Hurricane Ana south of Hawai’i

October 21st, 2014
Advanced Dvorak Technique (ADT) intensity estimate plot for Ana

Advanced Dvorak Technique (ADT) intensity estimate plot for Ana

A plot of the Advanced Dvorak Technique (ADT) intensity estimate for Ana (above) shows that the strength of the tropical cyclone fluctuated during its multi-day lifetime (storm track), initially becoming a strong Tropical Storm on 15 October, weakening on 16 October, and then slowly intensifying into a Category 1 Hurricane. A sequence of Suomi NPP VIIRS 11.45 µm IR channel images (below; click image to play animation) shows Ana at various stages during the 15-21 October period; the coldest cloud-top IR brightness temperature seen was -95º C at 00:40 UTC on 19 October.

Suomi NPP VIIRS 11.45 µm IR channel images (click to play animation)

Suomi NPP VIIRS 11.45 µm IR channel images (click to play animation)

After prior testing of additional Rapid Scan Operations (RSO) capabilities, the GOES-15 satellite was placed into RSO mode over the Hawai’i region to monitor Ana. During the 17-19 October period that Ana was classified as a Hurricane, GOES-15 10.7 µm IR channel images (below; click image to play a 75 MB animated GIF; also available as an MP4 movie file) revealed a number of robust convective bursts, with some exhibiting cloud-top IR brightness temperatures in the -80 to -85º C range (violet color enhancement). This was the first operational implementation of GOES-West RSO imagery over the Hawai’i region since the early 1990s, and the higher frequency of images — 8 images per hour, instead of the routine 4 images per hour — proved to be helpful for monitoring Ana (CPHC forecast discussion).

Longer-term animations of GOES-15 RSO imagery covering the lifetime of Ana are available in YouTube format (IR | visible). Much of the Hawaiian Islands received heavy rain, with amounts in excess of 11 inches reported on Hawai’i and Oahu.

GOES-15 10.7 µm IR channel images (click to play animated GIF)

GOES-15 10.7 µm IR channel images (click to play animated GIF)

Even after Ana was downgraded to a Tropical Storm early on 20 October, a Suomi NPP VIIRS 11.45 µm IR image showed that cloud-top IR brightness temperatures were still as cold as -90º C (yellow pixels within the violet-enhanced cloud tops). Also note the presence of cloud-top gravity waves propagating outward away from the storm center.

Suomi NPP VIIRS 11.45 µm IR channel image, with overlays of surface analysis and surface observations

Suomi NPP VIIRS 11.45 µm IR channel image, with overlays of surface analysis and surface observations

Late in the day on 20 October, GOES-15 0.63 µm visible channel images (below; click to play animation; also available as an MP4 movie file) revealed that the Low-Level Circulation Center (LLCC) of Ana became exposed as it moved to the southwest from beneath the canopy of deep convective cloud tops. This was a result of moderate southwesterly deep-layer wind shear across the region.

GOES-15 0.63 µm visible channel images (click to play animation)

GOES-15 0.63 µm visible channel images (click to play animation)

During the following night, the exposed LLCC was still very apparent on a Suomi NPP VIIRS 0.7 µm Day/Night Band image at 12:30 UTC or 2:30 am local time (below).

Suomi NPP VIIRS 0.7 µm Day/Night Band image

Record rainfall and flooding in Arizona

September 8th, 2014
GOES-15 10.7 µm IR channel images (click to play animation)

GOES-15 10.7 µm IR channel images (click to play animation)

McIDAS images of 4-km resolution GOES-15 10.7 µm IR channel data (above; click image to play animation; also available as an MP4 movie file) showed the merger of two large mesoscale convective systems (MCS) which produced an all-time record maximum calendar day precipitation amount of 3.29 inches at Phoenix Sky Harbor Airport (PHX) on 08 September 2014. Some locations in the Phoenix area received in excess of 5 inches of rainfall (NWS Phoenix event summary).

