Fog/stratus in the Strait of Juan de Fuca

May 20th, 2017 |

** The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing. **

As seen in a Tweet from NWS Seattle/Tacoma (above), a plume of fog/stratus moved rapidly eastward through the Strait of Juan de Fuca on 20 May 2017. A closer view of GOES-16 Visible (0.64 µm) images (below; also available as an MP4 animation) shows the formation of “bow shock waves” as the leading edge of the low-level fog/stratus plume encountered the sharply-angled land surface of Whidbey Island at the far eastern end of the Strait near sunset — surface observations indicated that the visibility at Naval Air Station Whidbey Island was reduced to 0.5 mile just after the time of the final 0327 UTC image in the animation.

GOES-16 Visible (0.64 µm) images, with hourly plots of surface reports [click to play animation]

GOES-16 Visible (0.64 µm) images, with hourly plots of surface reports [click to play animation]

A Suomi NPP VIIRS Visible (0.64 µm) image with RTMA surface winds (below) indicated that westerly/northwesterly wind speeds were generally around 15 knots at 21 UTC (just after the primary fog/stratus plume began to move into the western end of the Strait). Four hours later, there was a northwesterly wind gust of 27 knots at Sheringham, British Columbia (CWSP).

Suomi NPP VIIRS Visible (0.64 µm) images, with RTMA surface winds plotted in cyan [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) images, with RTMA surface winds plotted in cyan [click to enlarge]

During the following nighttime hours, a Suomi NPP VIIRS infrared Brightness Temperature Difference (11.45 – 3.74 µm) “Fog/Stratus Product” image at 0910 UTC (below) revealed that the fog/stratus plume covered much of the Strait (especially along the Washington coast), and that the leading edge had begun to spread both northward and southward from Whidbey Island. In addition, note the presence of a linear ship track (darker red enhancement) extending southwestward from Cape Flattery.

Suomi NPP VIIRS Infrared brightness temperature difference (11.45 - 3.74 µm)

Suomi NPP VIIRS infrared Brightness Temperature Difference (11.45 – 3.74 µm) “Fog/Stratus Product” image, with RTMA surface winds plotted in cyan [click to enlarge]

Bill Line (NWS Pueblo) showed the nighttime fog/stratus monitoring capability of a GOES-16 infrared Brightness Temperature Difference product:


On a side note, in the upper right portion of the GOES-16 (as well as the VIIRS) visible images one can also see the hazy signature of glacial sediment  flowing from the Fraser River westward into the Strait of Georgia. Longer-term changes in the pattern of this glacial sediment are also apparent in a comparison of Terra MODIS true-color Red/Green/Blue (RGB) images (source) from 20 April, 07 May and 20 May 2017 (below).

 

Terra MODIS true-color RGB images [click to enlarge]

Terra MODIS true-color RGB images [click to enlarge]

Fog/stratus dissipation: 1-minute GOES-16 vs 15-30 minute GOES-13

April 4th, 2017 |

GOES-16 0.64 µm Visible (left) and GOES-13 0.63 µm Visible (right) images, with surface reports of fog plotted in yellow [click to play animation]

GOES-16 Visible (0.64µm, left) and GOES-13 Visible (0.63 µm, right) images, with surface reports of fog plotted in yellow [click to play animation]

** The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing. **

Widespread fog and stratus had developed across southern Alabama and western Georgia during the pre-dawn hours on 04 April 2017. After sunrise, a comparison of 1-minute interval GOES-16 and 15-30 minute interval GOES-13 visible imagery (above) demonstrated the advantage of more frequent scans to monitor the dissipation of fog and stratus. The improved spatial resolution of the GOES-16 0.64 µm “Red visible” band — 0.5 km at satellite sub-point, vs 1 km for GOES-13 — also aided in the detection of smaller-scale river valley fog features.

Fog Detection using GOES-16 Channel Differences

March 6th, 2017 |

GOES-R IFR Probability Fields at 1230 UTC on 6 March 2017 (Click to enlarge)

Note: GOES-16 data shown on this page are preliminary, non-operational data and are undergoing on-orbit testing.

Here is what this blog post will show: It is vital to tweak the supplied default AWIPS Enhancements so that important atmospheric information is better highlighted.

GOES-R IFR Probability fields (Click here for a website that shows many examples), shown above, use present GOES Data and Rapid Refresh Data to forecast the probability that IFR conditions exist. (There are also Low IFR Probability fields and Marginal VFR Probability fields as well, data from this site). The inclusion of surface information via the Rapid Refresh Model output (that details low-level saturation) is vital to screen out false fog detection (regions where mid-level stratus does not extend to the surface) and to highlight IFR conditions that exist under cirrus that block the satellite detection of low clouds.

