As a compact but anomalously-deep middle/upper-tropospheric low approached Hawai’i on 29 November 2021, GOES-17 (GOES-West) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed complex lee wave structures southwest of the island chain. There were a few pilot reports of moderate turbulence in the general vicinity of these waves. 

Even though 250 hPa wind speeds along the western side of the upper-level low were relatively light (30-60 knots), those north-northeasterly upper-tropospheric wind speeds were higher than normal for Hawai’i during the end of November. A Turbulence Probability product (below) indicated that the risk for Moderate or Greater (MOG) turbulence was generally low (less than 50%), and focused along the dry slot near the axis of the 250 hPa jet streak. Note that the Turbulence Probability began to decrease later in the period, as those 250 hPa jet streak winds began to relax over the area. 

Training for this Turbulence Probability product is available here and here.

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GOES-16 True-Color imagery from the CSPP Geosphere site (link showing the data above) on 26 November, above, show features associated with strong flow through Chivela Pass in southern Mexico, gap winds often called Tehuano winds or Tehuantepecers. Strong descent associated with these events can often limit the presence of clouds that can be used as tracers. However, scatterometry (from this website) will show surface winds, and an MetopB overpass shortly after the end of the animation above, below, shows a core of strong winds over the ocean.

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The GOES-16 CONUS domain extends southward to the northern part of the Gulf of Tehuantepec (about 14.6 N Latitude). Visible imagery from 1516 UTC, below, is overlain with the Derived Motion Wind vectors (in the surface – 900 mb layer) at the same time. Strong northerly winds north of Chivela Pass are apparent, but the lack of clouds to track in the Gulf prevented the inference of winds there from the GOES-16 data.

The strong winds are also associated with a local increase in Aerosol Optical Depth (AOD), as shown below.

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Strong winds will cause significant mixing in the upper part of the ocean, which will result in cooling. Imagery from this website (shown below) shows cooling in the Gulf from previous events. Here is an animation from that website, courtesy Tim Schmit, NOAA/NESDIS/STAR

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In GOES-17 True Color images created using Geo2Grid (above), enhanced forward scattering during the morning hours helped to highlight the offshore transport of airborne dust.

Other blog posts discussing similar Tehuano wind events can be found here.

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30-second Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm), Dust RGB and Split Cloud Top Phase (11.2 µm – 8.4 µm) brightness temperature difference (BTD) images (above) revealed a plume of blowing dust propagating northward across the San Juan Province of western Argentina late in the day on 24 November 2021. The dust was being channeled through a gap in higher terrain along the foothills of the Andes (below).

A larger-scale view of hourly GOES-16 Visible images with plots of surface reports (below) suggested that this dust occurred in the vicinity of a strong cold front that was moving northward across Argentina.

Mendoza — located south-southwest of where the dust plume first became apparent in GOES-16 imagery — reported a thunderstorm with dust at 2000 UTC (along with a southeasterly wind gust to 32 knots), followed by a reduction of surface visibility to 0.5 miles at 2215 UTC as the air temperature sharply dropped with the cold frontal passage (below). About 260 miles (420 km) east of Mendoza at Rio Cuarto, a similar sharp temperature drop was seen as the cold front passed.

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GOES-17 Infrared imagery, above, centered on American Samoa, shows several low-level cloud lines from which showers are developing (and then rapidly dying, suggestive of strong shear, as noted in this 0600 UTC shear analysis taken from this website). There are also convective elements developing over/near some of the islands. What is the moisture/stability distribution around these showers?

GOES-17 Total Precipitable Water fields, below, show American Samoa at the edge of a moist (TPW exceeding 2″) band (associated with the South Pacific Convergence Zone), dryer air (TPW is around 1.2″) to the northeast and relatively dry air to the south (TPW is around 1.3-1.4″ in patches). There is an increasing amount of noise in this Level 2 product starting around 1200 UTC, manifest as horizontal lines, that arise because of the poor functioning of the GOES-17 Loop Heat Pipe. As the ABI instrument’s focal plane’s temperature increases, bands that are used in the computation of Total Precipitable Water (including Band 15), become noisy. (Note that Band 13 on this day is not obviously affected by the increase in the focal plane temperature).

GOES-17 ABI data can also be used to estimate atmospheric stability, as shown below. Lifted Index fields (also showing Loop Heat Pipe-related striping at the end of the animation) show strongest instability in the region where showers are most common to the north of American Samoa — in the moist band. The strongest instability is over the southwestern part of this domain (the diagnosed Lifted Index there is near -5). Level 2 products from GOES-17 can give hints as to where convection will form out over the open ocean where conventional observations are sparse, even when Loop Heat Pipe issues with GOES-17 start to become obvious.

Special note for the Lifted Index animation above: The bounds of the Lifted Index values have been changed from the AWIPS default — -10 to 20 — to -5 to 7; this was done to better differentiate between small variations in stability.

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