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2013 Atlantic Hurricane Season Birdseye Discussion #37

By: NCHurricane2009 , 7:55 AM GMT on July 09, 2013

...TUESDAY JULY 9 2013 3:55 AM EDT...
The Atlantic tropics are escalating with Tropical Storm Chantal to strike the Lesser Antilles later this morning...afterwards threatening Hispaniola (Haiti and the Dominican Republic) for a Wednesday strike. Chantal is likely to affect the Bahamas Thursday and Friday...and the southeastern United States coast by early next week. See Chantal special feature section for additional details on the tropical storm. In addition...watching a pair of vigorous tropical waves behind Chantal which are in the same favorable enviornment from which Chantal emerged and which are gaining computer model support...see paragraphs P10 and P11 for details. Should any of these waves become organized...I will be quickly upgrading them to special features on this blog.

Of note...it is unusual to have tropical cyclone activity east of the Lesser Antilles in early July. This indicates the potential for an active hurricane season ahead come August and September where a series of strong tropical waves emerging from Africa develop in the eastern open waters into potentially strong tropical cyclones.

 photo Jul_8_2013_2045Z_zpsd45f5309.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1917Z-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.

 photo Jul_8_2013_2045Z_zps7f57d1dd.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).

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 have been raised for various islands in the Lesser Antilles...Virgin Islands...Puerto Rico...the Dominican Repbulic...and Haiti. The latest status of these advisories can also be found at www.nhc.noaa.gov.

My short-term outlook continues to be focused on inverted upper trough just west of Chantal located between Caribbean upper anticyclone and low-latitude upper ridge axis mentioned in paragraph P8. Rather than tracking parallel to the isobars of low-level paragraph P5 ridge (as convetional wisdom for shallow/weak tropical storm dictates)...Chantal has tracked more northward than I thought at an angle against the isobars...perhaps while also steered by east side of the inverted upper trough (even though the inverted upper trough does not go below 500 mb as shwon in GFS model initialization). It appears Chantal's strengthening and t-storms have been supported by split flow upper divergence between southerlies west of the low-latitude upper ridge axis and eaterlies flowing across the inverted upper trough...with the t-storm latent heat release expanding the west extent of the low-latitude upper ridge and pushing off the inverted upper trough to the west. I expect that the inverted upper trough will stop moving west while jamming against the Caribbean upper anticyclone...forcing Chantal to pass below the inverted upper trough later today. As such...I forecast the track to remain WNW for the first 12 hrs...then bend more westward parallel to the forecast isboars of the paragraph P5 low-level ridge between 12 and 24 hrs after Chantal crests the axis of the stalling inverted upper trough. I keep the forward speed the same thru 24 hrs with the low-level ridge maintaining current strength. Chantal shows recent signs of struggling with a lack of t-storms in her NW half as she begins passing below the inverted uper trough and whatever Sahran dry air has collected belwo the upper trough. Therefore I show no additional strengthening for first 24 hrs...then show some brisk strengthening below favorable low shear and enhanced outflow of Caribbean upper anticyclone afterwards.

As we approach 48 hrs...I agree with a gradual bend toward the north into Hispaniola (Haiti and the D.R.) initiated by east side of paragraph P4 upper vortex which has vorticity down to the mid-levels capable of grabbing Chantal even if she never becomes stronger/taller (I also show a slight slowdown by 48 hrs with the paragraph P5 ridge weakening in advance of paragraph P1 weather system). Between 48 and 96 hrs...I generally agree with the NHC curving track into the Bahamas based on the GFS model's shape of the paragraph P5 ridge's western isobars as it continues eroding in advance of the paragraph P1 weather system. I also agree with a slowing track by 96 hrs due to blocking effect of a surface ridge due north forecast to develop below western convergent side of the north split of the paragraph P1 upper trough...but my Friday night position is a bit north of the NHC's based on the GFS's position of the blocking ridge which still shows enough room in my opinion for Chantal to still advance more northward. By Saturday...the southern split of the paragraph P1 upper trough looks to have evolved into a deep-layered vortex that tries to push Chantal north against the blocking surface ridge...but I think some westerly component in the track will develop on Saturday as the blocking ridge is shown in GFS to also intensify during that time.

Intensity forecast after Chantal moves into Hispaniola (Haiti and the D.R) is uncertain. My best guess is Chantal will weaken rapidly on Wed/Thu due to the combo of mountainous terrain and westerly vertical shear southeast of the paragraph P4 upper vortex. I show no additional weakening on Friday...modeling Chantal as a storm below the east side of the upper vortex where upper southerlies are more aligned & less shearing with respect to the forecast NNW track. By Saturday...I suggest some re-strengthening as Chantal could end up on the south reaches of an outflow enhancing/shear reducing upper anticyclone located in relatively higher pressures between the paragraph P4 upper vortex....forecast southern fracture of paragraph P1 upper trough...and another forecast cut-off upper vortex coming in from the east (associated with paragraph P3 NW Atlatnic upper trough fragment).

 photo Jul_09_2013_TS_Chantal_Forecast_zps49d344c1.png
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 (albeit I grow the swath a tad based on when I think she will reach max intensity). I also keep the swath biased to the right of my forecast track thru the forecast period because of her continued fast track early on...followed by the paragraph P4 upper vortex suppressing t-storms in her west half after she slows down later on. Note that at the end of my forecast (Saturday night)...my impact swath suggests Chantal will wrap in t-storms biased to the east as she escapes the paragraph P4 upper vortex and moves into southern reaches of more favorable upper anticyclone.

