Due to a tight pressure gradient between a high over the Yukon and a low over the Gulf of Alaska (surface analysis), strong offshore winds (with gusts as high as 78 mph) were lofting glacial silt from the northern portion of the Alaska Panhandle region and carrying it westward over the Gulf of Alaska on 01 February 2015. Hints of the narrow light grey plumes could be seen streaming southwestward then westward on GOES-15 (GOES-West) 0.63 µm visible channel images (above; click to play animation).

A closer look using a comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 0.64 µm visible channel images (below) showed that the areal extent of the airborne aerosols was much more evident on the DNB image (in part due to it’s more broad spectral response). However, other more intricate patterns were seen on the DNB image in the general vicinity of Middleton Island (station identifier PAMD) that did not appear to match the character of the airborne glacial silt features being blown westward from the Alaska Panhandle region.

A Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image from the SSEC RealEarth web map server (below) offers a clue to help explain the meandering features that stretched from the coast east of Prince William Sound toward the Middleton Island area: strands of phytoplankton, fed by nutrients in the river waters draining from the interior into the Gulf of Alaska. Sun glint along the edge of the VIIRS scan may have helped to highlight these features in the DNB image above. In fact, these water features were less obvious — and the airborne glacial silt more obvious — in a subsequent VIIRS DNB vs Visible image at 23:20 UTC.

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GOES-13 6.5 µm water vapor channel images (above; click to play animation) showed dry air (brighter yellow to orange color enhancement) moving across the Mid-Atlantic and Southeast regions of the eastern US in the wake of a strong cold frontal passage on the morning of 30 January 2015. There were also numerous pilot reports of turbulence, at both low altitudes (plotted in red) and high altitudes (plotted in cyan).

The most obvious feature seen on the GOES-13 water vapor images was the “rippled” signature of mountain waves, which extended far to the lee (southeast) of the Appalachian Mountains (the topographical obstacle to the strong northwesterly boundary layer flow that was causing the waves to initially form). A comparison of 4-km resolution GOES-13 6.5 µm water vapor and 1-km resolution Aqua MODIS 6.7 µm water vapor images (below) demonstrated the benefit of higher spatial resolution for diagnosing the areal coverage of such small-scale mountain waves. Of special note is the pilot report of “severe to extreme” turbulence at 4000 feet over South Carolina.

A comparison of the MODIS 6.7 µm water vapor channel image with the corresponding MODIS 0.65 µm visible channel image (below) showed that the severe to extreme reports in North and South Carolina were examples of Clear Air Turbulence (CAT), since no clouds were apparent in those areas at the time.

Regarding the numerous high-altitude pilot reports of moderate to severe turbulence, the NAM80 model depicted a 120-knot jet streak over South Carolina at 12:00 UTC, with another 120-knot jet streak approaching from the middle Mississippi Valley region (below). Note that there was strong wind speed shear to the north of the jet stream axis, which is where the bulk of the pilot reports of turbulence were located. Quite often there is an obvious moist-to dry gradient water vapor signature along or just poleward of a strong jet streak axis — but such a signature was not seen with this particular event.

In response to some of these pilot reports, at 16 UTC a SIGMET (SIGnificant METeorological advisory) was issued for occasional severe turbulence due to jet stream wind shear (below).

4-panel images showing the three GOES-13 Sounder water vapor channels (6.5 µm, 7.0 µm, and 7.4 µm) along with the conventional GOES-13 Imager 6.5 µm water vapor channel (below; click to play animation) showed how each channel helped to identify where the pockets of middle-tropospheric dry air were located.

The GOES-13 water vapor channel weighting functions plotted using data from the 12 UTC rawinsonde reports from Roanoke/Blacksburg, Virginia and Greensboro, North Carolina are shown below. Due to the very dry middle to upper troposphere, the water vapor channels were able to sense features farther down into the atmosphere than is usually the case — this is illustrated by the relatively low altitude of the water vapor weighting function peaks.

Compare the 2 examples above with the altitude peaks of the various GOES-13 Sounder and Imager water vapor channels under “normal” conditions, plotted using the US Standard Atmosphere as the sounding profile (below).

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GOES-13 was placed into Rapid Scan Operations mode during the evolution of the strong Nor’easter that affected much of New England (HPC storm summary), and the 10.7 µm IR imagery, above (available for download here as an MP4 and here as an animated gif) shows the development of the system over the 2-day period of 26-27 January. Of particular note in the animation is the southeast to northwest motion of cold cloud tops over central and eastern Long Island around 0500 and 0600 UTC on 27 January. Those cold clouds tops never quite made it to western Long Island or to New Jersey, where snow totals were less. The GOES-13 visible image animation for these 2 days is shown below (available for download here as an MP4 and here as an animated gif).

ASCAT microwave data continues to show the surface circulation. The METOP-A overpass at 1513 UTC, below, shows a center about 100 miles southeast of Nantucket, where gusts past hurricane force have been occurring. A large area of winds exceeding 50 knots (in red) is present over the northern Gulf of Maine.

