Most people in south-central Wisconsin have noticed how uncomfortable the weather has been over the past week. The unseasonably cool air from a week ago has been replaced with a hot, sticky airmass that feels more at home in late July than in early June. And while precipitation has been sorely lacking in parts of the area, at least the atmosphere is now more supportive of rain and numerous short-lived storms have been popping up (and quickly dying out) since the weekend.

The thermodynamic profiles observed by NOAA-20 and processed by NUCAPS help give insight into the impact of the atmosphere’s change from cool and pleasant to hot and humid. Three profiles are shown here: from Thursday the 3rd, to Sunday the 6th, to Tuesday the 8th.  All three are taken in the early afternoon, and all are in the general vicinity of Madison.  The surface temperatures on all three days are generally the same.  However, the surface dew points show a steady increase from one day to the next.  The conditions on the 3rd were warm, but they were not nearly as unpleasant to experience directly as they were on the 8th thanks to the substantial increase in surface moisture that took place during that time.

Of course, surface conditions don’t tell the whole story, and this is where the NUCAPS profiles prove their worth. The dashed line on each sounding represents the temperature of the near-surface mixed layer air parcels as they ascend. These air parcel trajectories were calculated by the SHARPpy package, a free and open-source tool designed to analyze atmospheric soundings.  As air rises, it cools, but it cools at different rates depending on if the air is saturated or not. If air from the surface is warmer than the air that surrounds it, it will continue to rise all on its own, but air that’s cooler is going to sink. Note that the 3rd and the 6th  are stable days. The dashed line is to the left of the red temperature line. On those days, an ascending air parcel is always colder than its environment and since cold air sinks, no surface-based air is going to rise high enough to make deep convective clouds and the rain that accompanies them.  However, on the 8th the situation is reversed: the dashed line is to the right of the temperature sounding which means that air from the surface is warmer than its environment and will rise on its own.

The instability isn’t that large on the 8th; rising air parcels aren’t substantially warmer than their environment and so there’s not a lot of buoyant energy.  Still, it has been enough to contribute to some localized showers.  That’s evident on the CMORPH satellite-based precipitation estimates over the midwest (available on SSEC RealEarth), which shows virtually no rain on the 6th but the results of some scattered showers in south-central Wisconsin on the 8th.  It’s not much, but right now the area needs every drop it can get.

 

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Moist air that moves from the mainland USA to over the north Atlantic will often induce fog formation over those cold shelf waters.  This is a common occurrence (see this blog post, for a recent example) GOES-16 True-color imagery, above, from 1301-2101 UTC on 7 June 2021, shows the northeastward progress of a fog bank. Note how Block Island, Martha’s Vineyard and Nantucket all served as barriers to the fog.  Such behavior is most likely when a strong inversion is in place.  The closest upper air sounding, Upton NY, over eastern Long Island, below, from this site, did show a very strong surface inversion at 1200 UTC as well as brisk southwesterly winds in the lower levels of the atmosphere. The strong inversion would also allow the bow wake to form upwind of the islands, as observed. (The inversion at Upton persisted through the day! 00Z 8 June sounding is here)

Fog does not penetrate far inland along the coast. Solar heating over land will induce vertical mixing that evaporates any fog over land. A similar event occurred on 8 June.

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GOES-16 (GOES-East) CIMSS Natural Color RGB images along with the Aerosol Optical Depth derived product (above) showed a cold front that was acting to focus a hazy band of aerosols — likely smoke — across the Upper Midwest on 06 June 2021. AOD values in the vicinity of the cold front were as high as 2.0 (darker blue).

The hazy signature of the cold frontal aerosols was better highlighted by GOES-16 True Color RGB images, created using Geo2Grid (below). One feature of interest was the group of long shadows cast by convective towers onto the top of the aerosol layer in central  Minnesota and northwestern Wisconsin.

In addition, 2 relatively long smoke plumes could be seen streaming rapidly east-southeastward across northern North Dakota.

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The lake breeze prevails! Visible on radar, a strong lake breeze off Lake Superior has given some quick heat relief to our coastal regions. Areas in Superior and Park Point have dropped into the upper 50s up on the hill we have fallen into the mid 70s. #mnwx #wiwx #lakebreeze pic.twitter.com/3RwKYtsvAH

— NWS Duluth (@NWSduluth) June 5, 2021

GOES-16 (GOES-East) Shortwave Infrared (3.9 µm) images (above) displayed the cooling effect of the lake breeze on land surfaces in Minnesota and Wisconsin on 05 June 2021.

A plot of surface data from Duluth Sky Harbor Airport (below) showed that the temperature dropped from 97ºF to 64ºF in 20 minutes as the lake breeze arrived. Large and rapid temperature drops were also seen at Superior, Wisconsin (plot | data) and Cloquet, Minnesota (plot | data).

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