2012 Atlantic Hurricane Season Birdseye Discussion #138
...SATURDAY OCTOBER 20 2012...6:00 PM EDT...
Tropical cyclone formation in association with a tropical wave interacting with an upper ridge appears to be occurring ahead of schedule in the central Caribbean Sea. The disturbance has been organizing...surface pressures are falling...and it has been recently upgraded to disturbance Invest 99-L. See Invest 99-L special feature section below for further details on this developing situation.
As mentioned in special update #137A...we are also monitoring disturbed weather in the central open Atlantic...associated with an upper vortex interacting with tropical waves passing by. This disturbed weather has recently been upgraded to Invest 90-L. Although their is presence of surface vorticity in the area...it is not well-organized. However...given the simultaneous presence of favorable upper winds...I have upgraded this system to a special feature. See Invest 90-L special feature section for additional details.
...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...CARIBBEAN DISTURBANCE INVEST 99-L...
At the surface...the eastern Caribbean tropical wave in paragraph P5 of previous discussion #137 has combined with western Caribbean/western Cuba surface troughing in paragraph P2 of previous discussion #137...resulting in a large surface tropical low across the Caribbean. In the upper-levels...east Caribbean upper ridge in paragraph P4 of previous discussion #137 and SE Mexico upper ridge in paragraph P1 of previous discussion #137 have combined into a singular upper ridge whose upper outflow is ventilating the system.
The most impressive response to the favorable upper outflow has been toward where the aforementioned surface tropical wave was located this afternoon. This location is in the Caribbean waters south of Haiti...where the t-storms are most concentrated and their are signs of curved cloud bands collecting around observed surface pressure falls. In the intro statement of previous discussion #137...we had expected possible tropical cyclone formation from this area by 120 hours as of that writing...which is now 84 hours as of this writing. Based on the level of organization on satellite...tropical cyclone formation could easily occur sooner than this timing.
Eastern solutions such as the GFS model first show gradual tropical cyclone formation while the SE US surface ridge in paragraph P2 builds to the north...and a cut-off upper trough from the paragraph P2 upper trough amplifies to the NW (thanks to equally amplifying paragraph P1 upper ridge). In such a scenario...tropical cyclone intensity is highly uncertain...as the tropical cyclone either suffers from SW vertical shear delivered by the cut-off upper trough...or thrives under the upper ridge...depending on exactly where the surface center consolidates. In the GFS solution...it appears the tropical cyclone is vertically coupled enough to be steered by the cut-off upper trough such that the tropical cyclone first drifts NNE toward Jamaica...eastern Cuba...the eastern Bahamas...and western Hispaniola while the tropical cyclone is trapped by conflicting steering between the surface ridge and cut-off upper trough. The long-range GFS solution then knocks out the surface ridge in between the paragraph P2 cyclone and paragraph P1 surface frontal system...sending the tropical cyclone ENE into the open Atlantic.
Solutions such as the current CMC model show a more western track toward Jamaica...Cuba...the Cayman Islands...the western Bahamas...and southeastern Florida. The CMC model also depicts a stronger tropical cyclone than GFS. A solution such as the CMC would verify if the cut-off upper trough is less influential. A less influential cut-off upper trough means less NE steering and more NW steering from the surface ridge...and also means a stronger tropical cyclone less affected by vertical shear from the upper trough.
Residents in Hispaniola (Haiti and the Dominican Republic)...Jamaica...Cuba...the Cayman Islands...the Bahamas...and southeastern Florida are urged to keep tabs on this developing situation until it is clearer which solution will play out.
...SPECIAL FEATURE...OPEN ATLANTIC UPPER VORTEX DISTURBANCE INVEST 90-L...
