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2012 Atlantic Hurricane Season Birdseye Discussion #15

By: NCHurricane2009 , 3:04 AM GMT on May 22, 2012

...MAY 21 2012...
Alberto moving northeast away from the US east shore...and could lose its tropical cyclone status in the next 24 hours. If so...this will be the last discussion until June 1 2012...or unless yet another round of early-season tropical activity occurs in the Atlantic.

This is the fifteenth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.

Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?

One improvement being attempted this evening is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-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 indicating surface lows, Hs indicating surface highs.


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).

Alberto...hmmmm....not much to elaborate on since it has been well-behaved with the previous forecast in discussion #14A. He moved SE...ESE...then NE while orbiting cyclonically about the adjacent upper low as expected...and will continue NE as the frontal system in paragraph P2 (mid-latitudes discussion below) approaches from the west. Alberto has just weakened to a tropical depression due to the southwesterly vertical shear developing across the system (as evidenced by the exposed center and weak convective clouds biased to the NE of that center). Southwesterly shear was initialized by the adjacent upper low now to Alberto's NW...and will be maintained by incoming frontal upper trough in paragraph P2. Updated track and intensity forecast in Figure 1 below.

Figure 1: My best guess forecast for Tropical Depression Alberto as of May 21, 2012 late evening. Trackwise...being to the right of the NHC's track last night was a good prediction...as indeed it did go to the right of the NHC's best guess track. In fact my rightward bias was not right enough. Recent ENE motion suggests the rightward bias to the NHC track will continue...and this is what I forecasted above.

P1...In the upper-left of the above charts...next upper ridge in the mid-latitudes has entered the western US. Upper convergence on its east side supports central US surface ridge centers...with a max of 1025 mb in NW MIssouri.

P2...Upper trough from western US has arrived to the central US Divergent southwesterly flow ahead of this upper trough continues supporting a complex surface frontal system. The 1006 mb frontal depression over SW Hudson Bay 24 hrs ago is now the dominant low pressure system of this front...and has zoomed to the NE corner of Hudson Bay such that it its not in scope of the above charts. This dominant frontal low is also driving another cold front pushing southward across Canada...marked in the upper-left in the above charts with a weak 1012 mb low.

P3...Upper ridge in NE US and E Canada will be pushing offshore into the far north Atlantic in the next hours. It has temporarily merged with the Caribbean upper ridge mentioned later in paragraphs P6 and P7. It continues to be supported by warm air advection ahead of the frontal system in paragraph P2. Convergence on the east side of this upper ridge supports a greater-than-1028 mb surface ridge that has moved offshore from E Canada.

P4...Upper low vortex south of the S tip of Greenland is tracking ESE...the southward component of its track likely caused by cold air advection behind its associated surface cyclone. This surface cyclone was weakening in the last 48 hours while beneath the non-divergent center of the upper low...and had reached 995 mb in its weakening phase 24 hrs ago. Animation of North American HPC surface analyses suggests this surface cyclone regenerated beneath NE upper divergence of the upper low...which has allowed it to re-strengthen to 990 mb. It is now again in a weakening phase beneath the upper low...rising back to 991 mb per latest HPC.

P5...Upper vorticity near TS Alberto seemed to perform a "fujiwhara-like dance" with Alberto per last night's special update #14A...which caused this upper vortex to make landfall in SC last night while tracking NW. This upper low is now well-inland in the SE US. The associated 1014 mb surface low that made landfall on the mid-Atlantic shore yesterday is now in the Ohio Valley at 1015 mb. Expect the upper trough associated with the system in paragraph P2 to absorb the upper low within the next 24 hours. Expect the surface frontal system in paragraph P2 to absorb the 1015 mb low in the next 24 hours. On a final note...northwesterly flow on the SW side of this upper low is converging heavily with westerly flow on the N side of Caribbean upper ridging (paragraphs P6 and P7)...continuing to maintain the dry air across the Gulf of Mexico and north Florida.

P6...Caribbean upper ridging persists...and its NE lobe has merged with the upper ridge mentioned in paragraph P3. Disturbed weather associated with this upper ridge is covered more extensively in the tropical belt discussion...paragraph P7. 24 hrs ago...a portion of the upper ridge's NE extension broke off into the NE atlantic....the break-off caused by locally intense warm air advection ahead of the cyclone in paragraph P4. The 1034 mb surface ridge center W of the Azores (24 hrs ago) now has dual centers...a 1030 mb western center supported by convergence E of the Caribbean upper ridge's NE extension...and an eastern 1026 mb center supported by convergence E of the NE Atlantic upper ridge.

P7...Upper vorticity near Alberto is now inland over the SE US (see paragraph P5 for details). Southwesterly upper flow has been directionally divergent between this upper vorticity and Caribbean upper ridging mentioned in paragraph P6...leading to T-storm activity in the W Caribbean...Cuba...Bahamas...and waters NE of the Bahamas. See paragraph P8 of discussion #14 on how I connected a few surface trough features into a "single surface trough" in the region (although technically it hasn't been analyzed as a single entity in TAFB). The 1009 mb W Caribbean disturbance at the west end of this "singular surface trough" has weakened slightly to 1010 mb. At the NE end of this "singular surface trough" is a new 1012 mb low west of Bermuda. Expect the weather in the W Caribbean...Cuba...Bahamas...and waters NE of Bahamas to remain unsettled due to the convergence of this "singular surface trough" and directional upper divergence between the Caribbean upper ridge and upper vorticity near Alberto. Because the upper vorticity near Alberto will be replaced by the incoming upper trough in paragraph P2...this will prolong this unsettled weather. Latent heat release from the clouds of this unsettled weather is also now supporting a good chunk of the Caribbean upper ridge and its NE extension.

P8...Upper low vortex persists NE of the Lesser Antilles. Animation of water vapor imagery suggests lift created by divergence at the boundary of the upper low (and perhaps also created by instability associated with its cold upper air temps) has eroded a lot of the dry air it had. Weak surface trough that it spawned yesterday is no longer in TAFB analyses.

P9...Upper ridging continues in the E tropical Atlantic. A tropical wave to the SW of the Cape Verde Islands 24 hrs ago is now midway between the Cape Verdes and Lesser Antilles (at the W end of this upper ridge)...and will soon enter a hostile shear environment created by the upper low in paragraph P8. An increase in clouds and t-storms is possible as this tropical wave interacts with the SE peripherial divergence of the upper low. Another tropical wave has moved offshore from Africa...beneath the E side of this upper ridge.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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1. nigel20
3:18 AM GMT on May 22, 2012
Thanks NC
Member Since: December 31, 1969 Posts: Comments:

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