Update - Saturday 07/12 11:30AM ET
Tropical Storm Warning is now in effect for the island of Bermuda.
Bertha is a ragged looking tropical cyclone. Like much of yesterday, Bertha is composed of a massive eye and outer band of moderate convection, with a remnant inner eye and occasional attempts at eye-wall rattling around inside. Her movement has been virtually stationary over the last 6 hours. Dvorak numbers have eased by 5kts and her intensity is being officiated at 75kts. The stationary nature of Bertha may mean that SSTs are beginning to cool due to upwelling, and if so this would also begin to contribute to weakening.
The forecast essentially is unchanged. There seems to somewhat better agreement among the models that she will stagnate for another 2 days, but then slowly...ever so slowly begin to move north and eventually northeast or east.
Update - Friday 07/11 7:00PM ET
Tropical Storm watch remains in effect for the island of Bermuda.
Bertha continues to go through structural changes in her core. As of 5pm, the new advisory has set Bertha's winds at 80kts. Since then, the inner eye-wall appears to be trying to close off again. Light shear and still amply warm waters could mean some intensification should that be the case. The rest (below) unchanged from the 4:30pm update...
All available guidance combined indicates that Bertha will likely become a west Atlantic meandering menace to shipping, and perhaps a threat to Bermuda. It is still unclear exactly what that threat will be and how long it will persist. Steering field remains weak and is only providing for a slow northwest motion over the last 24 hours, and this looks to continue right through day 3. Beyond that, there is divergence in the guidance, ranging from slow northeast motion...to turning back south in response to a stronger ridge. Intensity is forecast by NHC to very slowly decay over the 5 days. However, most of the 12z guidance I've evaluated suggest only modest weakening, and some depictions, such as the CMC, actually show a stronger system, days 4 and 5. It is extremely tedious to attempt to seek out minute factors that may or may not effect track and intensity, which is also susceptible to perturbation and emergence, therefore unknown at this time, to begin with. The over synoptic scheme, however, flexes the ridge north of Bertha in about 3 days, and this is really the impetus for showing the very slow, erratic, and even south motions day 4-6. Either is plausible at this time. In the nearer term, obviously interest in Bermuda need to monitor the progress of this storm.
Lastly, a growing consensus among the Global models is that a new CV tropical cyclone will get going between 60 and 72 hours, and do a similar act to Bertha in racing across the Atlantic at ludicrous speeds. This remains to be seen, but there is a TW nearby southeast of the CV islands.
Update - Thursday 07/09 7:00PM ET
Bertha went through some interesting structural changes during this afternoon. The cyclone now has concentric eye-walls, the outer most being 75 nautical miles in diameter, while the inner is 10 miles in diameter. As is usually the case, when this outer eye-wall was established it robbed some of the inflow into the inner eye-wall, causing it to weaken. For these reasons and a recession it the Dvorak numbers, Bertha's wind speed has been reduced to 75kts (~85mph). Just in the last 1 hour of IR imagery, however, the inner eye-wall appears to have strengthened, as a solid ring of cold cloud tops has just exploded and wrapped around. Overall, shear remains low and water temperature ample to sustain further strengthening. However, this is obviously predicated on Bertha's own structural morphologies not getting in the way of things. Probably fluctuations in intensity is the best way to go for now.
The forecast philosophy has also changed some. In the short term, a continued northwest to north drift is likely, in response a low amplitude trough currently moving by north of Bertha's location. There after the picture has become complicated. The last 24 hours of modeling have shifted the thinking away from the 2nd trough (day 4-5) being able to pick up Bertha and accelerate her finally to the graveyard. As far as I can tell there are no global model depictions during the day today that successfully pull Bertha north of middle latitudes, right through day 5. The National Hurricane Center confirms this thinking. Instead, the consensus has now shifted toward Bertha getting trapped amid a re-strengthened ridge axis nearby SE-E-NE of Bermuda, to rattle around for days. That said, it appears we will be tracking Bertha for quite some time.
