Virtual Chase Case

It's been quite a while since we've had one of these, and considering that the next couple of days will be rather quiet, this might be a good time to hone our forecasting skills with the height of severe weather season right around the corner.

This Virtual Chase Case is done in the style of the Tornado Forecasters Workbook by Tim Marshall and Tim Vasquez. Analyze the data and post your forecast along with a target town. This case may or may not be a day on which tornadoes occurred, and if you think it's a bust day, let that be known as well. Most of all though, have fun!

The solution to this chase case will be posted Monday night.

13z SPC outlook:
[Broken External Image]:http://www.onthefront.ws/case1/day1_1300.gif
...SYNOPSIS...
VERY ACTIVE FLOW PATTERN ACROSS THE CONUS THIS PERIOD. A COLD CORE
UPPER LOW WILL TAKE ON A NEGATIVE TILT AS IT IS EJECTED ACROSS NEW
ENGLAND AHEAD OF A STRONG IMPULSE DIVING SOUTH ACROSS ONTARIO
CANADA. SHORT WAVE RIDGING OVER THE PLAINS TODAY WILL BE REPLACED
BY A WELL DEFINED SRN STREAM TROUGH LIFTING NEWD ACROSS THE DESERT
SOUTHWEST THIS MORNING. A PACIFIC TROUGH AND ASSOCIATED COLD FRONT
WILL MOVE ACROSS WA AND THE NRN ROCKIES.

...HIGH PLAINS THROUGH SRN PLAINS...
SFC HEATING AND WLY MID LEVEL FLOW ACROSS THE HIGHER TERRAIN WILL
MAINTAIN LEE SIDE TROUGH...STEEP LAPSE RATES...AND PERSISTENT NWD
TRANSPORT OF RICH BOUNDARY LAYER MOISTURE ACROSS THE PLAINS. SHORT
WAVE RIDGE MOVING OVER THE CENTRAL U.S. IN THE WAKE OF MID LEVEL
WAVE RESPONSIBLE FOR OVERNIGHT MCS WILL PROVIDE FOR ABUNDANT
HEATING AND DEVELOPMENT OF VERY UNSTABLE CONDITIONS /CAPE 2000-4500
J/KG/ FROM NWRN TX NWD INTO SERN CO BY AFTERNOON.
A WELL DEFINED OUTFLOW BOUNDARY WAS SITUATED FROM HOU AREA NWWD
ACROSS CNTRL TX AND INTO WRN PORTIONS OF THE TX PNHDL THIS MORNING.
THIS OUTFLOW HAS FRAGMENTED THE SYNOPTIC SCALE FRONTAL ZONE
SITUATED ACROSS THE SRN PLAINS. AS SWRN UPPER TROUGH APPROACHES
LATER TODAY...LOW TO MID LEVEL SWLY FLOW ACROSS THE REMNANT
OUTFLOW...IN COMBINATION WITH HEATING ACROSS TX AND THE HIGHER
TERRAIN...WILL SHARPEN FRONTAL ZONE FROM NRN TX INTO SERN CO. LARGE
SCALE ASCENT AND UPSLOPE FLOW TO THE NORTH OF THE FRONT AND ALONG
THE LEE TROUGH/DRYLINE WILL AID STORM INITIATION ACROSS ERN CO AND
NERN NM BY LATE THIS AFTERNOON. 35-50 KT SFC-6KM SHEAR WILL
INITIALLY SUPPORT SUPERCELLS PRODUCING VERY LARGE HAIL...DAMAGING
WINDS...AND ISOLATED TORNADOES. ACTIVITY MAY BECOME MORE LINEAR
ALONG THE FRONTAL ZONE/INSTABILITY GRADIENT DURING THE LATE EVENING
AND SPREAD ESEWD AS 60-65 KT MID LEVEL WIND MAX MOVES FROM ERN NM
ACROSS THE TX PNHDL AND INTO OK. SEE PROBABILITY GRAPHICS FOR AREAS
EXPECTED TO EXPERIENCE GREATEST THREAT OF LARGE HAIL AND DAMAGING
WINDS WITH THIS SYSTEM.

...GULF COAST...
STRONG CONVECTIVE OUTFLOW HAS MADE IT ALL THE WAY TO THE GULF COAST
THIS MORNING. CONVERGENCE ALONG THE REMAINS OF THIS BOUNDARY
COMBINED WITH DEVELOPING GULF BREEZE AND INCREASING INSTABILITY
SUGGESTS RENEWED CONVECTION SHOULD DEVELOP DURING THE EARLY
AFTERNOON. PRONOUNCED MID LEVEL DIFLUENCE ACROSS THE GULF COAST MAY
PROVIDE LARGE SCALE SUPPORT FOR ASCENT WHILE 30-40 KT MID LEVEL
FLOW WILL AID UPDRAFT ORGANIZATION. A FEW STORMS MAY PRODUCE SVR
HAIL AND DAMAGING WINDS.

