A mid-Atlantic tropical wave worth watching; more on this year's steering currents
A tropical wave in the mid-Atlantic, near 9N 38W, has grown more organized since yesterday. This system has been labeled "96L" by the NHC. The wave has a small closed circulation, as seen on both visible satellite loops and this morning's 4:28am EDT QuikSCAT pass. Winds from QuikSCAT were as high as 35 mph. Wind shear is about 10 knots, and is forecast to fluctuate between 10 and 20 knots in the region over the next two days. This is low enough wind shear to allow some slow development. Sea surface temperatures are 27-28 C, which is above the 26 C minimum temperature tropical storms typically need to form. There is one cluster of strong thunderstorms near the center of circulation, but dry air to the north appears to be limiting the thunderstorm activity. The Saharan Air Layer (SAL) lies just 50-100 miles north of the storm's center of circulation, as seen in water vapor satellite loops. The GFS model does indicate a tropical depression might form here, but does not have a very good handle on it, since it is showing far too slow of a forward speed. Our other three reliable models, the NOGAPS, UKMET, and ECMWF, do not develop the system. Climatologically, formation of a tropical depression in this region of the Atlantic this time of year is quite rare. Given this fact, plus the presence of so much dry air near a relatively small circulation, I am not expecting this to become a tropical depression. Movement of 96L will be just north of due west over the next few days at 15 mph, as seen in the model forecast plots (Figure 1).
Figure 1. Computer model forecast tracks for 96L.
More on steering currents for this hurricane season
Yesterday, I posted my bi-monthly hurricane outlook, for the first half of July. Since it was getting a bit long, I presented only a short steering current analysis. More follows here. There are several ways to look at steering currents. I presented the position of the surface high pressure system known as the Bermuda High (or Bermuda-Azores High). Another way is to study how close to the surface a pressure of 500 millibars (mb) is found. When there is low pressure aloft, due to a trough of low pressure, the height at which a pressure of 500 millibars is found moves closer to the surface. If one plots up the "500 mb height anomaly"--the difference of where a pressure of 500 mb is found above the surface, compared to the average height from a climatology of the past 30 years--one gets a good measure of where above or below average storminess occurred. Higher than average 500 mb heights imply less storminess and possible drought conditions. The 500 mb height anomaly plot for June 2007 (Figure 2) shows higher than average heights across the southwestern U.S., where drought and high temperatures were observed in June. Lower than average 500 mb heights imply an above normal preponderance of troughs of low pressure and thus storminess. This was the case over Texas and Oklahoma in June. If these troughs are over the Atlantic, they act to recurve hurricanes out to sea at the longitude they are at. This only occurs if a hurricane penetrates far enough north to "see" the southernmost part of the trough of low pressure. Typically, this happens northward of about 20 degrees north latitude. Figure 2 shows lower than average 500 mb heights occurred over most of the Atlantic, meaning there were many more troughs of low pressure than usual. Had any hurricanes occurred over the Atlantic north of about 20 degrees north latitude, they would have gotten caught up in one of the troughs and recurved out to sea. The latest 2-week forecast from the GFS model shows a continuation of this above average frequency and intensity of troughs of low pressure over the Atlantic--much like we saw in 2006. Thus, we can expect any tropical cyclones that penetrate north of about 20 degrees north latitude to get recurved. This will very likely be the case for 96L, if it ever becomes a tropical storm.
Figure 2. Difference in height (in decameters, or tens of meters) from average of the 500 millibar height above the surface for June 2007. Image credit: NOAA/ESRL.
Jeff Masters
Reader Comments
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Found this at the NHC website.
Do you think NOAA is gonna call Proenza out because he does not project the "calm, clear and trusted voice in the eye of the storm" type image?
It is most certainly in a weakening phase. If that lasts or doesn't last, remains to be seen.
The main thing I wanted to say is according to Wikipedia, the wave that spawned Hurricane Bret in 1999, which was a 145 mph Category 4 hurricane at its peak, was barely discernable at times, yet it still developed. This wave is barely discernable as just a swirl with a few thunderstorms.
Just keep watching. If it develops it develops, if not, it doesn't. August and September is when the real action'll start anyway.
Link
I was going to predict that 96L would have a name by tomorrow's 11 PM update, as I think it still might, but I didn't want to be accused of wishcasting. I still say that, somehow, if it does, or gets TS status shortly thereafter, that it's a strong indicator of the season to come, along with the surprise of Barry, who somehow "created" itself in the face of strong shear and "no-no" forecasting, even by some of the more erudite members of this blog.
I honestly do believe that, and I thank you for taking the time to let me know that you read what I said, however correct or foolish it might have been. :-)
Jo
goodnight
There is no such thing as "wishcasting"; it is banned from this blog. lol
So, anyways, welcome to this blog, as I am looking foward to seeing you here. :)
copy and paste this comment and when it happens you can call me a hurricane expert.LOL
love all the links you guys post and the discussions are much better this than years passed. great job, guys
http://video.msn.com/v/us/msnbc.htm?g=f91ef032-b8cc-446f-96b1-b04e4bd96ba1&f=00&fg=copy
I'm not familiar with UTC time, so I looked it up. What I got was if the UTC time is 04:15, like the current floater is on 96L and add six hours to it, that's the time the image was taken. So, since I live in the central time zone, that would make the most recent satellite imagery 10:15 p.m/11:15 p.m. eastern. In other words, that makes the imagery two hours old.
Thanks weatherblog and again, moonlightcowboy.
This system is going to have trouble maintaining persistant convection, which should delay the attainment of depression status. The mid-level circulation looks good and convection has been flaring in the southern semi-circle.
This is the second invest already in the central/eastern Atlantic. I'm still seeing a situation similar to 1995, with perhaps a few less storms but more close calls for the East Coast.
Let's see how it is in the a.m...
Scenario 1:if there's more convection bursts (heavy) by the morning then a TD or TS is possbile in 24-48 hours...but nothing too significant.
Scenario 2:if it's looking the same it is now then maybe some slow development to a TD if it could escape from the dust and some shear...otherwise dissipation looks reasonable.
Scenario 3:if it is only a naked swirl by morning then dissipation has took place and any re-strenghthining would be very slow indeed...but most likely scenario is further dissipation...though the remenants could go in the caribbean and re-organize, something to watch, but probably not
Out of all the scenario's, scenario 2 seems most reasonable...we'll see in the morning.
In my heart, though, I still think that somehow 96L will see it through, become something stronger, and that we'll *really* be talking about it a week from now. But please do know that I hope I'm way more than wrong, as no one needs any more funky weather.
Jo
So it's a cyclone now right? There's a tropical cyclone formation alert on the NRL website.
if your talking about 96L...it is still an ivest. no TD or TS that I know of...or anybody frankly.
that model looks like a 3 ring circus
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