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By: NCHurricane2009 , 1:37 AM GMT on August 20, 2012
...AUGUST 19 2012...9:40 PM EDT...
As expected...Gordon weakens to a category 1 hurricane while bearing down on the southeasternmost Azores (Sao Miguel and Santa Maria) at this hour. Weather conditions in those islands are about to become severe...and not clear up until sometime tomorrow after Gordon passes. See first special feature section below.
The tropical Atlantic is extremely active at this hour. If two of any of the following areas of interest become tropical storms in the next three days...we will be tied or ahead of the record 2005 Atlantic hurricane season...with 10 tropical storms before August 22:
Area of Interest Invest 95-L...Even though Helene and its remnant low have become assimilated into the Mexican-SW US summer monsoon low pressure field...upper winds in the western Gulf of Mexico are still favorable for tropical activity...and we already have a new pop-up disturbance in the western Gulf that has been upgraded to Invest 95-L. See 2nd special feature section for further details.
Area of Interest Invest 94-L...see 3rd special feature section for latest info on this strong tropical wave.
Other Areas of Interest...Pop-up weather in the south-central Caribbean Sea has made landfall in Central America and can no longer develop (paragraph P7). Two additional tropical waves...one ahead of and the second behind Invest 94-L...also have organized t-storm activity and cyclonic turning (see paragraphs P9 and P10). Because the tropical wave in paragraph P10 was introduced into the National Hurricane Center (NHC) tropical weather outlook while I was writing this discussion...I have just granted it a special feature section on this blog. See 4th special feature section for details on paragraph P10 tropical wave.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1916Z-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).
...SPECIAL FEATURE...HURRICANE GORDON...
For Hurricane Gordon...my latest forecast (versus the NHC's) is shown in Figure 1 below.
Track-wise for Gordon...earlier in discussions #80 and #81...I had a track that passed Gordon's center over 30W-35N...then pass just south of Santa Maria. NHC at that time was proposing the center to pass north of 30W-35N and then pass between Santa Maria and Sao Miguel. My southward bias at the time was due to me giving more credence to low-level ridge intensification W of Gordon...as the low-level ridge S of Greenland (paragraph P2) merges with low-level Atlantic ridge (paragraph P4)...and by accounts in paragraph P2...this is beginning to occurr. Well tonight...it appears that Gordon's center is tracking in the middle between my and NHC's solutions from that time...which means Gordon's center will pass very near or over Santa Maria in the next hours. This is what is shown by the NHC at this hour...and I also agree with this solution. In Figure 1...the aforementioned intensification of the low-level ridging to the west is what causes the rightward bend in Gordon's track as a non-tropical remnant cyclone.
Intensity-wise...weakening continues as Gordon crossed the 26 deg C isotherm early this morning and into cooler waters. Gordon has been more-or-less following the weaknening rate shown in Figure 1 of discussion #82...so I maintain this weakening rate in Figure 1 below (this is a slightly slower weakening rate when compared to today's 5 PM EDT NHC advisory). This means I am forecasting Gordon to hit Santa Maria with category 1 hurricane force winds in the overnight...with the current track and current hurricane wind radius suggesting the east half of Sao Miguel also getting some hurricane force winds. The frontal zone draped near the Azores has been obliterated as Gordon has moved in...but Gordon's t-storms will gradual collapse over the more stable air over cooler waters such that it still becomes non-tropical.
Figure 1: Forecast for Hurricane Gordon this evening.
Impact swath in Figure 1 is drawn based on the tropical storm wind radius shown at the NHC 5 PM EDT advisory...then extrapolating that along the forecast track. I shrink it later on based on the forecast weakening.
...SPECIAL FEATURE...WEST GULF OF MEXICO DISTURBANCE INVEST 95-L...
While over east-central Mexico...Tropical Depression Helene and its remnant low quickly became ill-defined within the low pressure field of the Mexican/SW US summer monsoon. Despite this...outflow from W Atlantic upper ridge (paragraph P1) continues to enhance t-storms in this area...which have re-organized about a 1010 mb surface low that apperas to be centerd just offshore of Mexico and over W edge of Gulf of Mexico waters...judging by latest visible satellite imagery. Because of how well-organized the low is on visible...I am considering this a special feature on this blog. This low has been classified as Invest 95-L rather than ex-Helene...which tells us that officially this is not recognized to be the remnants of Helene.
Perhaps it is no surprise that disturbed weather in this area lingers...based on previous discussions on this blog (paragraph P6 discusion #81...paragraph P6 discussion #80...paragraph P5 discussion #79). All of these discussions suggested disturbed weather that would linger here for days...and perhaps that the frontal zone from paragraph P1 would dive south into this area and add to the activity...which indeed is beginning to occur. Satellite animations suggest surface low Invest 95-L is stationary...trapped in a low-level ridge weakness associated with this front...with the Atlantic low-level ridge (paragraph P4) to the east and west US/Canada low-level ridge to the north (paragraph P1) providing conflicting steering. Based on low-level GFS model animation...I expect 95-L to remain stationary thru 72 hrs...then begin drifting northward to south Texas beyond that time as the blocking low-level ridge to the north gets pushed eastward by the next frontal system in the mid-latitudes. With upper winds staying favorable thru this time...a tropical depression or storm is possible...so interests along the NE Mexico and south Texas coast shoudl monitor this system carefully. Even if no tropical cyclone development occurs...the risk of persistent rains could cause flooding problems.
