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By: NCHurricane2009 , 8:09 AM GMT on September 13, 2012
...SEPTEMBER 13 2012...4:15 AM EDT...
Tropical Storm Nadine strengthening a little faster than anticipated...and should become a hurricane later this morning. Nadine is then expected to recurve northward then eventually eastward...hence keeping the storm over open waters. The tropical cyclone could threaten the Azores beyond 5 days out...so interests in the Azores should monitor the progress of this system. See Nadine special feature section below for further details.
In the last 24 hours...the National Hurricane Center has introduced in their tropical weather outlook an area of loosely-clustered t-storms along a cold front over the western Bahamas. Currently...the outlook probability is at a low percent...and I do not see enough signs of development to consider this a special feature on this blog. See paragraph P2 in mid-latitudes discussion for details on this area.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-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...
In the last 24 hours....Nadine has strengthened faster than expected. My forecast on Nadine versus the NHC's is shown in Figure 1.
Track-wise...Nadine at 11 PM EDT is just north of 50W-20N rather than over that location...which means Nadine has been following the NHC's track better than my previous (which had an initial southward bias). The call on the initial southward bias was made based on projecting the NHC recorded storm track...and once again if the recorded storm track in Figure 1 is extrapolated...one would see an initial southward (or leftward) bias. However...I do not want to repeat another bias with the NHC forecasting doing well in the last 24 hrs...and moreover there has been a slight left shift in the NHC track in the last 24 hrs that I think is enough compensation for the fact that the current NHC recorded storm track seems to have a slight left angle with respect to the NHC forecast track. Therefore I agree with the NHC short-term forecast track.
With that said...in order for the short-term NHC track to verify...Nadine would soon have to turn NW. Nadine is probably strong/tall enough in structure to "feel" steering influence from the paragraph P4 upper vorticity...and computer models continue to support the gradual formation of a broad western Atlantic surface low along ex-Leslie's cold front (supported by cut-off upper vortex along paragraph P2 upper trough) in the next 24 hrs. Both of these facts suggest that a NW turn for Nadine should soon occur. Between 24 and 48 hrs...00Z GFS still develops a new low-level ridge west of Nadine which I surmise is from upper convergence as Nadine's upper outflow clashes with paragraph P7 Caribbean-to-C Atlc upper ridge. This low-level ridge prevents Nadine from progressing further westward between 24 and 48 hrs...so Nadine has no choice but to go straight north while attracted toward the western Atlantic low to the NW.
The steering picture still gets more complicated between 48 and 72 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 east US low-level ridge arriving to her north...the paragraph P6 low-level ridge to her southeast...and the ex-Isaac deep-layered vortex (paragraph P5) to her northeast. With all this conflicting steering...I honestly prefer to keep Nadine stationary between 48 and 72 hrs. But because the models and NHC forecast still want to move Nadine faster to the NE than what I am thinking for that timeframe...I have decided to conform a bit more and move Nadine more NE than I had done before.
After 72 hrs...00Z GFS shows a shortwave fragment of paragraph P2 upper trough re-enforcing the ex-Isaac deep-layered vortex...which helps drag Nadine eastward towards it. But what really inclines me to agree with an eastward motion by that time is that the low-level ridge to her north getting knocked out by the paragraph P1 upper trough. Coupled with paragraph P6 low-level ridge to her south and re-enforced ex-Isaac deep-layered vortex to her east...it makes sense to show eastward motion after 72 hrs. After 120 hrs (not shown in Figure 1)...models show Nadine catching up to what is now the paragraph P1 eastern US low-level ridge cell which would be NE of Nadine by that time. This catch-up to the ridge could deflect Nadine northward toward the Azores region later on.
Figure 1: My forecast for Tropical Storm Nadine created this morning.
