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)

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)

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

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)

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).

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

Small and intense Hurricane Gonzalo is moving north-northeastward out of the western tropical Atlantic towards Bermuda (Bermuda is located at 32.3º N, 64.8º W). (See the National Hurricane Center Website for the latest Advisories on Gonzalo) GOES-13 Visible imagery (0.63 µm) from the morning of 16 October shows a well-defined eye, intense convection and good outflow in all quadrants. Mesovortices within the eye are also apparent as shown in a storm-centered animation of the eye, below, until mid- and high-level clouds develop near the end of the animation.

GOES-13 0.63 µm visible channel images, centered over the eye of Gonzalo (click to play animation)

Gonzalo’s track will take it over sea surface temperatures that are not quite so warm (see image below, taken from the CIMSS Tropical Cyclones site). Cooler SSTs argue against any further strengthening, and the official forecast suggests peak intensity has already occurred.

Sea Surface Temperatures and Gonzalo’s projected (and past) track and intensity (click to enlarge)

Suomi NPP overflew the storm at 0536 UTC in 16 October. A toggle between the VIIRS Day/Night Band and the 11.45 µm imagery is below. Note that lunar illumination is dropping as the Moon phase wanes so visible features are less distinct than they would be during a more Full Moon. A ragged eye is obvious in both images, however, and there is evidence of a lightning streak well east of the eye in the Day/Night Band.

Suomi NPP Day/Night Band Visible Imagery and 11.45 µm Infrared Imagery (click to enlarge)

Both Aqua and Metop-A overflew Gonzalo during the day on 16 October. The Aqua True-Color image (from the MODIS Today website) shows the storm and the cold front that will help guide Gonzalo’s future path. The ASCAT scatterometer data from METOP-A, below, showed a tight region of hurricane-force winds.

METOP-A ASCAT Scatterometer winds, 1353 UTC 16 October 2014 (click to enlarge)

===== 18 October Update =====

Suomi NPP VIIRS 11.45 µm IR channel images

A sequence of Suomi NPP VIIRS 11.45 µm IR channel images during the 15-17 October period (above) showed the pattern of very cold cloud-top IR brightness temperatures at various times as the storm curved northward toward Bermuda (station identifier TXKF); IR brightness temperatures of -80º C and colder (violet color enhancement) were seen on 15 and 16 October. A close-up view shows Gonzalo on 2 consecutive VIIRS IR images (16:38 and 18:18 UTC) on 17 October.

The MIMIC Total Precipitable Water (TPW) product (below; click image to play animation) showed a plume of high TPW ahead of a cold front moving off the East Coast of the US, which was then reinforced by a northward surge of high TPW from Ganzalo. As a result, the TPW value calculated using the morning rawinsonde data from Caribou, Maine (station identifier KCAR) was 1.68 inches — the highest for so late in the season.

MIMIC Total Precipital Water product (click to play animation)

Around 20:15 UTC on 17 October, a comparison of GOES-13 10.7 µm IR channel and SSMIS 85 GHz microwave images from the CIMSS Tropical Cyclones site (below), the microwave image revealed the very large size of the eye of Hurricane Gonzalo (which was not as apparent on conventional IR imagery).

GOES-13 10.7 µ IR image and SSMIS 85 GHz microwave image

A close-up view of GOES-13 10.7 µm IR channel images with overlays of wind barbs and wind gusts from Bermuda International Airport (below; click image to play animation) showed the development of a convective burst just as the eye of Gonzalo was approaching the island. The airport observations (text listing | time series plot) showed a wind gust to 83 knots (96 mph) as the northern eyewall passed, and a stronger gust to 98 knots (113 mph) as the southern eyewall passed over Bermuda.

GOES-13 10.7 µm IR channel images, with overlays of wind barbs and wind gusts at Bermuda (click to play animation)

The Bermuda Weather Service launched a rawinsonde balloon at 00 UTC on 18 October; winds at the surface were from the southeast at 64 knots (74 mph), which carried the instrument package into the eye of Gonzalo for a portion of its ascent. Note the the temperature profile was nearly moist adiabatic from about 700 hPa to 200 hPa (below).

Bermuda rawinsonde report and GOES-13 10.7 µm IR image

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Suomi NPP 11.45 µm Infrared Imagery 13-15 October 2014 (click to enlarge)

Suomi NPP VIIRS 11.45 µm Infrared imagery, above, shows the slow strengthening of Ana as it approaches the Big Island of Hawai’i from the east-southeast. For further information on this storm, consult the Central Pacific Hurricane Center website. Ana possesses a noticeable central dense overcast, with overshooting tops that are very cold; brightness temperatures are as cold as -95º C! A similar loop as above, but of the Day Night band is here.

