Monday, April 29, 2013

Omega For The Block

A few days ago the concern for our upcoming weekend weather was the prospect of a cutoff low in the Mid Atlantic or Southeast, bringing rain and unsettled weather to the East Coast.  The slow moving cutoff low was brought to us by a large storm system that's over the Atlantic Ocean, slowing down the traffic of storm systems across the Atlantic and providing an opportunity for storms upstream to stall out or move slowly.  In the case of this cutoff low, modeling had projected its position as we entered Saturday to be near the East Coast.  By the time we got to today, the models have shifted the low's position steadily westward and have built in an omega block high pressure ridge overhead in the mid atmosphere.

It's called an omega block because of its resemblance to the Greek letter Omega.  Typically these ridges are either a cause or a byproduct of a stalled pattern -- in this case, the low in the Atlantic is more of a cause as the pattern to its west is stuck until something kicks this low out of the way.  You can see it on the satellite image below from this morning.  It's spinning around in the Central Atlantic and modeling projects it staying there for a few days, at least.

Since this low isn't moving much over the next week, the pattern over us remains somewhat stuck despite a strong low pressure system developing in the Midwest. It, too, gets stuck, and instead of pushing east it holds firm over Iowa and Missouri.  For us, our pattern should gradually improve as high pressure builds in and our current malaise of showers and dampness dwindles away later tomorrow and especially on Wednesday.   Assuming things hold serve with what is projected now, the result would be a sunnier latter half of the week and a very nice weekend.

That said, these blocking patterns are very, very fickle in the Spring.   That ocean storm will fire up a piece of mid level energy and gradually push it west and southwest later this week.  Models have inched this mid level low closer to the East Coast over the past couple of days. Compared to 24 hours ago, modeling has that piece of ocean energy about 300 miles farther west and southwest.  Its closer nudges are pushing everything farther back -- the center of the high ridge as well as the Midwest storm system.  While it may not directly impact us in the sense of bringing showers, it could set up a bit more of an onshore flow at the surface and throw additional cloudiness in for Friday and Saturday.  It's something we'll keep an eye on in case the forecast does take such a turn.

For now, note that the forecast isn't as certain as some are making it out to be and that a stalled storm is creating a Schuylkill-like slowdown in the atmosphere through the Northern Hemisphere.  Such blocking patterns aren't unusual in late April and early May and that their fickle nature could yield a surprise or twist before we get to the end of the week.  As of now, though, the Midwest storm does not appear to be a factor in our weather through the weekend.