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By: NCHurricane2009 , 8:45 PM GMT on May 20, 2012
...MAY 20 2012...
Alberto moves closer to the South Carolina and Georgia shores more than expected...but is also in unfavorable conditions and should weaken. See special feature section for further details.
This is the fourteenth 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 afternoon 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.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-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.
...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 ALBERTO...
Current synoptic situation of Alberto is summarized in Figure 1 below. The tropical storm moved more southwestward than shown by any forecast yesterday...and is now closer to southern SC and Georgia shores more than expected. General discussion paragraph P4 explains that the upper vorticity around Alberto has consolidated as a single upper low just south of Alberto. And with low-level ridging persisting to the northwest....one can see (with the blue and red arrows in Figure 1) the northeasterly flow that has been pushing Alberto southwestward since its birth. Alberto has also weakened...one big reason being that its anticyclonic outflow upper ridge (vital for maintaining Alberto's low surface pressure) has been absorbed by an upper ridge in the NE US (see paragraph P2). Alberto now looks more like a subtropical storm on satellite while beneath the cold core upper low centered to its south. This upper low has been wafting in dry air from the Gulf of Mexico into Alberto's circulation (source of dry air covered in paragraphs P2 and P4). In addition...this upper low perhaps is contributing to easterly shear which is keeping whatever remains of Alberto's T-storms biased to the west of center.
Figure 1: Alberto's current synoptic environment
Track-wise...my forecast yesterday was further southeast (i.e. more offshore) than the NHC. While being further south was perhaps more accurate...the more offshore aspect was not. I was expecting the 1006 mb cyclone NE of Alberto (now 1014 mb in the atmospheric features birdseye chart) to impart a drag that would keep Alberto more offshore than it has been...but it appers this cyclone is doing ditto when it comes to steering Alberto. My new track forecast (Figure 2 below) extrapolates the 3-hourly SW track seen in Figure 1 for the next several hours..then slows the track and bends it more west as the northwestern low-level ridge (in Figure 1) begins eroding in advance of the frontal system in paragraph P1. Southwesterly flow ahead of the frontal system will then begin the long-awaited acceleration towards the NE later in the forecast period. All-in-all...I am now left of the NHC forecast instead of to the right (as I was yesterday).
Intensity-wise...It was good yesterday to not predict additional strengthening (reasons cited were Alberto's anityclonic outflow getting squeezed out and adjacent dry air). However...it would have been better to suggest weakening as we have seen in the last day. Additional unfavorable factors (easterly shear...dry air advection covered in Figure 1 and the 1st paragraph of this special feature section) suggest additional slow weakening...and so that is what is shown in Figure 2 below. Note that a recent burst of T-storms covering the center prevents me from showing very immediate weakening (burst perhaps caused by some instability between the cold temp of adjacent upper low and warm Gulf stream surface waters).
Impact-wise...I was expecting in the previous forecast that the tangible weather (i.e. some rainfall) of Alberto would spread onshore in NE SC and SE NC. The rainfall never did reach SE NC...instead has been tracking SW with Alberto along the SC shore...and now the heaviest rains are moving southward across Georgia's coast and even into the NE corner of Florida using the latest radar animations. Using my track forecast...I don't expect these rains (which may contain some gusty winds) to go further south than NE Florida. See the drawn-in blue-dashed swaths and blue-text statements in Figure 2 for further details on Alberto's forecast impacts.
Figure 2: My best guess forecast for Tropical Storm Alberto as of May 20, 2012 afternoon. Note how the swath of impacts (blue-dashed swath) is leaned to the right-of-track late in the forecast...as an accelerating storm has its worst impacts to the right-of-center...and as Alberto is expected to be under southwesterly shear from the frontal upper trough steering it NE.
P1...In the above birdseye charts...an upper trough continues pushing eastward from the western US. Divergent southwesterly flow ahead of this upper trough continues supporting a disorganized frontal cyclone with fairly low pressures...right now the lowest pressure being 1009 mb in SE Minnesota. Southerly low-level flow ahead of this 1009 mb cyclone is in competition with northerly low-level flow behind a 995 mb cyclone mentioned below in paragraph P3...resulting in a southern Canada front dividing the air mass contrasts between the cyclones. This front had an east-west orientation during the past 48 hours...but is now being lifted north ahead of the upper trough and features an intensifying 1006 mb depression over SW Hudson Bay.
P2...Upper ridge that dominated central North America has shifted eastward into NE US and E Canada in advance of weather system in paragraph P1. It continues to be supported by warm air advection ahead of 1009 mb cyclone in Paragraph P1. This upper ridge has an anticyclonic center over W New York whose southern lobe has absorbed the remainder of Alberto's warm core upper anticyclone. Convergence on the east side of the upper ridge supports surface ridge centers of 1025 mb moving offshore from NE US...and 1028 mb that has moved east from Hudson Bay into the Quebec/Newfoundland border. 24 hrs ago...northwesterly flow on the east side of this upper ridge converged heavily with westerly flow on the north side of Caribbean upper ridging mentioned below in Paragraphs P5 and P7...which created a lot of dry air in the Gulf of Mexico/N Florida. This dry air is no longer associated with this upper ridge...but now with the upper voriticity near Alberto as mentioned at the end of paragraph P4.
