2013 Atlantic Hurricane Season Birdseye Discussion #18

By: NCHurricane2009 , 1:43 AM GMT on June 18, 2013

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...MONDAY JUNE 17 2013 9:45 PM EDT...
Caribbean tropical wave Invest 93-L has intensified into tropical depression two within the last 24 hours while moving into the western Caribbean Sea. The center has made landfall in southern Belize this past afternoon. See special feature section below for additional information on tropical depression two.

As promised in my final 2012 hurricane season birdseye discussion...I have begun to release post-storm reports for the 2012 Atlantic Hurricane Season which includes evaluations of how my storm forecasts (issued on these birdseye discussions) compared with that of the official forecasts from the National Hurricane Center. As these post-storm reports come out...they can be viewed on my other blog thread at www.wunderground.com/blog/MIHurricane2009.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...
 photo Jun_17_2013_2045Z_zpsfef9cf3b.png
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-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...
 photo Jun_17_2013_2045Z_zpsb56fb701.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...TROPICAL DEPRESSION TWO...
Tropical depression two has formed in the western Caribbean sea from tropical disturbance Invest 93-L...and has made landfall in southern Belize as of this past afternoon. See Figure 1 below for my forecast versus the NHC's as of this past afternoon/early evening. Visit www.nhc.noaa.gov for latest NHC forecasts and info on tropical depression two.

As the 12-hourly NHC recorded storm track in Figure 1 below shows...newly formed tropical depression two has taken a WNW track which is parallel to the isobars of the western extension of the paragraph P4 low-level ridge. Through the forecast period...computer models maintain low-level ridging with the same isboaric structure as a low-latitude upper ridge is expected to quickly build behind an Ohio/Tennesse Valley frontal cyclone as mentioned in paragraph P1. Therefore my track forecast philosophy is to simply maintain the current speed and heading of the current WNW track...which makes me a bit south of the NHC forecast track but generally keeping up with the forward pace in the NHC forecast track. Albeit...my forecast track beyond 48 hours is a bit faster than shown by NHC...perhaps due to the fact that computer models such as GFS build a blocking low-level ridge over Mexico and due west of the tropical depression (but while studying the simultaneous GFS upper wind forecast I see no source of upper convergence to support such a blocking low-level ridge...and therefore that is why I do not slow my forecast track beyond 48 hours). Note that midway in my forecast track I kink the circulation a bit more northward as I now believe their is a high probability that t-storms in the north half of the circulation will intensify over the Bay of Campeche when the system arrives there...with the circulation regenerating northward toward those t-storms. Previously I thought this system was going to track straight west and not emerge over the Bay of Campeche...but obviously now I have changed my mind given that the current track is more WNW rather than straight west.

The 12Z GFS model run from yesterday (shown in Figure 1b of my previous discussion) has done a good job of predicting the evolution of the Caribbean upper ridge...correctly predicting that the Honduras anitcyclonic center above this system would dissipate in the face of the paragraph P5 upper trough while also predicting that the dominate anticyclonic center would retrograde from northern South America to the waters south of Jamaica (indeed the blue H south of Jamaica in the above atmo chart confirms this). This has placed the tropical depression in southerly shear west of the Jamaica anticyclonic center and east of the paragraph P5 upper trough as evidenced by the thickest clouds being north of center. I was previously concerned the GFS model did not give enough credence to this system's t-storm latent heat release by dissipating the Honduras upper anticyclonic center...but by animating yesterday's 12Z GFS...the most recent GFS model run...and comparing to how well the runs match the current state of the upper atmosphere...I now trust the GFS has a good handle on what should happen. The GFS shows that the paragraph P5 upper trough should continue to get shoved off to the west perhaps due to warming provided by TD 2's t-storm latent heat release...but also due to the upper anticyclonic center south of Jamaica also getting displaced westward. The GFS shows a shortwave upper trough ejecting into the SE US by 72 hrs (originating from next mid-lat upper trough not yet in the scope of the above charts)...with a new Florida upper ridge forming in relatively higher pressures between the SE US shortwave and paragraph P1 upper vortex south of Bermuda. In turn...a fragment of that upper vortex retrogrades SW around the Florida upper ridge...which in turn is what pushes the upper anticyclone south of Jamaica to the west.

