Stereoscopic View of Tropical Storm Chantal

Tropical Storm Chantal, the third named storm of the northern Atlantic Tropical Season, has rapidly moved through the Lesser Antilles and into the eastern Caribbean Sea. The stereoscopic view of Chantal, above, uses visible (0.65 µm) imagery from GOES-12 (over the Equator at 60 W) and visible (0.63 µm) imagery from GOES-13 (over the Equator at 75 W) and shows Chantal as it moved between Martinique and St. Lucia. A similar image, but at 10.7 µm, is here. Coldest brightness temperatures northeast of the storm center were -78 C.

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

The GOES-13 satellite had been placed into Rapid Scan Operations (RSO) mode, providing images as frequently as every 5-10 minutes. Visible imagery (above; click image to play animation) revealed the presence of low-level convective outflow boundary cloud arcs along the western periphery of Tropical Storm Chantal. The appearance of these cloud arcs can be an indication that a tropical cyclone has encountered an area of dry air aloft (generally either from the Saharan Air Layer, or a mid-latitude dry air intrusion), which can sometimes have a negative effect on the rate of intensification of the storm. However, in this case, Chantal slowly intensified during the day as it moved over the warm waters and high ocean heat content that were in place over the western Caribbean Sea.

Morphed Microwave Estimates of Total Precipitable Water

The morphed animation of Total Precipitable Water (TPW) from the CIMSS MIMIC site, above, shows the maximum in TPW associated with Chantal moving west-northwestward across the tropical Atlantic on a path towards the Greater Antilles — likely Hispaniola. (Note to NWS WFOs: MIMIC TPW is available via LDM subscription and can be displayed in AWIPS). Several products available at the CIMSS Tropical Cyclones website (in addition to the MIMIC TPW above) can be used to diagnose the environment around the system and the strength of the system.

Objectively-determined Overshooting Tops over the Tropical Atlantic, 1915 UTC 9 July 2013

For example, Tropical Overshooting Tops (TOTs) (at this site), are related to the vigor of the convection that is sustaining the Tropical Storm. The objectively-determined TOTs, above, show a cluster within the convective envelope of the system. This suggests ongoing strong convection and a storm that is at minimum maintaining its strength at present. (Given that the upper-level environment can change rapidly, the presence of TOTs may not be well-correlated with strengthening, however). This line plot shows the maximum sustained winds of the system plotted with the number of TOTs near the system.

Saharan-Air Layer Analysis, Tropical Atlantic, 1800 UTC 9 July 2013

Dust from the Sahara Desert that is lofted by winds and transported into the atmosphere above the tropical Atlantic has a well-known suppressing effect on convection and therefore tropical cyclone development and strengthening. It is often visible from satellite. The SAL analysis, above, (from this site) shows little to impede Chantal as it moves into the Caribbean. Most of the SAL at present is behind the storm.

Wind Shear and Tendency, derived from Satellite data, 1800 UTC 9 July 2013

Other data at the CIMSS Tropical Cyclones site can be used to judge the environment that the storm is in. Around Chantal, for example, wind shear values (above, from here) are modest (and decreasing), but they increase in the direction that the storm is moving. Oceanic Heat Content in the Caribbean Sea surrounding Chantal is sufficient to support strengthening, and the current National Hurricane Center forecast does modestly strengthen the storm before landfall on or near Hispaniola.

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GOES-15 (left) and GOES-13 (right) 0.63 µm visible channel images (click image to play animation)

Severe thunderstorms developed during the afternoon hours across parts of northeastern Montana on 08 July 2013. A comparison of McIDAS images of 1-km resolution GOES-15 (GOES-West) and GOES-13 (GOES-East) 0.63 µm visible channel data (above; click image to play animation) showed the early stages of development of the storm that went on to produce large hail (up to 3 inches in diameter) and straight-line winds from a macroburst gusting as high as an estimated 95 mph (SPC storm reports | NWS Glasgow public information statement). Some locations also received heavy rain (as much as 1.87 inches in a 1 hour period) that produced flash flooding. One interesting feature seen on the visible imagery was a region of inflow feeder bands along the southern flank of the developing thunderstorm as it was northwest of Jordan (station identifier KJDN).

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data with overlays of SPC storm reports (below; click image to play animation) revealed the formation of an “enhanced-V” storm top signature with the storm as it was north of Jordan, which is usually an indicator that a storm is or is about to produce severe weather in the form of large hail, tornadoes, or damaging winds.

