Snow cover across the northeastern US

October 31st, 2008
MODIS true color + MODIS false color images

MODIS true color + MODIS false color images

MODIS true color and false color images from the SSEC MODIS Today site (above) showed the extent of snow cover that remained on the ground on the morning of 31 October 2008, following a large early-season snow storm that affected much of the northeastern US and southeastern Canada several days earlier. Snow cover appeared as lighter blue features on the false color image (in contrast to supercooled water droplet clouds, which appeared as shades of white on both the true color and false color images). Total snowfall amounts across the region were as high as 25.6 inches at Roxbury, New York and 16.0 inches at Mountain Lake, Pennsylvania — and 2-3 inches of snow were reported as far south as Kentucky and North Carolina (in the higher elevations of the Appalachians). Also, note the broad swath of lake-effect snow cover  that was oriented northwest-to-southeast across western Pennsylvania, downwind of Lake Erie.

AWIPS images of the GOES-12 water vapor channel (below) showed the development of the storm during the 28 October29 October period.

GOES-12 water vapor images

GOES-12 water vapor images

-40º F (-40º C) in Alaska

October 26th, 2008
NOAA-18 AVHRR 10.8 µm IR image

NOAA-18 AVHRR 10.8 µm IR image

October 2008 turned out to be the 4th coldest October on record at Fairbanks, Alaska. About 150 miles (240 km) to the north of Fairbanks, a minimum surface temperature of -40º F (-40º C)  was reported at Chandalar Lake, Alaska on 25 October, followed by a low of -42º F (-41º C) on 26 October 2008:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
1124 AM AKDT SUN OCT 26 2008

…VERY COLD TEMPERATURES OVERNIGHT IN THE NORTH EASTERN INTERIOR…

FOR THE SECOND NIGHT IN A ROW CHANDALAR LAKES OVERNIGHT TEMPERATURE WAS BELOW 40 DEGREES BELOW ZERO.

SATELLITE IMAGERY THIS MORNING INDICATED THAT SOME VALLEYS IN THE NORTH EASTERN BROOKS RANGE WERE APPROACHING 50 BELOW ZERO OVERNIGHT…BUT WITH NO OBSERVERS IN THE AREA WE HAVE NO OFFICIAL TEMPERATURE REPORTS FROM THERE.

THE PLACES THAT WERE 20 BELOW ZERO OR COLDER:

CHANDALAR LAKE…………………….-42
NORUTAK LAKE………………………..-30
BEAVER…………………………………..-29
COLDFOOT……………………………..-26
CHALKYITSIK……………………………-25
BETTLES AIRPORT……………………..-20

NOAA-18 AVHRR 10.8 µm IR imagery (above) showed numerous narrow fingers of very cold air draining into the valleys of the Brooks Range in northern and northeastern Alaska — many valleys exhibited IR brightness temperatures as cold as -40º C (darker blue color enhancement).

A magnified version of that same NOAA-18 AVHRR 10.8 µm IR image (below) revealed that some of the valleys located to the northeast of Chandalar Lake (station identifier PALR) were even colder, with IR brightness temperatures as low as -45º C or -49º F (violet color enhancement) — however, Eric Stevens (Science and Operations Officer at the Fairbanks AK NWS forecast office) informed us that the ASOS instruments at Arctic Village (station identifier PARC) were out of service at that time, so no surface temperature data was available to verify the cold values seen on satellite IR data. The coldest IR brightness temperature in the immediate PALR region was -44º C  (-47º F) at 12:38 UTC — a surface air temperature of of -39º C (-38º F) was reported at PALR about 2 hours after the time of the NOAA-18 IR image.

NOAA-18 AVHRR 10.8 µm IR image

NOAA-18 AVHRR 10.8 µm IR image

While the corresponding GOES-11 10.7 µm IR images (below) did show a similar area of cold IR brightness temperatures near -40º C (darker blue color enhancement) from Chandalar Lake (PALR) to Arctic Center (PARC), the fine detail of the cold air drainage into the valleys was lost (the effective resolution of the “4 km” GOES IR pixels increase in size to about about 20 km over northern Alaska, due to the large satellite viewing angle). The coldest IR brightness temperatures seen on GOES data in that region at 12:30 UTC was -39º C (-38º F), compared to -45º C (-49º F) indicated by the AVHRR data at 12:38 UTC.

GOES-11 10.7 µm IR images

GOES-11 10.7 µm IR images

According to the USA Today tabulation of daily national temperature extremes, this is the earliest -40º F temperature reported in the US during the 1995-2008 period (the earliest -40º date was 05 November in 1999, with the latest -40º date being 31 December in 2002). While this was also the earliest recorded -40º temperature for Chandalar Lake, it did not threaten the monthly October record low temperature for the state of Alaska (which was -48º F or -44º C at Clear Water, set in 1975).

