Fiona makes landfall in Nova Scotia

September 24th, 2022 |

GOES-16 Mid-level Water Vapor (6.9 µm) images [click to play animated GIF | MP4]

Hurricane Fiona transitioned to a strong post-tropical cyclone just prior to making landfall in Nova Scotia, Canada around 0700 UTC on 24 September 2022 — 1-minute Mesoscale Domain Sector GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed the storm during the 0200-1800 UTC period. This anomalously-deep cyclone may have set a new Canadian record (931 hPa) for minimum mean sea level pressure (surface analyses). The highest surface wind gust was 97 knots / 112 mph / 179 km/h at mesonet site Arisaig (along the northeastern coast of mainland Nova Scotia), with a peak wind gust of 90 knots / 104 mph / 167 km/h at METAR site Grand Etang CWGQ (just off the west coast of Cape Breton Island) shortly before 0800 UTC. A preliminary storm summary issued by Environment Canada is available here.

One feature of interest was a region of lee waves generated by strong easterly boundary layer winds interacting with the Highlands terrain in the northern portion of Cape Breton Island — these waves spread westward across parts of the Gulf of Saint Lawrence (below).

Comparison of GOES-16 Water Vapor image at 0700 UTC and topography [click to enlarge]

Sentinel-1A Synthetic Aperture Radar (SAR) water surface winds at 1015 UTC (below) captured lighter winds of 20 knots or less (brighter shades of violet) within the post-tropical cyclone’s eye as it was emerging northwestward from Cape Breton Island — along with an arc of strong winds within the storm’s eastern semicircle, where speeds of 100 knots or higher (shades of red) were seen. Off the north coast of Cape Breton Island, the peak wind gust at St. Paul Island (CWEF) just prior to 1000 UTC was 85 knots — and SAR winds at 1015 UTC in the vicinity of CWEF were in the 75-80 knot range.

Sentinel-1A Synthetic Aperture Radar (SAR) winds at 1015 UTC [click to enlarge]

The animation of GOES-16 airmass RGB, below, shows the evolution of Fiona and its interaction with a Potential Vorticity Maximum (orange/red in the RGB).

GOES-16 airmass RGB, 0000 UTC 21 September – 0000 UTC 25 September

Perhaps you are skeptical that the Orange/Red signature in the Airmass RGB is a Potential Vorticity Signature. Consider the animation below, downloaded from the TropicalTidbits website and showing 00 – 30 hour GFS model output from the run initialized at 0000 UTC on 23 September. A rich source of cyclonic potential vorticity air (in orange) wraps around Fiona as it moves north. Similar behavior is apparent in the Airmass RGB animation above. This cross section (also from the excellent Tropical Tidbits site) of the model data (24 h into the forecast run) also shows a classic stratospheric intrusion structure.

330 K Potential Vorticity color-shaded in PV Units at 6-h time-steps, 0000 UTC on 23 September 2022 through 0600 UTC on 24 September 2022 (Click to enlarge)

Center-fixing a storm with SAR and SSMIS data

September 23rd, 2022 |
Sentinel SAR Wind Speeds, 0757 UTC on 23 September 2022 (Click to enlarge)

Sentinel overflew Tropical Storm Gaston, northwest of the Azores Islands, shortly before 0800 UTC on 23 September 2022, as shown above. The Beaufort Scale enhancement suggest peak winds derived from SAR observations to be very close to 50 knots in curved bands to the east/southeast of the center, inferred to be just off the edge of observation swath, which is at 40.76o N Latitude, 29.3o W Longitude. GOES-16 satellite imagery spanning this time around sunrise shows an exposed low-level circulation center with stronger convection building along the northern perimeter of the storm (mp4 animation shown here, created with CSPP Geosphere — direct link is here). Because GOES-16 can view the low-level circulation, the parallax shift of the center in the animation is small (smaller than the parallax shift in the SAR/GOES-16 imagery shown here Fiona) even though Gaston is near the satellite limb.

DMSP-17 carries the SSMIS (Special Sensor Microwave Imager/Sensor) and overflew Gaston just after 0900 UTC, as shown below in an image from the SSEC Tropical Website. The estimated wind speed from this image is 52 knots, close to the SAR values shown above. The 0900 UTC update from the National Hurricane Center showed a center at 40.5oN, 29.6oW and maximum sustained windspeeds at 50 knots.

85 GHz observations from SSMIS-17, 0911 UTC on 23 September 2022 (Click to enlarge)

Fiona intensifies to a Category 4 hurricane

September 21st, 2022 |

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animated GIF | MP4]

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 4 Hurricane just east of The Bahamas at 0900 UTC on 21 September 2022. The coldest cloud-top 10.35 µm infrared brightness temperatures were around -81ºC.

GOES-16 Infrared images with and without a overlay of GLM Flash Extent Density (below) did reveal isolated brief periods of lightning in the eyewall region of Fiona — but most of the lightning activity was associated with convection well east of the eye.

GOES-16 “Clean” Infrared Window (10.35 µm) images, with and without a overlay of GLM Flash Extent Density [click to play animated GIF | MP4]

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 -84ºC, while maximum Cloud Top Height values were around 58,000 feet.

GOES-16 Cloud Top Temperature and Cloud Top Height derived products [click to play animated GIF | MP4]

In a time-matched comparison of Infrared Window images from Suomi-NPP and GOES-16 at 0700 UTC (below), the coldest cloud-top infrared brightness temperatures were -86ºC and -81ºC, respectively. A slight northwestward parallax displacement was evident with the GOES-16 image.

Infrared Window images from Suomi-NPP (11.45 µm) and GOES-16 (10.35 µm) [click to enlarge]

===== 23 September Update =====

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animated GIF | MP4]

As Fiona passed just to the northwest of Bermuda during the nighttime hours on 22-23 September, it was briefly downgraded to a Category 3 storm at 0600 UTC (before being again upgraded to Category 4 at 1200 UTC on the following morning) — but a wind gust to 81 knots (93 mph) was recorded at Bermuda Naval Air Station (station identifier TXKF) shortly before 10 UTC while that airport was temporarily closed to air traffic (Bermuda discussion). 1-minute GOES-16 Infrared images (above) showed Fiona during the 0000-1000 UTC period on 23 September.

A DMSP-17 SSMIS Microwave (85 GHz) image at 1053 UTC from the CIMSS Tropical Cyclones site (below) exhibited the eye and eyewall structure about an hour after the peak wind gust at Bermuda.

DMSP-17 SSMIS Microwave (85 GHz) image at 1053 UTC [click to enlarge]

Atmospheric Bore structure over Wisconsin

September 20th, 2022 |
GOES-16 Visible imagery (0.64 ), 1056 – 1911 UTC on 20 September 2022

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.

GOES-16 Visible imagery overlain with ProbSevere LightningCast probabilities and GLM Flash Extent Density, 1331 -1801 UTC on 20 September 2022 (Click to enlarge)

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.

Diagnosed 850-700 mb stability from NOAA-20 NUCAPS profiles, ca. 1915 UTC on 20 September 2022 (click to enlarge)

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.

Mammatus clouds over Madison WI, 19 September 2022 (Click t0 enlarge). Photo Credit: Bill Bellon

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.