GOES-13 10.7 µm IR channel images with hail and severe wind gust reports (click image to play animation)

A closer view using 375-meter resolution (projected onto a 1-km AWIPS grid) Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel data at 20:30 UTC (below) showed a textbook example of a well-defined enhanced-V signature on the IR image. The coldest cloud-top IR brightness temperature in the overshooting top region was -69º C (black color enhancement), while the IR brightness temperature in the nearby upstream “warm wake” region was -44º C (darker green color enhancement), making for an impressively large 25º C “delta-T” value. About 1 hour later this storm began to produce 1.75 inch diameter hail in Garfield county (north of Jordan).

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images

A comparison of the VIIRS IR image with the corresponding GOES-13 IR image (below) demonstrated the advantages of using polar-orbiter satellite date for severe storm identification: (1) with higher spatial resolution, severe storm signatures such as the “enhanced-V” are much better defined, and (2) there is minimal “parallax shift” associated with the large viewing angle from geostationary satellites positioned over the Equator.

Suomi NPP VIIRS 11.45 µm IR channel and GOES-13 10.7 µm IR channel images

===== 10 July Update =====

A comparison of before (04 July) and after (10 July) 250-meter resolution MODIS true-color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (below; displayed using Google Earth) revealed the extensive hail damage swath that was located from the north and northeast to the east and southeast  of Jordan, Montana (yellow arrows). At its widest point in far southern McCone county, the hail swath appeared to be at least 10-12 km (6-7 miles) wide.

Before (04 July) and after (10 July) MODIS true-color RGB images showing the hail damage swath

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GOES-13 3.9 µm shortwave IR images (click image to play animation)

A large fire resulted from a train derailment in the town of Lac-Mégantic, Quebec at around 05:15 UTC (1:15 AM local time) on 06 July 2013. According to the TSB investigation report and a CBC news story, the 72 rail cars were carrying crude oil. AWIPS images of 4-km resolution GOES-13 3.9 µm shortwave IR images (above; click image to play animation) revealed the fire “hot spot” (black to yellow to red pixels), with a maximum IR brightness temperature value of 36.5º C on the 05:30 UTC image.

The fire hot spot was no longer evident on the GOES-13 3.9 µm shortwave IR images after 06:15 UTC, perhaps because of partial obscuration by the approach of a patch of mid/high altitude clouds from the west — however, a well-defined fire hot spot (with a maximum IR brightness temperature value of 54.5º C) was still evident on a 375-meter resolution (mapped onto a 1-km AWIPS grid) Suomi NPP VIIRS 3.74 µm shortwave IR image at 06:21 UTC (below).

GOES-13 3.9 µm and Suomi NPP VIIRS 3.74 µm shortwave IR images

In addition to the fire hot spot on seen the VIIRS shortwave IR image, a very large and bright glow from the fire was apparent on the corresponding 0.7 µm VIIRS Day/Night Band image (below).

Suomi NPP VIIRS 3.74 µm shortwave IR and 0.7 µm Day/Night Band images

Hat-tip to Dan St. Jean of the National Weather Service forecast office at Gray/Portland, Maine for the heads-up on this event.

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

AWIPS images of GOES-13 0.63 µm visible channel data (above; click image to play animation) showed the development of pockets of thunderstorms across Iowa, eastern Nebraska, and northwestern Missouri  on 02 July 2013.  Several of these storms produced hail up to 1 inch in diameter (SPC storm reports).

Note the pronounced cyclonic spin across the region of thunderstorm development — this was due to the approach of a compact shortwave trough that was rotating around the western periphery of a larger-scale upper-level trough of low pressure that was centered over the middle Mississippi River valley on that day. This shortwave trough had a nice signature on GOES-13 6.5 µm water vapor channel images (below; click image to play animation).

GOES-13 0.65 µm water channel images (click image to play animation)

GOES-13 sounder Total Column Ozone product

In addition, the GOES-13 sounder Total Column Ozone (TCO) product (above; click image to play animation) revealed that a distinct maximum in TCO values (red color enhancement) accompanied this disturbance. NAM40 model overlays of the pressure of the Potential Vorticity (PV) 1.5 surface (a general indicator of the height of the dynamic tropopause) suggested that a PV anomaly was associated with the high TCO values (below) — and this PV anomaly was likely helping to dynamically force some of the development of thunderstorms seen across the region.

GOES-13 sounder Total Column Ozone product with NAM40 PV 1.5 pressure and 500 hPa geopotential height

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