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Tropical Storm Lorena Forecast Discussion Number 1
2019-09-17 16:56:09| National Hurricane Center (East Pacific)
Issued at 1000 AM CDT Tue Sep 17 2019 000 WTPZ45 KNHC 171456 TCDEP5 Tropical Storm Lorena Discussion Number 1 NWS National Hurricane Center Miami FL EP152019 1000 AM CDT Tue Sep 17 2019 First-light 1-minute visible imagery from GOES-W revealed that the disturbance just south of Mexico has developed a well-defined surface center. The system has also developed an extensive convective band stretching from the southwest to the northeast quadrant of the cyclone. A pair of ASCAT passes overnight showed winds were already at tropical-storm strength, so the system has been designated as Tropical Storm Lorena. The initial intensity is set at 35 kt, though the ASCAT data showed winds could actually already be higher than that, so that may be conservative. Lorena is moving quickly toward the northwest, or 305/13 kt. A mid-level ridge centered over the Gulf of Mexico should keep it moving generally northwestward for the next few days, at a slightly slower speed. Most of the dynamical models show this general scenario, however some of the typically reliable track models, including the GFS, seem to have trouble resolving the small cyclone, and show the cyclone moving more west-northwestward. This small difference is important and could be the difference between Lorena directly impacting Mexico or passing just to the southwest. The NHC forecast is on the far right side of the guidance envelope, favoring the ECMWF and UKMET solutions, which seem to have a more realistic current depiction of the tropical cyclone. The NHC forecast is on the far right side of the standard guidance envelope, but is near the middle of the ECMWF ensemble. This brings Lorena very near the Mexico coast in about 2 days and it is possible that the storm could make landfall. At longer ranges the GFS depicts the beginning of a binary interaction between Lorena and the newly-formed depression to the west, however at the moment this does not seem particularly likely, and the NHC forecast favors the ECMWF through day 5 which shows no such interaction. Moderate easterly shear appears to be affecting the tropical storm for now, and this will likely be the primary moderating factor for Lorena's intensity. SSTs are very warm and there is ample moisture, but only slow intensification is likely as long as the shear persists. The NHC forecast therefore shows slow strengthening for the next couple of days. Beyond that time, if the cyclone moves inland, it should quickly weaken or possibly dissipate, but if it stays farther offshore it may have an opportunity to strengthen further. The NHC forecast holds the cyclone nearly steady-state, but it is well below the consensus at day 4 and 5, since most of the intensity models are based on tracks that stay well offshore of Mexico. FORECAST POSITIONS AND MAX WINDS INIT 17/1500Z 13.7N 100.0W 35 KT 40 MPH 12H 18/0000Z 14.8N 101.0W 40 KT 45 MPH 24H 18/1200Z 16.3N 102.4W 45 KT 50 MPH 36H 19/0000Z 17.5N 103.6W 50 KT 60 MPH 48H 19/1200Z 18.5N 104.6W 55 KT 65 MPH 72H 20/1200Z 20.0N 106.3W 55 KT 65 MPH 96H 21/1200Z 21.0N 108.0W 55 KT 65 MPH 120H 22/1200Z 22.0N 110.0W 50 KT 60 MPH $$ Forecaster Zelinsky
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