MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #141
Updated: Oct 8
*******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.**********
...TUESDAY OCTOBER 6 2020 3:45 PM EDT...
See Major Hurricane Delta section below for the only currently active tropical cyclone in the Atlantic basin. See remnants of Gamma section below for a final statement on that system. See area of interest section below for a tropical wave of low pressure expected to move across the Caribbean Sea in the coming days.
REMNANTS OF GAMMA...Slow-moving Tropical Storm Gamma in the far southern Gulf of Mexico has drifted southwestward into the northwestern Yucatan peninsula while southwesterly shear and increasingly dominant low-level inflow of explosively intensifying Hurricane Delta to the east has caused it to weaken to a remnant low pressure over the last 24 hours. The eastern North America upper trough producing the shear has fractured into two halves that are moving away from the Gulf of Mexico...thus the shear over the remnant low pressure of Gamma has reduced. This has allowed the remnant low pressure to produce new thunderstorm flares much closer to the center with some signs of organization in satellite pictures...however with Hurricane Delta incoming from the east there is no room for the remnant low pressure of Gamma to develop its own low-level inflow needed for reformation into a tropical cyclone. Instead expect Gamma to become absorbed into Hurricane Delta's west side in the next 24 hours while adding to rainfall across the Yucatan peninsula of Mexico. This is my final statement on Gamma on this blog.
Infrared satellite image of Hurricane Delta as of 1726Z to highlight its eye primarily visible on the infrared color scheme:
MAJOR HURRICANE DELTA...While undergoing extraordinary and explosive intensification from a tropical storm to powerhouse category 4 hurricane in the matter of 24 hours...Delta's track is bending less northward and more westward across a path in the open western Caribbean Sea. The intensification is attributed to low shear and upper outflow of an expansive upper ridge in the region...rather warm 30 deg C waters...and also the compact structure of the storm as it has been noted in the past that compact tropical cyclones tend to have rapid intensity changes. For instance even though the central pressure of the hurricane is on the higher side for category 4 (in the mid-950s of mb)...the hurricane is now a category 4 nonetheless. This has to do with the fact the pressure gradient between the center and outside of the storm is quiet sharp because of its small size...thus not as much of a pressure drop is needed to bring winds up (explaining how a compact tropical cyclone has rapid intensity change). Delta is also a peculiar hurricane on satellite pictures...as true color visible schemes do not show an eye as one would expect in a category 4 hurricane. This is because the eye is cloud-filled...and so an infrared color scheme is needed to see the eye as shown in the above satellite image sample from 1726Z.
A very concerning observation for both Quintana Roo province and the Yucatan province in the northern Yucatan peninsula is the decrease in the hurricane's northward angle in its track. This is an indication the remnant low pressure of Gamma over the Yucatan has weakened enough such that Delta is now moving toward Gamma on a path to absorb it rather than dancing on a more northward track in a fujiwhara interaction with Gamma. Intersts in Qunitana Roo should now rush preparations for extremely damaging hurricane conditions (potential destruction of many structures near where the eye makes landfall...severe coastal storm surge flooding...and heavy flash flooding rains) by tonight as conditions will detereorate tomorrow. Wind damage will spread inland across Quintana Roo and arrive into the Yucatan province by late tomorrow...so although there is a little more time for preparations here for hurricane conditions...interests in the Yucatan provnice should also be preparing for Hurricane Delta in earnest! My intensity forecast for now assumes Delta will be just below the threshold of category 5 for tomorrow's landfall...but this could change if Delta begins an eye wall replacement cycle that causes a little weakening (but be warned that Delta will still be quiet strong and severe even with an eye wall replacement cycle at this point).