A comparison of 1-km resolution MODIS 0.64 µm visible channel, 11.0 µm IR channel, and 6.7 µm water vapor channel images from 17:49 UTC is shown below. One observation of interest is a ship report just southeast of the storm center: 50-knot winds from the south-southwest, with blowing spray reducing surface visibility to 2-3 miles.

===== 28 January Update =====

As the Nor’easter departed and the clouds began to clear over the northeastern US on 28 January, the Aqua MODIS true-color Red/Green/Blue (RGB) image shown above revealed the areas with significant snow on the ground. Note the thin areas of snow cover along the spine of the Appalachian Mountains, extending as far southward as Tennessee and North Carolina. Closer views of New York City and Washington DC are also available.

===== 29 January Update =====

The clouds had cleared from the Boston region on 29 January; a comparison of the Terra and Aqua MODIS true-color images (above) showed the changes in the offshore sediment patterns in the ~90 minutes between the overpasses of the 2 satellites. The strong winds of the storm caused upwelling of colder waters along the coast and nearshore areas, with the Suomi NPP VIIRS Sea Surface Temperature product (below) showing SST values as cold as the 30-33º F range (darker purple color enhancement).

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Forecasts have been consistent in the past days for a storm of historic proportions over parts of southern New England. What conditions that are present now argue for the development of a strong winter storm? The image above is the GOES Sounder Land Surface Temperature (or “Skin Temperature”) product; cold air is present over southeastern Canada, with surface temperatures near -30 C, associated with a surface high pressure system. The high pressure will act to reinforce the cold air at the surface, preventing or delaying any changeover to liquid or mixed precipitation (a MODIS Land Surface Temperature product at 1500 UTC on 26 January similarly shows cold air banked over southern Canada).

Winds over southern New England early on the 26th continued out of the north and northwest, maintaining cold air at the surface. The ASCAT (from METOP-A) imagery above shows brisk northwesterly winds south of southern New England just before 0100 UTC, with southwesterlies east of Georgia and South Carolina just before 0300 UTC. Those southwesterlies are helping moisten the atmosphere, and heavy snows require abundant moisture. MIMIC Total Precipitation (below; click image to play animation) testifies to the moistening that is occurring off the southeast coast as this system develops; the storm appeared to tap moisture from both the Gulf of Mexico and a pre-existing atmospheric river over the Atlantic Ocean.

[Added: The 1540 UTC ASCAT winds show the surface circulation east of Hatteras and the mouth of the Chesapeake Bay! Winds south of New England have shifted to northeasterly. The location of the circulation well off the coast suggests cold air can be maintained over land.]

Given that moisture and cold air are present, what features argue for the development of a strong storm? The GOES-13 water vapor images (below; click image to play animation; also available as an MP4 movie file) with cloud-to-ground lightning strikes superimposed show the potent system developing off the US East Coast and blossoming over the Gulf Stream as a secondary warm conveyor belt forms (a water vapor image with lightning animation from 25-26 January is available here). Strong sinking motion behind the system is indicated by the development of warm water vapor channel brightness temperatures (yellow color enhancement), and strong rising motion ahead of the system helps to generate widespread, strong convection. Convection also occurred over the Deep South late on 25 January in response to solar heating. The system depicted in the Water Vapor imagery is obviously quite vigorous.

Suomi NPP VIIRS 11.45 µm IR channel and 0.64 µm visible channel images (below) showed that there was a great deal of convective banding within the secondary warm conveyor belt.

Total Column Ozone is frequently used as a proxy of tropopause folding; tropopause folds accompany very strong storm development and the vertical circulation associated with the potential vorticity anomaly (maximum) associated with the folding draws stratospheric ozone down into the troposphere. GOES Sounder Total Column Ozone derived product images (below; click to play animation; also available as an MP4 movie file) show that the dynamic tropopause — taken to be the pressure of the PV1.5 surface, red contours — descends below the 400-450 hPa level along the southern gradient of the higher ozone values (green to red color enhancement) as the potential vorticity anomaly pivots eastward along the Gulf Coast states and then northeastward toward the intensifying storm. The presence of clouds prevented ozone retrievals over many areas, but some ozone values over 400 Dobson Units (red color enhancement) could be seen, which is characteristic of stratospheric air.

As the storm approached New England, a MODIS 11.0 µmIR channel image (below) revealed the presence of widespread embedded convective elements within the broad cloud shied, with some cloud-top IR brightness temperatures as cold as -65ºC (darker red color enhancement). These pockets of convection could enhance snowfall rates once they moved inland.

An overlay of the RTMA surface winds (below) helped to locate the position of the surface low east of the Delmarva Peninsula. That position agrees well with ASCAT winds from 0158 UTC on 27 January.

A comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 11.45 µm IR channel images at 06:39 UTC or 1:39 AM Eastern time is shown below. With illumination from the Moon in the Waxing Gibbous phase (at about 60% of Full), the DNB provided a “visible image at night” which showed the expansive offshore “comma cloud” of the storm, along with the locations of bright cloud illumination from dense lightning activity (note the bright lightning signature east of Cape Cod, which corresponded well with a cluster of positive cloud-to-ground lightning strokes). Numerous pockets of convective development were seen well off the coast of North and South Carolina, due to strong cold air advection over the warm waters of the Gulf Stream.

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