Monday, July 11, 2011

The ridge and the week ahead

Last night we had two MCCs merge together over Wisconsin to form a very potent MCC with a leading convective line in southern Michigan this morning.  Here's the IR satellite image from earlier this morning:
GOES-E IR image from 1015Z, July 11, 2011.
You can see the characteristic circular "blob" pattern in the IR imagery that is the hallmark of MCCs.  It seems that the particular route these storm complexes took--from the central and northern plains out through the central and upper midwest--is going to be the favored storm track for the next several days.  Here's today's SPC convective outlook:
SPC Day 1 convective outlook as of 1626Z, July 11, 2011.
And the day 2 convective outlook for Tuesday:
SPC ay 2 convective outlook as of 0600Z, July 11, 2011.
So what's causing the storms to move this way?

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.
Note the high-height area over the southeast with winds flowing clockwise around it.  The jet streaks within the jet stream are somewhat weak and well to the north--mostly in southern Canada.  You can visualize with clockwise flow around this ridge that the prevailing wind pattern would be a big arc stretrching from the desert southwest, up through the central and northern plains, across the central midwest, then out over the mid-Atlantic states.  It's no coincidence that that is the preferred storm path over the next few days.  These prevailing winds aloft carry the storms along with them.

Let's look forward in the GFS model to the forecast pattern on Wednesday morning:
GFS 48-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Wednesday, July 13, 2011.
The main jet stream is still pretty far to the north.  However, there is a strong, sharp ridge whose axis is forecast to stretch across the northern plains.  If you follow the wind barbs, you can still see that the prevailing flow is up out of the desert southwest, across the northern plains, and down through the central and upper midwest.  So I'd expect a similar storm track to continue.

Now on to Friday morning:
GFS 96-hour forecast of 300mb height (contours) and winds (barbs and colors) at 12Z, Friday, July 15, 2011.
It looks like the ridge has grown even more in this forecast.  Same predominant flow pattern, however.  I'm also interested to see what's going to happen with the trough that has been continually trying to move onshore in the Pacific Northwest all week.  It looks like this trough could be dampening the early summer glory here in the Seattle area by bringing more cloudiness and showers.  In fact, the longer this ridge sticks around the central part of the country, the longer the weather is going to be drizzly in Seattle...

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.
Note a broken stream of high relative humidities stretching up along the western Mexican coast, through the desert southwest, into the central plains and across the upper midwest.  It's that same pattern that we were seeing for the storm track.  So, storms forming in this region have both heat and deep moisture to work with.

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.
That upper-level low has been drifting northwestward overnight and it looks like it's about to start interacting with that plume of higher "monsoonal" mositure coming up from the desert southwest.  Will the added vorticity of this upper-level feature be enough to spin up a warm-core MCC?  What will happen?  We'll have to wait and see.

Ridges don't always have to be boring...

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