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 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #3

*******Note that forecasts 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.**********


...FRIDAY MAY 21 2021 12:45 PM EDT...

A subtropical or tropical storm is expected to form in the waters just east of Bermuda over the next day or so...see area of interest #1 section below for additional details. Elsewhere...a tropical low pressure has formed in the western Gulf of Mexico...see area of interest #2 for additional details on that feature.


AREA OF INTEREST #1...As expected...the current western surface frontal low pressure has intensified into the dominant low pressure due to the support of the eastern divergence zone of the cut-off upper vortex in the region...allowing it to absorb the secondary frontal low that was in the northwest Atlantic. The frontal low is also whirling westward toward towards the general direction of Bermuda and into the center of the upper vortex...with the aid of easterly flow on the south side of the deep-layered ridge to the north and west. As the frontal low has intensified into a frontal cyclone...it has become better defined and organized...especially as it lines up with the center of the upper vortex and its clouds. Despite being over lukewarm 20 deg C waters...the frontal cyclone is still likely to acquire tropical characteristics due to the instability provided by the cold 1195 dekameter height (200 mb layer) upper vortex. This instability is currently allowing for bands of showers and weak thunderstorms to appear on colorized infrared satellite around the center of rotation.


My forecast track is unchanged for the next 24 hours as the models have converged on the 34N-62W position shown in the previous forecast. Afer that time...the next frontal system/upper trough to exit from eastern Canada is expected to turn the upper vortex and surface cyclone northeastward. Due to the current strength of the deep-layered ridge to the north and west...the upper vortex is elongated north-to-south...therefore the southern lobe of the upper vortex may keep the surface cyclone tugged more southward as the whole system accelerates northeastward. Today's model consensus is in agreement on a more southward long-term track...therefore my updated forecast track is adjusted accordingly. Models also agree that the incoming front will absorb this system at 72 hours...and my updated forecast also shows this.


I have lowered the intensity forecast as the thunderstorm intensity has been on the low side thus far and as ASCAT passes only show winds of 35-knots (40 mph) on the north side of the surface cyclone. The forecast track will take the surface cyclone to the south edge of the Gulf stream where water temperatures are slightly warmer at 22 deg C. And thru 48 hours the upper-level temperature of the upper vortex is expected to remain cold (around 1195 dekameters in height at 200 mb). On October 27 2019 Tropical Storm Pablo was able to reach hurricane strength with a similar upper air temperature but with even cooler waters just below 20 deg C...so I still show in my forecast below a transition to fully tropical status...but have delayed the transition by about 24 hours.


On a final note...the forecast track keeps this system just east of Bermuda...therefore gusty damaging winds from this system are not expected over Bermuda. However...expect coastal sea swells to increase from now and through the weekend.

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

0 Hr Position (1200Z May 21)...Frontal cyclone centered at 35.5N-60W

IOH 24 Hr Forecast (1200Z May 22)...50 mph maximum sustained wind subtropical storm centered east-northeast of Bermuda at 34N-62W

IOH 48 Hr Forecast (1200Z May 23)...60 mph maximum sustained wind tropical storm centered at 35N-60W

IOH 72 Hr Forecast (1200Z May 24)...Absorbed by cold front near 36N-55W


AREA OF INTEREST #2...The tropical surface low pressure disturbance currently in the western Gulf of Mexico formed last evening with the aid of the eastern divergence zone of an upper trough that has stalled against the west side west side of the deep-layered ridge over the eastern US. The tropical low pressure is also being aided by low wind shear and upper-level outflow under the southwest quadrant of the deep-layered ridge. The NHC TAFB surface analysis at 1200Z had the surface low pressure center at 25.5N-95W...however satellite animation shows a well-defined swirl just to the east which is probably the mid-level spin. Despite a healthy structure on satellite...the system lacks thunderstorms near the center needed for tropical development. Water vapor satellite shows a trowal of dry air over Texas (supported by the western convergence zone of the upper trough) being ingested into the circulation...and this system is headed toward the northwestern Gulf of Mexico where water temperatures are below 26 deg C. Unlike area of interest #1...the upper-level temperatures are quiet warm due to the presence of the deep-layered ridge...therefore water temperatures of 26+ deg C are needed for instability and thunderstorms. Given the ongoing lack of thunderstorms and unfavorable thermodynamic picture that lies ahead...I forecast 0% odds of tropical cyclone formation as this system is steered into Texas by the deep-layered ridge. Given the dominant mid-level swirl is lopsided to the east of the surface position...I lean towards a landfall just east of Matagorda Bay Texas as opposed to over or south of the bay as some of the model runs show. I plan this to be my final statement on this system unless the situation changes...or if the NHC still has this system in their tropical weather outlook during my next update.

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

IOH 24 Hr Outlook (1200Z May 22)...0% chance of tropical cyclone formation (inland just northeast of Matagorda Bay Texas near 29N-95.5W)


...COMPUTER MODEL SUMMARY...

Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)


0000Z CMC Model Run...For area of interest #1...closest approach to Bermuda is at 36 hours when system briefly stalls at 34N-62W...accelerates northeastward and absorbed by cold front at 78 hours near 35.5N-57.5W. For area of interest #2...surface low makes landfall just south of Matagorda Bay Texas at 30 hours.


0000Z ECMWF Model Run...For area of interest #1...closest approach to Bermuda is at 24 hours when system briefly stalls at 34N-62W...accelerates northeastward and absorbed by cold front at 78 hours near 36.5N-59W. For area of interest #2...surface low makes landfall near Matagorda Bay Texas just after 24 hours.


0600Z GFS Model Run...For area of interest #1...closest approach to Bermuda is at 30 hours when system briefly stalls at 34N-62W...accelerates east-northeastward and absorbed by cold front at 72 hours near 35.5N-55W. For area of interest #2...no tropical cyclone formation shown.


0600Z NAVGEM Model Run...For area of interest #1...closest approach to Bermuda is at 42 hours when system briefly stalls at 35N-64W...accelerates east-northeastward and absorbed by cold front at 72 hours near 35.5N-60W. For area of interest #2...surface low weakens to trough at 12 hours (no tropical cyclone formation shown)

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