1-minute GOES-16 applications for the 8 May 2017 Colorado hail event

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.

By Ed Szoke and Dan Bikos

On 8 May 2017 severe thunderstorms hit northeast Colorado with a devastating hail storm across the Denver metro vicinity, causing more than $1.4 billion in damage.  This blog entry will look at GOES-16 imagery to show the synoptic setup, followed by the pre-storm environment, and then applications of the 1-minute imagery during the nowcast time period.

The 3 water vapor bands and 0.64 micron visible imagery from GOES-16:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8may17/colorado/4panel_early&loop_speed_ms=140

depict a closed low over far northern Baja.  The loop shows southerly flow in all 3 WV channels  bringing moisture northward across Colorado.  We can see this relatively deep moisture in the 12Z Denver sounding:

DEN_12z_sounding

Note the southerly flow extends to the surface.  Mid-level (700-500 mb) lapse rates are quite steep (8.1 degrees C / km).  Despite the fairly deep moisture, the dewpoint just above the surface was still fairly low (upper 30s F).  But this changed dramatically during the morning as a cold front surged southward across northeastern Colorado as shown in this visible loop with METARs:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8may17/colorado/conus_vis_metar&loop_speed_ms=140

The visible imagery shows low-level cumulus developing along portions of the Front Range behind the cold front, meanwhile at the end of the loop a thunderstorm develops at the leading edge of the cold front.  By 1800 UTC, the dewpoint at Denver had risen to 46 F with slightly higher values to the north.  Although 73 / 46 do not seem like very high values to support the severe hailstorm that would later occur, given the steep lapse rates, this temperature / dewpoint is actually quite unstable at this elevation.

What was the origin of the Denver hailstorm?  At the end of the last loop, we noticed a storm forming along the cold front southeast of Denver.  In the next loop, we look at several hours of 5 minute visible imagery:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8may17/colorado/conus_vis&loop_speed_ms=140

Notice the lines of south-north oriented convection southwest of the Denver area.   The origin of these lines appears to be collocated with regions of higher terrain.  The moist southerly flow impinging on these terrain features forms these lines of convection.  In the loop we can see that the Denver storm appears to develop at the north end of the mountain range indicated by the yellow arrowstatic_labeled

The animation also shows the first storm mentioned earlier that forms east of Denver around 1740 UTC and for a while looks fairly strong (although no severe reports were observed with this storm), but it dissipates quickly shortly after 1900 UTC.  We speculate that the air mass behind the cold front was still too cool and stable to sustain the storm.

Next, let’s focus on 1-minute visible imagery starting at 1900 UTC going through 2200 UTC:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8may17/colorado/meso_vis&loop_speed_ms=60

Early in the loop, extensive cumulus growth takes place over the stalled cold front, leading to convective initiation.  These storms remained southeast of Denver but do become severe (hail observed up to 2″ in diameter and funnel cloud).  We’ll discuss these storms in more detail later, however now we will focus on the Denver hailstorm.

Recall that the Denver storm develops on a cloud line with origins from the northern edge of a mountain range.  We can see this nicely in the 1-minute animation.  Following this particular storm, we notice that it rapidly develops an overshooting top around 2016 UTC as denoted in this image below:

annotated_denverstorm

This overshooting top expands rapidly and soon thereafter (2040 UTC) we get the first severe hail report west of Denver.  Shortly after this first hail report we see continued expansion of the overshooting top and what appears like anti-cyclonic rotation at cloud top.  This may however just be a strong divergence signature and not necessarily a circulation. Also during this time period, the storm motion changes from heading north-northeast to more east-northeast (turn to the right of the mean flow) coincident with numerous hail reports above 2″ in diameter and up to 2.75″ (baseball) in diameter.  Since this is a densely populated area that included numerous automobile dealerships, widespread damage occurred (latest estimates of at least $1.4 billion).  After passing through the Denver area, the storm motion reverts back to north-northeast once again, suggesting a downward trend in intensity (indeed, the frequency and size of the hail reports went down).

Early in the loop, at the southern end of our scene a thunderstorm produces anvil cirrus that moves to the north-northeast.  As that anvil cirrus approaches the cluster of thunderstorms southeast of Denver that we discussed earlier, they clearly act as an obstacle in the flow to the anvil cirrus coming up from the south (particularly evident as a ring of clear sky between the thunderstorm and approaching cirrus from 2015 – 2100 UTC).

Focusing our attention back on the cluster of thunderstorms southeast of Denver.  We observe a series of quasi-stationary waves oriented east-west just north of the overshooting tops, these are particularly evident in the 2030-2130 UTC range.  Are these waves induced by the thunderstorms or are they pre-existing?

