2014 Atlantic Hurricane Season Birdseye Discussion #74

By: NCHurricane2009 , 5:27 AM GMT on August 26, 2014

...TUESDAY AUGUST 26 2014 1:30 AM EDT...
Cristobal has intensified into the third hurricane of the 2014 Atlantic season earlier than expected this evening. The weather associated with Cristobal has finally lifted northward from the eastern Bahamas after dumping a few inches of rain over the islands...and now Cristobal is expected to accelerate northeastward over the western Atlantic. Because of its tendency to reform south and east toward the heavy thunderstorms sheared off to the south and east of the center...the forecast track guidance is now east enough such that Bermuda has issued a tropical storm watch. 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 midway between the Cape Verde Islands and Lesser Antilles is less organized than 24 hours ago and is struggling with the dry Saharan air to its north detected in the thermodynamics chart below. However their is continued attention from the NRL and NHC because conditions are generally favorable for development as the dry Saharan air layer is weaker relative to earlier this month and because this system will remain below favorable tropical upper ridging (currently marked by blue Hs in the eastern Atlantic in the atmospheric features chart below). The CMC model has withdrawn its support for developing this system in the vicinity of the Lesser Antilles in the timeframe that is now 96 hours away...instead developing it much later after crossing the Caribbean and into the Bay of Campeche (the ECMWF...or Euro...model also showed a similar solution today). However such a long range solution is unreliable. However the NAVGEM does suggest some development near the Lesser Antilles...and while the GFS also suggests some evolution near the Lesser Antilles it is only in the form of a vigorous tropical wave. I prefer to not upgrade this system to a special feature on this blog until their is more shorter range computer model support...or until this system shows signs of organization.

A vigorous tropical wave with widespread showers and thunderstorms over west-central Africa has been forecast to quickly develop when it reaches the eastern tropical Atlantic by the last three days worth of GFS computer model runs...predicted to occur in about 5 days. The Euro (ECMWF) meanwhile shows no development...while the CMC and NAVGEM show a broad tropical low as this system enters the Atlantic. I prefer not to upgrade this system to a special feature on this blog until we see how organized the wave is as it nears the west coast of Africa or until we see more computer models supporting the GFS solution. However interests in the Cape Verde Islands should monitor the progress of this tropical wave in case its circulation consolidates at a high enough latitude such that it passes over the islands instead of south of the islands.

 photo Aug_25_2014_2045Z_zps2fb011fd.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z and the 1922Z-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.

 photo Aug_25_2014_2045Z_Thermo_zpsf2e14018.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).

Current prognosis...As of the 11 PM EDT National Hurricane Center advisory...the center of Hurricane Cristobal was located at 25.1N-71.9W. In the past day Cristobal has been under northwesterly shear on the back side of the western Atlantic upper trough marked by a blue-dashed line north of Cristobal in the above atmospheric features chart. This caused the once-broad circulation of the storm to regenerate southeastward into intense storm bursts that have been sheared off...and with the center showing a more tight definition in satellite imagery additional reformation of the center is becoming less likely. The impressive intensity of the storm bursts appears to be due to very high upper-level divergence as the northwesterly upper flow over the storm splits while some of the flow turns westward across the south side of the eastern US upper ridge (marked by blue H and blue zig-zag lines in the above atmo chart) and the rest of the flow turns northeastward ahead of of the western Atlantic upper trough. Even though Cristobal doesn't look like a traditonal hurricane due to the presence of northwesterly shear across the storm...it appears the high amount of overhead upper divergence has allowed its surface pressures to fall enough to become a hurricane anyway as indicated by recon aircraft measurements.

