Unusually dry air over the Upper Midwest

April 20th, 2019 |

GOES-16 Low-level (7.3 µm, left) and Mid-level (6.9 µm, right) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm, left) and Mid-level (6.9 µm, right) Water Vapor images [click to play animation | MP4]

GOES-16 (GOES-East) Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images (above) revealed a southwest-to-northeast oriented band of anomalously-dry air along the northwest periphery of a slow-moving low pressure center over the eastern US on 20 April 2019. The hourly dew point dropped to 10ºF at Chicago O’Hare and to 6ºF at Chicago Midway and Chicago Executive airports. With the 10ºF dew point, the Relative Humidity was 12% — a new record low value for Chicago. Note that the coastline of part of the southern portion of Lake Michigan could be seen on the Low-level and Mid-level Water Vapor images.

AWIPS examples of the GOES-16 Low-level and Mid-level Water Vapor imagery are shown below.

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images [click to play animation | MP4]

A larger-scale view of GOES-16 Low-level Water Vapor (7.3 µm) image at 1201 UTC (below) showed that the Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX) rawinsonde sites were located within the elongated zone of dry air.

GOES-16 Low-level Water Vapor (7.3 µm) image, with plots of rawinsonde sites [click to enlarge]

GOES-16 Low-level Water Vapor (7.3 µm) image at 1201 UTC, with plots of rawinsonde sites [click to enlarge]

Plots of GOES-16 Water Vapor weighting functions for Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX) at 12 UTC (below) each showed significant contributions of Band 10 (7.3 µm, violet) radiation originating at the surface — this allowed the thermal signature of the outline of Lake Michigan to be easily seen in the 7.3 µm Water Vapor imagery. The contribution of Band 9 (6.9 µm, blue) was small near the surface, but still enough to allow a brief and subtle coastal signature on the 6.9 µm images.

GOES-16 Water Vapor weighting functions for Gaylord, Michigan and Lincoln, Illinois at 12 UTC [click to enlarge]

GOES-16 Water Vapor weighting functions for Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX)  at 12 UTC [click to enlarge]

Plots of Total Precipitable Water sounding climatology for Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX), with record minimum values for 20 April at 12 UTC highlighted within a red box (below) showed that the 0.10″ at KAPX and the 0.12″ at KILX set new records for that date/time.

Plots of Total Precipitable Water sounding climatology for Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX), with record minimum values for 20 April at 12 UTC highlighted within a red box [click to enlarge]

Plots of Total Precipitable Water sounding climatology for Gaylord, Michigan (KAPX) and Lincoln, Illinois (KILX), with record minimum values for 20 April at 12 UTC highlighted within a red box [click to enlarge]

Smoke in the Gulf of Mexico

April 18th, 2019 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with surface fronts plotted in cyan [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed some clearing of the dense pall of smoke across the far western Gulf of Mexico in the wake of a cold front that was moving southward/southeastward off the Texas coast on 18 April 2019. The parallel wave clouds of an undular bore were also evident ahead of the cold front from 13-16 UTC — the bore was also causing horizontal convective roll perturbations in the smoke about 20-40 miles ahead of the wave clouds (1506 UTC image).

The hazy signature of smoke was better defined in GOES-16 True Color Red-Green-Blue (RGB) images from the AOS site (below). This smoke was the result of widespread annual Springtime agricultural burning across southern Mexico, Guatemala, Belize and Honduras. Toward the end of the day, additional small plumes of smoke and blowing dust could  be seen moving back across the Gulf of Mexico into the “cleaner” air behind the cold front.

GOES-16 True Color RGB images [click to play animation | MP4]

GOES-16 True Color RGB images [click to play animation | MP4]

Thermal anomalies or “hot spots” (yellow to red pixels) associated with the larger fires in Mexico, Guatemala, Belize and Honduras could be seen in GOES-16 Shortwave Infrared (3.9 µm) images (below).

GOES-16 Shortwave Infrared (3.9 µm) images [click to play animation | MP4]

GOES-16 Shortwave Infrared (3.9 µm) images [click to play animation | MP4]

A map of fires detected by Suomi NPP VIIRS on the previous day is shown below, as viewed using RealEarth.

Fires detected by Suomi NPP VIIRS on 17 April [click to enlarge]

Fires detected by Suomi NPP VIIRS on 17 April [click to enlarge]

Ice in the Sea of Okhotsk

April 18th, 2019 |

Himawari-8

Himawari-8 “Red” Visible (0.64 µm) images [click to play animation | MP4]

JMA Himawari-8 “Red” Visible (0.64 µm) images (above) revealed circulations of ice within the Sea of Okhotsk (east of Sakhalin Island — station identifier UHSS is Yuzhno-Sakhalinsk, Russia) on 17-18 April 2019. Wind stress from an occluded Gale Force Low moving through that region on the previous day (surface analyses) likely helped to enhance some of the ice circulations.

In a comparison of Himawari-8 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (below), note that the ice appears much darker than cloud features (since ice is a strong absorber of radiation at the 1.61 µm wavelength).

Himawari-8 "Red" Visible (0.64 µm, left) and Near-Infrared "Snow/Ice" (1.61 µm, right) images [click to play animation | MP4]

Himawari-8 “Red” Visible (0.64 µm, left) and Near-Infrared “Snow/Ice” (1.61 µm, right) images [click to play animation | MP4]

Thanks to Thomas Birchard (NWS Honolulu) for bringing this interesting feature to our attention!

Mesoscale disturbance over northern Alaska

April 17th, 2019 |

GOES-17

GOES-17 “Red” Visible (0.64 µm) and Low-level Water Vapor (7.3 µm) images [click to play animation | MP4]

1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm) and Low-level Water Vapor (7.3 µm) images (above) showed a mesoscale disturbance that was moving northward over the eastern Brooks Range in far northeastern Alaska on 17 April 2019. The curved configuration of the associated cloud structure suggested that a closed circulation center was present (or had just recently developed) — while surface analyses showed an area of low pressure much farther to the south along the Alaska/Yukon border, there were no features moving northward across the region shown in the GOES-17 imagery.

Light to moderate snow was reported at Arctic Village as this mesoscale disturbance moved over the area (below).

Time series of surface weather observation data from Arctic Village [click to enlarge]

Time series of surface weather observations from Arctic Village [click to enlarge]

375-meter resolution Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images at 2131 and 2313 UTC (below) provided a more detailed view of this feature, in which the clouds exhibited an appearance suggestive of embedded convection. Cloud-top infrared brightness temperatures were as cold as -50ºC just southwest of Arctic Village on the 2313 UTC image — this corresponded to an altitude of 8.5 km on the 00 UTC Fairbanks rawinsonde data.

Suomi NPP VIIRS Day/Night Band (0.7 µm) images at 2131 and 2313 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) images at 2131 and 2313 UTC [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) images at 2131 and 2313 UTC [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) images at 2131 and 2313 UTC [click to enlarge]

13-km NAM model fields (below) showed no clear signature of either a closed circulation or a discrete vorticity center — so satellite imagery was depicting the presence of an important feature that was not captured by numerical models. While the 18 UTC model run did show an area of light precipitation moving northward toward the region, the 00 UTC model run scaled back the areal coverage of this precipitation.

3-km NAM 500 hPa height, wind and absolute vorticity [click to enlarge]

3-km NAM 500 hPa height, wind and absolute vorticity [click to enlarge]

3-km NAM Mean Sea Level Pressure and 1-hour accumulated precipitation [click to enlarge]

3-km NAM Mean Sea Level Pressure and 1-hour accumulated precipitation [click to enlarge]