There may be other areas of interest over the next week, as well. Both the ECM and GFS have been pegging another potential TC genesis of CV origin approaching the Windward Islands...just as an early heads up.
Update - Thursday 07/09 12:30PM ET
The only substantial change to note about hurricane Bertha this mid day is that the cloud tops have warmed slightly in the CDO region, and the eye has become less impressive. This has lead to Dvorak numbers that have settled back to 77kts. The TPC has officiated the winds to be 80kts (just over 90mph), but also suspect this to be generous. It is unclear what is causing this recent weakening trend (what's new in intensity guidance) as outflow remains fairly good, shear is low, and water temperatures are amply warm. The best that I, personally, can surmise is that there is dry air in the vicinity, and it may be that Bertha has been occasionally ingesting this stable air. The following is the WV image provided by NOAA:
Nevertheless, since the majority of parameters are in good shape it is more likely that Bertha will at least fluctuate in intensity over the next 3 days.
There is currently a trough pressing into the northwest Atlantic. As this passes the longitudes of Bertha, it will continue to create and impetus to move her north, albeit at slow rates of speed. In about 3 days, however, this trough will have left the longitudes of Bertha and a ridge will attempt to regain control north of Bermuda, prior to the next trough scheduled to arrive around day 4 or 5. The evolution of these events should ultimately force Bertha toward the graveyard. However, interests in Bermuda need to monitor this because as the latter ridge temporarily restrengthens in about 3 days, steering currents may drop off and/or even cause Bertha to turn left again. That may bring the TC quite close to the island of Bermuda.
Update - Thursday 07/09 12:30AM ET
As of the 11pm advisory, Bertha remains on a general 310 degree motion, but it has been noted that during that for about 2 hours, a discerned due west motion transpired. That has, however, corrected back toward the same apparent 310 degree motion as of 12AM. These kinds of wobbles are quite common in TCs moving at slow rates of speed, and are most likely due to permutations in the convective behavior more than anything else. Her Dvorak numbers were as high as 100kts but according to the TPC, they have recently come back to the 90kts (~105mph), making Bertha a category 2 hurricane.
Her presentation on satellite is intriguing as of the midnight hour. Her southwest quadrant is composed almost entirely of eye-wall, with limited outer features:
And, as you can see, her eye is still very well defined.
The forecast philosophy for Bertha has changed a little. Some of the more reliable models, including the GFS and the ECMWF, are conflicting with the NOGAPS, UKMET and GFDL about the position of Bertha after 72 hours. The former want to stall Bertha out just east or southeast of Bermuda, while the latter want to use an earlier trough to accelerate her toward the graveyard. Considering that the ridge north of Bertha has proven more resilient overall this warm season, a compromise is in order and the official call is for a near stall between 72 and 120 hours, while she awaits - perhaps - a more significant trough incursion from off the east coast, later on nearing day 5. Bertha could be moving rather erratically during that interval of time, and given to its relative close proximity to Bermuda, interest should pay close attention to the evolution of this system.
Update - Wednesday 07/09 12:30PM ET
The 11am advisory still has a 300-degree motion associated with Bertha. Her current intensity is set at 65kts, and some improvement in satellite presentation during the morning suggests at least modest re-intensification is taking place.
There has been some conjecture as to whether or not Bertha has found the means to resume more westerly track motion. I am not seeing a big enough deviation between her apparent axis of circulation to that of the clear NW track motion provided by the TPC - they seem to be aligning just fine. Moreover, there are no compelling synoptic reasons that assert Bertha should be resuming a west motion, for the time being. It is likely that convective idiosyncrasies and cloud patterns resulting have given some visual effects of west motion. However, the longer period observation still shows a general NW motion has persisted throughout the course of these "wobbles".
