2012 Atlantic Hurricane Season Birdseye Discussion #37
...JUNE 27 2012...2:40 AM EDT...
Debby weakens to a tropical depression while centered over northern Florida after landfall. See special feature section for updates and forecasts on this situation.
Watching two tropical waves in the Atlantic Ocean for possible development. See paragraphs P10 and P11 (in the tropical belt discussion) for further details.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1926Z-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 DEPRESSION DEBBY...
This continues to be a complicated situation...and therefore this current discussion of Tropical Depression Debby will be referring to quiet a few paragraph (P) numbers in the mid-latitude and tropical belt discussions below.
My updated track forecast versus the National Hurricane Center's (NHC) is shown in Figure 1 below. Since the previous discussion...Debby has continued eastward in response to the intensification of 998 mb surface low located on an eastern US front mentioned in paragraph P2. In fact...the track has recently been more ESE judging by latest nighttime shortwave infrared satellite...the more southward component perhaps caused by northwesterly flow behind the 998 mb cyclone...and ahead of the western US upper ridge (paragraph P1) and Great Lakes surface ridge (end of paragraph P2). The recent ESE motion makes me predict a forecast track to the right of (or further south) than the NHC's for most of the next 5-days. I maintain the current forward speed for the first 24 hrs (thru 11 PM Wed)...then begin to accelerate Debby faster as the west US upper ridge and Great Lakes surface ridge both get knocked out by incoming west coast frontal system mentioned in paragraph P1. The more northward deflection towards the latter half of the forecast is caused by west end of Atlantic surface ridge (paragraph P5) progged by the models to stay in place thru the forecast period. The very rapid acceleration to the NE by 11 PM Sunday is due to Debby getting caught in what models suggest to be vigorous southwesterly flow ahead of what is now the incoming west coast frontal system and its supporting upper trough.
Figure 1: My current 5-day Tropical Storm Debby forecast this early morning.
Intensity-wise...Debby continues coughing on dry air (source mentioned in paragraph P1)...and continues to be sheared by westerlies on the north side of its former warm core upper ridge (paragraph P7 feature). Combined with the recent north Florida landfall...Debby has weakened to a tropical depression. The only thing keeping Debby alive (as a trpoical cyclone) in this otherwise hostile environment is upper divergence between the northeast corner of the west Gulf of Mexico upper-level low (paragraph P2) and northwest half of the adjacent warm core upper ridge (paragraph P7). As Debby moves further away from the Gulf of Mexico upper low...source of upper divergence shortly will switch to split westerly flow located at the boundary of the warm core upper ridge (paragraph P7) and east US upper troughing (paragraph P2).
If Debby follows the forward pace shown in Figure 1...then the models suggest she will become just ahead of the upper troughing in paragraph P2...which will allow her low-level circulation to advect cooler air behind her and warmer air ahead of her. This would allow Debby to carve out her own amplifying upper trough (extension of the paragraph P2 upper trough) to her west...and amplify a cell of upper ridging (extension of paragraph P7 upper ridge) to her east. The upper wind field shown by the models is divergent at the boundary between the mini upper trough and mini upper ridge...and moreover Debby could easily ventilate herself into the eastern mini upper ridge...all of this suggesting further strengthening by Thursday thru Saturday timeframe. My intensity forecat in Figure 1 is more aggressive than the NHC's current version...as I am gambling she will take good advantage of all these dynamics at waters above or at 26 deg C. After she goes into cooler waters below 26 deg C (judging by above birdseye thermo chart) and gets strong southwesterly shear by Sunday...I weaken Debby at the end of the forecast. If Debby is maintaining strength more than shown by Sunday...this means she is doing so non-tropically while taking advantage of upper divergence ahead of the upper trough she is accelerating with.
5-day forecast impacts in Figure 1 are what I think if Debby follows my forecast. Impact swath is initialized using current Florida radar presentation...then I gradually mold it into the shape I think the storm will have by the 11 AM Saturday position. By 11 AM Saturday...I imagine Debby having clouds biased to her east side as the mini upper trough mentioned in the previous paragraph prevents her from building much t-storms on her west side. By 11 PM Sunday...the thickest clouds should shift to the northeast half of Debby under immense southwesterly shear...so the impact swath becomes symmetrical about the storm track at the very end because the track is also northeastward by that time. The forecast impact swath diameter also shrivels in size at the end as Debby fades over cooler waters/southwesterly shear.
The forecast track and impact swath in Figure 1 suggests that at this time...Debby should pass well west of Bermuda without direct impacts to the island. If the trajectory doesn't lift northward as quickly by late Thursday/early Friday...then maybe the far eastern fringes of her direct impacts could reach the island.
P1...Warm air advection ahead of surface frontal system hanging off west coast of North America continues supporting the upper ridge over the western US. After the Great Lakes surface ridge (in paragraph P2) has moved out of the way...the west coast frontal system has finally made its way into the western US with a 994 mb low pressure center in eastern Montana. Meanwhile...upper convergence on the east side of the west US upper ridge conitnues supporting dry air.
