ABBREVIATED UPDATE: SATURDAY, 22AUG15 / 7:45PM CDT
A Brief Tropical Update will be posted tomorrow with the
next ‘Limited’ Full Update scheduled for MON, AUG 24‘HURRICANE DANNY’ WEAKENS TO TROPICAL STORM – FUTURE NOT SO CERTAIN
DANNY continues to weaken at the rate I had suggested yesterday, with RECON data clearly indicating DANNY has fallen below Hurricane intensity with MAX sustained surface winds near 60Kts (probably lower...)– while the pressure in the storm’s center has risen sharply from 976mb yesterday to 994mb this afternoon.
The storm is located about 555NM east of the Leeward Islands, and is still heading WNW (290˚) at 12Kts. Wind shear and dry air have taken a major toll on the storm, with no indication of an eye wall, and the thermal eye-wall Temperature differential has decreased to less than 5˚C – and continues to decrease.
The cyclone should move across the Leeward Islands Sunday night and then move along or just north of Puerto Rico and Hispaniola on TUE and WED. It’s after this time period that major questions arise – assuming the cyclone still exists at all!
There are a number of varying model forecasts – with the GFS and EURO models calling for the system to dissipate (a very high probability scenario) while many of the Hurricane specific models maintain the vortex through the next 5 days. Considering the system is VERY SMALL, it wouldn’t have to pass all that far north of Puerto Rico and Hispaniola to avoid the disruptive impact of the mountainous terrain on these Islands. In addition, by DAY 5, there is a growing probability the high wind shear of ~30Kts that DANNY will have to deal with during the next several days will start to relax. Combine this with very warm SST’s (over 29˚C), a developing upper level anti-cyclonic flow and an approaching deep layered TROF over the eastern US – we end up with an environment that could (1) lead to re-intensification and (2) take a track that could threaten the SE US. To be sure – the odds are strongly in favor of the storm either completely dissipating or being so severely weakened it will not be able to recover – along with the realization that it is still too soon to completely write this system off.
TROPICAL CYCLONE KILO SLOWLY ORGANIZING – POTENTIALLY MAJOR THREAT TO HAWAII
After the first RECON into KILO last night indicated winds were below tropical storm intensity – the cyclone was downgraded to a Tropical Depression. But by this afternoon, SAT imagery and another RECON confirmed that the system is getting better organized - with the Low Level Circulation Center (LLCC) now located within the convective complex.
The developing cyclone is located about 500NM south of Honolulu and is heading WNW (295˚) at 13Kts. As expected, wind shear has fallen dramatically during the past 24 hours, and, upper level winds are taking on an anti-cyclonic pattern: all conducive to intensification.
The sub-tropical High to the north of the Islands which has been responsible for steering the system towards the West has begun to weaken – and this should lead to a slowing forward motion of the storm during the next few days as a developing TROF further to the W/NW will gradually turn the steering winds to a more south/southwesterly direction. This will result in turning the storm towards the North and then Northeast early next week. Just how soon the recurvature occurs will be critical to what – if any – impact the storm has on Hawaii.
The 12Z GFS run now shows the system tracking on a more westerly course away from Hawaii – while the hurricane specific models and the European ECMWF put the storm on a heading towards Kauai by WED and THU. In addition, 2 of the best intensity forecasting models ramp the storm up to CAT 4 intensity as the storm approaches Kauai! . Interestingly enough, the official forecast from the Central Pacific Hurricane Center (CPHC) based in Hawaii is forecasting the storm to make the sharp turn towards the Islands – BUT – only as a CAT 1 storm!. To be clear – this is a ‘classic’ SOP forecast. It is way too soon to ‘scare’ the public about a major hurricane that doesn’t even exist yet. And I don’t disagree in principal. But as a Weather Forecaster without the political restrictions inherent with official governmental 'responsibilities', I would be remiss to not provide all reasonable possibilities indicated by the numerical models - along with my 40+ years of professional forecasting. (Full Disclosure: Probabilities for the ‘worst Case scenario outlined above: 20%.)
