2012 Atlantic Hurricane Season Birdseye Discussion #91

By: NCHurricane2009 , 11:21 AM GMT on August 29, 2012

...AUGUST 29 2012...7:10 AM EDT...
In the last 24 hours...Isaac finally strengthened into a category 1 hurricane. Unforutnately...he has stalled as he makes landfall in southeastern Lousiana this morning. The large-sized wind field and slow motion of Isaac near the shoreline for the next several hours prolongs the storm surge flooding threat. For all US Gulf coast residents...mandatory evacuation orders should have been obeyed by now in storm surge prone areas. Latest watches/warnings can be found on www.nhc.noaa.gov. See Isaac special feature section below for additional details.

Open Atlantic tropical wave Invest 97-L has become Tropical Depression Eleven...then Tropical Storm Kirk...in last 24 hours...but is expected to recurve northeastward into the open Atlantic. See Kirk special feature section for details.

Strong tropical wave that emerged from Africa in previous discussion is already SW of the Cape Verde Islands this morning while becoming better organized. As a result...it has been upgraded to Invest 98-L...and I expect a 100% chance this will become our next Atlantic tropical cyclone. See Invest 98-L special feature section below for details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-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.

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).

My latest forecast versus the NHC's is shown in Figure 1 below.

Track-wise for Isaac...the NW track caused him to make landfall over the Missisippi River delta near Buras, Louisana as we have been expecting. This NW track was caused by surface ridge weakness of surface front mentioned in paragraph P1. Since that time...he has actually moved a little more left than expected under the influence of blocking low-level ridge that has built into the central US (cause of central US ridge mentioned in paragraph P1). This means my track and the NHC's in Figure 1 are adjusted leftwards from the previous. The blocking ridge has also slowed his track to a nearly-stalled to perfectly-stalled track for the time being...but the next surface frontal system and mid-latitude shortwave upper trough should pass to the north and weaken this ridge for the NW track to resume. That is why for the short-term I agree on a slow NW track into Lousiana in alignment with NHC's track.

Longer-term...it looks like the picture is more complicated than I previously thought...causing me to have a leftward-biased and much slower track inland than what the NHC shows. After examining the 00Z GFS model this morning...currently I am confident that the next shortwave upper trough and surface frontal system will pass too far to the north to recurve Isaac northeastward...unlike what we were previously promoting. In fact yet another blocking low-level ridge quickly builds behind the upper convergent back side of the passing shortwave upper trough...this second blocking low-level ridge causing me to show a slow NW track and fall behind the NHC's forward pace thru the first 72 hrs (thru 4 AM Fri).

By 72 hrs and beyond...I decide to keep my track further left than 00Z GFS...as the 00Z GFS by that time pushes Isaac rightwards directly into the second blocking low-level ridge to the NE...which doesn't make sense to me. I do see the 00Z GFS has zonal upper westerly flow over Isaac by that time...so perhaps it sees Isaac as an extratropical (non-tropical) low supported by and attracted to surface pressure falls of an upper westerly jet (maybe that's why 00Z GFS bends it rightwards directly into the low-level ridge). Instead at this time...I prefer moving Isaac NE into Iowa as it rounds the west side of the low-level ridge and recurves ahead of yet another frontal system/upper trough swinging in from the NW by 96 and 120 hrs.

Warnings remain spread across across US Gulf coast from Louisiana to ...parts of Florida panhandle...including all of the Alabama and Mississippi coastline in between. Mandatory evacuation orders in all storm surge prone areas along the US Gulf coast should have been obeyed by now. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.

Figure 1: My forecast for Hurricane Isaac this morning

Intensity-wise...Isaac became a hurricane finally...after mixing out the dry air mentioned in the Isaac section of previous discussion #90. Now as he moves into Lousiana...its all about forecasting the weakening rate. My intensity forecast for weakening due to landfall is upped from my previous due to him strengthening 5 mph above what I thought he would reach...and due to the delay in him moving inland. In short...I now have him a remnant low by 4 AM Fri instead of 4 PM Thu like I showed previously.

Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 10 PM CDT NHC advisory...later declining in size to represent weakening due to landfall. The latter part of the swath has more to do with heavy rain rather than gusty winds...as Isaac should be a tropical depression or remnant low later on. Some highlights to note about the impact statements of Figure 1. Impact statement (c) highlights the storm surge threat...which is going to be more significant than usual for a minimal hurricane....as the creation of the large tropical storm wind radius stirs up a large amount of water...so all mandatory evacuation orders along the US Gulf coast should continue to be obeyed. Impact statement (b) makes a point about an emerging flood risk when the storm slows and associated heavy rains persist over the same area. Impact statement (b) also emphasizes that heavy rains can be expected in squalls outside of the shown impact swath. I continue to expect these far reaching squalls to be biased to the east...thanks to frontal system moving into the eastern US in paragraph P1 spreading the moisture. Based on radar and satellite....these rains have already started and ended in last 24 hrs...as this batch of moisture is already racing into the Atlantic offshore of South Carolina and paralleling the NC coast on radar.

Tropical wave midway between the Cape Verdes and Lesser Antilles...in the open tropical Atlantic waters...has strengthened into TD 11 then TS Kirk in last 24 hrs. It strengthened in a southerly shear environment on the east side of paragraph P4 upper vortex located adjacent to this feature. Satellite image in Figure 3 shows when TD 11 became upgraded to Kirk...which shows the surface center is on the south edge of the storm canopy rather than at eye feature in the storm canopy that was visible at the time. Therefore this eye feature is believed to be the mid-level center.

