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Tuesday, September 23, 2008

Blocking Pattern and Retrograding Low

Now that fall is officially here, temperatures should begin steady drop. But the pattern we are in now will enhance that. The easterly wind has added some moisture and helped to hold our overnight temperatures in the 60s by the bay. The first signs of the fog season have begun as well.

Blocking Pattern
Typically weather systems in our part of the world, ride the westerlies and move west to east. Occasionally a 'block' in the upper level pattern can deflect that flow, or even reverse it for us.
Here is the 300mb (about 30,000ft) chart analysis for this morning showing the development of an upper level Low just to our south. That begins to deflect the flow, with the main branch of the jet stream in the Rockies and riding north in to Canada and clipping New England. This pattern in and of itself is a weak block, but our surface winds have been dominated by the surface High to our north dragging in a north east wind.

As we watch this pattern evolve, the upper level low gets stronger and instead of just blocking, it can actually steer weather systems the opposite way. In this case, from East to West. Here you can see the Thursday morning 300mb map with a more pronounced upper level Low and a distinct flow I highlighted coming off of the Atlantic and riding from southeast to northwest over Maryland.
That will help take a developing area of low pressure off of the coast, and bring it inland- instead of jumping out to sea.

The net result however will be wet for us as soon as Thursday and into the weekend. The surface low will follow this path just to our north. the Canadian positions it in the New Jersey coast then stalling for a Day in Northern PA. A few other models have slightly different tracks, but since this is not a winter snow or ice event, it's a matter of who will get more rain. Either way, it will get wet again and be a big chilly as this happens. We might spend a few days without reaching 70F.

1 comment:

Anonymous said...

Do you think we are officially clear of 80 degree weather? I am excited that it is fall because that means SNOW is in our not so distant future:)