Early-season winter storm in the Northern Plains

October 12th, 2019 |

GOES-16 Mid-level Water Vapor (6.9 µm) images, with hourly surface weather type plotted in red [click to play animation | MP4]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with hourly surface weather type plotted in red [click to play animation | MP4]

An early-season winter storm produced very heavy snowfall and blizzard conditions across the Northern Plains — particularly in central/eastern North Dakota and southern Manitoba — during the 10 October12 October 2019 period. GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) showed the long duration of precipitation across that region. A text summary of snowfall totals and wind gusts is available from the WPC, NWS Bismarck and NWS Grand Forks.

On 11 October, GOES-16 “Red” Visible (0.64 µm) images with an overlay of GLM Flash Extent Density (below) revealed intermittent clusters of lightning activity over northwestern Minnesota, northeastern North Dakota and southern Manitoba — while no surface stations explicitly reported a thunderstorm, NWS Grand Forks received calls from the public about thundersnow. The texture of cloud tops in the Visible imagery also supported the presence of embedded convective elements, which likely enhanced snowfall rates as they pivoted across that area (note that this convection was occurring along the leading edge of the mid-tropospheric dry slot seen in the Water Vapor imagery above). An animation of GOES-16 Visible imagery with plots of GLM Groups and surface weather type is available here.

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with an overlay of GLM Flash Extent Density [click to play animation | MP4]

One important aspect of this storm was the formation of a trough of warm air aloft or trowal (SHyMet | Martin, 1998) as the surface low began to enter its occluded phase on 11 October — contours of Equivalent Potential Temperature along the 295 K isentropic surface (below) helped to diagnose the axis of the trowal as it curved cyclonically across southwestern Ontario, southern Manitoba and then southward over North Dakota.

GOES-16 Mid-level Water Vapor (6.9 µm) images, with 295 K equivalent potential temperature contours plotted in yellow and surface fronts plotted in red [click to play animation | MP4]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with 295 K Equivalent Potential Temperature contours plotted in yellow and surface fronts plotted in red [click to play animation | MP4]

A similar animation with contours of 295 K specific humidity (below) also displayed the orientation of a west-to-east cross section B-B’ (green) across northern Northern Minnesota and northern Minnesota.

GOES-16 Mid-level Water Vapor (6.9 µm) images, with 295 K Specific Humidity contours plotted in yellow and surface fronts plotted in red [click to play animation | MP4]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with 295 K Specific Humidity contours plotted in yellow and surface fronts plotted in red [click to play animation | MP4]

The Line B-B’ cross section at 16 UTC (with and without contours of Equivalent Potential Temperature) is shown below. Note the deep column of upward vertical velocity (highlighted by color shading of Omega) centered over Langdon, North Dakota — the moist trowal airstream can be seen sloping isentropically upward and westward behind the 3 g/kg Specific Humidity contour, as it approached the region of upward vertical motion. Langdon received 27 inches of snowfall; the prolonged southward passage of the trowal over North Dakota likely contributed to this accumulation.

Cross section of RAP40 model fields along Line B-B' at 16 UTC [click to enlarge]

Cross section of RAP40 model fields along Line B-B’ at 16 UTC [click to enlarge]

As the storm was gradually winding down on 12 October, it exhibited a very broad middle-tropospheric signature on GOES-16 Water Vapor imagery (below).

GOES-16 Mid-level Water Vapor (6.9 µm) images, with surface frontal positions [click to play animation]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with surface frontal positions [click to play animation | MP4]

 

High Plains leeside cold frontal gravity wave

October 10th, 2019 |

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images, with plots of surface wind barbs and gusts in knots [click to play animation | MP4]

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images, with plots of surface wind barbs and gusts in knots [click to play animation | MP4]

GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) revealed the classic signature of a leeside cold frontal gravity wave (reference) moving southward across the High Plains on 10 October 2019. Peak wind gusts of 50-60 mph were reported at some sites in eastern Colorado and western Kansas — and impressive drops in surface air temperature accompanied the cold frontal passage.


On the corresponding GOES-16 “Red” Visible (0.64 µm) imagery (below), note the lack of clouds along the western end of the cold front (across the New Mexico / Texas border region).

GOES-16 "Red" Visible (0.64 µm, left), Mid-level Water Vapor (6.9 µm, center) and Upper-level Water Vapor (6.2 µm, right) images, with plots of surface wind barbs and gusts in knots [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm, left), Mid-level Water Vapor (6.9 µm, center) and Upper-level Water Vapor (6.2 µm, right) images, with plots of surface wind barbs and gusts in knots [click to play animation | MP4]

A plot of rawinsonde data from Amarillo, Texas at 12 UTC (below) showed how shallow the cold air was behind the cold front as it first moved southward through the Texas Panhandle.

