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Tropical Storm KARL Forecast Discussion Number 18
2016-09-18 22:59:25| National Hurricane Center (Atlantic)
Issued at 500 PM AST SUN SEP 18 2016 000 WTNT42 KNHC 182059 TCDAT2 TROPICAL STORM KARL DISCUSSION NUMBER 18 NWS NATIONAL HURRICANE CENTER MIAMI FL AL122016 500 PM AST SUN SEP 18 2016 Deep convection has gradually been increasing and becoming a little better organized since the previous advisory now that light upper-level southeasterly flow has replaced the previous hostile vertical wind shear conditions. Visible satellite imagery and ASCAT surface wind data from 1200Z-1300Z indicate that Karl's circulation remains well-defined, especially the inner-core wind field. The ASCAT data also revealed a large patch of 35-kt winds 60-100 nmi north and northeast of the center, and that is the intensity used for this advisory. The ASCAT data supports trimming back the 34-kt wind radius in the northeastern quadrant. Karl continues to move westward or 275/13 kt. Karl is expected to move westward along the southern periphery of the Bermuda-Azores ridge for the next couple of days, followed by a west-northwestward motion on days 3-5 as the cyclone approaches a weakness in the ridge located between Bermuda and the Bahamas. The global and regional models are in general agreement on this track scenario through 72 hours, After that time, the models diverge significantly with the GFS, Navy COAMPS, and GFDL models taking Karl farther west as a weaker and more shallow cyclone, whereas the ECMWF, UKMET, and HWRF models move Karl more northwestward as a stronger and deeper system. For now, the official forecast track remains a compromise between these extremes, close to the consensus model solutions, due to the lack of a stable, consistent run-to-run forecast by the models. The GFS and ECMWF models continue to forecast the vertical wind shear to decrease to less than 10 kt from now through the end of the forecast period, with near-zero shear values occuring at times between 72-120 hours. Such low shear values continue to favor a strengthening trend, especially since Karl will be moving over 30C SSTs by days 4 and 5. However, a somewhat dry mid-level environment is expected to temper the intensification process a little due to occasional intrusions of dry air into the inner-core region of Karl's circulation. Once the cyclone develops an eye feature, then strengthening will become more robust due to the low wind shear conditions and high sea-surface temperatures. Since these parameters can not be forecast very far in advance, the official intensity forecast continues to show slow strengthening throughout the period and remains close to the intensity consensus model IVCN. FORECAST POSITIONS AND MAX WINDS INIT 18/2100Z 18.3N 45.0W 35 KT 40 MPH 12H 19/0600Z 18.5N 47.0W 40 KT 45 MPH 24H 19/1800Z 19.0N 49.7W 40 KT 45 MPH 36H 20/0600Z 19.7N 52.2W 45 KT 50 MPH 48H 20/1800Z 20.5N 54.7W 50 KT 60 MPH 72H 21/1800Z 22.9N 59.7W 60 KT 70 MPH 96H 22/1800Z 25.4N 64.2W 70 KT 80 MPH 120H 23/1800Z 28.0N 67.0W 80 KT 90 MPH $$ Forecaster Stewart
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