To help address this question, we consider the 3 water vapor channels along with the 10.35 micron IR animation:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/8may17/colorado/4panel_gw&loop_speed_ms=100

In the water vapor bands, notice the east-west oriented quasi-stationary waves do exist prior to convective initiation.  These waves are most evident in the 6.95 micron (mid-level) water vapor band, somewhat evident in the 7.34 micron (lower-level) water vapor band and quite subtle in the 6.2 micron (upper-level) water vapor band.  It appears that once the anvil cirrus develops, the waves become visible within the anvil shield.  Perhaps these are gravity waves that are forming as the mid/upper level southerly flow is lifted over the stalled cold front.

One final feature to point out is in the 10.35 micron IR imagery in the lower right panel.  The overshooting top with the Denver hailstorm is quite vivid in the IR imagery, in particular at 2016 UTC in comparison with the visible imagery discussed earlier.  We can also easily follow the deviant storm motion and what appears to be the anti-cyclonic motion at cloud top around the time of the most significant hail reports.  The most pronounced enhanced-V signature is found in the storm further north of Denver where severe hail also caused damage.

For more information about the Denver hailstorm of 8 May 2017, see this link:

http://www.thedenverchannel.com/news/local-news/14b-may-hailstorm-on-track-to-be-colorados-most-expensive-ever

 

 

Posted in Convection, GOES R, Hail, Severe Weather | Leave a comment

Southern Georgia: West Mims Fire

As the spring season accelerates into summer, it is that time of year again for fires to occur all around the United States. A large fire that is a-brewing is the West Mims Fire located in southern Georgia, embedded in the Okefenokee National Wildlife Refuge. As of 11 May 2017, the fire, initially started by lightning, has burnt over 140,000 acres and is still raging on. To add insult to injury, southern Georgia has not received much precipitation over the past few months where the Okefenokee National Wildlife Refuge is currently in a D3, Extreme Drought as of 9 may 2017. A category D3 Extreme Drought is classified as an area that is susceptible to the listed impacts: major crop losses and potential widespread water shortages or restrictions, according to the US Drought Monitor. The latest updates of the West Mims Fire can be seen via the InciWeb link.

Here are the latest Day/Night Band (DNB) (0.7 um) and Imagery Band (I-4) (3.74um) animations from the Visible Infrared Imaging Radiometer Suite (VIIRS) on-board the Suomi-National Polar-orbiting Partnership satellite. For reference, the DNB utilizes a sun/moon reflectance model that illuminates atmospheric features, senses emitted lights, and assists in cloud monitoring the nighttime, while the I-4 band shows the locations of the hottest wildfires, known as ‘hotspots‘. The DNB is at 750 meter resolution while the I-4 band is at 375 meter spatial resolution. The animations are from 5-12 May 2017.

DNB Animation 

Animation highlights the evolution of the wildfires in southern Georgia denoted by the large white circle. Some of the features that are seen are the emitted city lights and the emitted lights from the fires, corresponding smoke, clouds and one can infer the location of the burn scar extent. Additionally, in the top-right hand corner shows the moon percent visibility and the moon elevation angle. A high moon percent visibility and a positive moon elevation angle imply the moon is above the horizon and adequate moonlight is present to see the atmospheric features via satellite.

DNB_animation

Imagery Band (I-4 ) Animation

Over the same domain, the IR animation shows the brightness temperatures of the fires from a range of 180-400 degrees Kelvin (K), where yellow and red colors imply the hottest parts of the fires. In contrast, the white, grey and black colors imply cold low-to-high clouds in the area. The evolution of the fire can be seen at a high spatial resolution at 375 meters.

IR_animation

Posted in Fire Weather, POES, Satellites | Comments Off

Tropical Cyclone Donna

Have you ever been to the Solomon Islands or the Republic of Vanuatu? They are both remote islands located in the southwestern Pacific Ocean, relatively close in proximity to Australia and Papau New Guinea. There is a tropical cyclone that is a-brewing in this area of the world….her name is tropical cyclone ‘Donna’. As of Friday morning, 5 May 2017, Donna is a Category 2 hurricane and is expected to reach a Category 3 on the Saffir-Simpson Scale.

Donna is unique in that it is considered an ‘out of season’ tropical cyclone where tropical cyclones are normally produced between the months of November and May in the Southern Hemisphere. Flooding, high winds and heavy rains are expected for islands that are in the path of Donna; a tropical cyclone warning has been issued for the effected areas.

