Snow in the southeastern US!

Due to strong, cold, upper-level low that swept through the southeastern United States last weekend (8-9 December 2017), there were variable snow totals that accumulated from southeastern Louisiana, all the way to the Appalachian Mountains.  Snow totals varied from a trace of snow to 10 inches plus in some areas.  The local NWS-Atlanta, GA, has some updated snow totals from this uncommon December snowstorm.

Even more fascinating, the large swaths/fields of snow that impacted the southeastern United States can be seen via satellite. The Suomi-National Polar-orbiting Satellite (SNPP), in which, the Visible Infrared Imaging Radiometer Suite (VIIRS), an instrument on-board SNPP is utilized here. VIIRS has 22 spectral channels, and the following satellite imagery is produced from one of those spectral channels; the Imagery Band (I-1) (0.64um) visible channel. This channel is at a high spatial resolution (375-m) and can see the snow swaths extending from Louisiana, Mississippi, Alabama, Georgia to the Carolinas, during the daytime (i.e. afternoon) hours.

Three separate, daily, visible images (9-11 December 2017) are provided showing the areal extent of the snow, and how the snow diminishes, due to solar heating, throughout the following days. It is important to note, that for 9 December 2017, the snowstorm just passed through the area hours before the satellite image was taken.

9 December 2017 @ 1906Z

10 December 2017 @ 1843Z

11 December 2017 @ 1826Z

For more information on the December snowstorm, click the flowing NWS -Peachtree City, GA link.

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Thomas Fire

Last week, more California wildfires had been initiated northwest of the Los Angeles Metropolitan area. Out of the fires that have spurred up in the last week, the Thomas Fire is the largest one. Over 230,000+ acres have been burned by the Thomas Fire, with approximately 15% of fire being contained so far. The cause of the fire is still under investigation and over 700+ structures have been burned as of 11 December 2017. Additionally, several thousands of people have been evacuated from the area. The latest updates on the Thomas Fire can be seen via the ‘Inciweb’ weblink.

Here is the latest Day/Night Band (DNB) satellite imagery, highlighting the evolution of the fire during the nighttime hours throughout the past week. Satellite imagery before the fire (4 December 2017), the day after the fire started (5 December 2017) and a week after the fire started (11 December 2017) are shown in the following images below. In the satellite imagery, emitted lights of the Thomas Fire and corresponding smoke can be seen, along with the nearby, emitted city lights of the Los Angeles metropolitan area. The Thomas Fire, initially developed near Santa Paula, California, and has migrated to the north, northwest of Los Angeles, throughout the past week.

4 December 2017 – Before the Thomas Fire initiated.

5 December 2017 –  The Day after.

11 December 2017 –  Approximately a week after fire initiated.

An animation of the fire from 4-11 December 2017 can also be seen, showing the evolution of the fire via the following link.

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December Wildfires in California

More wildfires are ravaging the California landscape again, as hot, dry and windy conditions persist over the west coast. Fires this time, have initiated and developed to the north and northwest of Los Angeles, California. Several fires have been identified and named such as the ‘Thomas’, ‘Creek’ and ‘Rye’ Fires. The Thomas fire started in the evening hours on Monday, 4 December 2017, while the others initiated thereafter. As of 6 December 2017, the fires have spread rapidly due the existing Santa Ana winds, destroying infrastructure and buildings along their paths. No percent of fire containment has been declared from firefighters and local emergency management officials.

Here is quick look at a satellite imagery product that highlights the magnitude of these fires. The use of the Near-Constant Contrast (NCC) product monitors atmospheric phenomena, senses emitted and reflected light sources and assists with cloud monitoring during the nighttime. Below is a comparison between two different days provided from two static NCC images.

The first image, is of 3 December 2017 @ 0935z (0135 local time), showing the Los Angeles metropolitan area located in southwestern California. This particular image is taken before the fires initiated on 4 December 2017. One can see the emitted city lights from Los Angeles and all the neighboring suburbs, along with the existing cloud cover, located to the south and east of the metropolitan area. In complement to the image, in the top-right corner, is the moon percent visibility and moon elevation angle, implying that the moon provided adequate moonlight and the moon was above the horizon when the satellite image was taken.

The next image, shown below, shows a few days later, on 6 December 2017 @ 1020Z (0220 local time) the emitted city lights from the metropolitan areas along with the location of the fires and and areas of smoke, which appear to be moving offshore in the south, southwest directions. These fires are ongoing and we’ll have updates on these fires in the near-future. Social media images and videos can be seen via the hyperlinks provided.

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Orographic cirrus / lee wave clouds as observed from GOES-16

The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing.  Users bear all responsibility for inspecting the data prior to use and for the manner in which the data are utilized.

