Supercells in the Southeast

May 6th, 2020 |

A cold front with ample moisture and instability ahead of it spawned numerous strong storms in the Southeast U.S. yesterday; particularly one long-lived supercell in South Carolina. A convolutional neural network model (CNN) was deployed in realtime on the 1-min GOES-16 mesoscale sector imagery. The model produces an “Intense Convection Probability” (ICP). The inputs for the model are the GOES-16 ABI 0.64 µm reflectance, 10.3 µm brightness temperature, and GLM flash extent density. It was trained to identify “intense” convection as humans do, associating features with intense convection such as strong overshooting tops, thermal couplets (“cold-U/V”), above anvil cirrus plumes (AACP), and strong cores of total lightning.

The animation below shows the ICP contours overlaid ABI 0.64 µm + 10.3 µm sandwich imagery, annotated with preliminary severe storm reports.


The long-lived supercell in South Carolina exhibited AACP and cold-U features, and produced numerous severe wind and hail reports (up to the size of tennis balls). While the NOAA/CIMSS ProbSevere models handled this storm well, the ICP ramped up on a couple of severe storms in northern Georgia before ProbSevere did. ICP for these cells exceeded 90% 15-18 min before ProbWind reached 50%. The ICP may be able to provide additional lead time and confidence to ProbSevere guidance for certain storms, utilizing spectral and electrical information from geostationary satellites. Incorporating ICP into ProbSevere is an active area of current research.

ProbSevere storm contours and MRMS MergedReflectivity for storms in GA and SC. The main or “inner” ProbSevere contour is shaded by the probability of any severe weather, while the outer contour is shaded by the probability of tornado, which appeared when that value was at least 3%, in this example.


An accumulation of ProbSevere storm centroids (white to pink squares, 50% --> 100%), NWS severe weather warnings, and SPC severe local storm reports from 12Z on May 5th to 12Z on May 6th [click to enlarge]

An accumulation of ProbSevere storm centroids (white to pink squares, 50% –> 100%), NWS severe weather warnings, and SPC severe local storm reports from 12Z on May 5th to 12Z on May 6th [click to enlarge]

Thunderstorm with abundant hail over southwest Oregon

April 5th, 2020 |

GOES-17 Band 02 (Red Visible, 0.64 µm) from 1500 UTC 5 April to 0300 UTC on 6 April 2020 (Click to play animated gif)

A thunderstorm over Jackson County, Oregon late in the day on 5 April 2020 produced abundant hail that accumulated to depths in excess of an inch over a wide swath of the county.  Visible animation from 1500 UTC on 5 April through 0300 UTC on 6 April show the region experiencing plentiful sun during the day, helping to destabilize the lower troposphere.  Much of the hail fell in the hour between 0030 and 0130 UTC on 6 April.  That time is captured in the visible loop above, and in the rocking animation from 00 to 02 UTC on 6 April 2020 (and back) below.  Similar animations (from 1500 UTC 5 April to 0300 UTC 6 April) are also available for GOES-17 Band 13 (10.3 µm) and the Day Cloud Phase RGB.

GOES-17 Band 02 (Red Visible, 0.64 µm) rocking animation from 2300 UTC/5 April to 0200 UTC/6 April 2020 (Click to enlarge)

The storms formed in a trough at 500 mb that was associated with abnormally cold temperature (image below courtesy Mike Stavish, WFO MFR). This feature is also discussed in this blog post.

500-mb Temperatures at 0000 UTC on 6 April 2020 (Click to enlarge)

The 0000 UTC Upper-Air sounding at Medford, below, (also courtesy Mike Stavish) shows modest Convective Available Potential Energy (CAPE) amounts and a low freezing level (less than a mile above ground) suggest the possibility of hail.

Medford Oregon Sounding from 0000 UTC on 6 April 2020 (Click to enlarge)

Indeed, the storm in question was a prolific hail producers, as shown in photographs below (photos courtesy Dan Weygand, WFO Medford)

 

NOAA/CIMSS ProbSevere display at 10-minute timesteps from 0000 to 0200 UTC on 6 April 2020 (Click to enlarge)

NOAA/CIMSS ProbSevere readouts in a timeseries, below (courtesy John Cintineo, UW-Madison CIMSS), shows very low values of parameters typically associated with severe weather.  ProbHail peaks at only 2%. The animation above shows very small values (Click here for a ProbSevere image at 0130 UTC with a Probe that shows values).   Such small values during this event amplify the message that local knowledge of conditions that are favorable for severe weather (and hail events like this that coat roads can lead to traffic crashes and fatalities) are important.

