2012 Atlantic Hurricane Season Birdseye Discussion #151
...SUNDAY NOVEMBER 4 2012...2:21 PM EDT...
Tropical cyclone development no longer possible in the southern Caribbean as highlighted in paragraph P5 of the tropical belt discussion. Slight chance of subtropical cyclone development in the open eastern Atlantic remains possible as highlighted in paragraph P4 of the mid-latitudes discussion.
See paragraph P2 in the mid-latitudes discussion for statement on what appears to be an imminent threat of a nor'easter impacting the same areas in the northeastern United States hit by last week's Hurricane Sandy. Formation of this nor'easter is expected at 72 hours...and while it will bring gusty winds...it is not expected to be as severe as Hurricane Sandy. This nor'easter is not expected to gain tropical characteristics.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1411Z-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).
P1...Intense 960 mb gale that affected southern Alaska sits offshore of the west coast of North America and currently out of the scope of the above birdseye charts. Currently...warm air advection ahead of the gale supports highly-amplified west coast upper ridge whose eastern convergence supports a 1027 mb surface ridge over the western US. For the next 72 hours...models show an eastern lobe of the gale moving into western Canada and to the north of the western US surface ridge...with cold air driven in by the back side of the eastern lobe energizing the paragraph P2 upper trough. See paragraph P2 for how the energized paragraph P2 upper trough creates a nor'easter. The models then pivot the remainder of the gale into the western US in the wake of the nor'easter.
P2...Upper trough over the western US in the previous discussion has been pushed east into the central US thanks to building west coast upper ridge mentioned in paragraph P1. The upper trough consists of an upper vortex now over the Great Lakes and upper trough extending to the SW. The Texas/Oklahoma 1010 mb frontal depressions supported by the eastern divergence of this upper trough have shifted into the SE US as a now singular 1013 mb depression...while the 1020 mb depression over South Dakota in the previous discussion has been left behind and is weakening to 1023 mb in eastern convergence of the paragraph P1 west coast upper ridge. See statement in paragraph P1 on how the upper trough is expected to be energized. By 72 hours...all models agree that strengthening upper divergence on the east side of the energized upper trough will support the rapid genesis of a surface non-tropical gale offshore of the Carolinas and along the front extending from what is now the 1013 mb depression. Because the strong gale is expected to track NE parallel to the US coast...it will be called a "nor'easter." Unfortunately...the nor'easter is expected to impact areas hit by last week's Hurricane Sandy with gusty winds...with rain and snow showers also possible. However...this nor'easter is not expected to be as strong as Sandy.
P3...Large upper trough pushing into the western Atlantic...mentioned in paragraph P2 of the previous discussion...has arrived except for its northern portion still over SE Canada. Vast convergence west of the upper trough supports a 1020 mb ridge in the Gulf of Mexico and 1025 to 1031 mb ridge diving SE from central Canada. Divergence on the east side of the upper trough supports 996 mb frontal depression over SE Canada...which has lifted NNE toward southern Greenland while strengthening to 995 mb. Relatively new 996 to 997 mb surface frontal depressions just south of Atlantic Canada in the previous discussion...supported by the same upper divergence...have intensified into a singular 990 mb depression that has moved northward into Newfoundland. Low-level warm air advection ahead of these frontal depressions are supporting an upper ridge that has moved offshore into the north Atlantic from the east coast of Canada. North Atlantic surface ridge offshore of Canada (currently greater-than-1024 mb) is becoming supported by the western convergence of the paragraph P4 upper trough.
P4...Large eastern Atlantic upper trough at all latitudes persists. Surface 1000 mb vortex south of the Azores in the previous discussion has weakened to 1010 mb while trapped beneath the non-divergent upper trough axis. Meanwhile...upper divergence east of this axis meanwhile supports a western Europe surface front extending from the 1010 mb low. Low-level warm air advection ahead of what is expected to be a nor'easter mentioned in paragraph P2 will keep the north Atlantic upper ridge mentioned in paragraph P3 amplified...causing the aforementioned east Atlantic upper trough to stay amplified east of the upper ridge axis...in turn strengthening western convergence on the back side of the upper trough axis such that the surface ridge currently in the north Atlantic (mentioned in paragraph P3) strengthens. In essence...the amplified north Atlantic upper ridge and strengthening surface ridge cuts-off a portion of the aforementioned 1010 mb low and east Atlantic upper trough that all retrogrades westward such that subtropical cyclone formation is possible.
...TROPICAL BELT DISCUSSION...
P5...Sprawling tropical Atlantic upper anticyclone continues to have one anticyclonic center east of the Lesser Antilles and another over the Caribbean Sea. Shortly after previous discussion #150 was written...the central Caribbean surface trough strengthened into a 1009 mb surface low with t-storms thanks to upper outflow enhacement of the upper anticyclone. However...the 1009 mb low has been steered westward into central America via the easterly flow south of the Gulf surface ridge in paragraph P3. Since landfall...the surface low has lost its t-storm activity and identity within the ITCZ between 0600Z and 1200Z today...and therefore tropical development in the southern Caribbean is no longer expected. As of 1200Z...there is another surface trough that has entered the environment below the sprawling upper anticyclone...this one located midway between the Lesser Antilles and Cape Verde Islands which is the fracture of the old cold front extending south from the 1010 mb low in paragraph P4. However as we saw with the aforementioned failure of development in the southern Caribbean...a surface trough below an upper anticyclone does not mean imminent risk of tropical development. Plus...this surface trough is suppressed by dry air. An animation of the above thermo chart over the last days reveals this dry air originated on October 31 when the current 1010 mb low of paragraph P4 was a deep-layered low in paragraph P2 of discussion #148. Western upper convergence of the deep-layered low triggered the sinking dry air which has since been advected anticyclonically within the aforementioned sprawling upper anticyclone.