*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...TUESDAY APRIL 11 2023 7:55 AM EDT...
Pre-season subtropical disturbance anticipated to materialize just offshore of southeastern Louisiana by tomorrow evening... however its potential for subtropical cyclone formation remains low as it is expected to lift northward and onshore relatively quickly. See area of interest #2 section below for more information.
As done on this site starting last year... I will be sequentially numbering up areas of interest for possible Atlantic tropical development throughout the year... resetting back to #1 at the start of each year. This scheme is so that each area of interest retains a numeric identity from update to update... which reduces confusion when simultaneous areas of interest begin and end when tropical activity increases during the Atlantic Hurricane Season. The current area of interest is tagged #2 as the first was tagged in late January (see earlier posts #1 through #4 on the home page of this site).
AREA OF INTEREST #2... Continuing to watch for possible tropical development in the northern Gulf of Mexico in the timeframe that now covers the next 48 hours. As is typical when dealing with pre-season development in the Atlantic basin... we look for the formation of a cut-off upper trough or vortex that will generate the surface low pressure area and thunderstorms. The upper trough that was previously over the central US is becoming cut-off from the mid-latitude westerlies and amplified over Louisiana and vicinity in response to an amplifying adjacent warm-core upper ridge over the western US centered over Wyoming (the upper ridge is being supported by northward warm air transport ahead of a broad and amplifed northeasern Pacific upper trough/surface frontal cyclone that has pushed into western Canada). This upper air pattern is expected to continue over the next day or so... which will cause the upper trough over Louisiana to amplify further into a cut-off upper vortex. By tomorrow evening the southeastern divergence zone of the upper vortex is expected to lower surface pressures just offshore of the southeastern Louisiana coast... with whatever low pressure system that forms lifting northward and inland not long after its formation while it orbits the east side of the upper vortex. A faster and more north-northeast track is expected beteween 48 and 72 hours as the blocking warm deep-layer ridge to approach from the western/central US becomes eroded from the approach of the amplified upper trough energy from the northeastern Pacific. The more reliable GFS and ECMWF models continue to position the upper vortex over Louisiana... but now forecast a slightly larger upper vortex which increases the probability of an offshore subtropical disturbance emerging. My updated forecast track in the outlook below remains similar to the previous as the track is consistent with an upper vortex over that position. Interestingly the CMC and NAVGEM models have recently trended with a further southeast position for the upper vortex while also suggesting a more elongated vortex with an elongated upper divergence zone... resulting in a larger/broader surface low pressure area located further east. For now favoring the GFS/ECMWF general consensus since those two models tend to be the more reliable in the global model suite.
Regarding odds of subtropical cyclone formation... I continue to go with a low peak of 10% as the emerging subtropical disturbance will have little time over water. Other factors that keep development odds low include the position of the upper vortex center over land instead of water... which will allow the south side of the upper vortex to impart some westerly shear over water where the disturbance would be trying to develop... and the 24 deg C water temperatures in the northern Gulf which are below the typical 26+ deg C threshold for tropical development. Although the cold temps of the upper vortex may help aid in generating instability and thunderstorms as indicated by forecast 200 mb heights being right at 1200 dekameters... would like to have seen even colder temperature (lower heights) for more confidence in tropical development given the below-26 deg C sea surface temperature environment.
Regarding impacts for tomorrow and Thursday... given that the reliable GFS and ECMWF have been consistent with the forecast track over the last couple of days the most likely impact zone will be southeastern Louisiana... southern Mississippi... southern Alabama... and the western Florida panhandle which will primarily see enhanced rainfall. Some coastal surf and breezy winds will only be possible... particularly toward the western areas of the impact zone... if the subtropical disturbance manages to ramp up relatively quickly during its short time over water. There is some chance of enhanced rainfall reaching as far east as the eastern Florida panhandle and southwestern Georgia... but those odds remain low as this idea is conveyed by the less reliable CMC/NAVGEM consensus.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Apr 12)... 0% chance of subtropical cyclone formation (just offshore of southeastern Louisiana near 28.5N-90W)
IOH 48 Hr Outlook (1200Z Apr 13)... 10% chance of subtropical cyclone formation (just offshore of southeastern Louisiana and southern Mississippi near 29.8N-89.5W)
IOH 72 Hr Outlook (1200Z Apr 14)... 0% chance of subtropical cyclone formation (Alabama/ Georgia/ Tennessee border near 35N-85.5W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z (Apr 11) CMC Model Run...
**For area of interest #2... broad surface low with multiple centers forms in the north-central Gulf of Mexico near 27.5N-90W at 54 hours... surface low lifts northward onto the northern US Gulf coast while dissipating through 90 hours
0000Z (Apr 11) ECMWF Model Run...
**For area of interest #2... surface low forms just offshore of south-central Louisiana near 28N-92W at 42 hours... drifts east-northeast toward the Alabama coast while dissipating through 78 hours.
0600Z (Apr 11) GFS Model Run...
**For area of interest #2... weak surface low forms just offshore of southeastern Louisiana near 28N-90W at 39 hours... lifts northward aross southeastern Louisiana and southern Mississippi while weakening to a surface trough through 66 hours.
0000Z (Apr 11) NAVGEM Model Run...
**For area of interest #2... broad surface low with multiple centers forms in the northeastern Gulf of Mexico near 27.5N-86W at 48 hours... makes landfall over the western Florida panhandle at 72 hours... weakens to an inland surface trough over northern Georgia by 90 hours.