It may not have been the "storm of the century", but it did grab the attention of the Storm Prediction Center. I was able to "chase" this storm from the safety of my desk at home. My "storm of the moment" was the subject of mesoscale discussion #402 and shortly after that, tornado watch #111. My storm raged along the border of Louisiana and Arkansas through the late evening hours of March 14th, 2008. You can clearly see the path on the SPC's Storm Reports graphic. Looking at the graphic it becomes obvious that this was not the only storm of the evening, but we will focus on this one and it will give us plenty to discuss.
SPC Storm Report for 12z on the 14th thru 12z on the 15th of March, 2008 (above)
Taking a look at the synoptic set-up for March 15th around 00z, which is the same time mesoscale discussion #402 was issued, we find a low pressure centered over the eastern portion of the Texas and Oklahoma border. The NOAA surface analysis (see below) also shows a short warm front (one red bump) attached to a cold front that basically follows the Texas and Oklahoma border east and on along the Arkansas and Louisiana border into Mississippi. There is a dryline that extends south from the low pressure all the way south to the tip of Texas. Looking at the mid level, you can also see the 500mb flow is fairly zonal, with a jet streak ending just prior to the border of Texas at Arkansas and Louisiana. You can also see a trough at this level. Also, right where the jet streak ends there is a very slight diffluent flow pattern. The 300mb flow is also zonal with a jet streak that ends just over the Texas border with Arkansas and Louisiana. There is a small area of lifting associated with this flow towards the eastern edge of the border of Arkansas and Louisiana. (Please click here to view the 500mb and 300mb charts.)
NOAA 0000z surface analysis for March 15th, 2008 (above)
Illustration of mesoscale discussion #402 from the SPC's mesoscale discussion archives
Issued at 1859CDT, 2359z on March 14th, 2008 (below)
Next we should look at the MLCAPE situation. You can actually see the MLCAPE highlighted in the mesoscale graphic above the previous paragraph. The smooth orange lines are the MLCAPE while the highest MLCAPE is noted with a red line and a “2000” notation. OK, so what is MLCAPE and how do we use it? MLCAPE is the Mean Layer Convective Available Potential Energy measured in Joules/kilogram (J/kg). We use the average dew point of the lowest 100mb to find the mean mixing ratio and the average temperature from the same area. Now we follow the mean mixing ratio up to where it meets the mean temperature (following the dry adiabat) and that gives us the Lifting Condensation Level or LCL. We now follow the moist adiabat until we cross the actual temperature sounding; this is where the parcel of air becomes warmer than its surrounding environment and therefore positively buoyant, also known as the Level of Free Convection or LFC. We then continue the line until it crosses the temperature line a second time. This is the Equilibrium Level, or EL. This is where the parcel is the same temperature as its environment. However, since it has some speed as it approaches this point, it doesn’t just stop there, it will actually oscillate a few times until it finally settles. The size of the area created by the line from the LFC to the EL and the actual temperature tells us the amount of CAPE, in this case the Mean Layer CAPE. The larger the CAPE is, the better our chances of convective fireworks. Of course, there is no magic number because that would make things too easy, so that is why we have some of these other forecasting tools!
SPC MLCAPE/MLCIN graphic from 0100z March 15th, 2008 (above)
The mesoscale graphic only shows part of the tale. We also have to overcome the Convective Inhibition or CIN. The CIN is the area on the cold side of the actual temperature as we trace the mean potential temperature to the LCL and then moist adiabatically to the LFC. It’s the area “behind the curve” as it were. This is a negative amount of J/kg. This area is proportional to the amount of energy required to lift a parcel to its LFC in order to “spring it.” Now, looking at the SPC’s graphic for our region of interest at 0100z (shown above), we can see an area of moderate MLCAPE (over 2000 J/kg) ringed by an area of higher MLCIN (in excess of -200 J/kg). However, the MLCIN is low (-100 J/kg or lower) under our area of high MLCAPE.
