Area of focus in the Eastern Aleutians in Southern Alaska

Due to the location of Alaska, geostationary satellite data can sometimes be difficult to work with. This is because the large satellite angle causes the satellite footprint to increase, thus decreasing the spatial resolution of the data. However, even though the spatial resolution is not ideal the temporal resolution allows geostationary satellite data to remain useful. The example below shows and area of fog/low stratus (FLS) moving northwestward toward the eastern Aleutians in southern Alaska.

GOES-R IFR probabilities computed using GOES-15 (left) and MODIS (right) around 08:20Z on May 30, 2013. Note that the images are rotated so true north is actually oriented from the lower left corner of the images to the upper right corner.

In the image above the orange and darker red colors indicate areas with a high probability of FLS over the northern Gulf of Alaska. The difference in spatial resolution stands out when comparing the images, however, it should be noted that the same areas of higher probabilities in the MODIS image are also picked up in the GOES image, just at a coarser resolution. Although the MODIS image looks more detailed than GOES, temporal trends can not be discerned from a single image. The next available MODIS pass over the area was at 12:26Z.

GOES-R IFR probabilities computed using GOES-15 (left) and MODIS (right) around 12:26Z on May 30, 2013. Note that the images are rotated so true north is actually oriented from the lower left corner of the images to the upper right corner.

In the 12:26Z image the FLS deck moved northwest over the eastern Aleutians as indicated by the higher probabilities that are now over the southeastern side of the Aleutians. This is confirmed by the surface station in Chignik, AK, which reported a ceiling of 400 ft at 12:00Z when it reported no ceiling at 08:00Z. Once both MODIS passes were available the NW movement of the FLS could be observed. However, in the 4 hours between the two MODIS passes it would be hard to forecast whether the IFR conditions would move into the area or stay out at sea.

On the other hand, GOES data was available about every 15 minutes. By animating the GOES images such as this, the slow NW movement of the FLS can be tracked and a better approximation of when IFR conditions will reach the Aleutians can be made. IFR conditions were first reported at Chignik, AK at 09:00Z, just as the higher GOES-R IFR probabilities reached the coastline. Also from the animation, it appears that the eastern Aleutians block the movement of the FLS deck from continuing over to the NW side into the Bering Sea. This blocking might be difficult to interpret from the two MODIS images alone.

Alaska has the largest coastline in the U.S. (more than twice the size of the entire lower 48, including Hawaii) where hazardous areas of FLS can move onshore. Although the high spatial resolution available from MODIS provides more detail in a single scene than GOES, the high temporal resolution that GOES offers makes finding trends in the movement of hazardous low clouds possible. This is another example of how looking at the GOES-R FLS products using GOES and MODIS together can be more useful than using only GOES or MODIS.

GOES-R IFR Probabilities from GOES-West over Alaska and the Bering Sea, half-hourly from 0000 UTC through 1500 UTC on 18 March 2013

Even though Alaska is at high latitudes, and GOES Imagery there is burdened with degraded resolution, the temporal aspect of the data can give useful information.  As an example, consider the animation of GOES-R IFR probabilities near Bethel Alaska along the west coast of Alaska.  Note that Bethel AK, in the center (nearly) of the image, is reporting MVFR/VFR conditions, with IFR conditions along the coast — at Kipnuk and Toksook — and offshore at Mekoryuk on Nunivak Island as well as St. George and St. Paul Islands.  The fused GOES/Rapid Refresh data is able to delineate correctly the regions with IFR conditions along the coast and offshore from the regions with MVFR/VFR conditions to the east.

GOES-R IFR Probabilities computed from MODIS data over western Alaska, 0826 UTC 18 March 2013

MODIS data can also give information over Alaska, and the 1-km resolution offers important information.  Bethel sits in the region of relatively low IFR probabilities west of the higher probabilities along the coast.

Toggle between GOES-R IFR Probabilities computed with GOES-West (at 1200 UTC) and with MODIS (1154 UTC) over the northern Gulf of Alaska and surrounding landmass

In regions where very small-scale terrain effects low cloud distribution — for example, in river valleys — the superior resolution of MODIS (on Terra or Aqua) or VIIRS (on Suomi NPP) allows a much better horizontal definition of cloud boundaries.  In regions of larger scale variability, such as over the ocean, differences between GOES and MODIS IFR estimates aren’t quite so noticeable, as in the imagery above.  Both fields show a large region of low stratus over the northern Gulf of Alaska moving inland.  The regions of lower ceilings and reduced visibilities match the stream of higher IFR probabilities moving northward over the Gulf, from Middlteon Is (PAMD) on the western edge to Sitka (PASI) and Petersburg (PAPG) on the southern/eastern edge.  Note also the break in high IFR probabilities where the Aleutian Peninsula attaches to the mainland, with higher probabilities, and IFR conditions, at PASV (Sparrevohn).

The character of the IFR probability field should alert you to the presence of a stream of high clouds over the western third of this image.  In that region, only model values are being used to compute IFR probabilities, and the total probabilities are therefore lower.