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2013 Atlantic Hurricane Season Birdseye Discussion #38
By: NCHurricane2009 , 6:17 AM GMT on July 10, 2013
...WEDNESDAY JULY 10 2013 2:20 AM EDT...
Speeding bullet Chantal has crossed the Lesser Antilles into the eastern Caribbean after delivering tropical storm conditions...and this early morning is showing signs of opening into a tropical wave due to her fast foward pace. See Chantal special feature section below for details. In addition...pair of vigorous tropical waves behind Chantal have become less organized and the computer model support has pulled back some...see paragraphs P9 and P10 for updates on these waves.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1925Z-released WPC analysis.
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 CHANTAL...
My updated forecast versus the NHC's 11 PM EDT forecast is shown in Figure 1 below...and visit www.nhc.noaa.gov for latest NHC forecasts. Tropical storm watches and warnings are in effect for the Virgin Islands...Puerto Rico...the Dominican Repbulic...Haiti...and some of the Bahamian islands. The latest status of these advisories can also be found at www.nhc.noaa.gov.
Based on the shape of the of the NHC recorded storm track in Figure 1...it appears my previous forecast accurately predicted a westward bend in track as Chantal crested the axis of inverted upper trough near the Lesser Antilles. Now this inverted upper trough has weakened as mentioned in paragraph P7...Chantal is on a more westerly course parallel to the isboars of low-level paragraph P5 ridge as a shallower/weaker tropical cyclone should be doing. Some other important observations to make are that Chantal has increased in forward speed from about 25 to 30 mph in the last 24 hrs...which appeared to have caused winds in her north half to increase to 60 to 65 mph maximum sustained within the last 24 hrs while simulatenously causing her south half to lose wind speed and even open up such that Chantal may now instead be a tropical wave. The t-storm activity in the circulation has become weaker (resulting in max winds dropping to 50 mph as of 2 AM EDT) and disheveled looking...a symptom of a system that no longer has a precise center of surface convergence where t-storms can maintain their strength and organize about. Last reconnaissance plane mission from NHC struggled to find such a precise surface center as noted in the NHC 11 PM EDT advisory. Furthermore...hot-off-the-press CMC and GFS models suggest Chantal opening into a tropical wave (although CMC later suggests a strong tropical cyclone after the wave passes Haiti...but I am skeptical as the CMC has a tendency to overdo tropical cyclones).
With these observations...my forecast in Figure 1 now suggests Chantal will dissipate into a tropical wave later this morning. I also forecast the winds coming further down to 45 mph maximum later this morning as the weakening state of the t-storms means the flight-level winds will not be as effectively mixed down to the surface. My updated track forecast is a leftward shift from my previous to account for the fact that tonight's 11 PM EDT position was a bit southwest of where I previously predicted. My updated track is also left of the current NHC as the NHC recorded storm track clearly has a leftward angle with respect to their forecast track. My updated track still agrees with a northward curve into Hispaniola (but moreso into Haiti with my leftward adjustment) as the deep-layered nature of the paragraph P4 upper vortex & surface trough pair should be able to leverage some northward pull on Chantal regradless of whether or not she is tall/strong or shallow/weak. My forecast suggests Chantal will not later re-generate into a tropical cyclone while the system gets heavily disrupted by one-two punch of land interaction with Haiti's rough terrain and southwest vertical shear ahead of paragraph P4 upper vortex.
In the small chance Chantal does not degenerate into a tropical wave later this morning...I will issue a special update with a new forecast on Chantal.
Figure 1: Forecast for Tropical Storm Chantal
Impact swath in Figure 1 is initialized with the tropical storm wind radius shwown at the 11 PM EDT NHC advisory...which is biased to the right side of the storm track due to Chantal's current rapid WNW motion. I keep the swath the same size thru the forecast with the assumption Chantal remains the same size. I also keep the swath biased to the right of my forecast track thru the forecast period because of her expected fast track within my short forecast period. I dissipate the swath after 24 hours with the assumption the remnant wave of Chantal becomes so disrupted that it degenerates into an ordinary tropical wave rather than staying a vigorous one.
P1...Next upper trough and attendant surface 1003 mb frontal depression centered over southern Hudson Bay is entering the upper-left corner of the above charts from western Canada and northwestern US.
P2...Cut-off upper trough that previously moved into the Azores has been absorbed by NW Atlantic upper trough fragment coming in from the west and mentioned in paragraph P3 below. Remainder of upper troughing from this system is beginning to move offshore from the NE US in between NW Atlantic upper ridge and western US upper ridge...as well as over the Texas/Mexico area as a weakening inverted upper trough to the south of the western US upper ridge.
