Charles E. Roop giving his own forecasts, weather discussions, photos and adventures for the Starkville, MS area, Florida, and beyond.

Thursday, July 22, 2010

Tropical Depression 3 Forms Near the Bahamas

Well, it appears that things kicked up a bit overnight. Invest 97L, the tropical wave that now sits over the southern Bahamas has slightly improved and now the National Hurricane Center says it has a closed circulation. To be considered a tropical cyclone, a storm has to have a closed center of circulation. At 11 a.m. EDT, it was upgraded to Tropical Depression 3.

From looking at satellite imagery this morning, it seems that convection has increased. Also, water vapor loops show that dry air isn't invading the circulation as much as it was. However, the heaviest of the thunderstorms remain to the northeast of the center, which indicates the upper-level low still has some influence on the depression, as it can also be seen on water vapor loops. The shear is expected to decrease as time moves forward.

Source: NOAA

Forecast models seem to track the center of circulation a little more to the south - more in the Florida Keys or even further south. For example, the HWRF (see here) has the storm making landfall in the southern Keys Friday morning with 37-knot winds (42 mph, which is tropical storm force).

With the decrease in wind shear and somewhat better development overnight, there is room for slight intensification today and tonight. However, I do not see this as a high-impact wind and surge system. It will mostly be a rainmaker for south Florida.


- South Florida should start to see affects from TD #3 as early as tonight.

- The Keys will likely get the strongest winds - around 40 knots according to NHC forecasts, with higher gusts possible.

- To the north (Miami-Dade, et al.), some gusts are possible but mostly rain is the main event from this. HPC QPC products show, at most, an inch of rain in some spots in southeast Florida on Friday. In a three-day period, some places can see 1 to 2 inches of rain in south Florida.

- Gainesville will experience better weather, but rain chances will be above normal Saturday as moisture is expected to be pulled in from the depression.

The ridge to the north should keep this storm to the south and not have too much of an impact on central and north Florida.

Interests in south Florida and the Gulf of Mexico need to keep an eye on TD #3. I'll probably make another blog post tonight.

UPDATE: I forgot to mention that there are watches and warnings in effect. From the NHC:

Wednesday, July 21, 2010

Invest 97L Not So Tough

Since last night, the tropical disturbance that is now located over the southeastern Bahamas has deteriorated a bit. It seems that wind shear from the nearby upper-level low, dry air intrusion, and the center of circulation moving over Hispaniola has affected this disturbance.

Earlier this afternoon, a NOAA G-IV jet inspected the region. Data indicated that upper-level winds are still not favorable for development, according to the National Hurricane Center. Sounding data also showed dry air at the mid levels to the west and north of the system.

A look at infrared and water vapor loops show convection starting to get a little better, but the water vapor clearly shows the upper low to the north.

The NHC thinks that upper-level winds will be "marginally favorable" for development in the next day or two. NAM and GFS models show the upper low moving ahead of 97L in time, but would it move far enough to reduce interfering the wave? Not sure.

The NHC has reduced its probability of reaching tropical cyclone criteria to 40 percent. It seems reasonable given how much of a hill this invest has to climb to make it to a depression, or even a storm.

The NAM, GFS, and other models seem to agree on a south Florida initial impact as early as Friday.


- At this time, it appears that it will mostly be an on-and-off rain event for south Florida. There could be some gusty winds, but nothing to freak out about.

- Rain should start to kick in sometime Friday and last through Saturday...maybe longer as a southerly flow should keep moisture levels high.


- NWS-JAX is calling for a 50 percent chance of rain for Friday and Saturday. The further north you go, the lower the rain chances. However, this would depend on how far north the tropical wave moves, or how strong it gets.

As time moves forward, it seems this thing might be nothing but a fizzle. Still, it is worth watching. This might be my last update on Invest 97L unless conditions change.

Quick update on Invest 97L...

Source: NOAA

This morning, it appears that the tropical wave that's north of Hispaniola and near the southeastern Bahamas has become disorganized. Wind shear analysis shows that the north part of the wave is still encountering at least 20-to 25-kt wind shear.

Because of the disorganized fashion of this disturbance, the US Air Force has cancelled their scheduled reconnaissance mission today. They may try again tomorrow if conditions warrant.