P3...Upper low vortex that moved from Labrador in the direction of S Greenland has passed south of Greenland's S tip. 993 mb surface cyclone (24 hrs ago) that was weakening with a lack of divergence beneath this upper low has weakened further to 995 mb.
P4...The cut-off upper low vortices 24 hrs ago....one over Georgia...and the other E of Virginia...have merged into a singular elongated upper low centered S of Alberto...extedning W into the Gulf of Mexico and NE into the W Atlantic (offshore of US). A 1006 mb cyclone 24 hrs ago was supported by divergence supplied from the upper vortex that was E of Virginia. This 1006 mb cyclone has now weakened to 1014 mb while beneath the less divergent axis of the now singular elongated upper low. This 1014 mb low is also tracking slowly west under the guidance of a 1025 mb ridge mentioned in paragraph P2...and could make landfall along the mid-Atlantic shore...but with weak (if any) weather. A final note about this elongated upper low is that it is now responsible for what has been dry air in the Gulf of Mexico/N Florida. Northwesterly flow on its SW side today is heavily converging with westerly flow on the north side of Caribbean upper ridging (paragraphs P5 and P7)...leading to sinking motion below for the persistence of this dry air.
P5...Caribbean upper ridging persists...and continues to extend NE into the open Atlantic for several-hundred miles. A portion of this NE extension has broken off and is a seperate upper ridge over the Azores. This break-off is caused by locally intense warm air advection ahead of the cyclone in paragraph P3. The remainder of this NE extension is now sustained by latent heat release of extensive T-storm activity in the W Caribbean...Cuba...and Bahamas as discussed in detail at the end of paragraph P8. The 1034 mb surface ridge center W of the Azores used to be supported by conergence on the E side of this NE upper ridge extension. Technically it is not beneath any such upper convergence at the moment...so this 1034 mb surface ridge may weaken in the next several hours.
P6...The upper trough from the E Azores to Europe 24 hrs ago is undergoing a split due to the strength of the NE upper ridge extension covered in previous paragraph P4. One split is an east-west upper trough S of the Azores...and the second split is an upper trough fragment venturing into W Europe that will soon leave the scope of the above charts. A surface trough over the Iberian peninsula was shown at 1200Z TAFB...probably supported by divergence on the E side of the W Europe upper trough fragment.
...TROPICAL BELT DISCUSSION...
P7...As mentioned earlier in paragraph P5...upper ridging continues across the Caribbean..and this upper ridge has been extended NE into the Atlantic for several hundred miles. The embedded weak upper low S of Jamaica has dissipated. Low surface pressures in the western Caribbean (with a 1009 mb center) used to be maintained by the outflow of this upper ridge...but is now being maintained differently as explained in paragraph P8.
P8...The upper vorticity near TS Alberto has consolidated into a single center just south of Alberto (see paragraph P4 for details). Southwesterly upper flow is directionally divergent between this upper voriticty and Caribbean upper ridging mentioned in paragraphs P5...P7...and P8...leading to T-storm activity in the W Caribbean...Cuba...Bahamas...and waters NE of the Bahamas. A surface trough formed just N of the Bahamas yesterday under this directional divergence...and the 1200Z TAFB has just added another surface trough in the W Bahamas. A 1009 mb W Caribbean disturbance used to be supported by the outflow of the Caribbean upper ridging...but is now supported by this directional upper divergence. One could almost connect this 1009 mb disturbance...the W Bahamas surface trough...and the surface trough N of the Bahamas into a "singular surface trough." 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 west US upper trough near paragraph P1...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's NE extension.
P9...New upper low vortex persists NE of the Lesser Antilles (its formation covered by paragraph P5 in discussion #13). It has begun mixing out some of the extensive dry air with divergence on its NE side. The lift created by this NE divergence is creating a new batch of T-storm clouds seen NE of the Lesser Antilles in satelllite imagery. This upper low's divergence has also created a new surface trough midway between the Cape Verdes and Lesser Antilles...added at 1200Z TAFB today. Expect this new surface trough to move west under the guidance of strong 1034 mb ridge mentioned in paragraph P5. Under such a west track...the surface trough will leave the upper low's divergence that spawned it...so it should weaken once it moves west.
P10...Upper ridging continues in the E tropical Atlantic. Upper outflow of this ridge coupled with surface convergence from a tropical wave (now SW of the Cape Verde Islands) has generated some lift to moisten away some of the dry Saharan air that was wafted into this region during the last 48 hours. However...the tropical wave's development is not expected as it will encounter hositle southweserly shear thanks to the upper low in paragraph P9.
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