GOES-16 Visible Imagery, in the mp4 animation above (click here for an animated gif), shows convection initially over central Wisconsin at sunrise moving eastward over Lake Michigan into lower Michigan. In its wake, cloud lines extending east-west move southward into southward Wisconsin. Parallel lines such as these are typically associated with atmospheric bores, previously discussed many times on this blog (link). A bore is usually associated with stable air; note how the convective line over southwestern WI at around 1500 UTC dissipates after 1600 UTC as it encounters the stable air associated with the bore.

LightningCast Probabilities are consistent with the southern convective line encountering air that is more stable, as shown above in an animation that pauses at 1506 UTC; lightning probabilities decrease with the southern line as they increase with the northern line that eventually sweeps southward through southern Wisconsin, producing hail.

1-minute Mesoscale Doman Sector @NOAASatellites #GOES16/#GOESeast Visible images showed an undular bore – generated by low-level thunderstorm outflow – which propagated southward across Wisconsin into Illinois: https://t.co/Vos4rmWhFI @NWSMilwaukee @NWSChicago #WIwx #ILwx pic.twitter.com/oxHqIixxIw

— Scott Bachmeier (@CIMSS_Satellite) September 20, 2022

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NOAA-20 overflew this region shortly after 1900 UTC on 20 September. The gridded 850-700 mb lapse rate, below, from this site, shows a region of more stable air over/around Chicago and southern Lake Michigan that is perhaps residual stability related to the bore feature.

Shout-out to Rebecca, a forecaster at WFO GRB, for also noticing these lines!

The thunderstorms were followed by mammatus clouds over Madison, as shown in the image below, courtesy Bill Bellon, UW-Madison SSEC/CIMSS.

TL;DR: Departing convection put down stable layer defined by atmospheric bore. Convection encountering this stable layer dissipated. Stronger convection moved in later, depositing hail.

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NOAA/NESDIS STAR is producing occasional SAR imagery over Hurricane Fiona (link). NetCDF files can be imported into AWIPS and displayed, as shown above (color-enhanced with Beaufort Scaling), in an image from late on 19 September when Fiona was just north of Hispaniola. The strongest winds were associated with the northern eyewall of the storm, and an asymmetry in the storm is apparent (The NHC discussion from 2100 UTC on 19 September (here), and the discussion at 0300 UTC on 20 September (here) will give more context). The Radial wind analysis at the SAR link (here) shows the strongest winds in the NW and NE quadrants of the storm, with a peak near 100 knots. The toggle below, from late on 19 September, compares GOES-16 ABI Clean window (10.3 µm) imagery and the Sentinel SAR wind estimates (with a different color enhancement). A parallax shift in the GOES-16 imagery means the two eyes do not overlay.

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Update on 23 September: RADARSAT-2 overflew Fiona at 2236 UTC on 22 September. The toggle below shows SAR winds estimated at that time, with peak values in the eyewall near 100 knots (for more information on this SAR pass, including a more complete image over the eye, click here). There is a parallax shift in the high clouds of the storm; Fiona at this time was near 30.5^oN, 69.3^oW, far from the sub-satellite point at 0^oN, 75.2^oW: the parallax shift for the high clouds will be away from the sub-satellite point. The time difference in the observations — 5 minutes — should not cause a big shift for a storm moving at 17 knots (i.e., 20 mph) to the north-northeast.

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Suomi NPP and NOAA-20 both overflew Hurricane Fiona (NPP flew overhead to the east, NOAA-20 flew overhead to the west) in the early morning of 19 September 2022, as shown above in imagery created at AOML (The Atlantic Oceanagraphic and Meteorological Laboratory) and displayed at the Direct Broadcast site there. The images appear to show an eastward motion of the eye — but GOES-16 animations, below, show a persistent west-northwest motion (landfall occurred in the Dominican Republic around 0730 UTC).

The apparent eastward motion of the eye also shows up in the infrared imagey, which rules out artifacts related to shadowing.

This might be an example of a Parallax shift in VIIRS imagery causing a shift in a feature. NOAA-20’s nadir was over Jamaica, considerably to the east of the Mona Passage where Fiona’s eye was developing. A parallax error may be responsible, because satellite navigation will place the tall clouds farther from the sub-satellite point than observed.

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The full-resolution Day Night band imagery from Suomi NPP, and from NOAA-20 (both available from the CIMSS ftp site here and here) show strong convection starting ca. 0530 UTC and continuing through ~0630 UTC near the eye.

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1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed Fiona as it intensified to a Category 1 Hurricane just south of Puerto Rico during the morning hours on 18 September 2022. The coldest cloud-top 10.35 µm infrared brightness temperatures were around -88ºC.

The corresponding 1-minute GOES-16 Cloud Top Temperature and Cloud Top Height derived products are shown below — the coldest Cloud Top Temperature values were around -91ºC, while maximum Cloud Top Height values were around 61,000 feet.

The highest wind gust at Buoy 42085 — located just south of Ponce (station identifier TJPS) — was 72 knots (83 mph) at 16 UTC (below).

Although Fiona was moving across relatively warm water, GOES-16 Infrared Window (11.2 µm) images with contours of deep-layer wind shear from the CIMSS Tropical Cyclones site (below) indicated that the storm was moving through an environment of moderate shear.

Hurricane Fiona later made landfall in far southwestern Puerto Rico around 1920 UTC — a DMSP-18 image at 2013 UTC (below) showed the eye as it was beginning to move into the Mona Passage.

A prolonged period of strong winds and heavy rainfall from Fiona led to widespread power outages and flash flooding across much of Puerto Rico.

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