October 20, 2010

Is a Major Pattern Change On the Way?

Filed under: Forecasting,Mid-latitude cyclones,Tropical — Erin @ 10:32 pm

After weeks upon weeks of dry weather, we have had two rain events in the past couple of weeks as fronts moved through. The rain has not made a noticeable difference in the drought situation, though it should be noted that we are not under fire weather watches every day anymore.

However, the models are starting to suggest a major pattern shift in the long range. It’s important not to focus on specific details this far out, because quite frankly, some of the details are almost certainly utter rubbish. The idea is that we are shifting into a precipitation-bearing trough pattern at last. The GFS 4-5 days out is showing a fairly robust system of about 1000 mb that is expected to bring rain. Jackson’s NWS office is already talking about this system and has given it a 40% chance of producing thunderstorms on Monday. This is potentially an interesting situation as far as severe weather is concerned, and it seems pretty likely that this particular rain event will play out in some form over the Deep South.

In runs from the past several weeks, the GFS has been indicating, for the most part, a return to dry conditions for as far as the model shows. More recently, it’s stopped doing this. After the Monday event, the GFS shows a potentially more extreme event developing over the 28th—and a freeze.


This image shows the storm event. The freeze would follow.

This is where you start to raise an eyebrow at the output and take it with a grain of salt. This would be early in the year for a first frost, but it has happened before. We’ll need to keep an eye on this and see if it stays in the runs and if other models start to pick up on it.

After the proto-event #2, the GFS then kind of goes off, showing a winter storm situation unfolding. It’s exactly what I mean when I say that the details are not what counts here, but the suggestions of cooler air and more precipitation. This is at the end of the run, which is notoriously poor in accuracy. The blue “0” line represents the freezing point.

As I said, the freeze aspect of this is almost certainly complete foolishness. A third rain event, however, is not out of the question at some point in the two-week range, and such a cyclone could easily pose a winter storm problem for areas along the East Coast, as a nor’easter has already done this year.

We are reaching a time of year when the gates start to close for tropical activity. In part, this is because the jet stream is reforming at a more southerly location. The high wind shear generated by the jet stream is a death knell for tropical activity, but it is encouraging of frontal and cold-core cyclonic development.

Tropical Depression 19
However, tropical storms will continue to form as long as they have the necessary conditions, and as I type this, I see that a new depression has been classified. TD 19 is located in the Caribbean and is forecast to intensify a bit and move over the Yucatan Peninsula. The circulation center of this depression is sheared and somewhat displaced from the main convection, but it seems that the convection and low-level center are moving closer together as the source of the shear moves out. The system is expected to get into the western Gulf. It probably won’t affect the central Gulf states, but it should be noted that the HWRF model explodes this into a major hurricane and sends it into the west coast of Florida. This is currently an outlier, but that is a good model, so it should be watched closely. The Gulf is cooling, but there is definitely enough heat in it to sustain a serious storm. Because of high shear, the Gulf currently cannot host a tropical storm, but the shear is supposed to largely disappear by the time TD 19 emerges from the Yucatan.

It will be interesting to see what type of weather system we get if TD 19 does move farther east and north than expected and the Gulf states get significantly cooler weather. The output of the GFS that shows those rain events does not take into account the possibility of tropical energy entrainment, because it favors a southern path for the tropical depression.

Keep an eye out; things could get interesting.

September 27, 2010

Fall at Last, But When Will We Have Some Rain?

Filed under: Drought,Forecasting,Tropical — Erin @ 10:41 pm

Over the weekend, an extremely welcome cold front passed through the Southeast, finally giving the record high temperatures and the persistent high pressure system that generated them a kick out the door. No 90s in the foreseeable future. According to Jackson NWS, this should finally begin a fall pattern in which troughs periodically pay the Southeast a visit. The front didn’t make a dent in the drought conditions that have developed over the area in the past month, however. It brought cool temperatures and rainfall to select areas, but most places just didn’t get much. Columbus AFB recorded less than a tenth of an inch for the entire frontal passage. With the ground as parched as it is, this meager water was soaked up immediately. It looks like we’ll have to wait a bit for a significant rain event.

That rain event unfortunately probably will not come from the tropics, though this would be a very fruitful source if it did happen. The Hurricane Center is giving a 40% chance of development to an area of low pressure in the west Caribbean, designated 96L, which is forecast to move into the Gulf.

However, models are all taking this disturbance into South Florida.

Things can change, of course, but it’s uncommon to have this much agreement that close to a landfall and to have a shift of that magnitude.

The next frontal passage will be this weekend, but this front is expected to be dry. It’s soon going to be tempting to have fall bonfires, but I don’t think it’s quite safe to do that just yet!

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