Antecedent Conditions for a Nor’easter

January 26th, 2015
GOES-13 Sounder Skin Temperature derived product image

GOES-13 Sounder Skin Temperature derived product image

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).

GOES_SkinT_1400_26January2015

GOES Sounder estimate of Skin Temperature, 1400 UTC 26 January 2015 (Click to enlarge)

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.]

MIMIC total Precipitable Water (click to play animation)

MIMIC total Precipitable Water (click to play animation)

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.

GOES-13 6.5 µm water vapor channel images (click to play animation)

GOES-13 6.5 µm water vapor channel images (click to play animation)]

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.

Suomi NPP VIIRS 11.45 µm IR channel and 0.64 µm channel images, with lightning, surface fronts and METAR reports

Suomi NPP VIIRS 11.45 µm IR channel and 0.64 µm channel images, with lightning, surface fronts and METAR reports

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.

GOES Sounder Total Column Ozone derived product images (click to play animation)

GOES Sounder Total Column Ozone derived product images (click to play animation)

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.

MODIS 11.0 µm IR channel image, with lighting strikes, METAR surface reports, and fixed buoy reports

MODIS 11.0 µm IR channel image, with lighting strikes, METAR surface reports, and fixed buoy reports

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.

MODIS 11.0 µm IR channel image, with RTMA surface winds

MODIS 11.0 µm IR channel image, with RTMA surface winds

Cold night in Labrador, Canada

January 22nd, 2015
Suomi NPP VIIRS 11.45 um IR channel image, with METAR surface reports

Suomi NPP VIIRS 11.45 um IR channel image, with METAR surface reports

With a cloud-free sky and light winds under a dome of high pressure, strong radiational cooling over a deep snowpack allowed the overnight low temperature to drop to -47º F (-43.9º C) at Wabush Lake (station identifier CYWK) in far western Labrador — this was quite possibly the coldest site in North America on 22 January 2015 (the coldest overnight low temperature in Alaska that morning was -39º F or -39.4º C at Galena). AWIPS images of Suomi NPP VIIRS 11.45 µm IR channel data (above) and MODIS 11.0 µm IR channel data (below) showed minimum surface IR brightness temperatures of -47º C or -52.6º F (darker blue color enhancement) in the western Labrador.

MODIS 11.0 um IR channel image, with METAR surface reports

MODIS 11.0 um IR channel image, with METAR surface reports

A comparison of 1-km resolution Soumi NPP VIIRS 11.45 µm and 4-km resolution GOES-13 10.7 µm IR images (below) showed the advantage of higher spatial resolution for more accurately locating the coldest regions.

Suomi NPP VIIRS 11.45 um and GOES-13 10.7 um IR channel images

Suomi NPP VIIRS 11.45 um and GOES-13 10.7 um IR channel images

Horizontal convective rolls: a satellite signature of blowing snow and ground blizzard conditions

January 8th, 2015
GOES-13 0.63 µm visible channel images with METAR surface reports (click to play animation)

GOES-13 0.63 µm visible channel images with METAR surface reports (click to play animation)

An Alberta Clipper disturbance quickly moved through the north-central US during the day on 08 January 2014, leaving only light amounts of snowfall (generally 1 inch or less). However, very strong winds in the wake of the system (with gusts as high as 59 mph at Bismarck, North Dakota and 69 mph at Bullhead, South Dakota) produced ground blizzard conditions as the newly-fallen light, fluffy snow was lofted and organized into long horizontal convective roll features. GOES-13 0.63 µm visible channel images with overlays of METAR surface reports (above; click image to play animation) and overlays of cloud ceilings and surface visibility (below; click image to play animation) distinctly showed the widespread horizontal convective rolls, along with their effect on the weather as they moved near or over various locations.

GOES-13 0.63 µm visible channel images with cloud ceilings and surfaces visibilities (click to play animation)

GOES-13 0.63 µm visible channel images with cloud ceilings and surfaces visibilities (click to play animation)

One question that arises is: are these horizontal convective roll features clouds, or simply highly-concentrated areas of blowing snow, or perhaps a little of both? A comparison of Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR, and 11.45 µm IR images at 19:00 UTC (below) might shed some light on the topic. As seen on the GOES-13 visible images, many of the roll features were tall enough to cast a shadow — this suggests vertical mixing to the top of the boundary layer, which appeared to be about 1 km deep on the morning Bismarck ND rawinsonde report. A few sites reported heavy snow (reducing visibility as low as 0.15 mile) as a horizontal convective roll moved overhead — however, the 11.45 µm IR brightness temperatures were barely colder than -20 to -25º C for even the most well-defined roll features (so their ability to produce heavy snow seems dubious). On the 3.74 µm shortwave IR image, if supercooled water droplet clouds had formed at the top of the roll features, they would appear darker (due to the reflection of solar radiation off the supercooled cloud droplets) — but this is not the case.

Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images

Suomi NPP VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images

Suomi NPP VIIRS true-color Red/Green/Blue (RGB) images from the SSEC RealEarth web map server (below) demonstrated the value of overlaying Google Maps information, for example to see which highways might be impacted by the larger and more well-organized horizontal convective rolls at that particular time.

Suomi NPP VIIRS true-color RGB images

Suomi NPP VIIRS true-color RGB images

Lake effect snow band from Lake Huron brings heavy snow to Pennsylvania

January 7th, 2015
GOES-13 0.63 µm visible channel images (click to play animation)

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

McIDAS images of GOES-13 0.63 µm visible channel data with overlays of surface weather type (above; click image to play animation) showed a large lake effect cloud band that had formed over Lake Huron, moved inland across southern Ontario, and then became further lake-enhanced as it moved over Lake Erie and across northwestern Pennsylvania on 07 January 2015. The report of heavy (4-star) snow in far northwestern Pennsylvania was at Meadville — it reduced surface visibility there to 1.25 miles at times, and produced at least 6 inches of snowfall at that location. To the north of Meadville, 8.5 inches of snow were reported at Edinboro.

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

AWIPS images of Suomi NPP VIIRS 0.64 µm visible channel data are shown with overlays of surface METARs, RTMA winds, and frontal boundaries at 17:39 UTC (above) and 19:19 UTC (below). The RTMA surface winds showed that there was low-level convergence in the vicinity of the weakening cold frontal boundary/surface trough that was sagging southward and southwestward across the region (animation) — this convergence may have helped to maintain the cloud band as it continued to move southeastward across Lake Erie during the afternoon hours. Meadeville PA is station identifier KGKJ.

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 0.64 µm visible image, with surface METARs, RTMA winds, and frontal boundaries

Suomi NPP VIIRS 11.45 µm IR channel images (below) showed that the coldest cloud-top IR brightness temperatures over both the Ontario and Pennsylvania portions of the band were -36º C (lighter green color enhancement) — at London, Ontario (CYXU), embedded towering cumulus (coded TCU EMBD) were reported at both 18 UTC and 19 UTC.

Suomi NPP VIIRS 11.45 µm IR channel images

Suomi NPP VIIRS 11.45 µm IR channel images

The Terra MODIS Sea Surface Temperature product around 18:11 UTC (below) showed that Lake Erie water temperatures were still as warm as the middle 30s F (blue) on either side of the blacked-out lake effect/lake enhanced cloud band — so moving an arctic air mass with 850 hPa temperatures colder than -20º C or -4º F over that water yielded a sufficient “delta-T” value to promote further enhancement/growth of the snow-producing cloud band which originated over Lake Huron.

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product