GOES-15 Infrared Imagery (10.7 µm), below, shows the storm making steady progress across the central Pacific. Very cold overshooting tops are also observed with GOES; values as cold at -91.5º C occur (in the animation at 0830 UTC). The very cold cloud tops indicate two different things: The tropopause is high and cold in the Tropical Pacific; there is a lot of latent heat being released in the developing storm.

GOES-15 10.7 µm Infrared Imagery 15 October 2014 (click to animate)

The rocking animation below shows that Ana appears to have emerged from the interaction between westward propagating and eastward propagating waves in the Intertropical Convergence Zone. (A similar animation using water vapor is here).

GOES-15 10.7 µm Infrared Imagery 10-15 October 2014 (click to animate)

The approach of Ana suggests that Rapid Scan Operations might be needed for Hawaii later this week. The most recent RSO Testing, on 15 October, showed that GOES-15 RSO data can be viewed in AWIPS. (Link, scroll to the bottom).

Added, 16 October 2014.

Strong convection near the center of Ana has become less organized early on 16 October, and the maximum estimated sustained winds of the storm have dropped. The animation of sequential Suomi NPP 11.45 µm IR imagery, below, does show deep convection reappearing at 1222 UTC.

Suomi NPP VIIRS 11.45 µm Infrared Imagery at 1047 and 1222 UTC 16 October 2014 (click to enlarge)

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GOES-15 6.5 µm water vapor channel images (click to play animation)

McIDAS images of GOES-15 (GOES-West) 6.5 µm water vapor channel data (above; click image to play animation) showed an upper-level low that moved from east to west over the Hawaiian Islands during the 13 October – 14 October 2014 period. This low forced the development of widespread showers and thunderstorms, especially over the Big Island of Hawai’i — and even produced some snowfall in the highest elevations around the summits of Mauna Kea and Mauna Loa. Some excerpts from Area Forecast Discussions issued by the National Weather Service at Honolulu on 13 October:

FXHW60 PHFO 131350
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
400 AM HST MON OCT 13 2014
[…]
FORECAST MODELS HAVE BEEN CONSISTENTLY CALLING FOR 500 MB TEMPERATURES BETWEEN -12 AND -13C WITHIN THE CORE OF THE COMPACT UPPER LOW. THIS IS EXCEPTIONALLY COLD FOR OCTOBER
[…]
FORECAST MODELS SHOW THAT THIS FEATURE WILL HOLD AS IT MOVES OVER THE BIG ISLAND LATER TODAY INTO TONIGHT…LIKELY PRODUCING ACCUMULATING SNOW OVER THE SUMMITS ABOVE 12000 FT. AS A RESULT…A WINTER WEATHER ADVISORY HAS BEEN ISSUED.
[…]

=====

FXHW60 PHFO 140152
AFDHFO

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HONOLULU HI
330 PM HST MON OCT 13 2014
[…]
THE SUMMITS OF THE BIG ISLAND HAVE BEGUN TO REPORT SNOWFALL ACCUMULATION…AND THIS WILL CONTINUE WITH A COUPLE OF INCHES POSSIBLE OVERNIGHT.
[…]

While examining a nighttime (11:21 UTC or 1:21 am local time) comparison of AWIPS II images of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel data covering Tropical Storm Ana (below), the main feature of interest was the inner core of cloud-top IR brightness temperatures as cold as -86º C (yellow color enhancement) associated with Ana — however, equally interesting was the appearance of a pair of bright white features in the middle of the Big Island on the DNB image (which highlighted the areas of snow cover that remained at the higher elevations).

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

A closer view comparing the VIIRS DNB and IR images centered over the Big Island (below) seemed to suggest that the 2 patches of bright snow cover (well-illuminated by a nearly Full Moon) were located along the western slopes of Mauna Kea and Mauna Loa. A similar comparison of the DNB image and high-resolution topography can be seen here.

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

An animation of GOES-15 0.63 µm visible channel images during the following daylight hours of 14 October (below; click image to play animation) revealed the gradual melting of the 2 patches of high-elevation snow cover as temperatures rose from around freezing into the 50s F near the summits (Cooperative observations).

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

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