A mid-level trough traveling east across the northern US brought widespread strong winds and high fire danger to much of the central US on 29 Mar 2020. GOES-East water vapor imagery captures the feature, ahead of which dry and warm conditions had developed. The cold front forced south by the associated Canada/US border shortwave is also evident in the moisture imagery.
Given the expected conditions, the NWS Storm Prediction Center had issued a very large Critical Fire Weather area in their Fire Weather Outlook for the day. Widespread RH below 20% and wind gusts over 40 mph would develop within the critical fire weather area during the day.
Deep vertical mixing along with a tightening surface pressure gradient resulted in strong surface winds developing across the region. 12Z Soundings from Denver and Rapid City captured the very deep dry air ahead of the trough. SPC Sounding Climatology indicated these atmospheres very close to their record Daily Min for TPW, with 0.1″ each.
GOES-East shortwave IR imagery from the day and evening reveals the evolution of the cold front south across the plains, along with the development of wildfire hot spots (black pixels).
Focusing on South Dakota, Natural Color Fire RGB imagery from the day captures a few large wildfires and their associated smoke plumes, along with the push of cooler air.
A comparison between ABI and VIIRS Fire Temperature RGBs over the hot spots provides an example of the additional spatial detail VIIRS provides over ABI regarding ongoing burning area. A Sentinel-2 pass provided even better detail around 1751 UTC, with the 10 m true color imagery showing the burned area and smoke plumes associated with the Rapid City wildfire.
Given the strong winds, blowing dust was of concern as well. 10.3 – 12.3 um Split Window Difference imagery from the northern plains shows relatively dark gray steaks developing behind the cold front under otherwise clear skies. These are areas of blowing dust, where the SWD is relatively low, or near to below zero. Algorithms and displays with set SWD thresholds had a more difficult time capturing these plumes of blowing dust as the signal wasn’t particularly strong. Sometimes, the basic SWD with lienar grayscale colortable will be ideal for detecting blowing dust.
Combining the SWD with SWIR and IR, we have a product that can be utilized to diagnose the wildfire hot spots and blowing dust plumes together.
Another VIIRS-ABI example shows the utility of VIIRS imagery for improved assessment of blowing dust plumes, including their exact sources. A South Dakota plume is barely recognizable in the ABI Day Land Cloud Imagery, while the plume and source region are clear in the same imagery but from VIIRS. Sentinel imagery form 1 hour earlier also captured the dust plume and source with even greater detail.
A thicker region of blowing dust developed within the San Luis Valley, and east over the southeast Colorado plains. This time, we view a VIS-SWD sandwich product to diagnose the denser dust plume. Surface obs indicated widespread wind speeds over 40 knots.
Finally, a couple areas of blowing dust also developed across Utah and Nevada during the afternoon in association with a smaller shortwave embedded within the base of the broader trough.
Bill Line, NESDIS and CIRA