...SOUTHEAST COAST...
LOW TO MID LEVEL BAROCLINICITY AND SLOPED ASCENT PERSIST WITHIN
SERN QUADRANT OF UPPER LOW AND ENTRANCE REGION OF MID LEVEL JET
STREAK MOVING ACROSS MID ATLANTIC. SEABREEZE ALONG THE SOUTHEAST
COAST MAY ENHANCE LOW LEVEL CONVERGENCE IN THE PRESENCE OF
INCREASING INSTABILITY /CAPE 1000-2000 J/KG/ AND STRONG DEEP LAYER
SHEAR. A FEW SVR STORMS ARE POSSIBLE WITH PRIMARY THREATS BEING
DAMAGING WINDS AND HAIL.

...NEW ENGLAND...
COLD MID LEVEL TEMPERATURES /-20 AT 500 MB/ AND ASCENT ON CYCLONIC
SHEAR SIDE OF 60 KT MID LEVEL JET WILL AID DESTABILIZATION ACROSS
INTERIOR NEW ENGLAND THIS AFTERNOON. HOWEVER...LATEST SOUNDINGS AND
SATL IMAGES SUGGEST THAT INSTABILITY MAY BE QUITE LIMITED. IF
BREAKS IN OVERCAST CAN DEVELOP AND PERSIST...A FEW TSTMS ARE
POSSIBLE. STRONG VERTICAL SHEAR WOULD SUPPORT UPDRAFT
INTENSIFICATION WITH POSSIBILITY OF ISOLATED HAIL AND PERHAPS SOME
STRONG WINDS.

12z Maps:
Surface obs centered over Denver
Surface obs centered over Amarillo
U.S. surface obs with frontal analysis and radar overlayed
925 mb analysis
850 mb analysis
700 mb analysis
500 mb analysis
300 mb analysis
250 mb analysis

12z Soundings:
Albuquerque, NM
Amarillo, TX
Dodge City, KS
Denver, CO
Fort Worth, TX
Midland, TX
Norman, OK

15z Surface Observations:
Centered over Denver
Centered over Amarillo

Jason
 
This is a tough one!

This is where I would target: http://www.waveformpc.com/my_target.GIF
Actually, this doesn't look like a standard MDT risk day at all, more like a bust, given placement of the low/mid level features, and rather weak low level flow. Mid level flow appears strong though, and low level moisture seems to be marching northwestward from the Gulf. I'm guessing that the colder mid level temperatures dipping down into the plains would help create more instability, but as of 15Z, things aren't lookin' so hot.

Anyway, I chose that target because of the energy diving into the west, as seen on the upper level maps. That would most likely help trigger some convection. Moisture also appears to be pooling all the way into that area, increasing slightly from the 12Z obs, winds also appear as though they want to turn more southeasterly, probably adding to some good helicities. The CAP doesn't appear to be all that strong over that area either. The 700mb isotherms and the orientation of the wind suggest that decreasing temperatures in the mid levels may take place.

Man, it sure is hard forecasting without models, satellite, and REAL radar.
 
With moisture sneaking up around the surface high left over from the previous night's MCS, and the upper level trough approaching from the Southwest - I'd probably head towards the Front Range somewhere between Colorado Springs CO and Las Vegas NM. I like the southeasterly surface winds and the fact that dewpoints along the front range are already in the upper 40s/low 50s at 15z with higher dewpoints upstream....low 50s dewpoints are pretty impressive at 6000 ft above sea level.

Another area worth watching is along the boundary and northeast of the surface low in western north Texas towards Abilene and Throckmorton - but that area still may be under the effects of the short wave ridge by afternoon.

Heading to the Front Range the day after a big MCS or cold front marches through the plains has yielded me good results before - so again I'd probably head to southeastern CO or northeastern NM in hopes of seeing at the very least some photogenic, sculpted ice machines.
 
With moisture sneaking up around the surface high left over from the previous night's MCS, and the upper level trough approaching from the Southwest - I'd probably head towards the Front Range somewhere between Colorado Springs CO and Las Vegas NM. I like the southeasterly surface winds and the fact that dewpoints along the front range are already in the upper 40s/low 50s at 15z with higher dewpoints upstream....low 50s dewpoints are pretty impressive at 6000 ft above sea level.