P1...Next upper trough and attendant surface 1006 mb frontal depression is entering the upper-left corner of the above charts from western Canada and Montana.

P2...Cut-off upper trough continues slowly east and has finally moved into the Azores and the waters southwest. Mid-ocean surface trough formerly supported by eastern divergence of this cut-off has been absorbed into 1012 mb surface low's trough mentioned in paragraph P3. Remainder of upper troughing from this system is over the NE US in between NW Atlantic upper ridge and western US upper ridge...as well as over the Texas/Mexico area as an upper vortex due south of the western US upper ridge.

P3...Upper trough and surface frontal system over NE Canada with a 1004 mb surface center is moving into the high seas between Canada and Greenland (western convergence of this upper trough system supports 1023 mb surface ridge centered over southern Hudson Bay...and in the last 48 hrs a fragment of this upper trough has split southeastward about the northeast side of the paragraph P2 NW Atlantic upper ridge and into the NW Atlantic where its eastern divergence supports a surface trough attached to a 1012 mb low SE of Newfoundland). Low-level warm air advection ahead of the 1004 mb center is supporting an amplifying upper ridge east of Greenland just outside of the above charts. Upper trough moving into the NE Atlantic high seas in paragraph P2 of the previous discussion as a result has weakened into an 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 1004 mb center is a 1010 mb frontal depression that has moved from Minnesota and into Lake Michigan 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 previously over the eastern Bahamas has retrograded westward into the western Bahamas while steered around the paragraph P2 NW Atlnatic upper ridge. As paragraph P9 highlights...upper divergence on the east side of this upper vortex is interacting with the north end of a surface tropical wave to produce storm activity in the vicinity of the Bahamas...the immensity of which caused this upper vortex to be breifly an area of interest in the NHC tropical weather outlook this past afternoon. This eastern upper divergence has atleast allowed for mid-level pressure falls with the current GFS model initialization showing vorticity as low as the 700 mb layer. The CMC computer model continues to develop a subtropical or tropical cyclone from this upper vortex as it retrogrades into the eastern Gulf of Mexico by 72 hrs. Still taking this CMC solution superficially at this time with no other models on board and with no signs of surface pressure falls.

P5...Surface ridge dominates much of the open Atlantic basin. A western 1025 mb center over Bermuda is supported by convergence between easterlies south of the paragraph P2 NW Atlantic upper ridge and southerlies on the east side of the paragraph P4 upper vortex. An eastern 1025 mb center SW of the Azores is supported by western convergence of paragraph P2 Azores cut-off upper trough. 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.

P6...T-storm activity below Gulf of Mexico upper ridging has considerably diminished in the last 24 hrs. Expect the upper ridging will become squaushed out by paragraph P4 upper vortex coming in from the east.

P7...Inverted upper trough previously in the western Caribbean is now over SE Mexico while persisting in relatively lower pressures south of the upper ridging mentioned in paragraph P6.

P8...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 an increase in south-central Caribbean t-storms in the last 72 hrs). This upper ridging still has an upper anticyclone that has shifted east to a location 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. 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 this upper ridging (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).

P9...Tropical wave previously over the north-central Caribbean is now moving into the western Caribbean. North end of this wave is interacting with eastern divergence of paragraph P4 upper vortex to produce activity in the vicinity of the Bahamas.

P10...Tropical wave previously south of the Cape Verde Islands is now southwest of the islands. It continues to have impressive storm activity while taking advantage of enhanced outflow of paragraph P8 low-latitude upper ridge axis over the Cape Verde Islands...as well as the moisture resurgence mentioned in the the latter part of paragraph P8. The storm activity appears to be sheared westward by the easterlies on the south side of the low-latitude upper ridge...but should the latent heat release of the t-storms locally inflate the upper ridging the upper flow directly over the tropical wave could become more anticyclonic and less shearing. If this tropical wave shows any signs of becoming better organized under less shearing...I will be upgrading it to a special feature on this blog.

P11...Satellite imagery suggests yet another impressive tropical wave is about to roll off Africa as marked in the lower-right of the above atmo chart. The storm activity appears to be sheared westward by the easterlies on the south side of the low-latitude upper ridge axis over the Cape Verde Islands mentioned in paragraph P8...but should the latent heat release of the t-storms locally inflate the upper ridging the upper flow directly over the tropical wave could become more anticyclonic and less shearing. If this tropical wave shows any signs of becoming better organized under less shearing...I will be upgrading it to a special feature on this blog.

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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2. WunderAlertBot (Admin)
6:17 AM GMT on July 10, 2013
NCHurricane2009 has created a new entry.
1. nigel20
9:48 PM GMT on July 09, 2013
Thanks NCH!
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