2012 Atlantic Hurricane Season Birdseye Discussion #119

By: NCHurricane2009 , 8:42 AM GMT on September 28, 2012

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...SEPTEMBER 28 2012...4:45 AM EDT...
An outage persists with GOES-E satellite imagery in the last 96 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next few days. See Nadine special feature section below for details.

Beginning in 48 hours...next upper trough in the mid-latitude westerlies (not yet in the scope of this discussion) is expected to produce high upper divergence with respect to SW US upper ridge in paragraph P1. This will drive the formation of a NE tracking surface low from the Gulf of Mexico...which could become quiet vigorous similar to Invest 93-L in discussion #110. Similar to 93-L...I expect this system to be non-tropical...but could still be capable of significant weather across the US Gulf coast...SE US...and along the east US coast. See details of 93-L in discussion #110 for the kinds of impacts a system like this could deliver.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1926Z-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...TROPICAL STORM NADINE...
Nadine has strengthened just a tad more than my intensity forecast showed in special update #118A. Strengthening continued as she has developed favorable upper anticyclonic outflow...becoming enhanced to the NE by departing paragraph P5 upper trough. It was speculated a few times that the upper vortex just SW of Nadine (currently mentioned in paragraph P6) would enhance the outflow to the SW...but it appears this feature has always been too close to Nadine and instead is providing some outflow blockage to the SW as evidenced by the storm canopy and upper outflow bias to the NE half of the storm seen on infrared satellite. This outflow blockage is why I maintain my special update #118A intensity forecast...despite Nadine being currently a little stronger than that intensity forecast. My intensity forecast shows weakening after 11 PM Fri (tomorrow night)...as Nadine turns northward over even cooler waters...and as the paragraph P6 C Atlc upper trough merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting vertical shear across Nadine with its east side. My current intensity forecast (shown in Figure 1) is in agreement with the 11 PM EDT NHC's for the first 24 hrs...then my weakening rate is more aggressive than the NHC's after that time. In fact...I have Nadine becoming a remnant low by 96 hours while the NHC asserts Nadine being a tropical cyclone thru the next 120 hours due to forecast uncertainty in the longer range.


Figure 1: My forecast for Tropical Storm Nadine generated this early morning.

Nadine is currently located west of the Canary Islands and south of the Azores...and has near-perfectly followed the previous short-term NHC (and my) track forecast. Curiously though...the NHC has done a slight rightward shift in the short-term track...while I choose to copy-paste my previous forecast points since the previous track forecast is doing so well. I also do not agree that Nadine will turn northward quiet as sharp as the NHC currently shows because the paragraph P4 ridge will be too close to Nadine from the NE...based on how the 00Z GFS model shows this ridge in its surface-level output this morning. All of this means I have a slight left bias in my forecast track for a period as seen in Figure 1. Northward hook by the end of the forecast is related to evolution of current E Canada 996 mb frontal cyclone and 1026 mb low-level ridge over the Great Lakes (both mentioned in paragraph P3). The straight north track eventually happens as Nadine gets pulled into the 996 mb frontal cyclone's narrow ridge weakness...located between paragraph P4 ridge by then located to the NE and what is now the Great Lakes ridge coming in from the NW. My straight north track remains slower than the NHC's current...and slower than the even faster 00Z GFS. These faster north-track solutions do not make sense to me as the two ridges are shown to be of equal strength and opposing influence. I think the GFS sees Nadine becoming a non-tropical cyclone supported by the eastern divergence of incoming paragraph P6 C Altc upper trough...in which case Nadine indeed would move faster to the north (and then potentially east into the Azores) while being coupled to and traveling with this upper trough. My vote for now is to dissipate Nadine in intense shear generated by this upper trough while she becomes trapped between the two low-level ridges.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track...then shrinking the swath away assuming Nadine follows my forecast for dissipation by 96 hrs. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Cut-off upper vortex from Utah..mentioned in paragraph P1 of previous discussion #118...has become deep-layered while stacking with broad surface vorticity its eastern divergence supports. Deep-layered vortex is positioned over SE Wyoming this evening...with its eastern upper divergence supporting a 1017 mb surface low in the W Dakotas and 1018 mb central US frontal depressions located along cold front trailing from 996 mb frontal cyclone in paragraph P3. In relatively higher pressures SE of this deep-layered vortex...SW US upper ridge (paragraph P1 of previous discussion #118) persists.

P2...Shortwave upper trough...and surface 1010 mb frontal cyclone supported by the eastern divergence of the upper shortwave...has entered the above atmo birdseye chart from Hudson Bay.

P3...Weather system remains anchored by upper trough over eastern North America gradually shifting into the NW Atlantic. The upper trough's eastern divergence continues driving E Canada frontal depression (cyclone)...which has intensified from 1007 to 996 mb in the last 24 hours. Upper trough's western convergence is driving intensifying Great Lakes surface ridge...which has strengthened from 1022 to 1026 mb in the last 24 hours. Western convergence of this upper trough also drives eastern US and NW Atlantic low-level ridge (which is now 1022 mb). Cut-off upper trough over the central Gulf of Mexico has shifted into the eastern Gulf and W Caribbean.

P4...Deep-layered ridge in the Atlantic...featuring an impressive 1030 mb center...persists. This deep-layered ridge is moving into the NE Atlantic while passing north of Tropical Storm Nadine.

P5...Amplified upper trough remains just offshore of Europe.

...TROPICAL BELT DISCUSSION...
P6...Upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell over W Atlantic...Cuba...and Bahamas has shifted east in advance of both upper troughs mentioned in paragraph P3...with the anticyclonic center of the cell now over Puerto Rico. Upper vortex just SW of Nadine (paragraph P5 discussion #118) has merged with upper vortex just NE of the Lesser Antilles (paragraph P3 discussion #118)...resulting in a central Atlantic upper trough. In relatively higher pressures southeast of this upper trough...upper ridge has built from the W coast of Africa as evidenced by upper-level cloud motions around paragraph P11 tropical wave.

P7...Surface troughing persists in Bay of Campeche...supported by split flow upper divergence between paragraph P3 east Gulf cut-off upper trough and paragraph P1 SW US upper ridge. Scattered t-storms across Central America are supported by this same upper divergence mechanism.

P8...Tropical wave continues across the central Caribbean Sea. It is producing scattered t-storms over the south-central Caribbean....Jamaica...and Cuba thanks to eastern upper divergence of paragraph P3 cut-off upper trough moving into the W Caribbean.

P9...Surface trough persists NE of the Lesser Antilles...which remains quiet enhanced by split flow upper divergence between the two merging upper vortices mentioned in paragraph P6.

P10...Eastern Atlantic surface ridge with a 1018 mb center persists NW of the Cape Verde Islands...albeit it has become eroded in its horizontal expanse thanks to Nadine's low-level low pressure field pushing in from the north. I currently speculate that this surface ridge is supported by upper convergence between northerlies from W Africa upper ridge (paragraph P6) and westerlies from paragraph P6 central Atlantic upper trough.

P11...Tropical wave W of the Cape Verde Islands continues producing a t-storm cluster while enhanced by upper outflow of upper ridge building from W Africa mentioned in paragraph P6.

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2. wxchaser97
10:13 AM GMT on September 28, 2012
Thanks NC09!
Member Since: March 16, 2012 Posts: 127 Comments: 7958
1. WunderAlertBot (Admin)
9:30 AM GMT on September 28, 2012
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