GOES-16 data in AWIPS includes pre-defined channel differences judged to have utility in Decision Support Services. One of these is Fog detection (the infrared Brightness Temperature Difference between 3.9 µm and 11.2 µm) that extracts information at night based on emissivity differences from water-based clouds at those two wavelengths. This is a product that can detect stratus clouds at night, if cirrus clouds do not block the satellite’s view. If those stratus clouds extend to the surface, then fog is a result. A GOES-16 Channel Difference field, shown below with the default AWIPS enhancement, contains information about the fog/low clouds that are present over North Dakota, and over Texas (click here for a graphic from the Aviation Weather Center that highlights regions of IFR conditions — Dense Fog Advisories were issued on 6 March over North Dakota).

The Fog signal in the Brightness Temperature Difference field at night occurs when the value is negative; the default color enhancement, below, contains a lot of color gradations that grab the eye in regions where the Brightness Temperature Difference is positive; for Fog Detection, those extra colors in regions of positive difference are needless visual clutter.

Brightness Temperature Difference fields (3.9 µm – 11.2 µm) over the United States, 1227 UTC on 6 March 2017 (Click to enlarge)

To get useful information from this field, alter the Brightness Temperature Difference enhancement to highlight negative values. That has been done in the toggle below with the IFR Probability field. Fog regions over North Dakota and Texas are apparent.  (Note that the scale for the Brightness Temperature Difference field here has also been flipped — click here to toggle between the two Brightness Temperature Difference field enhancements).

GOES-R IFR Probability fields and GOES-16 Brightness Temperature Difference fields, ~1230 UTC on 6 March 2017 (Click to enlarge)

GOES-R IFR Probability fields and GOES-16 Brightness Temperature Difference fields, ~1230 UTC on 6 March 2017 (Click to enlarge)

It is vital to tweak the supplied default AWIPS Enhancements so that important atmospheric information is better highlighted.

Tule fog in California

January 31st, 2017 |


The tweet shown above was issued by the NWS forecast office in Hanford, California — using an image of the GOES-15 Low Instrument Flight Rules (LIFR) Probability, a component of the GOES-R Fog/low stratus suite of products — to illustrate where areas of dense Tule fog persisted into the morning hours on 31 January 2017.

AWIPS II images of the GOES-15 Marginal Visual Flight Rules (MVFR) product (below) showed the increase in areal coverage of Tule fog beginning at 0600 UTC (10 pm local time on 30 January); the fog eventually dissipated by 2030 UTC (12:30 pm local time) on 31 January. Note that Lemoore Naval Air Station (identifier KNLC) reported freezing fog at 14 UTC (their surface air temperature had dropped to 31º F that hour). In addition, some of the higher MVFR Probability values were seen farther to the north, along the Interstate 5 corridor between Stockton (KSCK) and Sacramento (KSAC) — numerous traffic accidents and school delays were attributed to the Tule fog on this day.

GOES-15 MVFR Probability product [click to play animation]

GOES-15 MVFR Probability product [click to play animation]

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GOES-15 MVFR Probability and Aqua MODIS Infrared Brightness Temperature Difference (BTD) products [click to enlarge]

GOES-15 MVFR Probability and Aqua MODIS Infrared Brightness Temperature Difference (BTD) products [click to enlarge]

Legacy infrared Brightness Temperature Difference (BTD) products are limited in their ability to accurately detect fog/low stratus features if high-level cirrus clouds are present overhead. This is demonstrated in comparisons of GOES-15 MVFR Probability and BTD products from Aqua MODIS (above) and Suomi NPP VIIRS (below). Again, note the Interstate-5 corridor between Stockton and Sacramento, where the extent of the fog was not well-depicted on the BTD images (even using high spatial resolution polar-orbiter MODIS and VIIRS data).

GOES-15 MVFR Probability and Suomi NPP VIIRS infrared Brightness Temperature Difference (BTD) products [click to enlarge]

GOES-15 MVFR Probability and Suomi NPP VIIRS infrared Brightness Temperature Difference (BTD) products [click to enlarge]

Daylight images of GOES-15 Visible (0.63 µm) data (below) showed the dissipation of the Tule fog during the 1600-2200 UTC (8 am – 2 pm local time) period. The brighter white snow pack in the higher elevations of the Sierra Nevada was also very evident in the upper right portion of the satellite scene.

GOES-15 Visible (0.63 µm) images [click to play animation]

GOES-15 Visible (0.63 µm) images [click to play animation]

One ingredient contributing to this Tule fog event was moist soil, from precipitation (as much as 150-200% of normal at some locations in the Central Valley) that had been received during the previous 14-day period (below).

Total liquid precipitation and Percent of normal precipitation for the 14-day period ending on 31 January 2017 [click to enlarge]

Total liquid precipitation and Percent of normal precipitation for the 14-day period ending on 31 January 2017 [click to enlarge]