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Potential Tropical Cyclone Two Forecast Discussion Number 2
2019-07-10 22:59:56| National Hurricane Center (Atlantic)
Issued at 400 PM CDT Wed Jul 10 2019 000 WTNT42 KNHC 102059 TCDAT2 Potential Tropical Cyclone Two Discussion Number 2 NWS National Hurricane Center Miami FL AL022019 400 PM CDT Wed Jul 10 2019 Data from An Air Force Reserve reconnaissance aircraft, surface observations, and satellite imagery indicate that the broad low pressure system located over the northeastern Gulf of Mexico still lacks a well-defined circulation center. Multiple low-level swirls associated with individual convective cells were noted in the aircraft wind data. However, shower and thunderstorm activity has been increasing over the past couple of hours in the previously convective-free northern semicircle, an indication that low-level wind field is beginning to improve in that region of the cyclone. Upper-level outflow has become well established except to the north where modest northerly wind shear is inhibiting the outflow. The initial intensity of 25 kt is based on earlier scatterometer wind data and recent recon winds of 25-30 kt to the south and west of the center. The initial motion estimate remains 245/07 kt. The latest model guidance continues in good agreement on the cyclone moving generally toward the west-southwest or southwest for the next 24 hours or so, followed by a westward motion on Friday. Afterwards, however, the model guidance diverges significantly. The 12Z ECMWF, GFS, HWRF, and HMON models have shifted farther east and turn the cyclone northwestward to northward in 48-72 hours, moving it inland along the south-central and southeastern coasts of Louisiana. In contrast, the UKMET has shifted farther west and keeps the system on more of westward track, taking it inland along the central Texas coast. The main difference is how the models handle the ridge to the north, with the ECMWF, GFS, HWRF, and HMON rapidly eroding the ridge as a weak shortwave trough passes to the north of the cyclone, whereas the UKMET shows the ridge not weakening as much due to the shortwave trough weakening as it lifts out to the east, which allows the ridge to remain intact. Due to this significant bifurcation in NHC's most reliable track model guidance, the best course of action is to slow down the forward speed and only make minor adjustments to the overall tack, which has been shifted slightly to the east, but not as far east as the simple consensus and HCCA models. Only slow strengthening is forecast for the next 24-36 hours due to the lack of a well-defined center and inner-core wind field, along with some modest northerly wind shear. By 48 hours and beyond, however, the combination of very low vertical wind shear, an impressive outflow pattern forecast by all of the global and regional models, and anomalously warm sea-surface temperatures of 30-31C should allow for significant intensification to hurricane strength before landfall occurs after 72 hours. Given that the system is still in the formative stages, the official intensity forecast remains a little below IVCN consensus through 48 hours and trends higher toward the ECMWF-based SHIPS guidance at 72 hours. Key Messages: 1. A tropical depression is expected to form by Thursday over the northern Gulf of Mexico. Conditions appear favorable for this system to strengthen to a hurricane that will bring storm surge, rainfall, and wind hazards to the central Gulf Coast. 2. A dangerous storm surge is possible in portions of southern and southeastern Louisiana where a Storm Surge Watch is in effect. Additional storm surge watches may be needed later tonight or tomorrow. Residents in these areas should monitor the progress of this system and listen to any advice given by local officials. 3. A Hurricane Watch has been issued for much of the Louisiana coast and additional tropical storm or hurricane watches could be needed later tonight or tomorrow. Residents in the watch area should ensure they have their hurricane plan in place. 4. The slow movement of this system will result in a long duration heavy rainfall threat along the central Gulf Coast and inland through the lower Mississippi Valley through the weekend and potentially into early next week. Flash flooding and river flooding will become increasingly likely, some of which may be significant, especially along and east of the track of the system. FORECAST POSITIONS AND MAX WINDS INIT 10/2100Z 28.1N 87.4W 25 KT 30 MPH...POTENTIAL TROP CYCLONE 12H 11/0600Z 27.7N 88.1W 30 KT 35 MPH...POTENTIAL TROP CYCLONE 24H 11/1800Z 27.5N 89.0W 35 KT 40 MPH...TROPICAL STORM 36H 12/0600Z 27.6N 90.2W 40 KT 45 MPH 48H 12/1800Z 28.2N 91.5W 55 KT 65 MPH 72H 13/1800Z 29.3N 92.4W 75 KT 85 MPH 96H 14/1800Z 31.9N 93.0W 55 KT 65 MPH...INLAND 120H 15/1800Z 34.8N 93.1W 25 KT 30 MPH...INLAND $$ Forecaster Stewart
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