In the Atlantic tropics east of the Lesser Antilles...cut-off upper vortex mentioned in paragraph P6 of previous discussion #137 persists. The tropical wave interacting with the upper divergence on the east side of the upper vortex...also mentioned in the previous discussion...is moving westward toward the Lesser Antilles and exiting this area as shown in Figure 1 below and in the above atmo chart. The t-storm cluster sheared-off from the exiting tropical wave persists in split flow upper divergence on the NE quad of the upper vortex. The split upper flow...shown in Figure 1...is the result of flow whirling around the cold core upper vortex splitting with anticyclonic flow around the paragraph P2 W Atlantic upper ridge and around the paragraph P5 upper ridge. 1800Z ASCAT pass of surface winds reveals possible new surface low spinning up near 48W-20N while taking advantage of aforementioned split flow upper divergence. In addition...yet another tropical wave with 1012 mb low near 45W-17N is adding to the t-storm activity with its surface convergence. Although the 1012 mb low is also defined as a swirl in visible satellite imagery in Figure 1 and the ASCAT pass in Figure 2...the 1012 mb low is less-likely to develop due to SW vertical shear from the upper vortex as shown in Figure 1. As shown in the above atmo chart...the 1012 mb low/tropical wave was added within TAFB maps within the last 30 hours...and was not added beforehand due to its poor definition.
Figure 1: 1800Z synopsis of disturbance Invest 90-L using visible satellite imagery. Blue is upper air analysis...red is surface analysis.
Figure 2: 1800Z ASCAT of surface winds. Red Ls mark the location of the 1012 mb surface low near 45W-17N...and possible new surface low near 48W-20N.
I forecast possible subtropical or tropical cyclone formation in the next 48 hours from the new surface low spinning up near 48W-20N. Since the aforementioned split flow upper divergence supporting this new surface low is created by both the cold core upper vortex and adjacent warm core upper ridging...subtropical or tropical status will be dependent upon whether more credit is given to the upper vortex or upper ridging. Such a tropical or subtropical system would first track west on the north side of the upper vortex and south of the paragraph P3 1029 mb ridge...then track northward by 72 hours while getting swept into the paragraph P2 cyclone's cold front coming in from the west. Such a forecast track keeps this system well away from land areas and over open waters.
P1...Next surface frontal system and upper trough in the mid-latitudes is entering from SW Canada and the western US in the upper-left corner of the above atmo chart. Low-level warm air advection ahead of the frontal system supports upper ridge building across the western US.
P2...994 mb deep-layered cyclone/upper trough centered over Wisconsin in the previous discussion continues shifting eastward. While moving eastward across the Great Lakes...the surface center has weakened from 994 to 1001 mb in the last 36 hours due to lack of divergence beneath the upper vortex of the upper trough. Upper convergence on the west side of the deep-layered cyclone supports surface ridge (currently 1015 mb) that has moved from the western to the SE US in the last 36 hrs. Meanwhile...warm air advection ahead of the deep-layered cyclone supports an upper ridge that has built in the W Atlantic.
P3...Upper trough and surface gale activity in the vicinity of southern Greenland has undergone complex evolution in the last 36 hrs. The upper trough has absorbed secondary W Atlantic upper trough mentioned in paragraph P1 of previous discussion #137. NW Atlantic/SE Canada surface ridge supported by western convergence of absorbed secondary upper trough has intensified further to 1029 mb. The remnant surface gale of Rafael made an initial NW turn toward southern Greenland while absorbing the 977 mb gale supported by the eastern divergence of the upper trough. After nearing southern Greenland...the remnant gale of Rafael has reversed to a southeast track while entering deep-layer NW flow on the back side of the upper trough and east of the aforementioned 1029 mb ridge. Surface trough ENE of Bermuda...left behind by cold front extending SW from Rafael in the previous discussion...was analyzed as a weak 1017 mb low as of 1200Z TAFB analysis.
P4...Upper trough and surface frontal system entering western Europe in the previous discussion has exited the picture...except for a surface front extending into NW Africa seen in the upper-right corner of the above atmo chart. Western convergence of this upper trough formerly supported what is now a weakening 1021 mb ridge in the central and eastern Atlantic. This surface ridge is currently weakening in upper divergence ahead of the paragraph P3 upper trough.
...TROPICAL BELT DISCUSSION...
P5...Large-sized central Atlantic upper ridge in paragraph P4 of the previous discussion...and E tropical Atlantic upper ridge in paragraph P6 of the previous discussion...have merged into one large upper ridge across the tropical and subtropical eastern Atlantic.