2012 Atlantic Hurricane Season Birdseye Discussion #36
...JUNE 26 2012...2:10 AM EDT...
Tropical Storm Debby a flood threat to north Florida and south Georgia. See special feature section 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 1931Z-released HPC analysis.
In light blue is upper air anlaysis, 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 DEBBY...
This continues to be a complicated situation...and therefore this current discussion of Tropical Storm 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. Debby has continued going northeastward toward a ridge weakness created by the upper troughing and surface frontal systems mentioned in paragraph P2. Recently...nighttime shortwave infrared satellite imagery suggests an acceleration straight east instead of ENE as the NHC forecast shows...which is why my current forecast track is to the right (or south of) the NHC's. I surmise the recent eastward acceleration is Debby responding to the intensification of surface low pressures located on an eastern US front mentioned in pargraph P2. The earlier NE track and current eastward acceleration suggest the surface ridge diving SE from western Canada into the Great Lakes (end of paragraph P2) is not enough to numb Debby's eastward track toward north Florida and the Atlantic...so now I am depending on upper ridging in paragraph P1 to help later do this. After being initially faster than the NHC forecast track (to account for the recent east acceleration)...I become quiet slower than the NHC forecast track thru Thursday based on when the models think the upper ridge in paragraph P1 will be much closer to Debby (hence when I think this upper ridge will help numb her eastward progress in conjunction with the Great Lakes surface ridge). After the Great Lakes surface ridge and paragraph P1 upper ridge get knocked out by west coast frontal system mentioned in paragraph P1...there is deep-layered westerly flow left behind for Debby to accelerate faster to the east by Friday (which is what I show). By Saturday...I once again slow the forward speed of Debby based on what I think is weak surface ridging re-developing over the Great Lakes (caused by upper convergence on the back side of the shortwave upper trough that supports the incoming west coast frontal system).
Figure 1: My current 5-day Tropical Storm Debby forecast this early morning.
Intensity-wise...yesterday it was wrong to not worry about dry air mentioned in paragraph P1. I was expecting the tremendous upper outflow of Debby to keep low-level lift and moistening such that the dry air wouldn't penetrate...especially as the upper outflow would become more symmetrical as the west Gulf upper-level low (ULL) in paragraph P2 retrogrades westward and away (as the ULL moves about the anticyclonic center of the upper ridge in paragraph P1). But indeed...dry air has weakened Debby. Moreover...the warm core upper ridge/outflow of Debby (paragraph P7) has been pushed southward...as the eastern US frontal system and its upper trough (paragraph P2) appear to be amplifying to Debby's north at this hour. This has exposed Debby to westerly shear on the north side of the warm core upper ridge. I believe what is keeping Debby from collapsing under the dry air and westerly shear is supportive upper divergence located between the NE corner of the west Gulf ULL and NW edge of the warm core upper ridge.
I expect Debby to continue maintaining strength from said upper divergence thru landfall time...then weaken to a 35 mph max wind tropical depression over north Florida. The long time over land shown by my Figure 1 forecast would seem to suggest dissipation...but Debby has been a rather large tropical cyclone where I believe its offshore cloud bands over water will continue feeding latent heat release to Debby such that she is still a tropical cyclone all this time. Re-strengthening back to a tropical storm by Friday and Saturday (once she goes slowly into the Atlantic) is possible...with models suggesting the paragraph P7 warm core upper ridge re-building northward (near or over Debby)...after this upper ridge re-gains real estate behind the exiting paragraph P2 upper trough.
5-day forecast impacts in Figure 1 are what I think if Debby follows my forecast. Mention of isolated tornadoes is due to the many tornado warnings we have seen in south and central Florida in the last two days...but this is also a reminder that Debby could produce isolated tornadoes throughout the entire forecast period (as tropical cyclones are typically capable of). Impact swath is drawn based on the initial lopsided structure caused by westerly vertical shear...and then taking that sheared structure eastward along the forecast track. Width/shape of impact swath is a combo of looking at current radar and the NHC's 10 PM CDT tropical storm wind field. Current radar suggests potential for major flooding problems in northern Florida and southern Georgia...especially if Debby maintains her current radar presentation while following the rather slow eastward track I presented in Figure 1 above.
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. Eastward progression of west coast surface system is currently blocked by strong surface ridge entering the Great Lakes region and mentioned in paragraph P2. Meanwhile...upper convergence on the east side of the west US upper ridge supports dry air.
