Jeff Snyder
EF5
OK, so I'll largely consider today a bust. At least it wasn't a clear-sky bust, but it certainly was a bust in terms of the number of supercells and the lack of tornadoes. Here are my 2 cents...
All models had forecast Tds in the 63-68 range across southwestern OK by afternoon. When I looked at the 3pm obs, it was obvious that the models were way overdoing surface moisture. It's interesting to note that the same was true of 3/12 for the most part. When I looked at the FWD sounding and saw the 140mb deep moist layer, and noticing that shear wasn't terribly strong, I figured we wouldn't need to worry about mixing out the good juice. However, OK mesonet obs didn't really show any sites >63F dewpoint west or north of I44 by afternoon (and all areas from the 1st-tier of counties in western OK and westward into the TX panhandle were <58F td). So, it was quite obvious that the real nice dewpoints stayed down in southcentral OK (and southeastward from there), where places like Ardmore and Duncan saw 65-69F dewpoints most of the day. So, it appears that (a) the richer moisture couldn't advect far enough westward (into western OK and eastern TX panhandle), and ( B) the moisture that was there mixed out a bit. This led to much greater dewpoint depressions (and much lower boundary-layer RH) than were (was) forecast. So, in any particular storm's downdraft, there was likely plenty of evaporational cooling (with low RH present), which in turn likely lead to stronger cold pools. With strong cold pools, more convection was forced along its leading edge. Eventually, we saw rather quick upscale growth to a linear mode. With modest upper-level flow, along with plenty of potential for cold downdrafts, I feel the end was result were outflow dominant storms.
Now, I think we may have actually had too much insolation today. As strong diabatic heating occurred, the mixing depth increased (boundary layer deepened), which helped mix out some of the higher across western OK (dewpoints in many southwestern and west-central OK mesonet sites dropped 2-4F in Td between 1pm and 5pm). This likely allowed a very large area to near the convective temperature. With lowered Tds, and more potential for evaporative cooling in the low-levels, strong downdrafts developed. Since there was very little capping left over a relatively large area, the convergence along the leading edge of the downdrafts / cold pools was likely sufficient to initiate more convection. In addition, the RFD was likely strongly negatively buoyant (in fact, an OUN warning decision update even addressed the low theta-e air in the RFD). So, add all of this together, and I think that may explain why we saw a rapid evolution to a linear mode. Perhaps if we had a little less insolation, a little stronger capping may have persisted, and this issue may not have been as dominant.
It's also worth noting that low-level shear wasn't particularly strong. FWIW, RUC/SPC mesoanalysis didn't show >250m2/s2 0-3km SRH until about 2z. The relatively weak low-level shear in extreme western OK and the eastern TX panhandle, may have allowed the cold pools to spread away from the storm more than desired. The mid-level was actually stronger than the models had forecast (by about 10-15kts in most places of the target area), with 60kts at AMA and 35kts at OUN. I'm going to chalk this up almost entirely to the low-level thermodynamic and kinematic fields (moreso the thermodynamic environment, however).
EDIT: I continue to see some sometimes-large discrepancies in the dewpoints reported at ASOS obs and those reported at OK Mesonet sites. For example, Vernon was reporting a 70F dewpoin the late afternoon. Just a few 10s of miles away, Tipton and Grandfield Mesonet sites were only reporting 61-62F dewpoints. On the whole, it seems as the ASOS obs tend to be 1-5F higher in dewpoint than nearby Mesonet obs. Now, the ASOS sites directly measure dewpoint (with a chilled mirror), while the Mesonet sites calculate dewpoint from temperature and RH. So, I'm not entirely sure which one to believe, but there seems to be better spatial continuity with Mesonet obs compared to ASOS obs.
All models had forecast Tds in the 63-68 range across southwestern OK by afternoon. When I looked at the 3pm obs, it was obvious that the models were way overdoing surface moisture. It's interesting to note that the same was true of 3/12 for the most part. When I looked at the FWD sounding and saw the 140mb deep moist layer, and noticing that shear wasn't terribly strong, I figured we wouldn't need to worry about mixing out the good juice. However, OK mesonet obs didn't really show any sites >63F dewpoint west or north of I44 by afternoon (and all areas from the 1st-tier of counties in western OK and westward into the TX panhandle were <58F td). So, it was quite obvious that the real nice dewpoints stayed down in southcentral OK (and southeastward from there), where places like Ardmore and Duncan saw 65-69F dewpoints most of the day. So, it appears that (a) the richer moisture couldn't advect far enough westward (into western OK and eastern TX panhandle), and ( B) the moisture that was there mixed out a bit. This led to much greater dewpoint depressions (and much lower boundary-layer RH) than were (was) forecast. So, in any particular storm's downdraft, there was likely plenty of evaporational cooling (with low RH present), which in turn likely lead to stronger cold pools. With strong cold pools, more convection was forced along its leading edge. Eventually, we saw rather quick upscale growth to a linear mode. With modest upper-level flow, along with plenty of potential for cold downdrafts, I feel the end was result were outflow dominant storms.
Now, I think we may have actually had too much insolation today. As strong diabatic heating occurred, the mixing depth increased (boundary layer deepened), which helped mix out some of the higher across western OK (dewpoints in many southwestern and west-central OK mesonet sites dropped 2-4F in Td between 1pm and 5pm). This likely allowed a very large area to near the convective temperature. With lowered Tds, and more potential for evaporative cooling in the low-levels, strong downdrafts developed. Since there was very little capping left over a relatively large area, the convergence along the leading edge of the downdrafts / cold pools was likely sufficient to initiate more convection. In addition, the RFD was likely strongly negatively buoyant (in fact, an OUN warning decision update even addressed the low theta-e air in the RFD). So, add all of this together, and I think that may explain why we saw a rapid evolution to a linear mode. Perhaps if we had a little less insolation, a little stronger capping may have persisted, and this issue may not have been as dominant.
It's also worth noting that low-level shear wasn't particularly strong. FWIW, RUC/SPC mesoanalysis didn't show >250m2/s2 0-3km SRH until about 2z. The relatively weak low-level shear in extreme western OK and the eastern TX panhandle, may have allowed the cold pools to spread away from the storm more than desired. The mid-level was actually stronger than the models had forecast (by about 10-15kts in most places of the target area), with 60kts at AMA and 35kts at OUN. I'm going to chalk this up almost entirely to the low-level thermodynamic and kinematic fields (moreso the thermodynamic environment, however).
EDIT: I continue to see some sometimes-large discrepancies in the dewpoints reported at ASOS obs and those reported at OK Mesonet sites. For example, Vernon was reporting a 70F dewpoin the late afternoon. Just a few 10s of miles away, Tipton and Grandfield Mesonet sites were only reporting 61-62F dewpoints. On the whole, it seems as the ASOS obs tend to be 1-5F higher in dewpoint than nearby Mesonet obs. Now, the ASOS sites directly measure dewpoint (with a chilled mirror), while the Mesonet sites calculate dewpoint from temperature and RH. So, I'm not entirely sure which one to believe, but there seems to be better spatial continuity with Mesonet obs compared to ASOS obs.
