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Tropical Storm KARL Forecast Discussion Number 24
2016-09-20 10:59:42| National Hurricane Center (Atlantic)
Issued at 500 AM AST TUE SEP 20 2016 000 WTNT42 KNHC 200858 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 24 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 AM AST TUE SEP 20 2016 Karl's satellite appearance late yesterday gave the impression that it had increased some in organization, with the low-level center partially underneath a large mass of deep convection. Since that time, the low-level center appears to have outrun the convection, becoming exposed well to the west of the remnant convective mass whose cloud top temperatures have warmed significantly. The cloud pattern is typical of a tropical cyclone undergoing significant westerly shear. Dvorak classifications have decreased to T2.5 from TAFB and SAB, and on this basis, the initial intensity estimate is lowered to 35 kt. The initial motion is a faster and a more westerly 290/15. Karl should continue to be steered generally west-northwestward around the southwestern periphery of well-defined low- to mid-level subtropical ridge over the central Atlantic during the next couple of days. The cyclone should move into weakness in the ridge along 65W in 48 to 72 hours, which should result in a gradual turn toward the north with a considerable decrease in forward speed by day 4. Around 96 hours, Karl should become susceptible to the mid-latitude westerly flow after it crosses 30N as it begins to undergo a sharp recurvature. The models vary tremendously on how quickly Karl will recurve by day 5, with the GFS-based guidance whisking the cyclone rapidly northeastward in response to a deep-layer trough moving through the northeastern United States and Atlantic Canada. The ECMWF and the majority of its ensemble members show Karl lagging way behind before being ejected northeastward by a slowly amplifying trough. The new track forecast is shifted to the left and is slightly faster through day 3, largely because of the initial re-positioning of the cyclone. After that time, a compromise between the much faster GFS-based guidance and the ECMWF results in a slower and somewhat more eastward forecast track. Karl can be seen approaching an upper-tropospheric cold low in water vapor imagery. The unfavorable juxtaposition of Karl and this feature should not result in much intensification for about the next 36 hours, since the flow over Karl is convergent and there is some deep-layer westerly shear. This is the rationale for keeping the short-term intensity forecast low. By 48 hours and beyond, the shear is forecast to have finally decreased while the cyclone reaches near 30 deg C waters, and intensification is likely. With the flow becoming increasingly divergent over Karl around the time it recurves in 3 to 4 days, the intensification could occur more rapidly than this forecast indicates, and there is some chance that Karl could become a major hurricane. Late in the period, increasing southwesterly shear could become a mitigating factor and the intensity is shown to be leveling off by day 5. The new NHC intensity forecast is a little lower than the multi-model consensus initially and about the same or a little lower at days 3-5. FORECAST POSITIONS AND MAX WINDS INIT 20/0900Z 20.2N 52.8W 35 KT 40 MPH 12H 20/1800Z 20.9N 54.6W 35 KT 40 MPH 24H 21/0600Z 21.7N 57.0W 35 KT 40 MPH 36H 21/1800Z 23.0N 59.2W 45 KT 50 MPH 48H 22/0600Z 24.7N 61.5W 55 KT 65 MPH 72H 23/0600Z 27.4N 64.7W 70 KT 80 MPH 96H 24/0600Z 30.3N 64.5W 90 KT 105 MPH 120H 25/0600Z 34.0N 57.9W 90 KT 105 MPH $$ Forecaster Kimberlain
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