ELSEWHERE IN THE TROPICS…
Several significant Tropical Waves – with identified rotation – exist from near the Cape Verdes eastward across Africa. Some of these systems have large-scale rotation that ultimately may lead to cyclone formations during the next 2 weeks. Time restrictions mean I will not be covering these systems until they are on the verge of becoming cyclones – or until DANNY and KILO are no longer threats to the US.
**
Fig 1: Visible image of DANNY As confirmed by RECON – the center of the storm is located on the far southwest side of storm’s convection, and has a NE to SW elongated cloud pattern. The elongated axis is reflected in the vortex being ‘tilted’ in the same direction which has led to the rapid weakening of the past 24 hours. The ‘eye’ is no longer visible – confirmed by the RECON crew reports. Dry air has infiltrated the mid-levels in the southwestern semi-circle, helping to accelerate the ongoing weakening.
Fig 2: Deep Level (850mb-200mb – 5,000’ to 40,000’) Shear analysis from CIMMS Southwesterly wind shears are averaging 25Kts which is steadily ‘shredding’ the storm’s convective structure. Note the forecast storm track which will keep DANNY in a high shear environment for the next few days. However, shears do drop off considerably to the north of CARIB – which, according to the latest model runs – means DANNY will be moving back into a low shear environment early next week. Of course, it’s important to keep in mind that this is a static ‘snapshot’ of current shear values - and some models call for the shear to increase some north of the CARIB – while others show it dropping further across the Bahamas next week.
Fig 3: Steering Level winds in the 850mb-500mb layer (~5,000’ to 18,000’) There is a LOT of very valuable information in these charts. (But again - keep in mind this a ‘static snapshot’ of current conditions – not a forecast.) The sub-tropical high/ridge extends from the Azores westward to the southern US – with a ‘break’ in the ridge due to a weak upper Low near Bermuda with a weak TROF southward towards Hispaniola. The latest forecast calls for the upper Low/TROF to weaken and eventually dissipate. That will allow the ridge to build back across the southwest Atlantic, forcing DANNY to take a more westerly. But until that happens, DANNY will remain on its current WNW heading for a bit longer than had previously been expected. This results in the current track forecasts taking the storm NORTH of the CARIB Islands. Later in the week, the TROF that is shown over the Great Plains will strengthen somewhat and shift towards the eastern US and the sub-tropical ridge will begin to weaken again. When that happens, steering winds just off the US east coast (including the western Bahamas) will become more southerly, and DANNY will then begin to recurve to the NW and then north. (Of course, ALL this assumes there still ‘IS’ a storm!). And therein lies the growing forecasting dilemma for the week ahead.
Fig 4: SST’s along the projected Track for DANNY.SST’s are ~28˚C, and increase to 29˚ north of the CARIB. OHC (not shown) also increase dramatically – with some of the warmest and deepest (warm) water in the entire Atlantic. If DANNY is able to survive the dry and high shear environment during the next few days – it will end up traversing an area with a nearly unlimited supply of heat energy.
Fig 5: Hurricane model forecasts from earlier this morning. The forecast tracks are tightly clustered along the north coasts of Puerto Rico westward towards the lower Bahamas. The intensity forecasts have been a bit too slow to fall off given the current intensity of the storm - but most of them maintain Tropical Storm intensity thru 4 days out with the Official NHC and HWFI model calling for a weak tropical depression or complete dissipation by 5 days out. Notably – some of the better intensity models show a sharp re-intensification by Day 5, which, as discussed above, is still a distinct possibility.
Fig 6: A Late Arrival’ of model forecasts A large number of model forecasts – and some ensemble members show a large widening in projected tracks. While too many models to detail here – some of the forecasts that show a sharp turn to the north in 4 days are either climatologically based, shallow or medium level (which work well for very weak systems), or baratropic models that do not have sophisticated physics packages in their code. Nonetheless, there's a growing consensus that DANNY will track north of the CARIB islands.
