2014 Atlantic Hurricane Season Birdseye Discussion #75

By: NCHurricane2009 , 7:47 AM GMT on August 27, 2014

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...WEDNESDAY AUGUST 27 2014 3:45 AM EDT...
Cristobal to rapidly accelerate northeastward across the western and northern Atlantic over the next few days...at first as a large and vigorous tropical cyclone transitioning into a large and vigorous non-tropical cyclone. The forecast track guidance is has shifted west over the last 24 hours such that the threat to Bermuda is reduced and the threat to southeastern Newfoundland has increased. See special feature section below for additional details on Cristobal. Visit www.nhc.noaa.gov for up-to-the-minute the latest information on this system...including watches/warnings.

Tropical wave Invest 97-L currently east of the Lesser Antilles previously received attention from the NRL and NHC because conditions were expected to be generally more favorable for development as the dry Saharan air layer is weaker relative to earlier this month and with the expectation this system would take advantage of favorable tropical upper ridging (currently marked by blue-zig-zag in the lower-right in the atmo chart below). Instead this system continues to struggle with the dry Saharan air layer to its north. Moreover a central Atlantic upper ridge has been forming due to the latent heat release of Hurricane Cristobal and due to warm air advection ahead of a series of frontal cyclones supported by the western Atlantic upper trough. A tropical upper tropospheric trough (TUTT) has been forming between the central Atlantic upper ridge and aforementioned tropical upper ridge...with this system also battling unfavorable westerly shear on the south side of the TUTT. This system will likely continue dealing with unfavorable shear while following an upper vortex forecast to fracture off the western end of the TUTT. Furthermore with computer model support dropping for this system...development of 97-L is not expected.

The vigorous tropical wave with widespread showers and thunderstorms over western Africa remains organized as it nears the west African coast...and computer model support suggesting quick development as it enters the Atlantic tropics is increasing. Therefore this tropical wave has been upgraded to a special feature on this blog. See the second special feature section below for additional details.

The western Atlantic upper trough that Hurricane Cristobal is currently interacting with previously supported a lengthy surface front...the tail end of which has decayed into a surface trough over the Gulf of Mexico as marked by an east-west red-dashed line in the atmospheric features chart below. In the atmospheric outlook of Cristobal written during discussion #73...their was a southeastern Gulf of Mexico inverted upper trough retrograding westward around the eastern US upper ridge..and this inverted upper trough (marked as a blue-dashed line) is currently over southern Texas/northern Mexico. In addition...the vast upper anticyclone over Cristobal when it was a young storm has been split by the western Atlantic upper trough...with the western split evolving into a Gulf anticyclone as marked by a blue H. Split flow upper divergence between the norheast side of the inverted upper trough and Gulf upper anticyclone has been supporting intermittent bursts of disturbed weather over and offshore of Texas/Louisana and along the surface trough. However as outlined in the thermodynamic outlook of Cristobal written during discussion #73...this region was and still is dry as some dry air had collected below the inverted upper trough (and this dry air is still over the southern Gulf) and as dry air persits on the western convergent side of the western Atlantic upper trough. With the dry air and lack of computer model support...development is not expected along the Gulf surface trough.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Aug_26_2014_2045Z_zps76abb55a.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z and the 1930Z-released WPC analysis.

Features boxed in green...if any...are mentioned in the National Hurricane Center (NHC) traditional 48-hour outlook and or are considered an "Invest" on the Naval Research Laboratory site of the US Navy at the time the chart was generated. I do not box features in green if they are only included in the NHC's longer term 5-day outlook.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.

...THERMODYNAMICS BIRDSEYE CHART...
 photo Aug_26_2014_2045Z_Thermo_zps0fc21f05.png
This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

...SPECIAL FEATURE...HURRICANE CRISTOBAL...
Current prognosis...As of the 11 PM EDT National Hurricane Center advisory...the center of Hurricane Cristobal was located at 30.4N-71.5W...indicating Cristobal has tracked further to the left and a little faster than previous track guidance from 24 hours ago. My previous expectation during discussion #74 was Cristobal would lean a little more to the right in its northward track while consolidating below a strong storm canopy and associated small upper anticyclone that I was forecasting to form just southeast of the western Atlantic upper trough. Instead Cristobal and its strong thunderstorms have moved straight north directly into the upper trough...with the latent heat release of these storms punching out the upper trough such that it has weakened into two upper vortices as marked by a pair of blue Ls (one directly over Cristobal and another northeast of Cristobal). Remarkably Cristobal this past afternoon was able to maintain a ball of central thunderstorms while sliding below the southern upper vortex...perhaps suggesting the upper vortex was becoming so shallow that it was an aid to Cristobal instead of a suppressor. For instance shear has been reduced by the shallow upper vortex as its cyclonic flow matches the surface cyclonic flow of the storm...and widespread upper divergence between the north side of the upper vortex and west side of the upper ridge axis (marked by blue-zig-zag line to the southeast) has aided in widespread surface pressure falls supporting the additional strengthening and expansion of the wind field observed by aircraft reconnaissance. In recent hours since the 11 PM EDT NHC advisory...Cristobal has finally gained a more typical apperance of a tropical cyclone located below an upper vortex with the thunderstorms weakening and taking a broad ring shape around the center.