An AWIPS-II image of 375-meter resolution Suomi NPP VIIRS 11.45 µm IR channel data (below) showed the MCS pair at 09:07 UTC or 3:07 AM local time —  this was prior to the merger, and the southeastern storm exhibited a minimum cloud-top IR brightness temperature of -84º C (purple color enhancement), which was much colder than the -71º C seen with the northwestern storm. At the onset of the heavy thunderstorms at PHX, southerly to southeasterly winds  — likely outflow from the southeastern MCS — gusted as high as 31 knots (36 mph) and visibility was reduced to 0.8 mile (surface reports: text | graph).

Suomi NPP VIIRS 11.45 µm IR channel image

Suomi NPP VIIRS 11.45 µm IR channel image

As the circulation of former-Hurricane Norbert continued to spin over the Pacific Ocean west of Baja California, deep tropical moisture kept working its way farther inland — GOES sounder Total Precipitable Water (TPW) values in excess of 50-60 mm (2.0 to 2.4 inches) were eventually seen across the southwestern half of Arizona (below; click image to play animation).

GOES sounder Total Precipitable Water derived product images (click to play animation)

GOES sounder Total Precipitable Water derived product images (click to play animation)

The Blended Total Precipitable Water product (below; click image to play animation) also showed values of 50-60 mm working their way into southwestern Arizona during the 06-08 September period.

Blended Total Precipitable Water product (click to play animation)

Blended Total Precipitable Water product (click to play animation)

The Percent of Normal TPW product (below; click image to play animation) indicated that these TPW values were in excess of 200% of normal (yellow color enhancement) over large portions of the Desert Southwest. On the morning of 08 September, the TPW value of 2.03 inches derived from rawinsonde data at Tucson, Arizona set a record high for the month of September at that location.

Percent of Normal TPW product (click to play animation)

Percent of Normal TPW product (click to play animation)

MODIS false-color images show areas of flooding in Montana

August 26th, 2014
Before (12 August) and after (26 August) MODIS false-color RGB images

Before (12 August) and after (26 August) MODIS false-color RGB images

Some locations in northeastern Montana received up to 7-8 inches of rainfall over a 5-day period (NWS Glasgow rainfall map), which led to flooding in many areas. A comparison of before (12 August) and after (26 August) 250-meter resolution MODIS false-color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (above) showed the areal extent of flooding (cyan to darker blue areas on the 26 August image). The RGB images use MODIS bands 7/2/1 to help highlight the areas of of floodwater.

The maps below show the total observed rainfall, the departure from normal, and the percent of normal for the 7-day period ending on 26 August.

Observed 7-day rainfall

Observed 7-day rainfall

7-day rainfall departure from normal

7-day rainfall departure from normal

7-day rainfall percent of normal

7-day rainfall percent of normal

Can you use the VIIRS Day/Night Band to know where heavy rain is falling?

August 19th, 2014
Suomi NPP VIIRS Day/Night Band at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

Suomi NPP VIIRS Day/Night Band at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

The VIIRS Day/Night Band image toggle above shows nighttime cloud cover over southern Wisconsin associated with a line of deep convection; note how some city lights are not seen (Madison WI KMSN at 0659 UTC, and Milwaukee WI KMKE at 0838 UTC). There are several reasons this may have happened. For example, the convection could have knocked out power over a large region (this did not happen). Scattering associated with the thick convective clouds may have attenuated the city light so much that it could not be detected.

The toggle below of the corresponding VIIRS 11.45 µm Infrared imagery shows very cold cloud tops (-60º to -70º C, near the tropopause) over Madison at 0659 UTC (the observation at 0653 UTC at the Madison airport was Heavy Rain with a Thunderstorm) and over Milwaukee at 0838 UTC (when the Milwaukee airport was having Moderate Rain; they received a half-inch of rain between 0753 and 0853 UTC). The combination of the thick convective cloud and especially the heavy rain is very likely why city lights cannot be seen at certain times, as liquid water is an excellent absorber of visible light. This radar image (from this story) shows the areal extent of the heavy rain at 0745 UTC on 19 August.

Suomi NPP VIIRS 11.45 µm Infrared imagery  at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

Suomi NPP VIIRS 11.45 µm Infrared imagery at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)