2012 Atlantic Hurricane Season Birdseye Discussion #144

By: NCHurricane2009 , 8:21 PM GMT on October 27, 2012

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...SATURDAY OCTOBER 27 2012...4:20 PM EDT...
This is one of the most important birdseye discussions due to the ominous threat that Sandy brings to the mid-Atlantic and northeastern United States. Preparations in these areas should be pushed to completion this weekend as landfall is imminent on Monday. For coastal areas along the mid-Atlantic and northeastern United States...pay attention to local officials for mandatory evactuation orders associated with storm surge threat. For areas coastal and inland on the mid-Atlantic and northeastern United States...a large area of damaging 40+ mph sustained winds appears imminent...see blue-dashed impact swath in Figure 1 of the Sandy special feature section for current best guess of where these winds are likely to be. Be prepared for power outages that last for days in this region...and keep in mind methods to stay warm without power as it will get cold as Sandy wraps in cold air on her back side. For additional details...see the Sandy special feature section below...and visit www.nhc.noaa.gov...www.nws.noaa.gov...and listen to national media...local media...and local officials.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1328Z-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 SANDY...
Concerning the east coast of the United States...offshore western Atlantic waters...and interior sections of the mid-Atlantic and parts of the northeastern United States located inside the blue-dashed impact swath of Figure 1...impact statments (a) through (c) are fairly descriptive of what is likely ahead. Please take these hazards seriously and listen to local officials and media. Latest coastal watches and warnings are found under the public advisory for Sandy at www.nhc.noaa.gov. Latest local forecast statements...for the coast and inland areas...for those likely to be affected by Sandy are found at www.nws.noaa.gov.

Track-wise for Sandy...kudos to the GFS model which during the previous discussion predicted a brief period of an eastward track for today that has actually occurred in response to blocking effect of 1032 mb North America surface ridge (paragraph P1) and attaraction toward paragraph P2 deep-layered low pressure system. As of 2 PM EDT...Sandy has begun accelerating NE in response to steering from the incoming paragraph P1 upper trough. While the NHC track forecast in Figure 1 and the 12Z GFS model for today are in strong agreement for the short-term...and while the current direction of travel aligns with both of these solutions...the recent NE acceleration is occuring faster than shown by both of those solutions. Therefore my track forecast in Figure 1 agrees with the direction of travel shown by NHC...but I disagree with the forward speed and instead show a faster forward speed. My current track forecast in Figure 1 now has the center moving into New Jersey...while the NHC forecast track has the center moving into Delaware. The reason for my northward bias is that by the time Sandy hooks to the west...she should be further north due to her current faster-than-expected NE acceleration which I extrapolated.

What causes her westward hook into the US is her interaction with the incoming paragraph P1 upper trough. She will take maximum advantage of the eastern divergence of the incoming paragraph P1 upper trough and intensify non-tropically. Cool air advection on her back side in turn will amplify the upper trough into an upper vortex that would whirl her NW and eventually W.

Even though the 12Z GFS initially agrees with the too-sluggish NE track shown by NHC in Figure 1...the 12Z GFS between 48 and 60 hrs suddenly catches up to my landfall point shown in Figure 1...and so for now I rely on the 12Z GFS solution for what happens to Sandy after landfall. By landfall time...the 12Z GFS shows the paragraph P1 upper trough split into an upper vortex over North Carolina and shortwave upper trough moving towards the Great Lakes. It has the North Carolina upper vortex shoot NE and dissipate...which would mean Sandy would cyclonically loop on the NW side of the dissipating upper vortex as I show for Tuesday. By Wednesday and Thursday...Sandy would then be in a position to be lifted northward by the shotwave upper trough moving across the Great Lakes...but interestingly the 12Z GFS shows Sandy's cool air advection amplify this shortwave into a new upper vortex over western NY state by Thursday. Afterwards...this upper vortex shoots NE...and so I have Sandy make another small cyclonic loop on Thursday around the west side of NE-shooting upper vortex.

