2012 Atlantic Hurricane Season Birdseye Discussion #68
...AUGUST 1 2012...5:00 AM EDT...
Tropical disturbance Invest 99-L has become slightly better organized in the last 24 hours. Interests in the Caribbean Sea and Bahamas should watch this system. See special feature section for further details.
Elsewhere...t-storm activity that has entered the NE Gulf of Mexico not expected to develop...especially as it diminshes quickly this early morning (see end of paragraph P1). One of the Caribbean tropical waves has become an area of interest in the National Hurricane Center outlook...but is not expected to develop (paragraph P7). Impressive tropical wave with low pressure swirl is south of the Cape Verde Islands...but dry air appears to be inhibiting development (paragraph P9).
Of side note...July has ended with no Atlantic tropical cyclones for the month. This is the first July since 2009 that had zero Atlantic tropical cyclones.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1329Z-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 WAVE INVEST 99-L...
Tropical wave southwest of the Cape Verde Islands...designated as Invest 99-L...is continuing west and is now WSW of the islands. Its surface low pressure is still marked in NHC TAFB maps. This surface low and t-storms has favorable conditions thanks to established upper anticyclonic outflow as mentioned in paragraph P5 of tropical belt discussion. National Hurricane Center Outlook now suggests a 50% chance of tropical cyclone formation in next 48 hours. Given the increase of organization seen in last 24 hours...I am more confident that a tropical cyclone will develop and am now putting this as a special feature on this blog.
The CMC and GFS model are still the only two global models suggesting any development out of this system. The CMC continues to present a stronger tropical cyclone and more WNW track...while the GFS continues to present a weaker tropical cyclone and more W track. Surface and upper-level outputs of both models for the next 120 hours (5 days) are summarized in Figures 1 and 2. Features in those figures are marked with paragraph (P) numbers corresponding to paragraph numbers in the tropical belt and mid-latitude discussions.
As shown in Figure 1a...the CMC model is initialized with a position north of 10N latitude when in reality Invest 99-L is along 10N latitude judging by latest satellite image. This might explain some of its northward bias. CMC also suggests the upper vorticity in paragraph P2 (mid-lat discussion) splits over the next 5-days while a fragment retrogrades around the W Atlantic upper ridge (Figure 1b). Coupled with the fact that CMC calculates a more rapid strengthening rate (and hence a deeper-layered tropical cyclone capable of "feeling" upper features)...it suggests it will "feel" the western split of the upper vorticity and hence track more WNW. CMC model WNW track would take it over the northern Lesser Antilles and eventually the Bahamas.
The GFS model (Figure 2) is better initialized than CMC...showing Invest 99-L along 10N latitude rather than just north of 10N. It shows a weaker tropical cyclone and straight west track across the Lesser Antilles and into the Caribbean. Its more southward solution is due to calculating a weaker (and hence more shallow) tropical cyclone not as capable of feeling upper features. Even if 99-L did become stronger in the GFS solution...the GFS does not split the paragraph P2 upper vorticity like the CMC does such that it cannot track more northward toward a split.
Figure 1: 00Z CMC model output this morning. (a) is 0-hour...(b) is 120-hour. The left panes are surface pressure fields...the right panes are 500 mb (upper-level) pressure fields.
Figure 2: 00Z GFS model output this morning. (a) is 0-hour...(b) is 120-hour. The left panes are surface pressure fields...the right panes are 500 mb (upper-level) pressure fields.
Due to a more accurate initial position in GFS...coupled with slow rate of organization 99-L has had...I am leaning toward the weaker tropical cyclone and further south track shown by GFS. I think it is possible the current GFS output thru 5-days maybe too weak given the favorable widespread upper anticyclonic outflow currently over 99-L. So if 99-L becomes stronger and more deep-layered...and if the upper vorticity splits as shown in CMC...then it could "feel" a western split in the upper vorticity and track more WNW like CMC. I think such a WNW track would be further south than the current CMC since the CMC was initialized a tad too far north.
