With a large dome of high pressure centered over Iowa/Missouri/Illinois (above), the stage was set for a night of strong radiational cooling across much of the Upper Midwest on 17/18 October 2018. Minimum temperatures were generally in the 20-40ºF range, with the coldest being 15ºF at Champion in the Upper Peninsula of Michigan.

An animation of 5-minute GOES-16 (GOES-East) “Clean” Infrared Window (10.3 µm) images (below) suggested a lack of clouds across most of the region, except for some patchy mid/high-level clouds drifting slowly northward from Nebraska/Wyoming across western South Dakota and far southeastern Montana. However, a close inspection of the imagery revealed the subtle appearance of “motion” across northern/eastern North Dakota into eastern South Dakota and far western Minnesota. This was noted by Carl Jones (NWS Grand Forks), who further pointed out “strong winds just above the surface (45 kt at 900ft per KMVX VWP) mixing down a huge warm tongue at 900mb along with downslope influences”. A comparison of the GOES-16 Infrared images with topography showed the slightly higher elevation of the Coteau du Missouri (elevation around 2000 feet) across southwestern North Dakota and western South Dakota along with the more narrow Coteau des Prairies in northeastern South Dakota and southwestern Minnesota — and a subtle “downslope warming” effect could be seen on the Infrared images (warmer temperatures are darker shades of blue with the applied enhancement).

The same effect was evident on hourly images of the GOES-16 Land Surface Temperature product (below). However, with the Land Surface Temperature product enhancement, warmer temperatures appear as lighter shades of cyan.

Similarly, a sequence of higher spatial resolution Infrared Window images from Terra/Aqua MODIS (11.0 µm) and NOAA-20/Suomi NPP VIIRS (11.45 µm) (below) showed the slow propagation of the downslope warming. The lack of fog or low clouds was confirmed via this comparison of VIIRS Infrared Window and “Fog product” Brightness Temperature Difference at 0903 UTC; no airports were reporting any clouds or a surface visibility less than 10 miles.

12 UTC NAM40 winds and isotachs at a height of 0.5 km above ground level (below) verified the presence of broad southwesterly flow off the Coteau du Missouri and the Coteaus des Prairies, with subtle warming (darker shades of blue) immediately downwind.

Plots of 12 UTC rawinsonde data from Bismarck, North Dakota and Aberdeen, South Dakota (below) showed strong low-level temperature inversions that morning — and at the top of those temperature inversions, southwesterly winds of 27 knots at a height of 1056 feet over Bismarck and 26 knots at 728 feet over Aberdeen.

 

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More #highlatitudedust in #Greenland today! nice contrast of #MODIS and #VIIRS highlight the differences foam, sea-salt and dust aerosols , source is probabl mix of dust+snow @avoiland @jscarto @jobullard @TWMockford @HLCCD pic.twitter.com/xdm1RKoiBu

— Santiago Gassó (@SanGasso) October 14, 2018

As noted by Santiago Gassó, a long and very narrow plume of airborne dust was evident just off the southwest coast of Greenland on 14 October 2018. Terra MODIS and Suomi NPP VIIRS True Color Red-Green-Blue (RGB) images as viewed using RealEarth are shown below. An exposed (free of snow cover) glacial outlet between Qeqertarsuatsiaat and Paamiut was the point source of the dust plume — the change in water colors (shades of cyan) highlighted the offshore flow of meltwater from this glacier into the Labrador Sea, which then began to curve northward within the West Greenland Current. The strong pressure gradient between high pressure over southern Greenland and a low pressure southeast of the island (surface analyses) along with a passing trough axis caused brisk northerly winds, which lofted the aerosols into the boundary layer.

The plume of aerosols was also apparent on GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (below). The appearance of the plume on 1.61 µm imagery was due to the bright color of the “glacial flour” particles, which were efficient reflectors of incoming solar radiation — this brighter signature showed up well against the dark appearance of the water (which strongly absorbs radiation at the 1.61 µm wavelength).

The plume of airborne dust was also seen on GOES-17 Visible and Near-Infrared images (below), although the viewing angle was less favorable than from GOES-16.