Plot of rawinsonde data from Amarillo, Texas [click to enlarge]

Plot of 12 UTC rawinsonde data from Amarillo, Texas [click to enlarge]

However, note that GOES-16 Water Vapor weighting functions calculated using 12 UTC rawinsonde data from Amarillo (below) indicated that peak contributions were in the middle troposphere — in the 440-500 hPa pressure range — with no surface radiation contributions at 6.9 µm or 6.2 µm. It was the deep-tropospheric nature of the leeside cold frontal gravity wave that allowed its signature to be sensed by the 6.9 µm and 6.2 µm Water Vapor spectral bands.

GOES-16 Water Vapor weighting functions, calculated using 12 UTC rawinsonde data from Amarillo, Texas [click to enlarge]

GOES-16 Water Vapor weighting functions, calculated using 12 UTC rawinsonde data from Amarillo, Texas [click to enlarge]

Aircraft dissipation trails over southern Wisconsin and northern Illinois

October 6th, 2019 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animation | MP4]

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (above) revealed a series of aircraft “dissipation trails” drifting northeastward across southern Wisconsin and northern Illinois on 06 October 2019. These cloud features were caused by aircraft that were either ascending or descending through a layer of cloud composed of supercooled water droplets — cooling from wake turbulence (reference) and/or particles from jet engine exhaust acted as ice condensation nuclei to cause the small supercooled water droplets to turn into larger ice crystals (many of which then often fall from the cloud layer, creating “fall streak holes“).

A comparison of Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared (1.61 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images (below) helped to confirm the presence of ice crystals within the aircraft dissipation trails: a darker appearance in the 1.61 µm image (since ice is a strong absorber of radiation at that wavelength), and a colder (brighter white) signature in the 3.74 µm image. In the enhancement applied to the 3.74 µm and 11.45 µm images, colors are applied to infrared brightness temperatures of -30ºC and colder — and the shades of yellow represent cloud-top brightness temperatures in the -30 to -39ºC range.

Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared (1.61 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared (1.61 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Several of the “fall streak” clouds were seen in time-lapse videos of west- and east-facing AOSS rooftop cameras (below).

Time lapse of west-facing AOSS rooftop camera images [click to play YouTube video]

Time lapse of west-facing AOSS rooftop camera images (courtesy of Pete Pokrandt, AOSS) [click to play YouTube video]

Time lapse of east-facing AOSS rooftop camera images [click to play YouTube video]

Time lapse of east-facing AOSS rooftop camera images (courtesy of Pete Pokrandt, AOSS) [click to play YouTube video]

Orphan anvil over the Atlantic Ocean

October 3rd, 2019 |


The interesting east-to-west moving cold (brighter white) infrared signature mentioned above was determined by another Twitter user to be the convective debris of an isolated orphan anvil that developed over the Atlantic Ocean east of Florida (and north of the Bahamas) toward sunset on 03 October 2019. A comparison of GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images is shown below.

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

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

The convective tower producing the orphan anvil was still dimly illuminated by the setting sun at 2301 UTC (below), when cloud-top infrared brightness temperatures first became colder than -25ºC (darker blue pixel).

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images at 2301 UTC [click to enlarge]

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images at 2301 UTC [click to enlarge]

The coldest infrared brightness temperature exhibited by the orphan anvil was -29ºC at 2316 UTC — which closely corresponded to the 313 hPa pressure level in rawinsonde data from Cocoa Beach, Florida at 00 UTC (below). Wind speeds at that altitude were 42 knots; the 300 hPa analysis at 00 UTC showed a 50-knot wind speed maxima approaching the orphan anvil region from the northeast.

Plot of 00 UTC rawinsonde data from Cocoa Beach, Florida [click to enlarge]

Plot of 00 UTC rawinsonde data from Cocoa Beach, Florida [click to enlarge]

The orphan anvil signature was only apparent in Infrared imagery until about 2336 UTC — but since the surrounding atmosphere was fairly dry, the westward transport of moist convective debris could be tracked for another 3 hours using GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor imagery (below).

GOES-16 Low-level (7.3 µm, bottom), Mid-level (6.9 µm, middle) and Upper-level (6.2 µm, top) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm, bottom), Mid-level (6.9 µm, middle) and Upper-level (6.2 µm, top) Water Vapor images [click to play animation | MP4]

Orphan anvils often appear shortly before the onset of significant convective development — signalling that convective inhibition is weakening — as previously  discussed here, here, here, here and here.