|GOES-E IR image from 1015Z, July 11, 2011.|
|SPC Day 1 convective outlook as of 1626Z, July 11, 2011.|
|SPC ay 2 convective outlook as of 0600Z, July 11, 2011.|
It turns out that there is a big ridge aloft over the southeastern US--and it's not going anywhere. At least, according to the models it's not going anywhere. Here's this morning's GFS analysis at 300mb:
|GFS Analysis of 300mb height (contours) and winds (barbs and colors) at 12Z, July 11, 2011.|
Let's look forward in the GFS model to the forecast pattern on Wednesday morning:
Now on to Friday morning:
|GFS 96-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Friday, July 15, 2011.|
With weak jet streaks moving through, it's clear that storms that fire aren't going to have a lot of support of the flow aloft. In general, a ridge is not the most conducive area for storm growth and development--at least in a baroclinic sense. With weak synoptic-scale forcing, I'd continue to expect to see MCCs as the dominant form of storm as this pattern continues. Remember from my previous posts that MCCs tend to modify the environment more than the environment works to develop them. So, in the absence of strong forcing from the ambient environment, any instability is probably going to manifest itself in MCC form.
Why should we expect any instability at all? Two reasons--heat and moisture. As most people in the central part of the country already know, it's been hot this summer--and sinking, warming air under the prevailing ridge is not going to do much to diminish that. Heating at the ground level inherently works to destabilize the atmosphere.
The humidity is also a factor. Not only are we seeing muggy conditions near the ground, but the moist layer can extend to be rather deep. A lot of this is due to that same, clockwise flow pattern aloft. It turns out that this particular pattern (with a ridge over the southern US) helps drive the flow of "monsoon" moisture up from the southwest. This moisture doesn't originally come from the desert southwest--it actually has its origins in the tropical east Pacific. But the upper-air winds advect that moisture up and around. It's brought a lot of storminess to the southwest over the last few days. Here's the RUC analysis of 500mb relative humidity from this morning:
|RUC Analysis of 500mb relative humidity (greens) winds (barbs) and heights (contours) at 15Z, Wednesday, July 13, 2011.|
It will be interesting to see just how this week plays out. With such a dominant ridging pattern, the small-scale features will make all the difference. For instance--what about that small upper-level low analyzed over northwest Texas at 300mb in the first GFS image that I showed? It actually shows up on the water vapor imagery from this morning as a nice swirl over northwest Texas, so we know this isn't an error in the analysis:
|GOES-E water vapor image from 1645Z, July 11, 2011.|
Ridges don't always have to be boring...