In the very long range...both models show the next upper trough in the mid-latitudes along eastern North America by day 5. While currently over Alaska...this upper trough is not yet in the scope of the mid-latitudes and tropical belt discussion. From current GFS and CMC outputs...it looks as though this upper trough is not going to be far south enough to erode the surface ridge (paragraph P4 of mid-lat discussion). If the surface ridge stays enforced...99-L would continue its west or west-northwest track beyond 5 days.
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. Upper trough from Hudson Bay continues supporting a surface frontal depression...1008 to 1007 mb and now just SE of the bay. Convergence between this longwave upper trough regime and SW US upper ridge supports a 1020 to 1017 mb west US surface ridge extending to Minnesota...while a 1017 to 1022 mb surface ridge from West Virginia to Atlantic Canada is supported by upper convergence on the back sides of embedded shortwave upper troughs. Accelerationally divergent westerly jet on north side of SW US upper ridge appears to support a disorganized 1011 mb frontal depression over the central US. Surface frontal depression (extratropical low) heading toward Europe is still doing so as seen in upper-right of above atmo chart...and frontal depression offshore of Newfoundland is moving slowly NE. Finally...shortwave upper trough over SE US continues supporting a frontal depression along the coast of the Carolinas and Virginia...which is now heading slowly NE. Split flow upper divergence between this SE US shortwave upper trough and SW US upper ridge earlier had triggered strong t-storms over the SE US that have migrated into the NE Gulf of Mexico. There is a surface trough associated with this activity...but land interaction...high surface pressures from the surface ridge in paragraph P4...and recent decrease in this activity suggests no tropical development expected.
P2...Upper vortex just SE of the Azores has merged with shortwave upper trough supporting the surface frontal depression (extratropical low) heading toward Europe mentioned in paragraph P1. Mid-ocean upper vortex persists.
P3...Cut-off upper vorticity in the Caribbean Sea consists of an upper vortex over west Cuba.
P4...Atlantic surface ridge of 1018 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 appears to be reducing due to enhancement of tropical waves as described in paragraphs P5....P7....and P8. Within this surface ridge...mid-ocean surface low has weakened into a surface trough...but 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...East tropical Atlantic upper ridge finally has linked with south-central Caribbean upper ridge....and now it covers most of the Atlantic tropics. T-storm latent heat release from tropical waves in special feature section and paragraph P7 has locally inflated this upper ridge into cells of upper anticyclonic outflow. Upper vortex SW of Bermuda...once embedded in this upper ridge...has weakened into an upper trough and could soon merge with longwave upper trough regime in paragraph P1. Upper vorticity west of the upper ridge...located in the south Gulf of Mexico...is weakening to the south of the SW US upper ridge in paragraph P1.
P6...Tropical wave moving across central America is entering the eastern Pacific...and therefore this is the last statement on this tropical wave in this blog. After being enhanced by upper outflow of south-central Caribbean upper ridge in paragraph P5...its t-storm activity has reduced as it becomes suppressed by westerly shear south of the upper vorticity in paragraph P3.
P7...Tropical wave in the eastern Caribbean is pushing into the central Caribbean. Due to widespread vigorous t-storm activity that had overspread the Lesser Antilles...Puerto Rico...and Hispaniola...it was introduced into the National Hurricane Center outlook in last 24 hrs. Its vigorous activity has been supported by one of the cells of anticyclonic upper outflow described in paragraph P5. However...it is quickly heading into a much less favorable environment of westerly vertical shear associated with the upper vorticity in paragraph P3...so no tropical cyclone formation is expected here.
P8...Tropical wave WSW of the Cape Verde Islands...designated as Invest 99-L...has its own special feature section above. See special feature section for further details on this system.
P9...Tropical wave rolling off of Africa in the previous discussion has been added to NHC TAFB maps...at a location south of the Cape Verde Islands. Upper winds have become more favorable for this tropical wave as the northerly shear has reduced...thanks to anticyclonic outflow of Invest 99-L moving away from this system. This system has impressively developed a low pressure swirl also marked in TAFB maps. Despite better upper winds and the development of this swirl...there is little t-storm activity as of this writing...as if the dry air metioned in paragraph P4 is suppressing this tropical wave.