* GOES-17 images shown here are preliminary and non-operational *

Unfortunately, there were no surface observations in the vicinity of the plume source to indicate how strong the surface winds were blowing; the closest active reporting sites along the southwest coast of Greenland were Godthaab/Nuuk to the distant north and Narsarsuaq to the distant south (large-scale Near-Infrared image). However, Metop-B ASCAT winds (source) just offshore of the plume origin area were in the 30-40 knots range around 1440 UTC (below).

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20 days after Leslie initially formed (and 17 days after it underwent extratropical transition), an Aqua MODIS True Color Red-Green-Blue (RGB) image viewed using RealEarth (above) showed the storm at 1419 UTC on 13 October 2018, when it was still classified as a Category 1 Hurricane off the coast of Portugal. The southwest-to-northeast oriented cloud band just west of Leslie was associated with an advancing cold front (surface analyses), which soon began to absorb the tropical cyclone and aid in its extra-tropical transition a few hours prior to landfall.

EUMETSAT Meteosat-11 middle/upper-tropospheric Water Vapor (6.25 µm) images (below) exhibited a warm/drying trend (brighter shades of yellow) along the western and southern edges of Leslie as it moved inland across Portugal. Hourly Meteosat-11 Water Vapor images visualized using RealEarth are available here.

Along the coast of Portugal a thunderstorm was reported at Porto (LPPR) from 1930-2000 UTC (about an hour before landfall). Farther to the south, shortly after landfall the surface winds gusted to 55 knots (63 mph or 28.3 m/s) at Monte Real Air Base (LPMR) at 21 UTC and 42 knots (48 mph or 21.6 m/s) at Ovar Military Base (LPOV) at 23 UTC. The highest wind gust was 95 knots (110 mph or 49 m/s) at Figueira da Foz, located along the coast between LPMR and LPOV:

According to data from @ipma_pt a maximum wind gust of 176,4 km/h has been measured in Figueira da Foz near the coast during the passage of storm #Leslie! Two more hurricane-force gusts were observed in Coimbra and Aveiro (Image source: https://t.co/gOO95O2WUz). #wetter pic.twitter.com/T55ykhL8Vm

— Luca Mathias (@meteomathias) October 14, 2018

Meteosat-11 lower/middle-tropospheric Water Vapor (7.35 µm) images (below) revealed the characteristic “scorpion tail” signature of a Sting Jet (Monthly Weather Review | Wikipedia), along with a mesoscale region of warming/drying (darker shades of orange) driven by strong subsidence — this subsidence feature corresponded well with the report of strong winds at Figueira da Foz. Further discussion of this sting jet event is available here.

Radar composites from the Portuguese Institute for Sea and Atmosphere (IPMA) confirmed that post-tropical cyclone Leslie made landfall around 2100 UTC (below).

Although the view from GOES-16 (GOES-East) was very oblique, the warm/dry signature around the western and southern edges of the storm was still evident on Mid-level Water Vapor (6.9 µm) imagery (below).

The entire life cycle of Leslie — from becoming a named Subtropical Storm at 15 UTC on 23 September to making landfall as a post-tropical cyclone in Portugal at 21 UTC on 13 October — is shown with 15-minute GOES-16 “Clean” Infrared Window (10.3 µm)  and Mid-level Water Vapor (6.9 µm) images (below). Note that 5-minute imagery was available on 01 October, when GOES-16 was performing a test of the Mode 4 scan strategy.

The long and winding adventure of Hurricane #Leslie has finally come to a close. Over its 22.25 day existence as a coherent cyclone, it largely spun harmlessly over open ocean. It ultimately impacted #Portugal as an extratropical cyclone, producing wind gusts upwards of 110mph. pic.twitter.com/9vrOiBxQAy

— Brenden Moses (@Cyclonebiskit) October 14, 2018

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The VIIRS (Visible-Infrared Imaging Radiometer Suite) instrument onboard NOAA-20 includes a Day Night Band that can be used to assess, among other things, power outages after a strong storm such as Hurricane Michael. The toggle above shows Day Night Band imagery from 6 October (before Michael) and 12 October (after Michael), and the change in illumination from city lights over the Florida Panhandle northeastward into east-central Georgia is stark.

Of course, clouds can also mask city lights in the Day Night Band. However, 11.45 µm infrared imagery from VIIRS, below, on 12 October, shows scant evidence of clouds over the region where city lights are missing.

(NOAA-20 images here courtesy William Straka, CIMSS)

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