NOAA/CIMSS ProbSevere readout for the Radar Object 16917 in County Oregon

Rocking animations that cover the time of hail fall are shown below.  Because the storm formed/decayed as the sun was setting, use of visible imagery (at top) or Day Cloud Phase Distinction (that uses the visible 0.64 µm in the ‘green’ and the snow/ice near-infrared 1.61 µm in the ‘blue’) was a challenge.  Clean window imagery of course maintained a consistent signal through sunset.

Note the Day Cloud Phase Distinction does capture the glaciation of the cloud.  Sometimes this can be used to judge when lightning might become a hazard.  Lightning was a rare event on this day.

Rocking Animation of GOES-17 Clean Window IR (10.3 µm) from 2300 UTC/5 April to 0200 UTC/6 April (and back) (Click to enlarge)

Rocking Animation of GOES-17 Band 13 Clean Window Infrared imagery (10.3 µm) from 0000 UTC to 0200 UTC (and back) on 06 April 2020 (Click to enlarge)

Rocking Animation of GOES-17 Day Cloud Phase Distinction Red-Green-Blue composite imagery from 2300 UTC/5 April to 0200 UTC/6 April (and back) (Click to enlarge)


NOAA-20 overflew the West Coast, and NUCAPS soundings were produced, at 2047 UTC on 5 April.  (Click here to see NOAA-20 orbits on that day, from this site).  Gridded NUCAPS fields (from this site; update: from this site) of 500-mb Temperatures, Ozone Anomalies, and 850-500-mb Lapse Rates, below (imagery courtesy Emily Berndt and Frank LaFontaine, NASA Sport), show the cold air in the cutoff and the unstable environment that supported the convection.

Gridded NUCAPS estimates of 500-mb Temperatures, Total Ozone Anomaly and 850-500 mb lapse rate, data from NOAA-20 afternoon pass on 5 April 2020, ~2047 UTC (Click to enlarge)

(Thanks to Mike Stavish, Science and Operations Officer — SOO — in Medford for alerting us to this case)

Why 1-minute data matters: Beavertails

June 4th, 2015 |
GOES-14 Visible (0.6263 µm) Imagery, 04 June 2015.  1-minute imagery on the left, 5-minute imagery on the right (click to play animation)

GOES-14 Visible (0.6263 µm) Imagery, 04 June 2015. 1-minute imagery on the left, 5-minute imagery on the right (click to play animation)

Beavertails are ephemeral cloud features that form in the inflow of supercell thunderstorms. They are horizontally long and roughly parallel to the inflow warm front. Its appearance (and presence) is affected by and influences inflow into the storm, and by inference, it affects radar returns. That is — a change in the Beavertail cloud can precede a change in radar. Accurate detection of this cloud type, then, aids the understanding of evolving storm morphology. The animation above shows a severe convective system over southeastern Wyoming, viewed at 1-minute intervals (Left) and at 5-minute intervals. Beavertails that form persist for about 30 minutes, so 5-minute imagery will resolve them; however, the resolution of the 1-minute data is far better to monitor the small changes in size and shape that are related to storm inflow.

Do beavertail changes affect the radar? The animation below shows the ProbSevere product readout from 2000-2220 UTC (Courtesy John Cintineo, CIMSS) (Click here for a slow animation). (Click here for an animation (from 1918-2058 UTC) that includes warning polygons). The increases and decreases in the MRMS MESH appear unrelated to the formation/decay of the various beavertails.

NOAA/CIMSS ProbSevere Product, 2000-2020 UTC on 4 June 2015 (click to animate)

NOAA/CIMSS ProbSevere Product, 2000-2020 UTC on 4 June 2015 (click to play animation)

This storm was captured by different chasers. This YouTube video from Scott Longmore shows the evolution of the convective system from the ground. Hat/tip to Jennifer Laflin, NWS EAX and Chad Gravelle, OPG, for alerting us to this case.