However, I am now noticing a nice area of convection near the Lesser Antilles in the last few satellite images. This may be convection that's part of the wave - I'm not sure. If any mets are reading this, I am open to input on this.

Regardless, the NHC is still giving this a 60 percent chance of tropical cyclone development in the next 48 hours. Apparently, conditions are expected to improve for development.

Right now, it's expected to be a rain maker for the vicinity, including the Virgin Islands, Puerto Rico, and Hispaniola. The Bahamas should start to see rain starting tomorrow. The models continue to show a south Florida impact this weekend.

As for intensity, I am not sure if this will be developed and named within the next 48 hours. Still, interests in south Florida and the Gulf of Mexico need to watch this for potential development. Regardless of development, expect a wet weekend in south Florida.

I hope to have some time this evening to revisit the models and data to give another update. Stay tuned.

Tuesday, July 20, 2010

Watching the Tropics: Invest 97L

Wow, it's been a while since I have posted any weather analysis on this blog. I've been pretty occupied for the last few weeks. My load has lightened somewhat to the point where I can sit down and write a post on weather happenings. However, things will be busy again for me soon as I prepare to move to Starkville, MS for at least the next two years for graduate studies at Mississippi State University.

So far in the Atlantic-basin hurricane season, we have had one hurricane that broke some 40-plus-year records for the month of June (Alex). We had one tropical depression, but noting noteworthy on that.

We ("we," as in "weather people") are now watching what could be our next named storm - Bonnie. A tropical disturbance, dubbed Invest 97L, is located near Puerto Rico and Hispaniola and is producing heavy rainfall in the vicinity. NWS-San Juan, PR radar estimates are showing as much as 6 to 7 inches of rain in some parts of extreme eastern Puerto Rico since Saturday. This wave has been moving west-northwestward at about 10 mph and this track is expected to continue for at least the next day or two.

Right now there is at least one feature affecting 97L: Wind shear. Remember that too much wind shear can limit a tropical cyclone's development. There is fairly light shear in the south-end of the wave (as seen here in the analysis). On the north side of the wave, shear is relatively high at 20-25 knots. Satellite imagery picks this up fairly well. This shear is due to an upper-level low, and it can also be seen in initial model runs and in water vapor imagery (see image below from NOAA).

However, the National Hurricane Center is saying that the environment around 97L should improve for further development later on. This tells me that the shear is expected to decrease at least somewhat. About half of the intensity forecast models have the invest at tropical storm status in 12 to 24 hours.

As for the track, the model consensus earlier today has the disturbance moving towards south Florida by the weekend. However, new model runs seem to have it moving a little further south, more in the Florida Keys in about 72 hours.

An area of high pressure currently has a hold over the southeastern United States. The short-term outlooks seem to have this remain in place. This should keep the invest from moving too far north, sort of acting like a blockade.

Air Force hurricane reconnaissance aircraft is scheduled to investigate the disturbance on Wednesday, if conditions still warrant (and I think it will).

Also, as other meteorologists - such as Dr. Jeff Masters and Greg Nordstrom - point out on their blogs, we are in an phase known as the Madden-Julian Oscillation. It "is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days" (source, details: http://www.cpc.noaa.gov/products/precip/CWlink/MJO/MJO_1page_factsheet.pdf). We are in a phase now where enhanced convection can take place, and, in turn, higher potential for tropical development. Something to keep in mind.

In a nutshell:

- It has the potential in the next 48 hours to become Tropical Storm Bonnie and move west-northwest in this timeframe. As for Hurricane Bonnie, I think it's too early to say at this point.

- Its exact track is uncertain. It looks like south Florida could at least see some impacts from this disturbance - some wind and heavy rain. If it moves into the Gulf of Mexico, it could spell trouble for oil spill recovery efforts and have room to intensify. This invest needs to be closely monitored for the next few days, as there will be some impacts this weekend.

- Gainesville could only see a greater influx of moisture as the wave approaches and have higher rain chances. However, it depends on what path this wave takes.

- Wednesday's scheduled USAF reconnaissance mission should give us a better clue of what's happening, as well as better data to ingest into forecast models.

I will do my best to keep these updates going. I'm trying to get back into the weather blogging thing and work on my forecasting skills. Also, keep an eye on my Twitter feed (twitter.com/daweathaman) for the latest.