Tropical Storm Arthur forms east of Florida

July 1st, 2014
GOES-13 0.63 µm visible channel images (click to play animation)

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

The first tropical depression (update: Arthur was named as a tropical storm at 1500 UTC 1 July) of the season in the tropical Atlantic has formed just to the east of Florida. The visible imagery animation, above, shows persistent strong thunderstorms with overshooting tops in the area of disturbed weather over the Gulf Stream and the Bahamas. Refer to the National Hurricane Center and the CIMSS Tropical Cyclones sites for particulars on the future track of this system. Note that current forecasts have the system strengthening to a hurricane in the next few days, and close to the North Carolina coast on July 4th.

Metop ASCAT surface scatterometer winds at 1541 UTC, below, indicated that the strongest winds (green barbs, 30-39 knots) were found within the northeastern quadrant of the tropical storm.

GOES-13 visible images with Metop ASCAT surface scatterometer winds (click to play animation)

GOES-13 visible images with Metop ASCAT surface scatterometer winds (click to play animation)

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

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

The tropical Atlantic has lately been besieged by Saharan Air Layer (SAL) dust (see, for example, this post from last week, or this image from today); that dry air suppresses tropical cyclone formation. The animation of GOES-13 10.7 µm imagery, above, shows that this Tropical Depression formed out of an impulse that sank southward from the Carolinas over the past 6 days, so its gradual development has not been impeded by the SAL.

The VIIRS instrument on board the Suomi NPP satellite provided high-resolution imagery over this tropical system shortly after midnight on the 1st (see below). A large cirrus shield with brightness temperatures cooler than -70º C (Green in the enhancement) with a few overshooting tops that are colder than -85º C are present. An analysis of some NUCAPS Soundings from this overpass is here.

Suomi NPP VIIRS 11.35 µm infrared imagery, Day/Night Band imagery (0.70 µm) and lightning data at ~0715 UTC on 1 July 2014 (click to toggle through images)

Suomi NPP VIIRS 11.35 µm infrared imagery, Day/Night Band imagery (0.70 µm) and lightning data at ~0715 UTC on 1 July 2014 (click to toggle through images)

Arthur’s projected track moves the storm up the East Coast over very warm waters associated with the Gulf Stream. Both MODIS and VIIRS analyses of SSTs show widespread temperatures in excess of 80º F.

A comparison of Suomi NPP VIIRS 11.45 µm IR channel images at 0717 UTC and 1840 UTC, below, showed that the areal coverage of cold cloud tops was increasing during the day on 01 July, but the deep convection remained well to the southeast of Arthur’s low-level center of circulation.

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS 11.45 µm IR channel images

At 1840 UTC, a comparison of the Suomi NPP VIIRS 11.45 µm IR channel image with the corresponding 0.64 µm visible channel image with an overlay lightning data, below, revealed a large number of cloud-to-ground strikes within the 1-hour period ending at 1900 UTC.

Suomi NPP VIIRS 11.45 µm IR channel image and 0.64 µm visible channel image (with lightning data)

Suomi NPP VIIRS 11.45 µm IR channel image and 0.64 µm visible channel image (with lightning data)

===== 02 July Update =====

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

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

Arthur continued to slowly intensify on 02 July, and began to show hints of an organized eye structure on GOES-13 0.63 µm visible channel images (above; also available as an MP4 movie file).

A comparison of AWIPS-2 images of Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images (below) showed that the coldest cloud tops were north of the center of Arthur at 1822 UTC. A buoy just southwest of the center reported winds gusting to 52 knots (60 mph).

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

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

Even though an eye was not evident on GOES-13 10.7 µm IR channel imagery around 2045 UTC, a DMSP SSMIS 85 GHz microwave image at 2049 UTC did display a well-organized eye signature (below).