To track and monitor Donna, a forecaster or user can utilize the Day/Night Band (DNB) which is a sensor, and is one of 22 channels on the Visible Infrared Imaging Radiometer Suite (VIIRS) instrument which is on-board the Suomi-National Polar-orbiting Partnership (S-NPP) satellite. The DNB uses a sun/moon reflectance model that illuminates atmospheric features, senses emitted natural and anthropogenic lights, and assists with cloud monitoring during the night-time hours; consider it as a night-time visible channel. A DNB image of tropical cyclone Donna, provided from the CIRA TC Real-Time web-page at 1432Z, 5 May 2017, is seen below.

2017SH18_SRSNPPDV_201705051432

Users can see the high-resolution (750-meters) and the detailed cloud structures that the DNB provides during the night-time as Donna moves through the area. Users can also assess the magnitude of Donna, inferring how many islands are or will be impacted by the storm.

For more updates and current status on Tropical Cyclone Donna click the following link.

Posted in Miscellaneous, POES, Satellites, Tropical Cyclones | Comments Off

16 April 2017: 1.37 micron band (“Cirrus band”) features other than cirrus clouds

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.

By Dan Bikos, Louie Grasso, and Ed Szoke

For this blog entry, we are going to focus in on the state of Durango in Mexico during the mid-day hours of 16 April 2017.  Conditions during that time were warm and very dry:

TPW_metars

The sky cover was mostly clear throughout the period of interest (mid-day hours):

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B02&loop_speed_ms=80

A topographic map of the region reveals that the elevation (given below in thousands of feet) is quite high:

terrain

 

If we analyze the GOES-16 1.38 micron (“Cirrus band”):

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B04_default&loop_speed_ms=80

There are features that are moving that are approximately oriented southwest-northeast (ignoring the cirrus clouds later in the loop in the northern regions and also the low-level cumulus to the south).  These features are not clouds since we did not see them in the visible channel shown above.

Let’s look at the GOES-16 7.34 micron (“low-level water vapor”) band:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B10&loop_speed_ms=80

Features similar to those that were observed in the 1.38 micron band appear at 7.34 microns.  The 1.38 micron band can be displayed with a different color table to increase the contrast, thus bringing more clarity to the features that we observe:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B04_enh&loop_speed_ms=80

Recall at this wavelength, 1.38 microns, water vapor is the primary absorber.  If there is sufficient moisture to absorb incoming radiation, cirrus clouds show up rather clearly due to the large contrast between bright cirrus clouds and a dark background, hence the band being named the “Cirrus band”.  In the case discussed here, moisture is limited, particularly at higher elevations where we see the southwest-northeast oriented banded  features.  In fact, here is a comparison of the corresponding features at 1.38 and 7.34 micron band.

annotated1

We note that each feature labeled above has the following characteristics:

1) 1.38 micron band darker corresponds to 7.34 micron band cooler brightness temperature and

2) 1.38 micron band lighter corresponds to 7.34 micron band warmer brightness temperature.

Why?

In this relatively dry, higher elevation environment, the surface is not completely obscured by the intervening (and highly absorbing) atmospheric water vapor when viewed at 1.38 microns.  In this near-infrared band, regions that are darker correspond to more column-integrated water vapor (and a lower surface reflectance contribution), while regions that are brighter correspond to less column-integrated water vapor (and a  higher surface reflectance contribution).

The alignment of these features most likely associated with water vapor are oriented with the terrain:

annotated2

Note that the lower valleys at locations 5 and 6 can be seen as darker regions at 1.38 microns (recall, more water vapor is associated with darker regions at 1.38 microns).

Can we rule out that these features are associated with dust or smoke?  This will now be investigated.

The split window difference product (11.2 micron minus 12.3 micron band difference):

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/SWD&loop_speed_ms=80

would have negative values (brown in this color table) if lofted dust was present, since the values are positive, we can rule out lofted dust.

For assessing smoke, we look at the GOES-16 0.47 micron (“Blue”) visible band:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B01_midday&loop_speed_ms=80

There are no obvious smoke plumes during this time period.  However, if we look later in the afternoon when fires tend to be more pronounced and show up more clearly due to  favorable scattering associated with the view angle:

http://rammb.cira.colostate.edu/templates/loop_directory.asp?data_folder=training/visit/loops/16apr17/mexico/B01_later&loop_speed_ms=80

We do observe a few smoke plumes.  However, the orientation of the smoke plumes does not match with what we observed in the 1.38 or 7.34 micron bands and does not cover such a large area in bands that are oriented with the terrain.

In conclusion, the GOES-16 1.38 micron (“Cirrus”) band can observe features other than cirrus clouds and plumes of water vapor may be observed under specific circumstances.

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