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By: NCHurricane2009 , 6:57 PM GMT on August 03, 2014
...SUNDAY AUGUST 3 2014 3:05 PM EDT...
Tropical Storm Bertha is moving across the eastern Bahamas...and is expected to turn north and then northeast across the open northwestern Atlantic over the next few days while intensifying into a strong tropical storm or possibly a category 1 hurricane. Visit www.nhc.noaa.gov for the lastest watches and warnings with this system. See special feature section below for additional details on Bertha.
A surface trough formed over the western Bahamas yesterday with the support of split flow upper divergence between a Tropical Upper Tropospheric Trough (TUTT) upper vortex to the east and western Caribbean TUTT upper vortex to the southwest. The surface trough has moved westward into the Florida peninsula...but is producing a thunderstorm flare up just offshore of southeastern Florida in a favorable low shear-enhanced outflow environment beneath a small upper anticyclone that has developed between the two weakening TUTT upper vortices. Even though radar imagery is showing some rotation in the showers and thunderstorms offshore of southeastern Florida...the rotation is not showing signs of consolidation. Therefore it is currently assumed that this system is not developing fast enough to become a tropical cyclone before becoming assimlated in the next 48 hours into the approaching low pressure field of Tropical Storm Bertha to the east and frontal boundary to the north.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z and the 1317Z-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...
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...TROPICAL STORM BERTHA...
Current prognosis...As of the 2 PM EDT NHC advisory the center of Tropical Storm Bertha was located at 22.7N-72.6W...beginning to exit the eastern Bahamas and into the open western Atlantic while bending on an increasingly northward track. The thunderstorm activity of Bertha remains biased to the east side of the circulation center as this system continues to deal with westerly vertical shear imparted by the weakening upper vorticity associated with the Tropical Upper Tropospheric Trough (TUTT) mentioned in the atmo outlook section below...and also imparted by the upper anticyclone of the disturbance offshore of southeastern Florida. Ever since leaving the Dominican Repbulic early this morning...satellite imagery suggests this system is a bit better organized with the thunderstorm activity looking less blobbed and a bit more banded.
Atmospheric Outlook for the Forecast Period...This system will be taking an arcing track around the west side of what is now the 1024 to 1027 mb Atlantic surface subtropical ridge marked in the above atmospheric features chart. By connecting the western Caribbean blue L...central Atlantic blue L...and blue dashed lines north and south of Bertha in the above chart...a Tropical Upper Tropospheric Trough (TUTT) that is breaking apart spans the Caribbean to the central Atlantic. In the next 24 hours as the weakening TUTT upper troughs (blue dashed lines) north and south of Bertha dissipate...Bertha will find herself in a more favorable environment below a shear-reducing and outflow enhancing upper ridge (currently this upper ridge is present as marked by a blue H offshore of the eastern US). Towards the end of the forecast period...the eastern North America upper trough (marked by blue dashed line in the above atmo chart) will produce a surface frontal cyclone over southeastern Canada...with Bertha curving east-northeast between the north side of the surface Atlantic subtropical ridge and south side of the frontal cyclone. The track will then bend more northward around the east side of the frontal cyclone. The 990 mb frontal cyclone and upper trough southeast of Greenland in the above atmo chart is expected to evolve into a deep-layered cyclone in the northeast Atlantic during the forecast period...with upper convergence on the west side of the cyclone supporting a north Atlantic surface ridge to the east of Bertha that could also help deflect the track of Bertha more northward. Then at the very end of the forecast period...a non-tropical Bertha will bend more eastward in track while breaking through the north Atlantic surface ridge while becoming pulled into the northeast Atlantic deep-layered cyclone.
Thermodynamic Outlook for the Forecast Period...This system will track over 29 to 30 deg C water early in the forecast period. By 72 hours...if this system follows the forecast track shown below...this system will cross the sharp temperature gradient of the north wall of the Gulf stream and into waters below 26 deg C. This will encourage the weakening of this system as a tropical entity beyond 72 hours.
Track Forecast...Bertha is tracking very well with my previous track forecast shown in discussion #52. Moreover the NHC track forecast in the last 24 hours has become aligned closer to my previous...and therefore my updated forecast track below is a copy-paste of my previous with the exception of the 8 AM Thursday position which is further east. This is because computer models late in the forecast period now suggest Bertha's vigorous circulation will pull down some of the cool air associated with the eastern North America upper trough...which will form a shortwave upper trough and produce a tighter upper-level pressure gradient between the forecast shortwave and upper ridging to the south. A stronger upper-level pressure gradient will cause stronger upper-level winds that will push Bertha on a faster track as she becomes non-tropical...which is why I moved my forecast point for 8 AM Thursday further east. My forecast track line towards the end of the forecast is east-northeast...with a bend towards the north...and then a bend back to the east-northeast at the very end due to the forecast influences mentioned at the end of the above atmo outlook section.
Intensity Forecast...Because Bertha did not degenerate into a tropical wave...I have now bumped up my intensity forecast close to the NHC's most recent which suggests Bertha will breifly become a category 1 hurricane. I keep the intensification very slow for the next 24 hours as Bertha will have to deal with some westerly shear from the upper anticyclone of the tropical disturbance currently offshore of southeastern Florida. I then show more rapid strengthening as the disturbance is expected to have dissipated while becoming absorbed into the west side of Bertha's low pressure field and frontal boundary associated with the eastern North America upper trough. I forecast Bertha to become a hurricane a little earlier than when the latest NHC forecast shows...because I expect Bertha to begin weakening by 8 AM Wednesday while she crosses into cooler waters and begins to deal with westerly vertical shear on the north side of the upper ridge. However the weakening will be on the more gradual side as Bertha becomes a non-tropical cyclone supported by upper divergence on the east side of a forecast shortwave upper trough mentioned in the above track forecast section.
Impact Forecast...Impact swath in the above forecast graphic is extrapolation of the 11 AM EDT NHC tropical storm wind radius along my forecast track. Flooding from rainfall should not be an issue in the eastern Bahamas since the storm field of Bertha is relatively small and weak and since Bertha should remain on a brisk track...although isolated flash flooding can't be ruled out from a heavy shower. Sea swells early in the forecast period should not be much of an issue as Bertha is expected to be weak...but become more of a concern in the storm vicinity later in the forecast period if Bertha strengthens as forecast in the western Atlantic.
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