Severe thunderstorm strikes Virginia campground

July 24th, 2014 |
GOES-13 10.7 µm IR channel images (click to play animation)

GOES-13 10.7 µm IR channel images (click to play animation)

A supercell thunderstorm intensified as it moved eastward across the Chesapeake Bay (just ahead of an approaching surface cold front) on the morning of 24 July 2014 — as it reached the Virginia shore of the Delmarva Peninsula, it produced an EF-1 tornado and damaging straight line winds that were responsible for 2 fatalities and 36 injuries at the Cherrystone Family Camping Resort (located at the * symbol on the images). The storm also produced golf ball to baseball size hail (NWS damage survey | SPC storm reports). McIDAS images of GOES-13 10.7 µm IR channel data (above; click image to play animation; also available as an MP4 movie file) showed that the cloud-top IR brightness temperatures associated with the storm cooled quickly, from -45º C at 11:15 UTC to -64º C at 12:30 UTC. The temperature value was close to that of the tropopause (at a height of 15.4 km) on the 12 UTC rawinsonde data from Wallops Island, Virginia.

The corresponding GOES-13 0.63 µm visible channel images (below; click image to play animation; also available as an MP4 movie file) revealed the presence of an overshooting top at 12:30 UTC  (the time that the IR cloud-top brightness temperature values reached their minimum), which was also flagged by the automated Overshooting Tops detection algorithm.

GOES-13 0.63 µm visible channel images (click to play animation)

GOES-13 0.63 µm visible channel images (click to play animation)

AWIPS-II images of the NOAA/CIMSS ProbSevere product (below) followed the radar feature associated with the supercell thunderstorm. Around 11:30 UTC, the ProbSevere value was low, around 5-10%, a result of weak satellite-detected growth (and moderate glaciation) early in the storm’s life, along with low values of MRMS Maximum Expected Size of Hail (MESH). Environmental parameters from the Rapid Refresh model that were supportive of convection: MUCAPE exceeded 2200 J Kg and Shear values were greater than 30 m/s. As the cell tracked to the east and began to move over Chesapeake Bay, both MUCAPE and Shear gradually increased, to values near 2400 J/kg and 35 m/s, respectively. MRMS MESH was oscillating as the cell approached Chesapeake Bay, from 0.44 inches at 11:42 UTC (ProbSevere value of 10%) to 0.37 inches at 11:46 UTC (ProbSevere of 7%) to 0.65 inches at 11:48 UTC (ProbSevere of 29%) to 0.56 inches at 12:00 UTC (ProbSevere of 18%). As the storm moved over the Bay, MESH sizes jumped, to 0.86″ at 12:04 UTC (ProbSevere of 58%, the first crossing of the 50% threshold), to 1.02″ at 12:06 UTC (ProbSevere of 71%), to 1.86″ at 12:12 UTC (ProbSevere of 92% , the first crossing of the 90% threshold), and to 3.09″ (!) at 12:16 UTC (ProbSevere of 91%). At 12:20 UTC, when the Tornado Warning was issued, MRMS MESH was 3.51″ and ProbSevere remained at 91%. Thus, the warning was issued 16 minutes after ProbSevere exceeded 50%, and 8 minutes after ProbSevere was greater than 90%. The NWS storm survey indicated that the campsite fatalities occurred around 12:33 UTC, or 13 minutes after the issuance of the tornado warning.

NOAA/CIMSS ProbSevere product

NOAA/CIMSS ProbSevere product

The rapid intensification of the system as it moved over the Chesapeake begs the question: was instability diagnosed? In the animation below, GOES-13 sounder Derived Product Images (DPI) of Lifted Index (top panel) and CAPE (bottom panel) showed a rich source of instability just south of the cloud-obscuring convection (and ahead of the southward-moving cold front). Lifted Index values derived at 1147 UTC were around -6 at the mouth of the Chesapeake Bay (bright yellow enhancement); CAPE values were around 2500 J/kg (yellow and red enhancements).

GOES-13 Sounder DPI estimates of Lifted Index (top) and CAPE (bottom) [click to play animation]

GOES-13 Sounder DPI estimates of Lifted Index (top) and CAPE (bottom) [click to play animation]