows how the gradual “right-turning” nature of some supercell tracks
can be anticipated via changes in the vertical wind shear over space and time.
42) The westerly flow was relatively stronger over the beginning portion of the track
(note the 20-kt westerly mean wind), while the southerly flow was relatively stronger
over the end of the track (note the 10-kt westerly mean wind).
43) Four weak tornadoes, eighteen severe hail reports (three > 2”), and one severe wind
report. [Demonstrate B2K method.]
44) Here is another event a forecaster worked. Often times you can deduce you have a
supercell (vs. a strong multicell) simply by comparing the observed and forecast storm
motions for the strongest cells.
45) The split is apparent in the visible satellite images. Question 7: Do you notice any
differences in stability based upon cloud character? Stable billow clouds are noted
northeast of a surface boundary/cloud line, while horizontal convective roles are noted
toward the southwest of the cloud line.
46) Two severe hail reports (one 1.75”). Notice that the left-mover formed as result of
a split, and the observed motion matched the predicted motion rather well, allowing
the warning forecaster to have more confidence that this was a true left-moving supercell
(despite having no velocity data). As the left-mover died, it started moving with the
mean wind. Other left-movers also occurred later on in the event.
47) Here is a 0-3km hodograph from BUFKIT. Note: although BUFKIT does employ
the B2K method, the mean wind is not calculated as prescribed, but rather pressureweighting produces a mean wind that is biased toward the winds in the lowest few
kilometers AGL (most noticeable in the RUC). Also, the only curvature in this profile is
below 500 m AGL, typical of environments with splitting storms. If you don’t have
BUFKIT at your office, bother your SOO until he/she installs it.
48) This is the longest example, and is intended to show how storm motion can be
calculated and monitored. Continuous propagation – supercell moves in a more or less
steady-state fashion. Discrete propagation – supercell moves in a discontinuous fashion
(daughter cells…multicell model). Quasi-discrete or quasi-continuous propagation –
supercell moves as an individual entity, but cells feed the main storm (feeder cells). Use
cell motion for storm-relative calculations.
49) RM supercell motion predicted to the south-southeast; LM to east. Upper-level
storm-relative flow only 11-12 m/s – suggestive of HP supercells.
50) Storms forming along well-defined boundaries can present a challenge when
forecasting supercell motion – i.e., boundary layer convergence propagation.
51) Note these cells are not isolated from one another, increasing the chance for mergers.
52) Note the two primary circulations. Question 8: Do you see a second clockwise
circulation? Another weaker clockwise circulation is evident to the north of the primary
mesocyclone (which is manifest in a subtle split in the reflectivity animation – next
slide).
53) Continuous or quasi-continuous cell motion is tracked during this time. Note the
small “element” break off RM1 at the onset.
54) Additional cells quickly form along the boundary.
55) Two mesocyclones are observed in close proximity.
56) Continuous or quasi-continuous cell motion is tracked during this time, but
considerable discrete development is also observed. Careful analysis of radar data
reveals the separate circulations.
57) Is this one supercell or three (this is a rhetorical question)?
58) Two circulations remain apparent (especially in the volumetric data).
59) Continuous or quasi-continuous cell motion is tracked during this time, but
considerable discrete development is also observed.
60) This loop is a combination of the previous three, but in this case discrete storm
motion is tracked. Doesn’t this look like just one big storm?
61) This animation shows several overshooting tops with discrete propagation to the
southwest.
62) Left hodograph: Storm motion farther away from the shear vector than predicted.
Propagation due to boundary layer convergence is a likely candidate for this.
Right hodograph: Huge storm motion error when discrete storm motion is calculated.
This brings up the notion of “cell” vs. “system” motion.
Question 9: What other propagation mechanism(s) was(were) dominant in this case?
63) Discrete storm motion not observed for left-moving supercell – mostly likely since it
was moving away from the strong boundary.
64) HP supercells along strongly convergent boundaries will be the hardest to predict in
terms of storm motion. Knowledge of potential propagation due to gust fronts,
convergence zones, and mergers is especially important in these situations. Cell motion
is more relevant for SRH calculations than is system motion.
65) Note the long track of severe reports (mostly hail) in eastern Arkansas. This was
from just one storm.
66) Clearly this is an environment favoring elevated convection (vs. surface based).
MUCAPE = 1121 to 2434 J/kg. Lifting level = 1300 to 850 m.
67) Note the predicted motions. Question 10: Why do you think left-moving supercells
occurred in this kind of environment?