Intensity-wise...Nadine has strengthened more than expected in the last 24 hours with a 30 mph max wind increase during that time. If this continues...Nadine should become a hurricane later this morning. Despite the faster than expected intensification...the NHC still forecasts a peak of 85 mph max winds as they did 24 hrs ago...but I prefer to make Nadine have a higher peak than previously given the higher initial intensity. However...I do not prefer extrapolating another 30 mph max wind rise for the next 24 hrs...nor significantly overshooting the NHC intensity forecast...because Nadine's western upper outflow looks to be becoming increasingly choked by the paragraph P4 upper vorticity. Because of my westward bias in position from 48 hrs and onwards...Nadine would be closer to westerly shear generated by what will be an incoming paragraph P1 upper trough...so I weaken Nadine faster than the NHC shows during that time with the assumption that Nadine follows my forecast track. However...my weakening rate is slower than shown before because my forecast track positions during that time are now closer to the NHC track positions.
The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius. The impact swath is gradually grown in size until peak intensity...and then shrunken in accordance with weakening late in the forecast period. Note the rightward bias in the impact swath (with respect to my forecast track) beginning Friday...associated with westerly vertical shear that should be starting by that time.
P1...Next upper trough and surface frontal system in mid-latitude westerlies is entering the upper-left corner of above atmo birdseye chart. The surface front has advanced eastward into the central US...which curls into a 983 mb frontal cyclone over Hudson Bay in Canada. Warm air advection ahead of the front supports upper ridge wave over the NE US and E Canada. Upper convergence of the upper trough supports a low-level ridge building over Montana.
P2...Upper trough has advanced into the western Atlantic...and its eastern divergence supports the still-strong remnant low of Leslie which has zipped NE by the south tip of Greenland. Even though Ex-Leslie is exiting the picture...it should be noted she will continue to bring elevated winds and surf to the Atlantic high seas shipping lanes as she heads toward northern Europe. The associated surface frontal zone tailing from ex-Leslie is stretched across the central Atlantic and Gulf of Mexico. A portion of the frontal zone (over the W Bahamas) is beneath a favorable upper anticyclonic center of paragraph P7 Caribbean upper ridging...an area the National Hurricane Center has highlighted in their tropical weather outlook in the last 24 hrs. So far there are no signs of tropical development on satellite imagery...and there is no computer model support for this area. Albeit the closest support is from an outlier NOGAPS solution...which is the only model that retrogrades a fragment of the western Atlantic surface low (mentioned in the Nadine special feature section) westward to the north of the Bahamas...but the NOGAPS positioning places it north of the favorable upper anticyclonic center such that it would get ripped by unfavorable westerly vertical shear. Elsewhere...the western upper convergence of the upper trough supports a large area of dry air and a 1029 mb surface ridge over the eastern US and W Atlantic. A fragment of this upper trough remains cut-off...now located over Arkansas/Louisiana/Gulf of Mexico area.
P3...To the southeast of ex-Leslie...the remnant of Michael has continued NNE and merged with the cold front extending from ex-Leslie. Evidence of this is found by archived 0000Z and 1200Z September 12 infrared satellite images found on UNISYS (http://weather.unisys.com/archive/sat_ir/1209/). Ex-Michael was positioned ESE of the south tip of Greenland as of 1928Z HPC analysis...but was then dropped in further HPC analyses as if ex-Michael has become indistinct along the front.
P4...Cut-off upper vorticity remains in the open Atlantic...now established as SW-NE upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7). An embedded upper vortex is currently over the northern Lesser Antilles.
P5...Remnant deep-layered low of Isaac is retrograding westward to the south of the Azores...steered by south side of P6 surface ridge supported by eastern convergence of amplified Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P7.
P6...Atlantic surface ridge has been eroded out of the western Atlantic as ex-Leslie's Atlantic cold front continues advances in. Its remaining eastern Atlantic portion is supported by the SE convergence of lengthy paragraph P7 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.
...TROPICAL BELT DISCUSSION...
P7...Upper ridging across the tropical Atlantic persists. The Caribbean to Central Atlantic upper ridge cell has been stretched into the NE Atlantic toward Europe thanks to low-level warm air advection ahead of the ex-Leslie (paragraph P2) system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air across the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa and above Tropical Storm Nadine.
P8...The tropical wave south of the Cape Verde Islands in the previous discussion is now SW of the islands. Its t-storm cluster remains weak...and using the above thermo birdseye chart...this appears to be due to paragraph P6 dry air.
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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