On 22 November 2017, lee wave clouds (also referred to as orographic cirrus) developed downwind of various mountain ranges in Montana and Wyoming.  GOES-16 10.3 micron imagery does an excellent job of capturing these lee wave clouds as regions of exceptionally cold brightness temperatures.  The following 4 panel display of GOES-16 imagery depicts:

Upper left: 10.3 micron (band 13)

Upper right: 3.9 micron (band 7), same color table as band 13

Lower left: Nighttime microphysics RGB (Red: 12.3 – 10.3 microns; Green: 10.3 – 3.9 microns; Blue: 10.3 microns)

Lower right: Fog product (10.3 – 3.9 micron)

This loop spans 1122 to 1757 UTC, therefore the appearance of the imagery transitions between nighttime and daytime, which is very important to note for any imagery/products that contains the 3.9 micron band.  One prominent feature of the 3.9 micron band is that all clouds, both liquid and ice, will exhibit an approximate 20 degrees Celsius increase in temperature when the sun rises over them during the early morning hours. That is, during the daytime, a significant solar reflected component increases 3.9 micron brightness temperatures.  The terminator can be seen best in the fog product centered around 1417 UTC.

There are a mix of orographically induced lee wave clouds (easy to spot since they are locked to the terrain) along with what may be referred to as “synoptic” scale cirrus that is advecting along and is not locked to the terrain, an example of the different types of cirrus is shown here annotated on the 10.3 micron image at 12:27 UTC; the other 3 images are there for reference.

Prior to sunrise,  lee wave clouds in Montana exhibited their initial development.  Lee wave clouds in Montana began to develop around 11:47 UTC.  By 12:27 UTC, brightness temperatures at 10.3 microns were very cold, around -70 degrees Celsius, as the lee wave clouds expanded (see top left image above).  The non-orographic cirrus, annotated in 10.3 microns, exhibit cold brightness temperatures, around -45 degrees Celsius (see top left image above).  Prior to sunrise, the solar contribution to 3.9 micron brightness temperatures is missing; consequently, brightness temperatures at 3.9 microns (top right image above) are similar to those at 10.3 microns.  In sharp contrast, the appearance of the lee wave clouds, in Montana, in both the nighttime microphysics and fog products appear  contrary to clouds composed of ice.  As indicated above, green in the nighttime microphysics RGB comes from the 10.3 minus 3.9 micron temperature difference, which is the fog product.  Therefore, green in the nighttime microphysics RGB (bottom left) and light blue in the fog product (bottom right) is a liquid cloud signature.  However, brightness temperatures are between -45 and -70 degrees Celsius; temperatures that are much colder than the homogeneous freezing temperature.  An apparent dilemma exists: both 10.3 and 3.9 micron brightness temperatures suggest ice clouds while both nighttime microphysics RGB and fog products suggest liquid clouds.  How could this be?

Unfortunately, both the greenish color in the nighttime microphysics RGB and the bluish color in the fog product over Montana are “false signals” (i.e., anomalous from what is expected).  There are 2 possible explanations for the “false signal” in each product.  First, temperatures at 3.9 microns are cold enough to allow noise to appear in imagery.  Secondly, the existence of relatively small ice particles.  Appearances change however when the terminator passes by and the sun shines on the scene.

At 16:47 UTC, the terminator passed by and the sun is shining on the scene shown in all 4 panels in the figure below.

Due to solar reflection off of all types of clouds, brightness temperatures at 3.9 microns have increased approximately 20 degrees Celsius.  Consequently, brightness temperatures at 10.3 microns (upper left) are much colder than brightness temperatures at 3.9 microns (upper right); therefore, the appearance of 10.3 and 3.9 microns are quite different during the daytime compared to nighttime.  In particular, the false signature disappears as a consequence of brightness temperatures increasing at 3.9 microns relative to 10.3 microns (lower 2 panels).  Hence, ice clouds appear black in the fog product (lower right).  There is a difference, however, in the brightness temperatures of the lee wave clouds in Montana and Wyoming compared to all other ice clouds at 3.9 microns.  Research has shown that smaller ice particle sizes reflect more solar energy than larger ice particle sizes.  Thus, the existence of relatively small ice particles can explain the warmer brightness temperatures at 3.9 microns of the lee wave compared to all other ice clouds (including the non-orographic cirrus).  GOES-16 allows us to observe these characteristics of lee wave clouds, which are important forecast considerations in temperature forecasting.  When very cold clouds exist at night, use 10.3 microns rather than 3.9 micron imagery (or product that uses the 3.9 micron band) due to significant noise in the 3.9 micron band.


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