While crossing the northeastern Yucatan peninsula and entering the southwestern Gulf of Mexico...I expect the increased land interaction on the new forecast track will cause the tight core of Delta to potentially dissolve into a broader core over land...with the hurricane potentially growing notably larger in size while absorbing Gamma's remnant low pressure. Thus my updated intensity forecast calls for Delta to be a slowly-recovering category 2 hurricane over the western Gulf of Mexico by 48 to 72 hours as a larger and broader system needing time to re-consolidate and re-develop an organized inner core. The next feature to interact with Delta during that time will be the current upper trough fragment over Mexico which will swing eastward toward Delta...and so shear induced by the upper trough by 72 hours is why I also do not show an intensity above category 2 during the later part of the forecast period. With my southwestward adjusted track forecast points in the shor term...I am left to conclude that Delta will interact with the southern part of the upper trough where the steering flow is more westerly and less southerly as opposed to the northern part of the upper trough where the steering flow would otherwise be more southerly...thus I forecast a sharp eastward curve in the track by 96 to 120 hours. So the ironic thing is that even though I have adjusted the track forecast westward in the shorter term...I am left with my longer term forecast track still having a landfall over southeastern Louisiana and southern Mississippi....just a little later and at a different angle more parallel to the US Gulf coast such that points further west such as south-central Louisiana and points further east such as southern Alabama and the western Florida panhandle are now more in play with seeing notable wind and rain impacts from Delta. Regarding intensity for the US Gulf coast landfall...it is possible that despite seeing increasd wind shear and lower water temperatrues that Delta hangs on to category 2 status as the divergence zone of the incoming upper trough may aid Delta's thunderstorms. I expect by 120 hours Delta will transition into a remant frontal low pressure supported by the upper trough while moving well inland over the southeastern United States. See the home page bulletins for more information on expected impacts from Delta given these forecast updates.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Oct 6)...140 mph maximum sustained wind major hurricane centered in the northwestern Caribbean Sea at 18.5N-83.5W
IOH 24 Hr Forecast (1800Z Oct 7)...155 mph maximum sustained wind major hurricane centered on the east coast of the Yucatan peninsula at 20.2N-87.5W
IOH 48 Hr Forecast (1800Z Oct 8)...105 mph maximum sustained hurricane centered over the western Gulf of Mexico at 22.5N-92.5W
IOH 72 Hr Forecast (1800Z Oct 9)...105 mph maximum sustained wind hurricane centered over the northwestern Gulf of Mexico at 26N-93W
IOH 96 Hr Forecacst (1800Z Oct 10)...75 mph maximum sustained wind hurricane centered over southeastern Mississippi at 31N-89W
IOH 120 Hr Forecast (1800Z Oct 11)...Remnant frontal low pressure centered over northern Georgia at 34.2N-84.5W
AREA OF INTEREST #1...The tropical wave of low pressure in the open central Atlantic is currently approaching the Lesser Antilles islands...but the islands and eastern Caribbean Sea are already seeing a drastic increase in thunderstorms thanks to the southeastern divergence zone of an upper vortex retrograding southwestward and breaking away from the main branch of central Atlantic upper vorticity. With this cold core upper vortex expected to continuously retrograde westward ahead of the tropical wave aroud the sprawling west Atlantic upper ridge thru 96 hours...and dissipate by 120 hours while remaining cut-off from higher latitude cold air...the upper wind pattern could increasingly become more conducive to tropical development as the western Atlantic upper ridge with its low shear and upper outflow overspreads the tropical wave in the wake of the upper vortex. It is possible that eastern divergence zone of the upper vortex aids in initial development of the wave...then dissipates to allow more favorable upper winds to take over...a similar setup to how Hurricane Delta formed. Therefore I have raised peak 5-day odds of tropical cyclone formation to 30% despite there being a lack of computer model support showing development. Forecast track in the outlook below is based on the tropical wave moving west-northwest in the east-southeasterly flow to setup between mid-latitude surface ridging and the east side of Hurricane Delta...followed by a more northward bend in track by 120 hours as what will be the remnant low pressure of Delta over the southeastern United States creates a surface ridge weakness. Given this forecast track...I am forced to mention Jamaica and the Cayman Islands in regards to this tropical wave on the home page bulletins of this site.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 7)...0% chance of tropical cyclone formation (just west of the Lesser Antilles near 12.5N-61.5W)
IOH 48 Hr Outlook (1800Z Oct 8)...10% chance of tropical cyclone formation (eastern Caribbean sea near 13N-66.5W)
IOH 72 Hr Outlook (1800Z Oct 9)...10% chance of tropical cyclone formation (central Caribbean sea near 13.5N-71W)
IOH 96 Hr Outlook (1800Z Oct 10)...20% chance of tropical cyclone formation (central Caribbean sea near 14N-75W)
IOH 120 Hr Outlook (1800Z Oct 12)...30% chance of tropical cyclone formation (southwest of Jamiaca near 16N-79W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run... For Major Hurricane Delta...passes over the northeastern Yucatan peninsula at 30 hours while absorbing the remnant low pressure of Gamma...located over the western Gulf of Mexico as a larger and still intense hurricane at 60 hours...turns northward and makes landfall at 96 hours over the Texas/Louisiana border...located over the Louisiana/Arkansas/Mississipi border as a weakening inland remnant low pressure at 120 hours. For area of interest #1...no tropical cyclone formation shown.
0000Z ECMWF Model Run....For Major Hurricane Delta...passes over the northeastern Yucatan peninsula between 24 and 48 hours while absorbing the remnant low pressure of Gamma...
located over the western Gulf of Mexico as a larger and still intense hurricane at 72 hours...turns northward and makes landfall just after 96 hours over the Texas/Louisiana border...located over southwestern Louisiana as a weakening inland tropical cyclone at 120 hours. For area of interest #1...no tropical cyclone formation shown.
1200Z GFS Model Run...For Major Hurricane Delta...passes over the northeastern Yucatan peninsula at 24 hours while absorbing the remnant low pressure of Gamma...located over the western Gulf of Mexico as a larger and still intense hurricane at 60 hours...turns northward and makes landfall just at 84 hours over south-central Louisiana...located over the Tennesee/Mississippi/Alabama border as a weakening inland remnant low pressure at 120 hours. For area of interest #1...no tropical cyclone formation shown. Elsewhere...a tropical wave with low pressure spin emerges from western Africa by 162 hours with weak tropical cyclone formation suggested.
0600Z NAVGEM Model Run...Today's 0600Z run not available from source.