2012 Atlantic Hurricane Season Birdseye Discussion #70
...AUGUST 3 2012...7:15 AM EDT...
Tropical depression five has strengthened into Tropical Storm Ernesto during the previous 24 hours...and is now crossing the Lesser Antilles into the Caribbean Sea. Ernesto is expected to strengthen further...and interests in the Caribbean Sea should continue monitoring this system very carefully. See first special feature section below for further details.
Tropical wave rolling off of Africa has quickly organized over the last several hours. See second special feature section below for further details on this system.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-released HPC 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 ERNESTO...
Tropical depression five became Tropical Storm Ernesto of 50 mph max sustained winds just before crossing the Lesser Antilles...and is now weakening to 45 mph as it crosses the islands into the Caribbean Sea. As advertised in the previous discussion...Ernesto is beginning the delicate process of having its upper anticyclonic outflow merge with the anticyclonic cell of the paragraph P6 tropical wave (located to the NW). The tropical wave's upper anticyclonic cell is blocking outflow and hence limiting t-storm activity in the NW half of Ernesto. This has allowed dry air mentioned in paragraph P4 to work its way into the NW half of the storm. The strength of Ernesto's upper anticyclonic outflow is keeping the E Caribbean inverted upper trough (paragraph P5) at bay to the west...but still watching carefully to see if Ernesto catches up to the inverted upper trough in which case vertical shear out of the south would also become a problem for the storm.
My intensity forecast is shown in Figure 1. Thru today and into tomorrow morning...I expect Ernesto to slightly weaken further from the unfavorable effects described above. Beyond that...I expect Ernesto's upper anticyclone to have finished merging with the anticyclonic cell to the NW...which would create a more symmterical and robust upper outflow environment over Ernesto that would squash out the E Caribbean inverted upper trough and dry air. My intensity forecast toward the latter part of the forecast is dampened as I expect possible southerly vertical shear from the paragraph P3 upper vorticity...which the GFS model sluggishly retrogrades out of the way. However...I think the center of Ernesto would be in split flow upper divergence between its upper anticyclonic cell and this upper vortex...and also have plenty of eastern upper outflow dump into the anticyclonic cell...so I still show a slow strengthening rate by then. My intensity forecast by the end of 5-days in Figure 1 is a category 2 hurricane.
Trackwise...I see no reason to disagree with the steady west-to-WNW track shown by NHC for the next three days...given there will be deep-layered easterly flow on the south sides of the W Atlantic upper ridge (paragraph P1) and Atlantic surface ridge (paragraph P4). Upper trough over W Canada previous discussion is about to enter the upper-left corner of above atmo birdseye chart. Based on model presentations of this upper trough (and another upper trough that digs right behind it from Hudson Bay)...it now appears it will erode the Gulf portion of the Atlantic surface ridge beginning day 3 and later. However...there will be another surface ridge supported by convergence behind the upper trough...so this surface ridge in conjunction with the Atlantic surface ridge would create more of a NW track rather than a total turn to the north. I am a little north of the NHC forecat track in Figure 1 by day 5 as I am expecting a vigorous hurricane that is deep-layered enough to feel some steering influence from the paragraph P3 upper vortex.
Blue-dashed impact swath in Figure 1 is initially based on tropical storm wind radius shown at 5 AM EDT NHC advisory...coupled with the small t-storm canopy presently over and just south of the center. The swath is then drawn generally symmetric about the forecast track in a gradually growing size to represent the strengthening storm I forecast. Notice the swath becomes increasingly biased to the north of my forecast track by the end....to represent possible southerly vertical shear from the paragraph P3 upper vortex later on. Interests in the Caribbean Sea should monitor this system very carefully.
Figure 1: Forecast for Tropical Storm Ernesto this morning.
