August 1, 2011

Emily Is a Threat To the U.S.

Filed under: Forecasting,Tropical — Erin @ 8:20 pm

After days of teasing weather watchers (and the National Hurricane Center), a tropical wave in the Atlantic has formed into Tropical Storm Emily.  The storm is rather disorganized and not at all “attractive” in the tropical cyclone sense, an artifact of its having had multiple competing vortices for several days that prevented its consolidation into a single system.

Because of its delay in getting organized, Emily is a threat to the United States.  I am going to blog regularly about this system as long as that remains the case.

 

What’s the synoptic setup?

Emily is going to move mostly west, slightly WNW, along the southern end of the Bermuda High.  Its strength will depend primarily on possible land interaction during this time.  The National Hurricane Center forecasts an impact on the island of Hispaniola, which would weaken the system.  How much remains to be seen; many a major hurricane has been reduced to a tropical storm by this island, but some systems that are much weaker have survived passage.  It is terribly difficult to forecast how much effect the mountains will have on any particular storm.  A lot depends on how well-organized the system is when it reaches the island (I do not mean its intensity; intensity and cyclonic organization are not the same thing), how long it stays there, and whether there are any additional destructive factors such as dry air intrusion and wind shear that are hitting the storm at the same time.  It is arguable that there’s not a lot of point in making a forecast for Emily after its interaction with Hispaniola at all, because what happens to it after that will be heavily influenced by its strength at that point.  I’ll discuss the various possibilities, however.

The high is going to be weakened on its left flank by a trough coming off the Atlantic coast in a couple of days.  This should pull the storm to the north.  How much depends on how weak Emily is and how strong the trough has managed to become.  The stronger Emily is, the more northward it is expected to move, all things being equal with respect to the trough.  I think that the trough will be the most important player here, though, and should be watched at least as closely as the tropical system.  It is very uncommon to have a strong trough coming off the East Coast at the beginning of August, and there has been a pattern this year of the GFS (the U.S.’s long-range weather forecast model) overdoing the strength of lows in the days before they arrive.  I am not inclined to buy into a strong trough unless I see it materialize, but it’s always best not to count anything out, either.

Emily is probably too far south and west to have a land-free recurvature (“fish storm”) path.  It’s not impossible, but it is unlikely.  It simply took too long to develop for that to be the most likely track.

 

What’s the model spread?

Models generally have clustered around the state of Florida as of Monday evening, with the NOGAPS (U.S. Navy model) farthest west and the GFDL farthest east.  The NOGAPS is showing an implied strike on the Florida panhandle (it has Emily stalling in the Gulf and not making final landfall within a 7-day period), and the GFDL shows a “fish storm” recurvature.  It is important to observe that this trend for the GFDL is relatively new; until the past 24 hours or so, that model was showing a strike on the East Coast of Florida and the HWRF model was showing a recurvature.  Now that has reversed itself.  In the meantime, the NOGAPS has been consistent in its Gulf track.  Consistency alone is not a reason to support a model’s output, but it is generally indicative of a model’s having a better grip on the environment than one that is prone to the “windshield wiper effect.”

 

Is the Gulf Coast at risk?

Short answer:  yes, but it’s not set in stone.  As of now, I still would say that the Florida peninsula is most likely to get hit, but the Gulf is a definite possibility, especially if Emily is weakened by interaction with land and/or the trough is weak.

Several forecasts indicate that the storm will remain weak for long enough to stay south and get into the Gulf of Mexico before making the recurvature.  This is not a fluke, or a one-off from some model; it has been a solution for the NOGAPS, Canadian, and UK several times over the past two days.  Furthermore, models such as the HWRF have been hinting at a Florida East Coast strike at a perpendicular angle, indicating a strengthening ridge that would force Emily westward again.  While these models do not go out far enough yet to indicate what would become of Emily after the Florida strike, entry into the Gulf (in a weakened state) is certainly possible in this scenario.

 

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.

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