68) The left-mover raced north-northeast, while a right-mover traveled east across
northern Louisiana. There was a warm front across northern Louisiana separating the
two environments. [Jeff Craven has developed a WES case for this event.]
69) Note the supercell moved faster than predicted by 5 m/s (not too extreme given these
kind of situations). Question 10: Do you think the supercell is “seeing” all of this shear
& SRH. [Richard Thompson (and others) have relevant papers in the 22nd SLS
Conference regarding the “effective” shear depth and the “effective” SRH depth.]
70) In this case, the lowest 1000 m has been truncated, and the “SFC” actually represents
1000 m AGL. This is a more reasonable shear profile for the left-moving supercell, with
about a 3 m/s error in storm motion.
71) Storm motion for elevated supercells can be significantly different that the forecast
given by the standard B2K method. This is where you really need to be on your guard
(can have a 20kt difference in observed vs. predicted storm motion if you use the wrong
mean wind).
72) This example is used to show a potential weakness of the B2K method. With any
method (or algorithm) you use, it is important to understand the limitations (as we saw in
previous examples).
73) Bulk shear = 65 kts/6km. Note predicted supercell motion to the east-northeast.
74) These are very large and wide supercells. Note that continuous propagation
appears to dominant (discrete propagation not observed).
75) About a 6 m/s error is evident—farther away from the shear vector than predicted.
Question: What do you think is the cause of this error? Updraft-shear propagation is
most likely – due to the strong shear and large/wide updrafts.
76) Large and strong updrafts may propagate significantly farther away from the shear
than relatively small and weaker updrafts—especially when the shear is large. DaviesJones (2002) noted that off-hodograph propagation increases with updraft width.
77) A shear-relative perspective of supercell motion is especially needed for situations
like these.
78) RM’s moving west; LM’s moving south. What is going on here?
79) “Erratic” storm motion could be explained from the vertical wind shear
perspective. Right-movers were more dominant than left-movers – consistent with the
clockwise turning shear vectors in the lowest 3 km of the hodograph. Vertical wind shear
relatively “weak” for supercells [0-6km bulk shear = 14.5 m s-1 0-6km total shear = 33.5
m s-1] Note: you need to make sure you have enough shear before you apply the
concept of predicting supercell motion.
80) This is the last example. This is the University of Maryland tornadic event.
81) Moist and unstable profile – thermodynamic depth about 12km (e.g., MW depth
about 6km). Note predicted supercell motion to the northeast.
82) Note this supercell (SC1) moved about 10 knots slower than the mean wind or the
forecast supercell motion. Question 12: Why do you think this supercell is moving
slower than predicted? Echo tops were 25,000 to 30,000 feet.
83) Note this supercell (SC2) moved about 10 knots faster than the previous one – much
closer to the forecast supercell motion. Echo tops were 45,000 to 50,000 feet.
84) About a 4 m/s error is evident in the upper-left hodograph – due to an incorrect mean
wind. When the 0-4km mean wind is used (lower center), the predicted supercell motion
is much closer to the observed. Moreover, sometimes mini- or low-topped supercells
don’t deviate the full 7.5 m/s from the mean wind.
85) This example shows how the mean wind can vary whether you have low-topped or
“tall” supercells.
86) The benefit of overlaying the mean wind and forecast supercell motion on radar data
cannot be overstated.
Assignment: Develop a procedure to overlay radar data, supercell motion forecasts, and
the mean wind (RUC40, state scale). Use it in operations, and I’m convinced you will
see improvement in your warnings and short-term forecasts.
87) Use this as a general outline. A solid understanding of the storm propagation
mechanisms will allow you to anticipate supercell motion, and as a result, do some true
forecasting (models cannot do this, yet). In addition, it might be useful to overlay the
supercell motion vectors on satellite imagery to better understand how the various
propagation mechanisms may be affecting storm motion.
88) Bunkers et al. (2000) provides the theory and statistics behind the B2K method.
Zeitler and Bunkers (2005) covers much of what is within this presentation. Doswell
(1991) and UCAR (2003) contain good background material on hodographs.
89) Inputs are in the yellow boxes. Allows for varying mean wind depths as well as
varying deviations from mean wind. Contact matthew.bunkers@noaa.gov with any
questions. Don’t worry about “breaking” the spreadsheet (just get another copy if
needed).
90) If you have any questions whatsoever, please don’t hesitate to contact me.