A widespread sea stratus event evolved across the Gulf of Alaska and into adjacent inner channels from 3/15 – 3/16 as broad high pressure established itself over the region above favorable low level moisture. Forecasters at the NWS WFO Juneau office noted their use of GOES and VIIRS imagery together to aid in tracking the evolution of low clouds during this event, along with an associated drizzle threat at the surface beneath the stratus.
GOES-West full disk water vapor imagery revealed an omega block setup over the Gulf of Alaska, with low pressure on either side of the Gulf of Alaska high pressure (Fig 1).
Both GOES-17 imagery and VIIRS imagery were used by forecasters in decisions of whether or not to include lower CIGs/VIS conditions in the 18Z TAFs. These decisions impacted local pilots whose ability to fly depended on the extent of the lower cloud bases. Forecasters also used GOES and VIIRS imagery in combination with other datasets to provide DSS to core partners regarding low cloud evolution. For example, Forest Service called the office inquiring about if and when the low clouds were going to lift in a certain area as they needed to take a helicopter to a mountain top to service infrastructure. Forecasters were able to give them some guidance on if it would lift and what the ceiling could be if it did by using a combination of area cameras, recent trends in satellite data, and model data.
Analysis of GOES-West full disk Nighttime Microphysics RGB imagery at night transitioning to Day Cloud Phase distinction RGB imagery during the day on the 16th reveals the wide swath of low cloud cover over the Gulf, and expansion of clouds east into the inner channels (Fig 2). The IR components to the RGBs were modified slightly to account for the cooler airmass (lower the warm end by 10-20 C). At nadir, the ABI bands in the nighttime RGB have 2 km spatial resolution, while the Day RGB components have 0.5, 1, and 2 km resolutions. However, at the latitude of the Gulf of Alaska, pixel size is approximately 3-4x larger.
Overnight 375 m I band VIIRS fog difference (11.4 um minus 3.7 um) imagery provides a much higher resolution (spatially) of the low clouds, with three subsequent passes showing expansion of the low clouds east into the inner channels (Fig 3). Cloud edges and smaller scale cloud features are more easily diagnosed in the more detailed VIIRS imagery compared to GOES. During this 1.5 hour period of time, low stratus spread around PAGS and into PASI and PAGN weather observation sites. Recall the VIIRS I bands (0.64 um, 0.86 um, 1.6 um, 3.7 um, 11.4 um) and associated multispectral products provide the highest resolution (375 m), while the M bands and associated products provide a lower 750 m.
Day cloud Phase Distinction RGB imagery from VIIRS provides a similar higher resolution look at the extent of the low clouds during the day (Fig 4). Localized low cloud cover is diagnosed spreading south over PAPG during this 1.5 hour time frame. This RGB utilizes three I bands, so provides 375 m resolution.
Forecasters specifically noted the value of the periodic high resolution and low parallax VIIRS imagery for this type of event in order to get a better representation of cloud type. In AWIPS, they will view the GOES imagery with VIIRS overlaid, taking advantage of the strengths of both data sources.
Bill Line (NESDIS and CIRA) and Aaron Jacobs (NWS WFO Juneau)