October 26, 2010

Major Storm Unfolding in the Midwest

Filed under: Mid-latitude cyclones,Severe — Erin @ 10:56 am

The megastorm that weather people have been talking about for several days has materialized, and it is already bringing very high wind and long lines of severe squalls and tornadic supercells to the Midwest and upper Tennessee Valley. Rain and thunderstorms associated with the cyclone extend as far south as the Gulf states. Blizzard conditions are expected for areas on the cold side of the storm, which is expected to bottom out its pressure in the 960 mb range. The low, centered over Minnesota as of this writing, is already at 966 mb. The factors causing this powerful system are many: Very unseasonable heat in the Southeast has led to a powerful warm sector for the cyclone, and a surge of cool air to contrast with this has caused development of a strong jet stream (>100 knots) in the upper levels of the atmosphere. Had Hurricane Richard’s remnant low been drawn more to the east, as the models were suggesting a couple of days ago, the system would have become even more powerful. A mid-latitude cyclone of this magnitude is not common. It is a credit to modern technology that our computer models were able to accurately predict this rare of a system several days in advance.

The Storm Prediction Center issued a high risk for today, a rare occurrence, but with dozens of tornado warnings already called for the Midwest where the storm’s warm sector meets its area of highest vorticity advection, I expect that this will end up verifying.

October 21, 2010

Well, This Is Certainly Interesting

Filed under: Forecasting,Mid-latitude cyclones,Tropical — Erin @ 10:43 am

Since I posted last night, the range of possibilities for our coming weather in the Deep South has expanded quite a bit, and the “interesting” scenario that I hinted at towards the end is suddenly looking a lot more likely to actually unfold. I am talking about the possibility that Tropical Depression 19, now upgraded to Richard, gets into the central/east Gulf of Mexico and interacts with one of the coming troughs rather than dissipating in Mexico or extreme south Texas.

The HWRF model, which was alone last night among the well-known models in showing Richard going to the northeast, has been joined now by the GFDL, ECMWF, and the majority of other models, as this Google Earth screencap shows.

The National Hurricane Center has made note of this trend as well and is expecting to move its forecast path to the east if the trends hold up. They tend to err on the side of consistency, avoiding what has been dubbed “the windshield wiper effect” when models make dramatic shifts in their forecasts. But it seems likely right now that they are already leaning in favor of an eastern track for this tropical system. The two storm-centric models, the HWRF and GFDL, turn Richard into a major hurricane and slam it into Florida, but the Google graphic indicates that there is actually quite a wide range of possible landfall locations. If the shear in the Gulf of Mexico drops off as forecast, there is really no reason why a major storm couldn’t happen (though I think there are some limits on just what is possible). Ida last year almost became a major in the Gulf in November during an El Nino autumn, after all.

(No, I do not believe Richard will actually approach Category 5.)

What about our cold-core cyclones, then? And that possible early freeze?

The GFS has (and this should surprise exactly no one) backed off its screwball idea of winter precipitation for Mississippi in the first week of November. As of 06Z’s run, it was not on board with the eastern path for Richard, which throws a major wrench into matters, but let’s look at the evolution of the trough before Richard might enter the picture. If he does get into the east-central Gulf, it’s going to be about a week from now before we can consider a landfall.

The first trough, the one expected to bring rain and thunderstorms to Mississippi on Sunday and Monday, is still on schedule. This model has increased the amount of rainfall that we are apparently going to get out of this, showing up to 2 inches in a small area and widespread totals over 1 inch. This is the trough that would pull Richard up, up, and away, sending it somewhere into the Gulf Coast and adding even more rain. (Remember, the GFS rainfall totals as of this run assume that Richard does not get into this part of the Gulf and is not picked up by this trough.) I’m having a hard time accepting a hurricane of major-plus intensity (let’s say Category 4) in the Gulf in a strong trough situation in late October, though stranger things have happened. It seems likely to me that if this trough gets it, Richard will begin to transition into post-tropical and lose some of its intensity. Cold-core cyclones do not get as strong as tropical cyclones, either in minimum pressure or in wind speeds. (There have been some non-tropical lows that went down to the 920 mb range, but these were at extremely northern latitudes. It’s much less unusual to see sub-980 mb cold lows close to the poles.)

When the energy of a tropical cyclone is entrained into a trough, the trough benefits from it. These types of systems have spawned infamous nor’easters, such as the storm created by the transitioning Hurricane Noel in 2007. The GFS already turns the trough into a 990 mb low and possible nor’easter, and as I said, that run doesn’t even take into account the possibility of tropical-origin vorticity being advected into the dominant low. The South could very easily be in for a major flooding rain event (it is unbelievable that I could realistically use the word “flood” when we are technically in a drought), but if we have the situation of a former major hurricane being pulled into a strong autumn trough, the Northeast could get a monster storm. AccuWeather.com is well-known for hyping weather events, but I think they may be onto something with their current news story about this possible storm. Let’s just say that, while nothing is definite, the potential definitely exists.

The GFS still predicts the first freeze to occur just before Halloween. This freeze would follow the second trough, which would dump even more rain on us. The freeze would be a dry freeze (in other words, a believable one) and the day following it, Halloween, would possibly not lift out of the 50s for highs. This has happened before; I distinctly recall a Halloween in the mid-90s (I am thinking 1993 or 1994) in which it did freeze overnight. After that, the cold air lifts out. As I said before, this model run has dropped the “early November Southern winter storm” foolishness of the third trough it shows.

I am going to be very mindful of the evolution of Richard and the strength of the trough that the NHC and models are increasingly convinced will get it. Things are getting interesting here, folks.

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.

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