GOES-13 0.63 µm visible channel image and DMSP SSMIS 85 GHz microwave image

GOES-13 0.63 µm visible channel image and DMSP SSMIS 85 GHz microwave image

Mesoscale Convective Systems over the Upper Midwest, and a Mesoscale Convective Vortex over Wisconsin

June 18th, 2014
Suomi NPP VIIRS 11.45 µm IR channel and 0.7 µm Day/Night Band images, with cloud-to-ground lightning strikes

Suomi NPP VIIRS 11.45 µm IR channel and 0.7 µm Day/Night Band images, with cloud-to-ground lightning strikes

A comparison of AWIPS images of Suomi NPP VIIRS 11.45 µm IR channel and 0.7 µm Day/Night Band data (above) showed very large areas of cold cloud-top IR brightness temperatures associated with Mesoscale Convective Systems (MCSs) over the Upper Midwest region of the US at 08:00 UTC (3:00 AM Central time) on 18 June 2014. The coldest IR brightness temperature was -88º C over far southern  Minnesota.  Numerous bright white “streaks” were seen on the Day/Night Band (DNB) image, which indicated portions of the cloud that were illuminated by intense lightning activity. Cloud-to-ground lightning strikes are also plotted on the DNB image, showing how electrically-active these storms were at the time. The western MCS initially formed over eastern South Dakota during the previous evening, producing a few tornadoes there (SPC storm reports). The eastern MCS began to form later along the Wisconsin/Illinois border region — one aircraft flying near the northern edge of a rapidly-developing thunderstorm encountered severe turbulence.

Shortly after the time of the Suomi NPP satellite overpass, a 08:21 UTC overpass of the NOAA-19 POES satellite provided AVHRR-derived CLAVR-x Cloud Top Temperature (CTT), Cloud Top Height (CTH), and Cloud Type products (below). The minimum CTT value was -84º C, and the maximum CTH value was 14 km; much of the MCS cloud shield was classified as the Overshooting Top type (magenta color).

POES AVHRR Cloud Top Temperature, Cloud Top Height, and Cloud Type products

POES AVHRR Cloud Top Temperature, Cloud Top Height, and Cloud Type products

After sunrise, McIDAS  images of GOES-13 0.63 µm visible channel data (below; click image to play animation; also available as an MP4 movie file) showed that the eroding MCS cirrus shield aloft exposed a middle-tropospheric Mesoscale Convective Vortex (MCV) which continued moving eastward across Wisconsin during the day.

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

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

Consecutive overpasses of the Terra and Aqua satellites provided MODIS 0.65 µm visible channel images of the region (below). The convective outflow boundary from the earlier MCS activity had acted to push the warm frontal boundary (which had been acting as a focus for convective development) south of the Wisconsin/Illinois border, leaving a relatively stable boundary layer with a weak capping inversion aloft over Wisconsin — as a result, the MCV circulation did not play a role in initiating any new convective development.

MODIS 0.65 µm visible channel images, with surface reports and surface fronts

MODIS 0.65 µm visible channel images, with surface reports and surface fronts

Blowing dust in the central Plains, and a severe weather outbreak in the southern Plains and Lower Mississippi River Valley regions

April 27th, 2014
Suomi NPP VIIRS true-color RGB image

Suomi NPP VIIRS true-color RGB image

A Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image visualized using the SSEC RealEarth web map server (above) showed large areas of blowing dust across parts of the central Plains states on the afternoon of 27 April 2014. Surface winds were gusting in excess of 60 mph at some locations.

McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (below; click image to play animation) displayed the formation of a well-defined “blowing dust front” that moved eastward across Kansas.

 

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 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) showed the cold cloud top IR brightness temperatures (as cold as -81º C, violet color enhancement) associated with severe thunderstorms that produced widespread hail, damaging winds, and tornadoes (which were responsible for 16 fatalities).

GOES-13 10.7 µm IR channel images, with overlays of SPC storm reports (click to play animation)

GOES-13 10.7 µm IR channel images, with overlays of SPC storm reports (click to play animation)

A 1-km resolution POES AVHRR 12.0 µm IR channel image with SPC storm reports at 02:13 UTC or 9:23 PM local time (below) showed the large mesoscale convective system that was responsible for a number of tornadoes in Arkansas a few hours earlier. Note the appearance of subtle cloud-top gravity waves propagating northwestward away from the storm center.