Another area worth watching is along the boundary and northeast of the surface low in western north Texas towards Abilene and Throckmorton - but that area still may be under the effects of the short wave ridge by afternoon.

Heading to the Front Range the day after a big MCS or cold front marches through the plains has yielded me good results before - so again I'd probably head to southeastern CO or northeastern NM in hopes of seeing at the very least some photogenic, sculpted ice machines.

Yeah, I had originally picked the TX panhandle, but then decided to back it up into the CO area, as it looks like moisture was already pooling in that area, and the winds were turning more southeasterly between 12Z and 15Z. Certainly not a clear cut day, and I don't think I would be issuing a MDT risk...
 
Looks like a crapfest to me, but it's only April and I'm hungry so I'm driving to Boise City, OK to set up and see what happens.....
 
Ok, Jason, I'll bite! I appreciate the work you're putting into this.

In RL I'd lke to have a better idea at how fast the mesoscale upper features have been progressing. The sense of the SPC outlook is that they have been moving right along, so the vortmax over extreme SE AZ/northern Mexico is probably translating ENE at 40kts or so. This should deepen the low near Midland in the afternoon and sharpen the dryline south from Midland.

There's already pretty impressive backed, deep moist flow being drawn up through San Angelo and Abilene, good surface heating likely, and some capping.

Short answer: Go to Sweetwater and prepare to follow isolated developments as they book northeastward into the convergence between there and Childress. Then curse as the best action fires upslope near Boise City because the cap was too strong further south. Such is life. :wink:
 
Well, for fun I'll go ahead and say this looks like a potential bust for tornadoes. Features that catch my eye are as follows:

Positive tilt upper trough axis off the CA coast with inflection near Tuscon. Rather unremarkable upper flow features upstream with northwest flow over the region of interest. Diffluence associated with morning MCS and shortwave sweeping across OK leaving subsidence in wake across the region, as noted in the forecast discussion by SPC. Upstream diffluent flow (and shortwave - poorly shown in analysis) near the 4 corners region could be expected to sweep into the northern portions of the outlook region, and less so further south. Alarming is the strength of the 700 mb ridge along the lee of the Rockies, as well as the consdierable 700 mb RH. In fact, the AMA sounding shows a red flag scenario with two inversions, the higher of which capping a saturated layer which sould lead to persistent clouds limiting surface heating in the Texas panhandle later in the day. The 850 - 925 mb pattern shows a deformation axis south of the Red River - arching WNW with a sharp thermal gradient at 850 mb between AMA and MAF. Surface analysis shows a 1002 mb low just north of MAf with trough extending NE and then ENE south of the Red River, and another nearly N-S trough extending northward. A weak ridge extends from nw New Mexico to GDP, as well as from MCS in se TX outflow just south of the aforementioned trough. The main surface moist axis extends along the ridge axis from se TX toward Roswell, NM. By 15Z, sfc low has weakened, but trough axis from sw TX to sw OK has sharpened, with cool and cloudy conditions persisting in the TX panhandle region.

Target(s): If I have to pick somewhere to be - How about Spur, TX. I'd go with Throckmorton, just because it seems you can often pick that as a target and it just happens, but maybe this is close enough. Unclear if sfc based convection evers happens this day though - particularly given persistent cloudiness north of the boundary along with very warm 700 mb temps. Disorganized sfc flow in CO may not matter for a DCVZ day - but I know very little about those events so will not attempt to forecast for that area.

Glen
 
Ok, I know what case this is. Suppose I shouldn't ruin it for everyone else still guessing. Edited to remove the date.

I could see some potential in se CO - certainly the cap is breakable there and there is some support for better wind fields later in the day.

Glen
 
Just looking quickly as I don't have much time.

I'm a sucker for CAPE and southern targets so I'd have to go sit at the Sonic at Sweetwater TX. I get the impression that the cold front is diving somewhat, which begs more investigation. But the cap is not as weak over FWD as I had anticipated, and those winds are nicely backed... as long as the patterns aren't running away the southern targets might shape up nicely.

Lots of moisture piled up along TX Panhandle / NM border and into Colorado, so there's some definite potential there too for picturesque supercells coming off the rangeland and mesas.

My other impression is the SPC outlook badly needs revision... it's smells like a warmed over Day 2.