P2...Frontal system and longwave upper trough over the eastern US is currently complicated. Surface front that was pushing into the NW Atlantic yesterday has dissipated thanks to intensifying 1024 mb center of open Atlantic ridge mentioned in paragraph P5. 24 hrs ago...an east Canada 1000 mb frontal cyclone delivered a surface front reaching into the eastern US. The 1000 mb low is no longer the dominant feature on this front...becoming replaced by a rapidly intensifying 998 mb cyclone moving NE into coastal Maine and Nova Scotia. Locally strong cold air advection behind this 998 mb surface cyclone has carved out an upper vortex that is beginning to align with the surface cyclone. Once this happens...a much less divergent environment beneath the upper vortex will begin weakening the surface cyclone. Cut-off upper vortex (interacting with Tropical Storm Debby) in the western Gulf of Mexico was left behind by this longwave upper trough regime three days ago...but now the cut-off is exiting the Gulf of Mexico while retrograding around the anticyclonic center of the upper ridge in paragraph P1. Of final note...upper convergence on the back side of this longwave upper trough regime supports a SE-moving surface ridge over the Great Lakes who is currently at 1018 mb.
P3...Upper ridge over the western Atlantic and south Greenland remains supported by warm air advection ahead of the complex frontal system outlined in paragraph P2 above. The eastern convergence of this upper ridge supports surface ridging in the vicinity of Greenland that has recently merged with the 1024 mb center mentioned in paragraph P5.
P4...Large upper trough in the north-central Atlantic is moving NE toward Europe...and the surface frontal cyclone it once supported is decaying with a lack of divergence beneath the upper trough axis. From the isobars drawn on the 1800Z TAFB analysis...the surface cyclone is currently below 1016 mb. Cut-off upper trough east of Bermuda in the previous discussion continues to be pushed eastward by growing upper ridge mentioned in paragraph P7.
P5...Open Atlantic surface ridge has a 1021 mb center SE of the Azores and newly-intensifying 1024 mb center SW of the Azores...with the cold front extending from the north-central Atlantic surface cyclone (paragraph P4) spliting the two centers. The 1024 mb center has supportive upper convergence...as upper southwesterlies ahead of the cut-off upper trough E of Bermuda (paragraph P4) converge with upper northwesterlies behind the north-central Atlantic upper trough (also mentioned in paragraph P4).
P6...Highly-amplified cut-off upper vorticity in the NE Atlantic (over the Canayr Islands) has undergone changes in the last 24 hours. The north half has merged with north-central Atlantic upper trough mentioned in paragraph P4...but not before upper divergence ahead of it touched off a 1013 mb surface low and cloud swirl near Spain and Morocco. The southern half has retrograded southwestward around an anticyclonic center of the upper ridge mentioend in paragraph P9.
...TROPICAL BELT DISCUSSION...
P7...Upper ridge over the southern Gulf of Mexico continues to be expansive....and its inflated state is attributed to previous latent heat release from Debby's t-storm clouds. This upper ridge has gone further eastward expansion thanks to warm air adveciton ahead of complex frontal system outlined in paragraph P2.
P8...Cut-off upper vorticity persists in a SW-NE elongation...from the eastern Caribbean to the tropical waters east of the Lesser Antilles. Westerly flow going into the west flank of this vorticity heavily diverges with mainstream easterly flow on the south side of the upper ridge in paragraph P7...resulitng in an increase in t-storms near and over Panama as well as a new surface trough.
P9...Expansive east Atlantic upper ridge continues. In conjunction with the surface ridge in paragraph P5 and upper ridge in paragraph P7...deep-layered easterly flow exists across much of the Atlantic tropics that is advecting African desert dry air (brown shading in the above thermo birdseye chart) westward. A northeastern lobe of this upper ridge had been greatly amplified in the last 48 hours thanks to warm air advection ahead of north-central Atlantic surface cyclone mentioned in paragraph P4. This northeastern lobe has now consolidated into an anticyclonic center southeast of the Azores...and some of the NE Atlantic upper vorticity (paragraph P6) has dived southwestward while moving around this anticyclonic center. The result is that a westward-tracking upper low is now embedded in this east Atlantic upper ridge.
P10...Tropical wave crossing the Lesser Antilles in the previous discussion is now crossing the eastern Caribbean Sea. It remains suppressed by dry air mentioned in paragraph P9 above...but is about to enter the moistening environment and surface trough near Panama (paragraph P8). This tropical wave in the short-term will continue to battle northeasterly shear from the strong upper ridge mentioned in paragraph P7. However...if this tropical wave interacts with the Panama activty and then later gets closer to the favorable low shear/enhanced outflow center of this upper ridge...then we may have something to watch for tropical development later on.
P11...Tropical wave SW of the Cape Verde Islands has been successful in fighting off dry air mentioned in paragraph P9...because it has received enhanced poleward upper outflow thanks to upper vorticity mentioned in paragraph P6. See paragraphs P6 and P9 for how some of this favorable upper vorticity is following the tropical wave as a westward-tracking upper low. The tropical wave has gained some cyclonic banding features (albeit small in size) in its t-storm cloud pattern...and the National Hurricane Center gives this a 10% chance of developing in the next 48 hours while introducing it into their tropical weather outlook in the last 24 hours. If these trends continue...I will be writing a special brief update or be upgrading this to a special feature in my next full discussion.