Intensity-wise for Sandy...normally I do not predict intensity forecast points for the non-tropical phase of a tropical cyclone...but this event is going to be so significant for the mid-Atlantic and NE US that I feel I must. After weakening into a strong tropical storm (70 mph max winds) early this morning...she regained hurricane status (75+ mph max winds) after her tremendous cloud field's latent heat release punched out what has been an upper vortex near west Cuba that delivered dry air and southerly shear into the system. Due to her dramatically improved satellite appearance...my intensity forecast in Figure 1 for the next 12 hours suggests possible additional strengthening using tropical methods (t-storm latent heat release enhancing her anticyclonic upper outflow). Alternatively...the NHC prefers to maintain her current strength for the next 12 hours. After 12 hours...I re-join the NHC intensity forecast of 75 mph max winds as southwest vertical shear should increase from the paragraph P1 upper trough. The NHC and I agree that Sandy will have potential to strengthen a bit as she makes landfall...but this is through non-tropical processes as Sandy takes maximum advantage of upper divergence on the east side of the upper trough. I delayed transition to non-tropical by 12 hours from my previous...due to her currently-impressive and more tropical satellite appearance. She will then whirl beneath the less-divergent axis of the amplifying paragraph P1 upper trough...causing her to weaken as a post-mature non-tropical gale as she moves inland...and I use the NHC's weakening rate since I am inexperienced with precisely predicting the max wind speed of a decaying non-tropical gale. However by Thursday..I prefer to stay above the NHC intensity guidance as she may maintain strength or even re-strengthen a little if Sandy interacts with the eastern divergence of a shortwave upper trough moving across the Great Lakes as the 12Z GFS shows.


Figure 1: Hurricane Sandy Forecast

My impact swath initialization in Figure 1 is based on extrapolating the 11 AM tropical storm wind radius along my forecast track. This wind radius is larger than yesterday's...because Sandy's low pressure field size got another boost while taking advantage of mass split flow upper divergence between the dissipating upper vortex over west Cuba and her own warm core upper anticyclone. Therefore...my impact swath includes more areas than it did previously...and is also adjusted northward due to my northward adjustment in forecast track from my previous. Notice that late in the forecast...I expand the wind (impact swath) radius only in the NE half of the storm as she intensifies non-tropically. This is because as Sandy becomes non-tropical...her SW half will whirl in dry and cold air beneath the upper trough axis...while her NE half stays very moist. It is the rain bands in the NE half that will then mix down the strong upper-level winds of the paragraph P1 upper trough's jet stream to the surface. The storm surge threat is emphasized in impact statement (a)...because even though Sandy is only a category 1 hurricane by max winds...her wind field diameter is one of the largest ever for such a hurricane such that she will push far more water than a usual category 1 hurricane does.

...MID-LATITUDES DISCUSSION...
P1...Surface frontal system across central North America is working its way into eastern North America in the last 36 hrs. In the upper-levels...the system continues to be anchored by a large upper trough which has moved from the western to the central US. Vast divergence east of this upper trough has developed an intense 974 mb frontal gale over Hudson Bay in northern Canada....while vast convergence west of this upper trough supports a strong 1032 mb ridge centered over the central US. Low-level warm air advection ahead of the gale supports an upper ridge axis which has mostly joined Sandy's gigantic warm core upper anticyclonic outflow in the last 36 hrs.

P2...Deep-layered low pressure system/upper trough is moving from the western to central Atlantic. At the surface...their continues to be a broad low pressure spin in the low 990s of mb supported by eastern divergence of the upper trough. Meanwhile....the non-tropical remnant of Tony has conitnued ENE into the NE Atlantic...currently located midway between the Azores and Canary Islands at a location where it has escaped the aformentioned eastern upper divergence such that it has been weakening. Cool air advection of the aforementioned broad surface low continues to keep the upper trough amplified into an upper vortex. Meanwhile...eastern North America surface ridge supported by western upper convergence of this upper trough has become increasingly eroded in advance of 974 mb frontal gale mentioned in paragraph P1. Lastly...warm air advection ahead of the aforementioned broad surface low is maintaining the full-fledged NE Atlantic upper anticyclone.

P3...Upper trough in the NE Atlantic is moving ever so slowly eastward into western Europe while cut-off from the mid-latitude jet stream at a location SE of the upper anticyclone mentioned in paragraph P2 above. Upper convergence on the west side of the upper trough supports 1015 mb surface ridge in the eastern Atlantic. 1000 mb low associated with this system in the previous discussion has moved into western Europe over the last 36 hrs...and trailing from it is a cold front marked in the upper-right corner of the above atmo chart.

...TROPICAL BELT DISCUSSION...
P4...Upper ridge in the eastern tropical Atlantic persists. Pair of tropical waves WSW of the Cape Verde Islands in the previous discussion is now midway between the Cape Verdes and Lesser Antilles. Shower and t-storm activity with these features have declined in the last 36 hrs while leaving the favorable upper outflow of the east tropical Atlantic upper ridge...and while entering an area of dry air from the upper convergent SE quad of the massive upper anticyclonic flow associated with Hurricane Sandy.

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