Therefore with the paragraph P5 upper trough moving westward and away from TD 2...and with the upper anticyclone south of Jamaica rapidly advacing westward over TD 2 thru the forecast period...low shear and enhanced upper outflow from the upper anticyclone keep TD 2's conditions favorable for development. Therefore the only inhibiting factor against strengthening will be land interaction. My intensity forecast in Figure 1 below assumes TD 2 will soon weaken to a remnant low as it advances further inland...and is below the NHC forecast which predicts re-development to a minimal tropical storm when it emerges over the Bay of Campeche (I prefer to keep it below tropical storm force (below 40 mph winds) as my more southerly forecast track has less exposure to Bay of Campeche water than the NHC's forecast track does). TD 2 will then make landfall again in the heart of Mexico...when it then ultimately dissipates.

 photo Jun_17_2013_TD_2_Forecast_zpsb3239c4f.png
Figure 1: Forecast for Tropical Depression Two

Current colorized infrared satellite shows a ball of convection a bit sheared northward from the center with an outer rain band along the west coast of the Yucatan peninsula. I predict that the ball will be generally persistent with TD 2 thru the forecast....and so my impact swath in Figure 1 is based on extrapolating that ball along my forecast track. It is entirely possible we will see outer rain bands fling outward from the ball and outside my impact swath region...but with the rainfall to be heaviest in the ball....I believe the most significant impacts with this sytem will be where my impact swath region is drawn.

...MID-LATITUDES DISCUSSION...
P1...East-west trowal of upper vorticity originating from western Canada is gaining a more SW-NE tilt as shown by blue-dashed line in upper-left corner of above atmo chart. The trowal is expected to soon swing east with its eastern divergence to soon support surface frontal cyclogenesis over the Ohio/Tennesse Valleys within the next 12 to 24 hours (in fact the 1012 mb surface low over NE Oklahoma in the above atmo chart appears to be the early phases of this cyclogenesis). Western convergence of this upper trowal supports 1022 mb surface ridge (also marked in upper-left corner of above atmo chart) that will be building rapidly behind the forecast Ohio/Tennessee valley surface frontal cyclogenesis. To the south of the trowal...former United States upper ridge has been reduced to a Texas upper ridge due to the swining in of this trowal. Meanwhile...a fragment of the upper trowal that dove around the upper ridge 10 days ago is now an upper trough and associated 1005 mb surface frontal cyclone zooming across the northeastern Atlantic and currently moving into western Europe. 5 days ago...yet another fragment of the upper trowal had dove around the upper ridge...where we now have an upper trough in the western Atlantic that has recently broken into an upper vortex south of Bermuda...a shortwave upper trough and attendant 1022 mb surface frontal depression SE of Newfoundland...and north-central Atlantic upper trough and attendant 982 mb surface frontal cyclone that has weakened to 992 mb as it has settled (like all mature frontal cyclones) directly below the upper trough axis where their is a lack of divergence.

P2...Upper anticyclone NW of the Cape Verde Islands...mentioned in paragraph P2 of the previous discussion...has reversed into a brand new upper vortex N of the Cape Verde Islands due to relatively lower pressures between upper anticyclone mentioned in paragraph P4 and eastern Atlantic low-latitude upper ridge mentioned in paragraph P6. Some of this new upper vortex's vorticity maybe a cut-off of the NE Atlantic upper trough mentioned in paragraph P1.

P3...Cut-off upper vortex persists in the open central Atlantic.

P4...Surface ridge dominates much of the open Atlantic basin. Its main center...currently 1034 mb...is now supported by eastern upper-level convergence of a north-central Atlantic upper anticyclone moving into the Azores. This upper anticyclone is a result of warm air advection ahead of the 992 mb frontal cyclone mentioned in paragraph P1 above. This surface ridge has had a westward extension into the Gulf of Mexico over the past days due to what is now southeastern convergence of what is now the Texas upper ridge mentioned in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Cut-off Upper trough over the Yucatan peninsula in the previous discusison has been pushed west into SE Mexico and the Bay of Campeche thanks to the warming provided by tropical depression two's latent heat release. As the above special feature section discusses in detail...expect this upper trough to continue to get pushed westward.

P6...Elsewhere in the Atlantic tropics...Caribbean Sea continues to be dominated by upper-level anticyclonic ridge in part associated with the outflow structure of Tropical Depression Two. Low-latitude tropical upper ridging meanwhile remains in the eastern Atlantic. Tropical wave east of the southern Lesser Antilles in the previous discussion is now crossing the southern Lesser Antilles into the southeastern Caribbean Sea tonight. It appears the tropical wave's t-storms have recently been enhanced by divergent northwesterly upper flow at the boundary between the Caribbean upper ridge (mentioned earlier in this paragraph) and relatively new upper vortex located south of Bermuda (mentioned in paragraph P1). As the above special feature section discussed in detail...expect a portion of the upper vortex south of Bermuda to have a southern fracture that will retrograde west across the Caribbean....and as a result this tropical wave will soon become suppressed as it tracks through the Caribbean.

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