Snow cover over the northeastern US and southeastern Canada

October 23rd, 2008
MODIS Visible, Snow/ice, Land Surface Temperature, and NDVI

MODIS Visible, Snow/ice, Land Surface Temperature, and NDVI

AWIPS images of the MODIS visible channel, 2.1 µm near-IR “snow/ice channel”, Land Surface Temperature (LST) product, and Normalized Difference Vegetation Index (NDVI) product (above) showed an area of snow cover over parts of upstate New York, Vermont, New Hampshire, and Maine in the northeastern US, as well as Quebec and New Brunswick in southeastern Canada on 23 October 2008. Snowfall amounts over that region were generally in the 2-6 inch range (NOHRSC), with as much as 7 inches reported at Killington in Vermont. This snow cover was also very evident on true color imagery from the SSEC MODIS Today site.

Some items to point out on the AWIPS MODIS imagery above:

  1. the snow cover appears as brighter shades of white on the visible image, as do the cirrus cloud features over the far northwestern corner of the image and the stratocumulus clouds found over the eastern and southeastern portions of the image
  2. the snow cover appears darker on the 2.1 µm near-IR “snow/ice channel”, since snow (or ice) is a strong absorber of radiation at that wavelength
  3. the LST values were about 10º F colder over the snow cover (upper 20s to low 30s F, darker blue colors) compared to adjacent bare ground areas
  4. NDVI values over the snow cover were also significantly lower (0.1 to 0.2) compared to the adjacent areas that still had green vegetation in place

Even though the MODIS LST values over the snow cover were about 10º F lower,  the actual instrument shelter air temperatures reported across the region were only a few degrees F colder over the areas with snow on the ground (below), in part due to  the relatively high October sun angle.

MODIS LST product with METAR reports

MODIS LST product with METAR reports

As an aside, an examination of the patch of thick cirrus clouds in the far northwestern corner of the images helps to demonstrate the value and accuracy of the MODIS Cloud Top Temperature (CTT) product (below). The coldest  brightness temperature seen on the MODIS IR window channel image in the vicinity of the cirrus feature was -45º C (darker green enhancement), while the MODIS CTT product indicated temperatures as cold as -57.8º C (darker blue enhancement) over that same patch of cirrrus. Looking that the 12 UTC rawinsonde report from Maniwaki, Quebec (CWMW), assuming that the tops of the cirrus were in the 33,000-35,000 foot range, the air temperatures at those altitudes were around -55º to -58º C (closer to the coldest MODIS CTT value).

MODIS 11.0 µm IR window + Cloud Top Temperature product

MODIS 11.0 µm IR window + Cloud Top Temperature product

Lee-side cold frontal gravity wave

October 22nd, 2008
GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

AWIPS images of the 4-km resolution GOES-12 6.5 µm water vapor channel (above) showed a southward-propagating  lee-side cold frontal gravity wave over New Mexico and Texas on 22 October 2008. This gravity wave was caused by a surface-based cold frontal boundary that was moving southward across the region.

MODIS 6.7 µm water vapor image + fog/stratus product image

MODIS 6.7 µm water vapor image + fog/stratus product image

A comparison of the 1-km resolution MODIS 6.7 µm water vapor channel and the MODIS fog/stratus product (above) indicated that there were narrow cloud bands along the leading edge of the frontal boundary / gravity wave, as well as more extensive patches of fog and/or stratus behind the front in the Texas panhandle.The MODIS Land Surface Temperature (LST) product (below) depicted LST values dropping into the 40s F (green colors) behind the front, with much warmer LST values in the 50s and 60s F (yellow to orange colors) ahead of the front.

MODIS 6.7 µm water vapor channel + Land Surface Temperature product

MODIS 6.7 µm water vapor channel + Land Surface Temperature product

NOAA wind profiler data from Jayton, Texas (below) showed the deepening of the cold northerly flow after the cold front moved through the area — the top of the cold air appeared to be close to the 700 hPa level (around 10,000 feet above ground level).

Jayton, Texas NOAA wind profiler data

Jayton, Texas NOAA wind profiler data

GOES-12 water vapor channel weighting functions calculated for the rawinsonde profiles at Amarillo, Texas (below) demonstrated a significant lowering of the layer being detected by the water vapor channel in the 12 hours between 00 and 12 UTC on 22 October. With the drier air mass in place at 12 UTC, the GOES-12 water vapor channel was able to detect a substantial amount of energy originating from within the 500-700 hPa layer, allowing the signature of the frontal gravity wave to appear on the GOES-12 water vapor imagery. The wave structure was better-defined on the MODIS water vapor image, due to the improved spatial resolution and the more direct satellite viewing angle.

GOES-12 water vapor channel weighting functions for Amarillo TX

GOES-12 water vapor channel weighting functions for Amarillo TX