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Tropical Depression Ten

UPDATED
500 AM AST Mon Aug 30 2021
LOCATION
775 MI ENE OF THE LEEWARD ISLANDS
WINDS
35 MPH
PRESSURE
1008 MB
MOVING
N AT 8 MPH

Tropical depression continues with little change in intensity under strong upper-level winds.

ALERTS
HAZARDS
SUMMARY
DISCUSSION

There are no coastal watches or warnings in effect.


None.


At 500 AM AST (0900 UTC), the center of Tropical Depression Ten was located near latitude 20.8 North, longitude 50.6 West. The depression is moving toward the north near 8 mph (13 km/h). A general northward motion is forecast to continue through Monday followed by a gradual turn toward the northwest by Wednesday. Maximum sustained winds remain near 35 mph (55 km/h) with higher gusts. Little change in strength is forecast during the next couple of days. Slow strengthening is forecast to begin in the latter part of this week. The estimated minimum central pressure is 1008 mb (29.77 inches).


The deep convection with Tropical Depression Ten is in a sheared bursting pattern this morning, associated with overshooting cloud top temperatures below -80 C. A 0518 UTC AMSR-2 microwave pass did show a bit of banding associated with this activity on the 37 GHz low-level channel. However, Proxy-Vis satellite imagery indicates this convective activity remains located downshear of the low-level center thanks to very strong 30-50 kt northwesterly flow associated with a subtropical jet at 200 mb. The latest round of subjective Dvorak estimates were 25 kt from SAB and 35 kt from TAFB. Taking a blend of these estimates and the earlier ASCAT wind data supports keeping the intensity at 30 kt for this advisory.

The depression has resumed a northward motion this morning, with the latest estimate at 360 degrees at 7 kt. A deep-layer trough passing by well to the north is continuing to provide a weakness in the subtropical ridge, allowing the cyclone to escape northward. The system could even move a bit east of due north over the next 24-36 hours if down-shear convective bursts help to drag the low-level center a bit right of the steering flow. After 36 hours, the deep-layer trough moves eastward, allowing the subtropical ridge to build back in. The net result is that the depression should turn leftward and begin a more northwestward motion by the latter part of this week. The latest track guidance has once again made another westward shift this cycle after 36 hours, and the NHC track forecast has been nudged in that direction as well. However, the latest track is still not as far west as the GFS & ECMWF models, and further westward adjustments may be needed in subsequent forecasts.

Strong upper-level flow is the primary hindrance for the depression currently. In fact, both GFS & ECMWF-based SHIPS guidance shows the vertical wind shear remaining above 30 kt for the next 24 hours as the cyclone moves through the core of a subtropical jet streak. Interestingly, this shearing flow seems to be mostly based in the upper-levels, with much lower mid-level shear diagnosed by UW-CIMSS. This lower mid-level shear may help explain why deep-convection has not yet been completely stripped away from the low-level center. After 36 hours, most of the guidance agrees that an upper-level low will cut off to the southwest of the depression, providing a more favorable upper-level environment over the system. However, it remains unclear what will be left of the depression by that time, and the latest 00z ECMWF, HWRF, and HMON runs suggest the vortex will be too weak and diffuse to take advantage of the more favorable conditions. For now, the latest NHC intensity forecast will maintain the current intensity through 48 hours, with only modest intensification beginning after that time assuming the circulation is coherent enough to take advantage of the more favorable environment. The latest intensity forecast is just a bit lower than the previous forecast, and is also lower than the HCCA and IVCN consensus aids. It remains distinctly possible that the depression could become a remnant low if its convection is completely stripped away.

Although strong upper-level westerly shear continues to plague the cyclone, its satellite presentation improved early this morning as its center moved closer to the edge of the convective cloud mass to its east. An ASCAT-A pass from 1100 UTC revealed an area of 30 to 40-kt winds in the eastern semicircle of the cyclone, with some slightly stronger winds possibly rain contaminated underneath the deep convection. Additionally, UW-CIMSS ADT objective estimates have risen to around 40 kt within the past few hours, and TAFB gave a T2.5/35 kt subjective Dvorak classification at 12 UTC. These data support upgrading the depression to Tropical Storm Kate. Its initial intensity is set at 40 kt for this advisory, although that could be a bit generous given recent satellite trends.

A weakness in the subtropical ridge is allowing Kate to move just west of due north, or 355/7 kt. This general motion should continue for the next day or so before the subtropical ridge becomes reestablished over the central Atlantic Ocean. Thereafter, the cyclone should move northwestward on Wednesday and Thursday along the southwestern periphery of the ridge. By Friday, an approaching deep-layer trough should cause the cyclone to accelerate northward or north-northeastward through the rest of the forecast period. The track guidance has shifted a little left of the previous NHC track, and so the official forecast has been adjusted in that direction to bring it closer to the TVCA and HCCA consensus aids.

The near-term intensity forecast is tricky, as the subtropical jet stream will maintain strong west-northwesterly shear over Kate during the next 24 to 36 h. In fact, recent satellite imagery of the cyclone shows the center is already more exposed than earlier this morning as the convection is waning. Kate is likely to continue exhibiting a bursting convective pattern over the next couple of days, which would likely result in some intensity fluctuations that hover around the tropical-storm-force threshold. The official NHC intensity forecast shows Kate as a 35-kt tropical storm during the first 36 h of the forecast. If Kate survives the hostile shear conditions, some modest intensification will be possible while the cyclone remains over 28 deg C waters. However, Kate will encounter a drier mid-level environment as it gains latitude, so significant strengthening does not appear likely at this time. The official intensity forecast is similar to the previous one beyond 48 h, as it shows only modest strengthening with time. By day 5, the global models suggest that Kate could be becoming absorbed by a larger extratropical low expected to form and deepen near Atlantic Canada.

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