...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Tropical wave currently south of the Cape Verde Islands has been well-organized since leaving west coast of Africa yesterday. Its t-storm driven latent heat release has been mainly south of the paragraph P5 upper ridge axis...which has locally increased the speed of upper easterly winds as it tries to locally inflate this upper ridge. This has slightly increased the easterly vertical shear over the tropical wave...as evidenced by the organized t-storm bands taking a westward bias in satellite pictures. If the strong t-storms continue to locally inflate the upper ridge...then the upper flow over the tropical wave will become less easterly and more anticyclonic...making tropical cyclone formation more likely.
Over the next 5-days...mid-ocean upper vortex mentioned in paragraph P2 will be the inhibiting factor for development. GFS model 200 mb (upper) wind forecast suggests the upper vortex may stay parked long enough to begin imparting unfavorable southwesterly shear on this tropical wave. After 5-days...the upper vortex is forecast by GFS to lift out while it re-links with the mid-latitude westerlies...returning upper winds to a favorable state for tropical development. At this time...no computer models develop this tropical wave into a tropical cyclone.
P1...Longwave upper trough regime continues across eastern North America and Atlantic high seas...with low-level warm air advection ahead supporting a west Atlantic upper ridge...while another upper ridge builds behind over the SW US. Deep-layered low remains parked just offshore of the British Isles (upper-left corner of above atmo birdseye chart). Surface ridge E of Atlantic Canada has strengthened from 1020 to 1026 mb while supported by upper convergence on the back side of deep-layered low. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1002 mb frontal depression over the central US...while 1004 mb depression has accelerated from south-central Canada to E Canada while becoming supported by divergence on the east side of longwave upper trough regime. Upper convergene behind the longwave supports a 1015 mb surface ridge just SW of Hudson Bay. Frontal depression approaching Azores has amplified its supporting shortwave upper trough into a small upper vortex thanks to local cool air advection on its back side. Nova Scotia/Newfoundland frontal depression has weakened into surface troughs stretching into NW Atlantic.
P2...Mid-ocean upper vortex persists. Western fragement has broken off and is passing south of Bermuda while quickly orbiting strong W Atlantic upper ridge mentioned in paragraph P1. It will soon merge with longwave upper trough regime discussed in paragraph P1.
P3...Cut-off upper vorticity over west tip of Cuba persists.
P4...Atlantic surface ridge of 1014 to 1025 mb centers is supported by a few upper convergent sources while stretching from the Gulf of Mexico to the waters offshore of west Europe....including convergence SE of the of the SW US and west Atlantic upper ridges in paragraph P1. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Tropical Storm Ernesto...and the tropical waves in paragraphs P6 and P8. Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P2 and W Atlantic upper ridge in paragraph P1.
...TROPICAL BELT DISCUSSION...
P5...Upper ridge continues to cover most of the Atlantic tropics. T-storm latent heat release from paragraph P6 and P8 tropical waves...as well as from Tropical Storm Ernesto....continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with inverted upper troughs forming between these cells. Inverted upper troughs are over E Caribbean Sea and WSW of Cape Verde Islands.
P6...Tropical wave in the central Caribbean continues has moved west into Central America. Its t-storm activity...and associated upper anticyclonic cell of paragraph P5 upper ridge (driven by t-storm latent heat release)...remains displaced to the east thanks to upper vorticity in paragraph P3. Divergent southwesterly upper jet between paragraph P3 upper vorticity and upper anticyclonic cell is also helping to support these t-storms...as well as a surface trough over Cuba and the Bahamas that appears to be a north fracture of this tropical wave.
P7...Tropical wave SW of the Cape Verde Islands is struggling while ingesting dry air described in paragraph P4 while simultaneously competing for low-level inflow with strong tropical wave to its east (strong tropical wave discussed in 2nd special feature section above and paragraph P8 below).
P8...Tropical wave rolling off of Africa in the previous discussion (now south of the Cape Verde Islands) has been introduced into the National Hurricane Center Tropical Weather Outlook in last 24 hours. See second special feature section above for further details on this system.