An analysis of the Saharan Air Layer, above (from this website), shows dry conditions stretching from Africa to the southwestern Atlantic, wrapping around the clouds associated with a strong tropical disturbance that is forecast to move northwestward over the Lesser and Greater Antilles in the next couple days.  (Both the dry air and the presence of high terrain in the Antilles will likely affect the development and structure of this storm).

The Saharan Air Layer is accompanied by an Elevated Mixed Layer (EML) that was apparent in the 0000 UTC Upper-Air sounding from San Juan (TJSJ), as shown below (from this site).  (At 0000 UTC, Puerto Rico was entrenched within the SAL air as shown in this analysis).  Note the steep lapse rate from 800 to 550 mb and the strong east winds in the layer.  By 1200 UTC, moist air moving in from the east had altered the EML.

NOAA-20 overflew this region just after 0600 UTC (orbits, from this website), and its thermodynamic profiles also gave evidence of the EML.  NUCAPS profiles in the region were produced by infrared/microwave retrievals that converged to a solution, as shown in the map below.  (The region of the tropical disturbance, over the Leeward Islands, shows red sounding dots where rain is likely falling).

Two soundings from this overpass are shown below, along 65º W at ~21º N and ~20º N, also show evidence of an Elevated Mixed Layer in the same region of the atmosphere.  (It is more apparent at 21.15º N/64.73º W)

Gridded NUCAPS fields are available in AWIPS, but they are also available outside of AWIPS at this site from NASA SPoRT.  850-500 mb and 700-500 mb Lapse rates, shown below, show a region of steep lapse rates as you might expect from an EML to the north of Puerto Rico (yellow in the color enhancement applied).

Gridded NUCAPS fields also confirm the dry air associated with the SAL air that contains the EML.  Total Precipitable water is around 30 mm, and 850-mb relative humidity is under 40% in the region where the EML is indicated by lapse rates.

For the latest information on Potential Tropical Cyclone #9 (should this system be named, the next name in the Atlantic alphabet is Isaías), refer to the National Hurricane Center website.

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JMA Himawari-8 “Red” Visible (0.64 µm) images (above) showed trains of vortices extending downwind (southeast) of the Kuril Islands on 28-29 July 2020 (surface analyses). One unusual aspect of these features was the development of small cloud elements within the cloud-free “hole” portion of some of the vortices.

H/T to Santiago Gassó for alerting us to this interesting case.

Himawari animation (fwd/bkwd) confirms it, condensation starts inside the clean holes which remain as cloud after the surrounding dissipates. @CIMSS_Satellite @GOESguy https://t.co/BLx2hqKdEo pic.twitter.com/PW6xNHQsWd

— Santiago Gassó (@SanGasso) July 29, 2020

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MIMIC TPW rocking animations from 17-27 July 2020 (and back) [click to play mp4 animation]

Morphed Microwave Imagery at CIMSS (MIMIC) estimates of Total Precipitable Water are derived from microwave sensors such as AMSU and ATMS on different polar-orbiting platforms. MIRS retrievals are used to estimate Total Precipitable Water from each polar swath, and those swaths are then advected forwards and backwards by GFS model winds. In this way, global coverage is achieved; each point on the globe is influenced most by the closest polar pass that most recently sensed the atmosphere. In some cases, that closest pass might occur after the time of the image. Thus, final images in this animation will change with time until about 16 hours after the time of an image. (You can find a training video on this product here, and data are available online here).

The animation above (click here for an animated gif) shows hourly data for ten days ending late on 27 July; during this time, Hurricane Douglas formed and moved just north of the Hawai’ian island chain. The rocking animation clearly shows the initial impulse for Douglas emerging out of the Intertropical Convergence Zone south of Mexico and moving west-northwestward towards Hawai’i. One can speculate on the effect of the moisture associated with Pacific Tropical Depression 7E, which disturbance formed just before Douglas and to its west, had on Douglas’ structure as it moved north of Hawai’i. Did that extra moisture help?

Ten-day rocking animations of MIMIC are routinely available for the eastern Pacific, for North America and for Australia.

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1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed  Category 1 Hurricane Douglas as it moved just north of Hawai’i during the day on 26 July 2020 (the boundary of the Mesoscale Sector was abruptly shifted westward at 1658 UTC). The coldest cloud-top infrared brightness temperatures were around -80ºC.

The apparent storm center as seen in satellite imagery was shifted north of the actual surface center location, due to the presence of southerly/southwesterly deep-layer wind shear as shown by a 20 UTC analysis from the CIMSS Tropical Cyclones site (above). This offset was also evident in a comparison of a DMSP-17 SSMIS Microwave (85 GHz) image at 1711 UTC with the analyzed surface position of Douglas at 1800 UTC (below).

===== 27 July Update =====

GOES-17 Visible and Infrared images (above) showed that the low-level circulation became exposed from the deep convection of Douglas a few hours after sunrise on 27 July, as it moved southwestward near the island of Nihoe. An analysis of deep-layer wind shear at 22 UTC (below) indicated the presence of 25-30 knots of southwesterly shear over that area — an unfavorable environment for tropical cyclone intensification/maintenance, in spite of the fact that the storm was moving over favorably-warm water.

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