Steering field remains weak, but with a trough "denting" the Bermuda ridge as it presses off-shore the NE U.S. over the next day and half, if anything this offers at least a small argument for continue northwest or perhaps even eventually a north motion...albeit slowly. Speed of motion does not appear to have any means to really accelerate until perhaps very late in the 5-day forecast period, at which time a stronger trough pushing off the Mid Atlantic and northeast coasts may very well finally accelerate Bertha seaward.
Until that time, Bermuda can not be ruled out as being impacted and should continue to monitor the progress of Bertha. And, it may be that 4 or 5 days of monitoring will be required.
Update - Tuesday 07/08 10:30AM ET
Bertha is undergoing a great deal of shear and is weakening.
The eye itself remains barely intact beneath sheared clouds, but even that will not last long. Today it will likely gradually weaken more and later it may "decouple" from the system and then weaken more rapidly. Shear and Dry Air are taking its toll on Bertha this morning cloud tops are warming and other indications of weakening are all there.
What happened yesterday, and again this morning shows just how poorly intensity forecasting is understood. The lack of Recon because of the distance offshore and general lack of other data also had a lot to do with it.
The future track may slow down and con the track with speed and potential drifting later may force Bermuda to watch it for a while, although the NHC forecast and models suggest it probably will pass well to the East of the island. It has a slight chance to have minor restrengthening after it gets through the first round of shear, but water temperatures in that area suggest it won't get close to the levels it had overnight. The official forecast just shows the system weakening.
The storm remains no threat to the United States. Bermuda will need to watch it for a bit longer.
Update - Monday 07/07 5PM ET
Bertha is now a Major Hurricane with sustained winds of 100 knots (115mph) and a central pressure of 948MB. Movement over the past six hours has been to the northwest at about 10 knots and a slower forward motion is likely as the storm has reached a weakness in the Atlantic Ridge. Bertha is probably close to (if not at) her peak intensity as upper level wind shear begins to influence her strength and a slow decline in wind speed is likely over the next few days. The folks in Bermuda still need to maintain a careful watch on this hurricane as the weekend approaches.
Elsewhere, the basin is quiet - for the moment - with another tropical wave likely to exit the west African coast on Tuesday.
11:20 AM EDT 7-July-2008 Update
Bertha is now nearly a Category 2 hurricane, it will likely strengthen to be at or around a Category 2 hurricane today and part of tomorrow. After that it could enter a higher shear region and weaken, there are some signs it may fight that, however, and stay fairly strong.
The National Hurricane Center keeps it weakening, albiet slowly over the next several days. The current forecast track takes it East of Bermdua, but Bermuda itself is well within the cone. Interests in Bermuda should keep watch on this system.
The pressure as issued at 975mb, corrected from the first 11AM Advisory.
The GFS still hasn't "found" the system and a lot of the track models take that into account, once it gets it we see a better handle on the shear and track.
Bertha has become the atlantic basin 2008's first hurricane, and is still racing along fairly quickly at 17mph toward the west-northwest. It formed on July 7th, the same day that 1996's Bertha formed into a hurricane.
Bertha, as forecast, has become a hurricane with wind speeds of 75 MPH and the forward movement slowing down to 17 MPH.
In the past twelve hours, Bertha has started to turn more toward the west-northwest. Currently, it appears that Bertha is not a threat to the US mainland nor the Caribbean, but this is not completely solid yet so its still worth watching. The most likely scenario, by far, remains that it stays on the track and curves north.The newest trend suggests that Bermuda may feel the effects of Bertha this weekend. This according to the current National Hurricane Center forecast. Conditions probably will keep Bertha from gaining too much strength (right now it's looking pretty healthy), and it does not appear Bertha will gain much more than a Category 1 or 2 hurricane, in fact it is more likely that Bertha will weaken in the longer term.
Outside of Bertha, nothing appears to be forming over the next few days.
Event Related Links
Edited by typhoon_tip (Sat Jul 12 2008 11:16 AM)