Atmospheric Outlook for the Forecast Period...Northwesterly vertical shear induced by the western Atlantic upper trough has the potential to relax in the next 24 hours as the high intensity of Cristobal's thunderstorm mass maybe releasing enough latent heat to produce upper-level warming for the formation of a small upper anticyclone just southeast of the upper trough. Now that Cristobal is a strong/tall tropical cyclone...I currently expect Cristobal to track slowly north-northeast early in the forecast period while feeling the mid to upper flow on the east side of the western Atlantic upper trough. By second half of the forecast period...the 1003 mb frontal cyclone and its vigorous shortwave upper trough currently over south-central Canada will charge east across eastern Canada and into the north Atlantic...which will knock out the 1023 mb ridge to the north such that Cristobal rapidly accelerates 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 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 circualtion 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 is currently over warm watrers in the 30 deg C range very supportive for additional strengthening in the early part of the forecast. This system will be tracking over waters above 26 deg C until Friday...when its expected to cross the 26 deg C isotherm into cooler waters. Dry air to the northwest of Cristobal has intensified as shown in the above thermodynamics chart...I believe in part due to the western Atlantic upper trough becoming more amplified due to the formation of an anticyclone over Cristobal immediately to the southeast of the upper trough (as discussed in the early part of the above atmo outlook section). With the upper trough more amplified...this promotes stronger upper convergence/sinking motion on the back side of the upper trough...hence the strengthening of the dry air. However I expect Cristobal to maintain strong storm activity with the support of the aforementioned anticyclone and instability brought on by the warm sea surface temperatures such that I do not expect this dry air to be an issue early in the forecast period...and by late in the forecast period the dry air will be irrelevant since Cristobal's strength will be maintained by non-tropical processes.

 photo Aug_25_2014_H_Cristobal_Forecast_zpsb452971f.png

Track Forecast...The continual eastward component of Cristobal's center reformation (as opposed to a pure southward or southwestward reformation) means that the storm no longer has the chance to bend west in track along the south side of the low-level ridge to the north before this ridge gets knocked out by 48 hours by the frontal cyclone mentioned in the above atmo outlook section. Therefore my updated track forecast above is a big rightward adjustment from my previous...and supports the idea of a north-northeast track turning and accelerating to the east-northeast as detailed in the above atmospheric outlook section. My updated track is a bit to the right of the NHC's 11 PM EDT as satellite suggested some additional eastward drifting of the center as it continues to sneak/reform beneath the strong thunderstorm canopy to the southeast. My updated track also remains behind the NHC's as Cristobal's southastward center reformation that has been going on over the last 24 hours has caused it to be behind schedule in regards to northward progression displayed in previous NHC forecasts (and yet the current NHC forecast shows the same rate of northward progression shown previously).

Intensity Forecast...Forecasting Cristobal to strengthen a bit more than the NHC showed at 11 PM EDT due to warm sea-surface temps in the 30 deg C range and with the expectation Cristobal forms a small shear-reducing/outflow enhancing upper anticyclone that covers at least the eastern half of the storm (details regarding this upper anticyclone mentioned in the early part of the above atmospheric outlook section). Later in the forecast period (Thursday and onward) I become below the 11 PM EDT NHC intensity guidance as I believe the westerly shear it encounters on the south side of the frontal cyclone's upper trough will be too high to support anything at hurricane force by Thursday. The rate of weakening after Thursday is shown to be slow as Cristobal transitions into a non-tropical entity supported by upper divergence on the southeast side of the frontal cyclone's upper trough.

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 Bermuda which is right on the edge of the swath. It is therefore wise for Bermuda to be under a tropical storm watch as any additional rightward shift in track would allow Bermuda to be in the swath...which would mean a brief period of tropical storm force winds on Wednesday. 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.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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3. WunderAlertBot (Admin)
7:47 AM GMT on August 27, 2014
NCHurricane2009 has created a new entry.
2. NCHurricane2009
5:54 AM GMT on August 26, 2014
Quoting 1. HurricaneAndre:

Very good blog, lots of great information.

Thanks very much! It took me longer to get this one out because I came home late today (was test driving a sports car was thinking about buying)....
Member Since: December 31, 1969 Posts: Comments:
1. HurricaneAndre
5:51 AM GMT on August 26, 2014
Very good blog, lots of great information.
Member Since: December 31, 1969 Posts: Comments:

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