Atlantic surface ridge north of Kirk (paragraph P3) is steering this system westward...while the adjacent upper vortex is currently just an agent of southerly vertical shear. As I generated my track forecast in Figure 2...I had quiet a rightward bias relative to the NHC 11 PM EDT forecast from last night. The new 5 AM EDT NHC track forecast has come much closer into agreement with mine...but I still have a tad of a rightward bias. This gives me more confidence in my track forecast...which was genereated before the 5 AM NHC forecast was released. The following three reasons are why I have a rightward-biased track forecast

(1) The 1021 mb ridge center to the west of Kirk seen in the above atmo chart would slow his westward progress.

(2) In as little as 24 hrs out...00Z GFS from this morning shows the cold front from the 988 mb vortex (paragraph P1) creating a break in the paragraph P3 subtropical ridge...pulling Kirk northward.

(3) The surface center may regenerate northward into the storm canopy.

By 96 hrs in my track forecast...I expect Kirk to become non-tropical as it merges with a cold front of the next system to shoot into the Atlantic right behind the system in paragraph P1.

Figure 2: Forecast for Tropical Storm Kirk this morning

Figure 3: Shortwave Infrared Early morning satellite image of when TD 11 became TS Kirk. Blue clouds are higher in the atmosphere...and black clouds are lower in the atmosphere. The low cloud motions are used to deduce where the storm is centered at the surface.

For intensity...I chose to stay below the 5 AM EDT NHC forecast which predicts a 75 mph hurricane (my suggested max intensity is a 60 mph tropical storm). I presume the NHC and models expect that Kirk will distance himself from the shearing upper vortex as he turns northward...and hence allow the surface center to slide into his favorable t-storm canopy whose latent heat release is providing good upper outflow as seen in Figure 3. However...I am more shy with my intensity forecast because of the above thermo chart...which shows paragraph P3 dry air covering the south half of the storm...which is inherently more stable. My thinking is that the surface center could pull this dry air into the t-storm canopy and weaken it...and in fact the t-storm canopy has already been steadily weakening since the time of Figure 3 and now. The reason max intensity is held thru 96 hrs is that he would get supportive upper divergence from accelerating southwesterly flow above the cold front he is merging with.

Impact swath in Figure 2 is initialized based on a combo of the tropical storm wind radius shown at 5 AM NHC advisory and the orange shaded t-storm clouds in the AVN infrared satellite color scheme. It becomes more symmetric about the storm track as Kirk turns more northward into his t-storm canopy...and alothough he may get southwesterly shear as he gets non-tropical...the NE track by that time still allows the impact swath to be symmetric about the storm track.

Vigorous tropical wave is emerging from Africa in previous discussion is now SW of the Cape Verde Islands. Its cloud pattern is still signatory of easterly vertical shear on the south side of paragraph P4 upper ridge. However...its t-storm latent heat release is locally inflating this the upper ridge into an upper anticyclonic center that enhances the outflow and reduces the shear. Given the recent string of tropical wave development and computer model support this wave has...I am assuming now a 100% chance it will eventually become a tropical cyclone. For the shorter-term...computer model runs suggest that this system will not get swept up in the paragraph P3 ridge weakness that Kirk will...and therefore show a more westerly track with this system than Kirk.

P1...Upper trough in the mid-latitude westerlies has entered the eastern US and eastern Canada. Upper divergence east of this upper trough continues supports a surface frontal that has moved into the eastern US that curls into a vigorous cyclone moving into NE Canada (marked with 988 mb center in above atmo chart). Upper convergence behind this upper trough supports a surface ridge building into the central US and central Canada...with multiple centers above 1020 mb becoming marked in the area.

P2...Main of upper trough persists in the Atlantic high seas southeast of Greenland...leaving behind a shortwave along the NE US shore that has pushed into the NW Atlantic in last 24 hrs. Surface frontal cyclone supported by divergence from this upper trough has moved into Europe from the British Isles...with a second frontal cyclone seen on satellite trailing behind and also about to head into the British Isles. 1020 mb frontal low...supported by divergence from the NE US-NW Atlantic shortwave mentioned above....has passed north of Bermuda. Actually...this frontal low is no longer shown in HPC or TAFB maps as if it dissipated...but there is a band of clouds NE of Bermuda that is a continuity of this system. North-central Atlantic 1018 mb low mentioned in paragraph P2 of previous discussion is now 1010 mb and moving into the Azores...its intensification supported by divergence east of the Atlantic high seas upper trough.

P3...Atlantic surface ridge with 1021 to 1022 mb centers persists. Easterly flow on the south side of this ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge is supported by northwesterly flow (on back sides of paragraph P2 upper troughs) converging with westerly flow (on north side of paragraph P4 upper ridge).

P4...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm and across the Gulf of Mexico/west Caribbean. The remainder of the upper ridge is in east half of the Atlantic. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone...currently located in the central Caribbean. Tropical wave Invest 97-L has strengthened into Tropical Storm Kirk...resulting in local inflation of the upper ridge which in turn is resulting in a persistent upper vortex SW of and adjacent to Kirk. Another embedded upper vortex in this upper ridge appears N of the Cape Verde Islands...perhaps remnants of upper vortex near the Canary Islands mentioned in paragraph P3 of previous discussion #90.

P5...Upper vortex in the Bay of Campeche is dissipating near the Yucatan as seen in the above atmo birdseye chart...getting squahsed out by the sprawling upper anticyclone of Hurricane Isaac.

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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