Fig 7: Hurricane DANNY Track and Intensity Forecast chart from the US NAVY The latest forecast from NHC (which the NAVY follows exactly) calls for DANNY to track just north of CARIB – in line with the model forecast consensus. The red ‘arrows’ are MY current track forecasts which is north of the NHC forecast. By the end of my forecast on Thursday – my 2 northernmost tracks have the highest probability of the 3 shown. (I’m still debating whether to resurrect my original Hurricane Forecast chart package from several years ago. For now – I’ll just annotate the NAVY charts if time permits.)
Fig 8: Visible image of KILO The most striking feature of the above image is the location of the surface vortex which now lies beneath the eastern most portion of the cirrus canopy. Just 24 hrs ago, the surface circulation was several hundred miles to the east of the main convection. This is a clear sign of KILO’s increasing organization.
Fig 9: Deep Level (850mb-200mb – 5,000’ to 40,000’) Shear analysis from CIMMS The vertical wind shear that was about 15Kts yesterday, has fallen to 5Kt’s or less – an extremely favorable development for intensification. The strong shears of 40-50Kts near/north of the Islands will be decreasing as well during the next 48 hours.
Fig 10: Upper level satellite derived wind analysis. An anti-cyclonic wind pattern with the best outflow to the north and over the western semi-circle - is still relatively modest, but should improve by Monday when an increase in intensity is most likely to begin. (Outflow is CRITICAL to how intense a tropical cyclone can get. (I’m planning to post a small set of ‘slides’ later this week covering the structure of a tropical storm to help people get a better understanding of what makes a Hurricane ‘tick’.).
Fig 11: Upper level satellite derived wind analysis. Similar to the chart provided above for DANNY – we find a strong High pressure/ridge to the north and east of the Islands which has begun to weaken as a TROF extending from a strong storm in the Gulf of Alaska (GOA) southward to a weak upper level Low/TROF far to the west of the Hawaiian Islands that's slowly advancing to the east. The light steering level winds near KILO will weaken further during the next 36 hours – slowing the forward motion of the storm to near a crawl for about 12 hours- allowing it plenty of time to begin intensifying. By mid-week, the TROF to the west will strengthen some and advance further to the east – forcing KILO to make a ‘hard right turn’ just as it begins to intensify at a faster rate. (Again – keep in mind intensity forecasts are extremely problematic – especially more than 48 hrs out in time).
Fig 12: Detailed Forecast for 850mb Winds and surface Pressure from the very good HWRF Hurricane Forecast model valid next THU. This is one of the 2 models calling for KILO to become a CAT 4-5 Hurricane as it tracks towards Kauai. The model shows the center of the 940mb Hurricane some 180NM southwest of Kauai – with MAX winds of over 140Kts. Yikes….
Fig 13: Same as above – but from the GFDL model In case 940mb and 140Kts isn’t impressive enough – this model is forecasting a 922mb storm with 160Kt winds. Worthy of 2, Yikes!
Fig 14: Tropical Storm KILO Track and Intensity Forecast chart from the US NAVY We come back down to earth with the NAVY forecast from the CPHC (which the NAVY follows exactly) calls for KILO to slowly intensify during the next few days, becoming a CAT 1 Hurricane by FRI when the storm is shown ~250NM southwest of Kauai. Note the size of the error ‘cone’. I don’t think I’ve ever seen an error cone that large before… But what really doesn’t make sense to me is that they went along with the track forecast from the very models calling for the most intensification – but barely give a nod to their intensity forecast numbers. Even with the ‘politically required’ notion of being very conservative with these long range forecasts – if they accepted the Track forecasts – they should have at least brought the storm up to CAT 2 intensity. (Stepping off the soapbox now...)
✭ A BRIEF Tropical Update will be posted tomorrow ✭
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Steve
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