P3...Upper trough and surface frontal system with a 1009 mb surface center is moving eastward across the high seas between Canada and Greenland (western convergence of this upper trough system supports 1023 mb surface ridge between Nova Scotia and Newfoundland...and in the last 72 hrs a fragment of this upper trough has split southeastward while moving across the NW Atlantic and now into the north-central Atlantic approaching the Azores...with its eastern divergence supporting a surface trough attached to a 1016 mb low ESE of Newfoundland as well as a southwest fragment of it becoming a small upper vortex SE of the paragraph P2 NW Atlantic upper ridge). Low-level warm air advection ahead of the 1009 mb center is supporting an amplifying upper ridge east of Greenland just outside of the above charts. Upper trough in the NE Atlantic high seas in paragraph P3 of the previous discussion as a result remains a weakened NW-SE tilted upper trough within this upper ridge and due north of the Azores. Along the lengthy front extending south and west of the 1009 mb center is a 1013 mb frontal depression that has moved from Lake Michigan and into Lake Superior in the last 24 hrs...and this depression is supported by split flow upper divergence between northerlies on the east side of the paragraph P2 western US upper ridge and mainstream mid-latitude westerlies.
P4...Upper vortex persists over the western Bahamas. Its eastern upper divergence...as well as divergence between northerlies on its west side and easterlies on the south side of the western US upper ridge...are generating showers and t-storms in the western Bahamas and Florida. This divergence during the previous discusison appeared to at least trigger mid-level pressure falls...and now it appears the pressure falls have made it to the surface with NHC TAFB declaring a surface trough moving into Florida in its recent surface maps (this surface trough may also be associated with fracture of north end of paragraph P8 tropical wave). Despite these developments...the CMC computer model has stopped showing subtropical or tropical cyclone development from this system...and other computer models continue to not support development of this system.
P5...Surface ridge dominates much of the open Atlantic basin. A western 1026 mb center over Bermuda is supported by convergence between easterlies south of the paragraph P2 NW Atlantic upper ridge and southwesterlies ahead of the paragraph P4 upper vortex. This surface ridge continues to have a northeastern lobe extending into western Europe supported by eastern convergence of upper ridge east of Greenland mentioned in paragraph P3.
...TROPICAL BELT DISCUSSION...
P6...Inverted upper trough persists over SE Mexico...as well as parts of the Gulf of Mexico...while persisting in relatively lower pressures south of the western US upper ridge mentioned in paragraph P2.
P7...Anticyclonic upper ridging in the eastern half of the Atlantic persists and remains expanded into the eastern two-thirds of the Caribbean (the enhanced poleward outflow at the west lobe of the Caribbean portion supports t-storms in the south-central Caribbean and Central America area). This upper ridging still has an upper anticyclone SE of the Azores. This upper ridging also continues to have a low-latitude axis present over the Cape Verde Islands extending into W Africa...with the west end of this upper ridge axis inflated into an anticyclone due to recent latent heat release of Chantal's t-storms (eastern convergence of this upper anticyclone is supporting sinking motion that re-enforces Saharan dry air midway between the Cape Verde Islands and Lesser Antilles). Chantal's recent latent heat release has also weakened inverted upper trough near the Lesser Antilles...and pushed some of the vorticity of this inverted upper trough westward into the south-central Caribbean such that the Caribbean upper anticyclonicity has split into one anticyclone E of Nicaragua and another anticyclone north of Puerto Rico. Embedded SW-NE oriented string of upper vorticity persists over W Europe's Iberia peninsula...through the Canary Islands...and into the waters NW of the Cape Verde Islands. Days ago the south end of all the upper ridging mentioned in this paragraph (in conjunction with the south end of the paragraph P5 surface ridge) advected dry Saharan air from Africa...large swaths of which still remain as shown by brown shading in the above thermo chart. Since this upper ridging developed a low-latitude axis over the Cape Verde Islands that created upper outflow and resulting moistening lift over the ocean surface...the supply of dry Saharan air has ended as indicated by the current shades of white and blue toward Africa in the lower-right of the above thermo chart (see paragraph P7 of discussion #32 for development of low-latitude upper ridging over the Cape Verde Islands).
P8...Tropical wave previously over the western Caribbean has moved into the Yucatan peninsula and is becoming suppressed by paragraph P6 upper vorticity.
P9...As mentioned in paragraph P7...west end of low-latitude upper ridge axis has inflated into an upper anticyclone. Tropical wave souhtwest of the Cape Verde Islands has become less organized while suppressed by dry Saharan air midway between the Cape Verdes and Lesser Antilles re-enforced by convergent east side of the anticyclone also mentioned in paragraph P7. The tropical wave is also becoming suppressed by new inverted upper trough forming in relatively lower pressures southeast of this anticyclone.
P10...Tropical wave rolling off Africa in the previous discusison has been added to NHC TAFB maps in last 24 hrs at a location SE of the Cape Verde Islands. Its t-storm mass has become less organized and sheared southwestward from the wave axis by the easterlies on the south side of the low-latitude upper ridge axis over the Cape Verde Islands mentioned in paragraph P7...but should the latent heat release of the t-storms locally inflate the upper ridging the upper flow directly over the tropical wave could gradually become more anticyclonic and less shearing. In the event this tropical wave shows any signs of becoming better organized under less shearing...further development could become possible.
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.