...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Strong tropical wave now WSW of the Cape Verde Islands is still showing impressive t-storm activity and cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has generated a warm core upper anticyclone aloft....which has reducing the easterly shear and enhanced the upper outflow. As a result...tropical cyclone formation is still likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system eventually. I predicted tropical cyclone formation from this system sometime today...but it appears this has been delayed due to some level of dry air ingestion (source of dry air in paragraph P4).
In special update #82A...I had provided a 5-day (120 hr) outlook for this system...which is now a 4-day (96 hr) outlook. The paragraph P numbers in that outlook still fit with the mid-latitude and tropical belt discussion P numbers in this dicsussion as well. As a current 4-day outlook...I still stand behind that forecast for now...but I would say that the threat of a major hurricane in the NE Caribbean area is reducing due to the delay in this system becoming a tropical cyclone. So far...the track forecast shown in that outlook is doing quiet well. The more WNW track I show (which brings the storm into Hispaniola by day 4) has a northward bias compared to the main GFS model run...but on the other hand is in the midst of the spread of GFS's ensemble members.
...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE...
Another tropical wave has emerged from Africa recently...located well east of Invest 94-L...southeast of the Cape Verde Islands...and featuring cyclonic turning and organized t-storm activity. As I was writing this discussion...it has been introduced into the NHC tropical weather outlook...and likewise I have voted to give this a special feature section due to its organization.
As mentioned in the Invest 94-L forecast (special update #82A)...there is an upper vortex that is expected to form east of 94-L...associated with relatively lower pressures east of 94-L's upper anticyclone. Latest 200 mb wind barbs in above atmo chart reveal this upper vortex is already forming. Will have to carefully monitor if this emerging upper vortex suppresses or enhances the outflow of this tropical wave...depending on its relative position to this wave. If outflow enhancement occurs...tropical cyclone formation risk from this system is going to increase.
P1...Upper trough over the eastern US/Canada persists. Strong surface frontal cyclone supported by divergence from this upper trough...located over southern Hudson Bay...has weakened to 1004 mb under the less divergent upper vortex that is a product of its local cool air advection. Warm air advection ahead of this surface cyclone now supports W Atlantic upper ridge. W Atlantic upper ridge still has a SW-NE tilt...now stretching from E Mexico (where it supports the upper outflow of 95-L) to the central Atlantic waters E of Bermuda. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with 1020 to 1022 mb centers over the western US/Canada as seen in the upper-left of the above atmo chart.
P2...Upper trough regime over Atlantic high seas persists. Frontal cyclone just offshore of Europe in previous discussion is still performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Western upper convergence of Atlantic high seas upper trough supports a 1023 mb ridge just south of Greenland. Shortwave upper trough from Atlantic Canada is merging with Atlantic high seas upper trough...therefore losing its identity. Surface cyclone supported by this shortwave has moved from SE of Newfoundland to the waters W of the Azores while chasing Hurricane Gordon. Upper covnergence on back side of high seas upper trough is causing the 1023 mb ridge south of Greenland to merge with Atlantic surface ridge in paragraph P4...so either this surface cyclone will become squashed out of existence from this surface ridge development...or alternatively will dive southeastward on the east side of this surface ridge development.
P3...Elongated upper trough in the central Caribbean has been pushed westward into central America in last 24 hrs as it retrogrades about W Atlantic upper ridge mentioned in paragraph P1.
P4...Atlantic surface ridge with 1020 mb to 1023 mb centers is supported by a few upper convergent sources while stretching from the central Gulf of Mexico to the waters offshore of W Europe....including convergence SE of the of the W Atlantic upper ridge (paragraph P1)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.
...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. Hurricane Gordon continues to be near the north edge of this upper ridge. T-storm latent heat release from Gordon continues to locally inflate this edge of the upper ridge...which has resulted in an embedded shortwave upper trough south of Gordon. T-storm latent heat release from tropical wave Invest 94-L is causing this upper ridge to concentrate into an anticyclonic center over Invest 94-L.
P6...Upper vortex in central tropical Atlantic is retrograding westward...thanks to growing W Atlantic upper ridge (paragraph P1) to its NW. The upper vortex is currently passing north of the Lesser Antilles.
P7...Visible satellite imagery shows that yesterday's disturbed weather in the south-central Caribbean with light cyclonic turning has made landfall in Central America. The landfall and suppression beneath the paragraph P3 upper vorticity means that tropical cyclone formation is no longer possible with this system. Relatively new E Caribbean upper ridge that enhanced this disturbance yesterday still persists.
P8...Tropical wave that was in the eastern Caribbean in the previous discussion is now in the central Caribbean. It is currently beneath split flow upper divergence between the E Caribbean upper ridge in paragraph P7 and upper vorticity in paragraph P3...so an upheaval in t-storms with this tropical wave is possible during the next hours.
P9...Tropical wave midway between the Cape Verdes and Lesser Antilles continues producing t-storms extending hundreds of miles west of Invest 94-L. There remains enough cyclonic turning is in these clouds such that the NHC TAFB analyzed a 1010 mb low on the south end of this tropical wave as of 1800Z TAFB (and as shown in above atmo chart). Its t-storms remain supported by outflow beneath the E Atlantic upper ridge (paragraph P5)...coupled with enhanced poleward outflow streaming into paragraph P6 upper vortex.
P10...Another tropical wave has emerged from Africa recently...located well east of Invest 94-L...southeast of the Cape Verde Islands...and featuring cyclonic turning and organized t-storm activity. As I was writing this discussion...it has been introduced into the NHC tropical weather outlook...and likewise I have voted to give this a special feature section due to its organization. See 4th special feature section above for details on this system.
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