Atmospheric Outlook for the Forecast Period...The 1023 mb low-level ridge to the north will be knocked out in the next 24 hours by the 998 mb frontal cyclone and its vigorous shortwave upper trough currently racing east across eastern Canada and headed for the north Atlantic. This will cause Cristobal after 24 hours to rapidly accelerate east-northeast in deep-layered westerly flow south of the surface cold front trailing from the frontal cyclone and south of the frontal cyclone's vigorous upper trough. By the end of the forecast period...Cristobal is expected to transition into a large and strong non-tropical system supported by divergence on the southeast side of the upper trough....with the potential for the track to be rather fast and bend increasingly north when the back side of Cristobal's circulation pulls down the cooler air associated with the upper trough which makes the upper trough stronger/more amplified.

Thermodynamic Outlook for the Forecast Period...Cristobal has moved northward from waters of 30 deg C to waters that are 28 to 29 deg C. The updated faster forecast track shown below now predicts Cristobal will cross the 26 deg C isotherm into even cooler waters by Thursday. Dry air to the west of Cristobal remains strong due to strong upper convergence on the west side of the highly amplified western Atlantic upper trough Cristobal is currently interacting with. Perhaps thermodynamic parameters have become less favorable due to the above-noted arrival to lower sea surface temps and ingestion of some of this dry air...perhaps explaining why Cristobal has begun to struggle below the upper vortex as mentioned at the end of the above current prognosis section. These unfavorable thermodynamic parameters will no longer matter at the end of the forecast period since Cristobal's strength will be maintained by non-tropical processes by then.

 photo Aug_26_2014_H_Cristobal_Forecast_zps5be2780b.png

Track Forecast...Perhaps the 1023 mb low-level ridge to the north finally got a bit of influence on Cristobal as the storm has tracked a bit more leftward than yesterday's track guidance. Cristobal has also tracked a bit faster than previously forecast...causing me and the NHC to adjust the forecast track points to the north and west over the last 24 hours. I am a bit left of the 11 PM EDT NHC track forecast as Cristobal remains on a dead north course whereas the NHC forecast suggsested that Cristobal was supposed to be leaning north-northeast. I bend the track more northward than the NHC at the very end of the above-shown forecast in response to the likelihood of Cristobal amplifying an upper trough as mentioned at the very end of the above atmo outlook section. It is the presentation of the upper trough amplitude in model runs that caused me to bend the track more northward than the NHC at the end of the forecast period.

Intensity Forecast...I am below the 11 PM EDT NHC intensity guidance as it appears Cristobal is beginning to struggle with less favorable thermodynamic parameters and struggle below an upper vortex as mentioned in the last sentence of the above current prognosis section and as mentioned in the above thermodynamic outlook section.

Impact Forecast...The above-illustrated impact swath suggests that no land areas are forecast to received sustained winds of tropical storm force due to the forecast track...with the exception of southeastern Newfoundland which could experience tropical storm force winds on Friday morning. The swath is based on extrapolation of the 11 PM EDT NHC tropical storm wind field along my forecast track. Surf and rip currents will also be another effect of this storm as noted in impact statement (c) in the above forecast graphic.

...SPECIAL FEATURE...STRONG TROPICAL WAVE OVER WESTERN AFRICA...
Current Prognosis...Based on Africa satellite imagery...the tropical wave axis is estimated to be at 5W longitude with associated showers and thunderstorms covering eastern Guinea...eastern Sierra Leone...Liberia...Cote D'Ivoire...southwestern Burkina Faso...and the southwestern edge of Mali. The organization of the thunderstorms suggests the vorticity maximum of the tropical wave is located along 8N latitude. Comparing to satellite pictures 24 hours ago suggests the troipcal wave is moving at a brisk pace of about 12.5W longitude per day. However the track forecast below shows the westward pace slowing down to not stray too far from computer model runs which show a slower westward pace than this.

Atmospheric Outlook for the Forecast Period...The tropical wave is expected to take advantage of favorable tropical upper ridging (currently marked by blue-zig-zag in the lower-right in the atmo chart above). A central Atlantic upper ridge has been forming due to the latent heat release of Hurricane Cristobal and due to warm air advection ahead of a series of frontal cyclones supported by the western Atlantic upper trough. This central Atlantic upper ridge will be doubly enforced by warm air advection ahead of what is expected to be a large/vigorous non-tropical Cristobal later in the forecast period. A consequence of the central Atlantic upper ridge is that a tropical upper tropospheric trough (TUTT) has been consolidating in relatively lower pressures between the central Atlantic upper ridge and aforementioned tropical upper ridge. This system is expected to stay southeast of the unfavorable TUTT thru the forecast period.

Thermodynamic Outlook for the Forecast Period...This system will be tracking over favorable 28 to 29 deg C waters upon entry into the Atlantic from Africa. On the forecast track below...temps will decrease to the 27 to 28 deg C range by the end of the forecast period...but this is still in the favorable range. This system is expected to develop in the moist air presently seen in the above thermo chart southeast of the dry Saharan air layer...and if it strengthens into a strong enough tropical cyclone prior to approaching the dry Saharan air it could develop a moisture field that fights off the dry air.

24 Hr Forecast (0000Z August 28)...Tropical wave emerging from western Africa with a tropical low located near 8N-17.5W

48 Hr Forecast (0000Z August 29)...Organized Tropical low located near 9N-25W...or south of the Cape Verde Islands

72 Hr Forecast (0000Z August 30)...35 mph maximum sustained wind tropical depression centered near 10.5N-31W...or southwest of the Cape Verde Islands

96 Hr Forecast (0000Z August 31)...45 mph max sustained wind tropical storm storm centered near 11.5N-36W

120 Hr Forecast (0000Z September 1)...60 mph max sustained wind troipcal storm centered near 13N-41W

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