Tim
 
It's a really difficult call without visible satellite... We can only assume that there is widespread low cloud-cover (a soupy muck) in central, northern, and western Texas, though this may be deceptive given the relative lack of observations for such a huge area. I like the northwestern OK panhandle and southeastern CO IF there indeed is widespread cloudcover in TX. If there is some clearing going on, which there very well may be given the ridging working into the area ahead of the upper-level trough in the western U.S. So, with this said, I'd target somehwere near Childress, TX.
 
Since Lamar and La Junta are already staked out, I'd have to go to Toonerville, Co. The road structure stinks, but you can see for miles.
 
Since Lamar and La Junta are already staked out, I'd have to go to Toonerville, Co. The road structure stinks, but you can see for miles.

Virtual chaser convergences are allowed as long as you pull well off the road and don't block traffic. :D
 
so do we get the answer????!!! :D

Thanks to all who participated. My apologies for not getting the solution posted earlier tonight, but as promised, here it is:

This day, May 28, 2001, was a classic example of why simply chasing the SPC risk areas will not always pay off. Those chasers who eyed the upslope flow and 50+ dewpoints along the Colorado front range scored big today.

The mid-level shortwave ridge kept significant convective development across the moderate risk area to a minimum while the upslope flow regime along the front range in northeast New Mexico and Colorado led to the development of two tornadic supercells - the most significant of these being the Ellicott, CO tornadic supercell. If I remember correctly, only one chaser observed this event, but I'm not aware of who it was or whether or not any images of this event are online. More information regarding this event can be found at:

http://www.crh.noaa.gov/pub/elcodmgsvy.html

The other area of significant convective development was near the Raton, NM/Trinidad, CO area. A few chasers were in this area and documented the event. Four supercells formed over this area and trained off the front range before eventually dying as they moved into the Colorado Plains.

The first supercell formed at ~3:30 p.m. MDT and was rather marginal in appearance but left much small hail in it's wake. The second supercell was a nicely striated LP.

LP Supercell near Trinidad, CO (Scott Blair)
Base Refelctivity Image 6:14 p.m. MDT

Once the LP moved off the mountains and dissipated, the third supercell developed and took on HP characteristics as it produced a large wall cloud, which eventually became wrapped in rain and hail.

Mothership supercell wrapping precip. around the wall cloud (Chris Kridler)
Base refelctivity Image 7:30 p.m. MDT

As the third storm fell apart, the fourth supercell moved off the mountains at dusk, this one taking on a more classic appearance. The cell produced a large, low-hanging wall cloud, which went on to produce a tornado 1 mile east southeast of Hoehne, CO at 8:31 p.m. MDT. The tornado was fairly weak but did do $5000 worth of damage to a garage.

Wall Cloud with fourth supercell (Dave Lewison)
Tornado produced by the supercell (Dave Lewison)
Base Reflectivity Image 8:31 p.m. MDT

While the moderate risk area busted, one severe storm did develop near Midland, TX and produced hail up to 1.75".

Chase accounts from this day:
Scott Blair
Dave Lewison
Chris Kridler
Bobby Eddins
Sam Barricklow
 
Ok, I know what case this is. Suppose I shouldn't ruin it for everyone else still guessing. Edited to remove the date.

I guess Google ended up being the most useful forecast tool for this past event.

Jason
 
Ok, I know what case this is. Suppose I shouldn't ruin it for everyone else still guessing. Edited to remove the date.

I guess Google ended up being the most useful forecast tool for this past event.

Jason

While I didn't use Google, there were several things that tended to give it away...

First, I was actually drawn to CO, since OK and TX looked like crap... Northerly winds and decreasing T's/Td's between 12Z and 15Z just don't sit well with me... And SFC obs did indicate quite a bit of clouds. The SPC outlook looked like crap, I knew for sure the MDT risk wasn't going to verifiy, so that went out the window.

I then noticed the southerly winds in CO, and increasing Td's, which made me wonder. Then, moving on up, 925-850mb did show some nice moisture streaming into that area... The setup at 700mb showed the shortwave over CA nicely, which would probably be the trigger for ANY sort of convection that day. Temperatures at 700mb also seemed like it would decrease through the day, given the orientation. So, I had CO stuck in the back of my head.

I then thought about central TX, where Td's were actually climbing, as well as temps. Then, I had second thoughts, as the shortwave in CA was much too far away to produce convection in TX for the time being, and that weak front/boundary didn't appear as though it would be enough to break the CAP...

Then, I so coincidentally noticed that one of the SFC maps was centered on Denver, so then I knew the risk must have been near CO... So, I picked the southeast section.

But, that setup REALLY sucked... I still can't believe that a MDT risk was in effect with that kind of a setup. Heck, I know I wouldn't have chased that day even if I was located in OK...
 
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