P2...Frontal system and longwave upper trough over the eastern US is currently complicated. Surface front near the east coasts of the US and Canada is pushing into the NW Atlantic...with a 1010 mb center over SE Canada. 1004 mb frontal cyclone on the east coast of Hudson Bay is now 1000 mb over eastern Canada while delivering another front reaching into the eastern US. 48 hrs ago this longwave upper trough regime had left behind a cut-off upper vortex (interacting with Tropical Storm Debby) in the western Gulf of Mexico...and this cut-off feature still persists. Of final note...upper convergence on the back side of this longwave upper trough regime supports a strong west Canada surface ridge that is diving southeast into the Great Lakes region with a 1023 mb center.
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 a 1023 mb surface ridge in the vicinity of south Greenland.
P4...Large upper vortex in the north-central Atlantic has weakened into an upper trough...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 1200Z TAFB analysis...the surface cyclone is currently below 1012 mb. Cut-off upper vortex south of Bermuda in the previous discussion continues to be pushed eastward by Debby's sprawling warm core upper ridge...and is now east of Bermuda while it too has weakened to an upper trough.
P5...Open Atlantic surface ridge has a 1022 mb center NE of the Azores that is supported with upper convergence NE of the E Atlantic upper ridge (paragraph P9) and W of the NE Atlantic upper vorticity (paragraph P6). This surface ridge also has a 1019 mb center south of the north-central Atlantic surface cyclone (paragraph P4)...and a 1021 mb center east of Bermuda.
P6...Highly-amplified cut-off upper trough in the NE Atlantic (over the Canayr Islands) has amplified even further into upper low vortices. The amplification of this cut-off fragment is due to equal amplification of the east Atlantic upper ridge (paragraph P9) to its southwest. Divergence on the east side of this cut-off upper voriticity continues to support cloudiness that has expanded from the Moroccan coast in the last 24 hours...and now this cloudiness has the signature of a surface low pressure swirl and also reaches into southern Spain. This swirl of clouds has been upgraded to a surface trough per 1800Z TAFB analysis.
...TROPICAL BELT DISCUSSION...
P7...Upper ridge over the southern Gulf of Mexico continues to sprawl with the support of Tropical Storm Debby's thunderstorm latent heat release. This upper ridge used to support the outflow of Debby...but now Debby's surface center is on the north side of this upper ridge as Debby has been moving northeastward per special feature section above. The result is Debby is now under westerly vertical shear on the north side of this upper ridge.
P8...Cut-off upper vorticity persists in a SW-NE elongation...from the eastern Caribbean to the tropical waters east of the Lesser Antilles.
P9...Expansive east Atlantic upper ridge continues. In conjunction with the surface ridge in paragraph P5...deep-layered easterly flow exists south of this upper ridge such that dry air from the African desert is being advected westward across the Atlantic tropics (brown shading in the above thermo birdseye chart). A northeastern lobe of this upper ridge has greatly amplified in the last 24 hours thanks to warm air advection ahead of north-central Atlantic cyclone in paragraph P4. As explained in paragraph P6...this amplified northeastern lobe has caused equal amplification of NE Atlantic upper vorticity...and in turn the amplifying upper vorticity has dug southward and pushed the main body of this upper ridge westward.
P10...Tropical wave midway between the Cape Verde Islands and Lesser Antilles in the previous discussion is now crossing the Lesser Antilles in this discussion. It remains suppressed by dry air mentioned in paragraph P9 above...but is about to enter the southern reaches of the upper ridge mentioned in paragraph P7. This upper ridge was a manifestation of Debby's warm core upper outflow...and as a result some of the moist air from Debby's cloud tops have been advected southward and into the environment this tropical wave is about to cross. Although moistening air is "good news" for this tropical wave...it will in the short-term have to battle northeasterly shear from this same upper ridge. However...if this tropical wave 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 south of the Cape Verde Islands in the previous discussion is now SW of the islands. If the tropical wave in paragraph P10 is any indication...this tropical wave should also succumb to the dry air mentioned in paragraph P9. However...this tropical wave has been better fighting the dry air than its predecessor in paragraph P10...because it has received enhanced poleward upper outflow thanks to upper vorticity mentioned in paragraph P6. As the tropical wave moved away from this upper vorticity...I expect it to begin suffering from the dry air.