POES AVHRR 12.0 µm IR channel image, with overlay of SPC storm reports

POES AVHRR 12.0 µm IR channel image, with overlay of SPC storm reports

Several hours later, a comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 07:15 UTC or 2:15 AM local time (below) revealed the presence of ripples of mesospheric airglow waves propagating northeastward across Missouri and far southern Illinois and Indiana. These mesospheric airglow waves were likely caused by intense overshooting cloud tops associated with the severe thunderstorms as they were producing tornadoes over Arkansas. Several bright “lightning streaks” could also been seen on the VIIRS DNB image, as these electrically-active thunderstorms were responsible for over 2000 cloud-to-ground (CG) lightning strikes in the 15-minute period and over 8000 CG lightning strikes in the 1-hour period preceding the VIIRS images. The SPC storm reports plotted on the IR image cover the 5.5 hour span from 02:00 to 07:30 UTC.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images

===== 28 April Update ======

A comparison of “before” (25 April) and “after” (28 April) 250-meter resolution Aqua MODIS true-color RGB images from the SSEC MODIS Today site (below) showed much of the southwest-to-northeast oriented damage path resulting from the EF-4 rated Mayflower/Vilonia/El Paso tornado in Arkansas (NWS Little Rock storm summary).

Before (25 April) and after (28 April) Aqua MODIS true-color RGB images showing the damage path from the Vilonia, Arkansas tornado

Before (25 April) and after (28 April) Aqua MODIS true-color RGB images showing the damage path from the Vilonia, Arkansas tornado

Lunar Eclipse Effects on VIIRS Day/Night Band Imagery

April 15th, 2014
Suomi NPP VIIRS Day Night Band images, times as indicated

Suomi NPP VIIRS Day Night Band images, times as indicated

A Total Lunar Eclipse occurred over North America early in the morning of 15 April 2014. The resultant lack of lunar illumination had a profound impact on the “visible image at night” quality of the VIIRS 0.7 µm Day/Night Band imagery during the Suomi NPP satellite overpass at around 0800 UTC over the midsection of the United States. The animation above shows excellent illumination from the Full Moon at 0619 UTC (2:19 AM Eastern time) along the East Coast, and also at 0940 UTC (2:40 AM Pacific time) along the West Coast; however, the image at 0800 UTC (3:00 AM Central time), in the middle of eclipse totality, resembles Day/Night Band images seen during a New Moon. The three images can also be combined into a collage, as shown below, to illustrate the changes in the Day/Night band that occur as lunar intensity changes.

Collage of three successive Suomi NPP VIIRS Day Night Band images, times as indicated

Collage of three successive Suomi NPP VIIRS Day Night Band images, times as indicated

A comparison of three similar Day/Night Band images from the day before is here.

Even though the 08:00 UTC VIIRS Day/Night Band image exhibited a comparatively dull and “washed-out” appearance (due to very little illumination from eclipsed moonlight), there were still features of interest that could be seen. For example, a closer look over the southeastern US using the 0.7 µm Day/Night Band and 11.45 µm IR channel images (below) showed the effect of lightning activity associated with a pre-cold-frontal squall line over the Florida panhandle and the adjacent offshore waters of the Gulf of Mexico: a pair of long, narrow bright streaks (caused by cloud illumination from intense lightning activity as the sensor was rapidly scanning from northwest to southeast), abruptly followed by dark black streaks that eventually faded away. These dark black streaks represent post-saturation “recovery periods” after the sensor scanned the extremely bright cloud features. Incidentally, the coldest 11.45 µm IR brightness temperature over the Florida panhandle was -80º C (very near the area of dense cloud-to-ground lightning strikes).

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images, with surface frontal analysis and 1-hour cloud-to-ground lightning strikes

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images, with surface frontal analysis and 1-hour cloud-to-ground lightning strikes