Sam in the Atlantic

September 23rd, 2021 |
GOES-16 True-Color Imagery over the tropical Atlantic Ocean showing Tropical Sam, 1810 UTC on 23 September 2021 (Click image to view in CSPP Geosphere)

Tropical Storm Sam is poised to strengthen in the tropical Atlantic. The True-Color image above shows the cyclone just to the south of a region of dust that is apparent in the true-color imagery as a greyish/brown region extending from the storm towards Africa. A Saharan Air Layer (SAL) analysis, below (from this site), also shows this region of dry air to the east of the storm. Sam is not projected to be influenced by the dry air that is following it.

Saharan Air Layer analysis, 1500 UTC on 23 September 2021 (Click to enlarge)

Both 200-850 mb shear and SSTs, shown in a toggle below, will not inhibit strengthening. Shear is weak, and waters are warm. An ASCAT pass from MetopC shortly before 1200 UTC on 23 September (from this NOAA OSPO site) is shown here. A slightly later pass from MetopB (from the OSI SAF site) is shown here.

Sea-surface temperatures at 1200 UTC on 23 September 2021, and 200-850 mb shear at 1600 UTC on 23 September 2021 (Click to enlarge)

The National Hurricane Center projects Sam to become a Major Hurricane. Refer to the National Hurricane Center for more information.

Strong winds over Lake Michigan

September 23rd, 2021 |
GOES-16 Band 3 (0.86 µm) imagery at sunrise (1151 – 1246 UTC) on 23 September 2021

GOES-16 “Veggie Band” near-infrared imagery (Band 3, 0.86 µm), above, (from the CSPP Geosphere website, click here for a link to the animation at that website) shows an early-season Fall cyclone over lower Michigan. During the animation, RADARSAT Constellation Mission Satellite 1 (RCM-1) was in a descending pass down Lake Michigan. Synthetic Aperture Radar winds from that satellite (from this website) produced a complex windfield over extreme eastern Lake Superior (just before 1152 UTC) and over Lake Michigan (just before 1153 UTC), as shown below.

SAR-derived winds from RCM1, 1151-1153 UTC on 23 September 2021 (Click to enlarge)

It is interesting to consider the influence of prominences along the western shore of lower Michigan, such as Big Sable and Little Sable points, as well as Sleeping Bear Dunes on the distribution of winds. There seems to be lighter winds in the lee of those regions in this northerly wind regime. Also, consider the wind funneling through outlets over Lake Huron, and being blocked by the islands in northern Lake Michigan.

SAR winds are affected by ice in clouds. That might be the cause of the very strong winds indicated to be just offshore or southwestern lower Michigan, and over the Lake Michigan south of 43 N and east of 87 W. GOES-16 Cloud-top phase does show ice over the entire Lake, as shown below.

GOES-16 Cloud Top Phase, 1151 UTC on 23 September 2021 (Click to enlarge)
National Data Buoy Center buoy locations over the western Great Lakes (click to enlarge)

The National Data Buoy Center oversees a large portfolio of moored buoys (and C-MAN sites) that monitor the winds, and these buoys can be used to see how the SAR winds compare to observations. For example, the winds at buoy 45007 around 1100 UTC on 23 September, as shown in the plot below, were around 25 knots with gusts to almost 35 knots.

Winds at NDBC Buoy 45007. The SAR winds above shows values near 1200 UTC On 23 September
Winds at NDBC Buoy 45002. The SAR winds above shows values near 1200 UTC On 23 September

Winds at buoy 45002 (above) peaked at around 1200 UTC: 27 knots with gusts to 35 knots.

Strongs winds in early Autumn act to cool the lake. The Lake Surface Temperature plot, below, from this site, shows the cooling at buoy 45007 that occurred with the winds.

Lake Surface temperatures, 20-24 September 2021 (Click to enlarge)