00z Sounding for KSHV (Shreveport, LA.) courtesy of the University of Wyoming (above)
So how does this all come together? Glad you asked! Take a look at the sounding above from KSHZ, Shreveport LA. Shreveport is in the northwest corner of Louisiana, right in the middle of that 2000+ J/kg MLCAPE reading. Looking closely you can see that the wind barbs in the boundary layer indicate the winds are from the south-southwest, bringing moisture from the Gulf. This increasing moisture is coming into an area that has been warming all day so we have increasing moisture and increasing temperature which give us increasing Theta-E! Pretty cool, but wait, there’s more! As the moisture increases the mean mixing ratio increases causing the LCL to rise closer to the LFC. The closer together the LCL and LFC are, the less space “behind the curve” and that means that our MLCIN is going to decrease while our MLCAPE increases. I’ve colored in the MLCAPE on my sounding; there wasn’t enough space for me to color in the MLCIN.
0135 NEXRAD radar base reflectivity snapshot courtesy of wunderground.com (above)
0135 NEXRAD radar storm relative velocity snapshot courtesy of wunderground.com (below)
So what’s the result of all of this mumbo-jumbo? How about some thunderstorms? If you take a look at the base reflectivity NEXRAD radar snapshot from 0135z on March 15th, you will see that I have outlined Claiborne County in northwest Louisiana. The precipitation in that area qualifies as heavy with colors indicating returns in excess of 50dBZ. The next snap shot is the storm relative velocity for the same area. Under the area of heavy precipitation you can see a large area of green that indicates winds blowing back towards the radar site. Just past this to the northeast is the cream colored area which suggests no winds and then further away we have the orange colored area indicating winds blowing away. This looks like an area of divergence possibly caused by heavy rain creating a downdraft. As further proof that there were strong winds at the surface, there was a report of a downed tree in that county from a wind gust of an unknown speed, this information comes directly from the SPC’s storm report and was logged at 0140z; only 5 minutes after the snapshots were taken.
*************************************************************************************************************************************
Reflective Essay
You want me to identify regions at risk of severe weather without first consulting the SPC? Right now that feels like trying to run an obstacle course with a blindfold. Don’t get me wrong, I’ve learned a lot in Meteo 101 and so far in 361, but I think I’d like to try a few more runs through the course before I go “Jedi” and “use the force.”
At this time, the first thing I need to do is get a feel for the synoptic set-up. I need to pull out the HPC surface analysis and take a look to see where the synoptic weather drivers are. I need to find the low pressure systems and the associated frontal systems. I need to see if there are troughs or drylines that can drive my weather towards the convective. I’m not above reading the HPC discussions either because sometimes other people can help lead me to the right information.
Once I have an idea what is happening on the synoptic scale I would head over to the PSU e-wall. I’m a fan of the four panel GFS models. Taking a look at the upper left hand corner to see what is going on at the 500mb level, looking for troughs and vortices that might give me hints. Next to the lower right, not so much to see where the precipitation is going to be, but looking for the areas of stronger lifting that might help drive convective activity. Another place I like to use is the NCEP page of analysis and models. Once again I like to start with the GFS model (although I would never limit myself to one model!) The NCEP page has the 200, 250 and 300mb charts and you can loop them to watch the possible progression. I also like the 500mb heights and vortices chart here for ease of reading.
I would then find myself a page for soundings. I’d want to look at the soundings to see if I could determine CAPE, CIN, LFC and CCL to see if we were in for some fireworks later in the day.
My final stop would be the NEXRAD site at the wunderground.com page. I can check out the base reflectivity before checking into the base or storm relative velocities.
My weaknesses would be identifying some of the more subtle upper level troughs on my upper air charts and determining where important features are on the base and storm relative velocity radar returns. Trying to pick out down drafts, gust fronts or actual rotation from the subtle changes in color from one pixel to the next is a challenge. Improvement will come from plenty of practice in the Mesoscale Prediction Centre. I have tried to push myself beyond my comfort level when discussions revolve around things that I am not positive about and contribute to the discussion. I am still eagerly anticipating my chance to actually lead the discussion as well.
No comments:
Post a Comment