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BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

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MY 2023 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #9

*******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.**********


...WEDNESDAY APRIL 12 2023 7:53 AM EDT...

Pre-season subtropical disturbance now materializing in the north-central and northeastern Gulf of Mexico... some potential for subtropical cyclone formation remains with this disturbance over the next 24 hours before it moves northward and onshore. 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 24 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. An upper trough that was previously over the central US has evolved into a cut-off upper vortex over Louisiana due to the passage of a warm core upper ridge to the north (this ridge is being supported by northward warm air transport ahead of a broad and amplified upper trough/surface frontal system now sliding across western North America). As of this writing the southeastern divergence zone of the Louisiana upper vortex is generating a new surface trough of low pressure in the Gulf of Mexico along 90W longitude along with increasing showers and thunderstorms across the north-central and northeastern Gulf of Mexico. In the next 24 hours a more defined surface low pressure center is anticipated to form along the surface trough and lift northward toward the southeastern Louisiana/Mississippi coastal regions while orbiting the east side of the upper vortex. A faster and more north-northeast track is expected beteween 24 and 48 hours as the blocking warm upper ridge to the north and blocking southeastern US surface ridge to the northeast become eroded by the approach of the amplified upper trough from western North America.


Regarding odds of subtropical cyclone formation... I have raised peak development odds to 20% due to the recent increase in shower and thunderstorm activity noted in the prior paragraph and as the usually-conservative ECMWF model has recently hinted at potentially rapid subtropical cyclone formation in the next 24 hours. The global models over the last 24 hours have converged on an elongation of the Louisiana upper vortex while also pushing it east-southeast toward the US Gulf coast... due to the force of the passing warm upper ridge to the north... which results in an elongated upper divergence zone on the southeast side of the upper vortex that will tend to keep the surface low pressure area larger/broader with an eastward extension into the northeastern Gulf of Mexico. A larger/broader structure with multiple competing centers may go against subtropical cyclone formation which is one incentive for keeping the odds of subtropical cyclone formation still low. Other incentives include the position of the upper vortex which will tend to be 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 tonight and into tomorrow... expect the impact zone to include southeastern Louisiana... southern Mississippi.... southern Alabama... and the western Florida panhandle which will primarily see periods of heavy rainfall. Periods of heavy rainfall now appear likely further east... across the eastern Florida panhandle... the Florida peninsula... and southern Georgia due to the early presence of increasing activity in the northeastern Gulf of Mexico and also the models converging on a broader surface low with a northeastern Gulf of Mexico extension as discussed above. 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 as suggested by the recent 0000Z ECMWF model run.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Apr 13)... 20% chance of subtropical cyclone formation (just offshore of southeastern Louisiana and southern Mississippi near 29.8N-89.5W)

IOH 48 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 12) CMC Model Run...

**For area of interest #2... broad surface low forms in the north-central Gulf of Mexico near 27.5N-90W at 24 hours... makes landfall on the Mississippi coast at 48 hours... dissipates near Mississippi/ Alabama/ Tennesse border by 66 hours.


0000Z (Apr 12) ECMWF Model Run...

**For area of interest #2... broad surface low forms in the north-central Gulf of Mexico near 27.8N-90.5W at 24 hours... possible subtropical cyclone formation just offshore of southeastern Louisiana at 30 hours... center moves across southeastern Louisiana at 36 to 42 hours... weakens to a remnant low on the Mississippi coast at 48 hours... remnant low dissipates over southern Alabama at 60 hours.


0000Z (Apr 12) GFS Model Run...

**For area of interest #2... broad surface low forms in the eastern Gulf of Mexico by 18 hours with one center just offshore of southeastern Louisiana and another near 26.2N-85W... both centers merge east of southeastern Louisiana at 30 hours... broad merged center makes landfall at the Mississippi/ Alabama border at 39 hours... dissipates over northern Mississippi at 60 hours.


0000Z (Apr 12) NAVGEM Model Run...

**For area of interest #2... rapid subtropical cyclone formation offhsore of southeastern Louisiana near 28N-91.5W at 18 hours... subtropical cyclone moves north across the southeastern Louisiana coast through 36 hours and makes landfall on the Mississippi coast at 42 hours... inland remnant low dissipates over northwestern Alabama at 66 hours.

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