* This is a printable version of the case exercise. A multimedia version is also available.*
This case presents a blowing snow forecast challenge over north-central Canada. The time period of interest is from 04 to 10 February 2003. The case addresses specific low-level wind and snow conditions over the central mainland region of Nunavut. You'll be asked to assess the potential for blowing snow and blizzard conditions for the Baker Lake region of Nunavut.
Location of Interest: Baker Lake, Nunavut
Case Time Frame: 04 Feb - 10 Feb 2003
Phenomenon of Interest: Blowing Snow
Forecasting Challenges:
Nunavut spans across a vast stretch of northern Canada, covering 2 million square kilometers—a fifth of Canada's total landmass. Most of its land lies north of tree line. This sparsely populated territory stretches from the coast of the Beaufort Sea in the west, across the Hudson Bay to Baffin Island in the east, and from Manitoba in the south, to the northern-most Queen Elizabeth Islands. With all its islands and open stretches of tundra, there are few roads connecting the 29,000 residents. Air travel is the primary means of transport between communities, while snowmobiles and all-terrain vehicles are commonly used for local travel.
The specific area of interest for this case is Baker Lake, which is near the geographic center of Canada. Baker Lake is Canada's only inland Inuit community and has around 1,500 full time residents. Located in the central mainland region of Nunavut, it sits at the mouth of the Thelon River on the northwestern edge of Baker Lake. The lake stretches eastward towards an inlet of Hudson Bay, 320 km away. In the summer, the town is a jumping off point for hunting, fishing, and canoe trips. Year around, the town is dependent on air travel for supplies.
Located as far north as it is, Baker Lake experiences cold and dark winter months. The climatology listed here provides an overview of what to expect in Baker Lake during the winter months.
During the winter, Baker Lake residents deal with fairly constant winds and frequent blowing snow conditions. Average winds in February blow at more than 12 knots.
See Thresholds for Blowing Snow and Blizzard Conditions in the Supporting Topics area for more information.
It is safe to say that Baker Lake is the blizzard capital of Canada. Between 1980 and 1999, residents there had to cope with an average of 91 blowing snow days and over 20 blizzard events per year!
The MSC's Arctic Weather Centre in Edmonton issues forecasts for all of Nunavut. Given the sparse population and remote location of most communities, the forecasters in Edmonton have to deal with a number of limitations. Foremost, the network of surface stations is much sparser than in the south. A few of these remote stations can be viewed using a Webcam. The example shown here is located on the southeast coast of King William Island, where for much of the winter, the sun is low or below the southern horizon.
Given the latitude of the region, geostationary satellite (GOES) data is of limited use. Verification is much more dependent on the use of polar-orbiting sensors (POES). It's important to keep in mind that POES data does not come in as regularly as GOES data, and the availability of POES imagery is tied to the orbital paths of the satellites.
With the Baker Lake area setting and forecast limitations in mind, let's take a look at conditions early on 04 February 2003. The GEM Regional model is used throughout this case. The 00-hour analysis will be used to show current conditions, while 48-hour forecasts are used to support forecast questions. Be sure to take note of the timeline as you progress through the case.
The upper-level pattern at 0000 UTC shows a high-amplitude ridge over the Yukon Territory that extends southward over the Pacific off the coast of British Columbia. Downstream of the ridge is a major trough. A branch of the polar jet stream is rounding the base of the trough over southwestern U.S., and another branch of the polar jet stream defines the trough farther north over Hudson Bay, northwestern Ontario, and the midwestern U.S.
The MSLP pattern shows high pressured centered off Vancouver Island and another large area of high pressure over the eastern Canadian prairies and Nunavut. A 988 hPa low lies over southwestern Alaska, while a weak low has formed over the Beaufort Sea. Relatively low pressure extends from the southern Arctic Ocean across the Northwest Territories and into Alberta.
The temperatures in mainland Nunavut are very cold, in the -30 to -40ºC range.
Take a look at the GEM Regional forecast for MSLP and the 1000-500 hPa thickness. Note that the previously mentioned ridging, centered from Nunavut southward, is being squeezed from the north by a developing surface frontal system. From southern Nunavut northward, the MSLP gradient is generating westerly geostrophic winds and is tightening. MSLP contours are becoming cyclonically curved ahead of a cold front located approximately over Victoria Island, just north of mainland Nunavut.
A look at a recent snow-water equivalent analysis shows snow coverage throughout most of Nunavut, including significant amounts to the north and southwest of Baker Lake.
Not much has changed 8 hours later. Although aloft some amplification of the upper ridge and trough system is evident, with troughing in southeastern Canada intensifying and slowing. The gradient over western Canada is strengthening somewhat behind a short wave. Fairly strong north-northwesterly to north-northeasterly flow is present over the Northwest Territories, Alberta, and Saskatchewan.
At the surface, the intense low pressure system has moved over Lake Huron and is heading northeastward. The Beaufort Sea low pressure is still evident but relatively weak, and lee troughing is developing over the western Northwest Territories. Moderately strong high pressure dominates British Columbia, and weak high pressure extends up into southern and central Nunavut.
The time is now 0400 UTC on 05 Feb. Twenty-four hours ago, based on the model, it looked like things were setting up for a frontal passage across the Baker Lake area sometime on 06 February.
Take a look at the GEM Regional forecasts for the next 48 hours. Compare the forecast to the following four generalized MSLP and 500 hPa synoptic patterns. Each of these patterns represents a potential for blowing snow over the yellow area of interest. In this case, our area of interest is northern mainland Nunavut, northwest of the Hudson Bay. Which of these synoptic patterns is most likely to set up over that area in the next 24-48 hours?
CMC 4-Panels GEM Regional 48-hr fcst initialized 00 UTC 05 Feb 2003
Pattern A Pattern B Pattern C Pattern D
As a surface trough approaches Baker Lake, do you anticipate blowing snow for the 0000-0600 UTC 06 Dec period? What about later in the forecast period, between 0600-1800 UTC on 06 Feb?
UMOS Winds - interactive map GEM Regional 48-hr fcst initialized 00 UTC 05 Feb 2003
MSC Snow-Water Equivalent SSM/I derived SWE valid 02 Feb 2003
MSC Snow-Water Equivalent Change SSM/I derived SWE Change from 23-30 Jan 2003
Surface Station Observations Surface station observations across Nunavut for 2100-0300 UTC 04-05 Feb 2003
Conditions for Baker Lake: 0000-0600 UTC 06 Feb
No blowing snow or very minor blowing snow until the front passes Blowing snow with long sustained time periods No blowing snow conditions as the data indicates that the upstream trough will quickly weaken and winds will diminish
Conditions for Baker Lake: 0600 -1800 UTC 06 Feb
No blowing snow as the winds will not strengthen significantly behind the front Blowing snow beginning shortly after the frontal passage and becoming significant No blowing snow conditions as the front is weak when it passes through Baker Lake
As the day progresses, the anticyclonic flow around the ridge aloft continues to strengthen. 70 to 90 knot northwesterly to northerly flow at 500 hPa now extends from the Beaufort Sea through the Northwest Territories and Alberta. A wave and associated westerly jet streak are apparent over the Beaufort Sea. The cyclonic flow in the large long-wave trough extends from Saskatchewan eastward.
At the surface, an intensifying trough has developed in association with the wave aloft and is moving southeastward from Victoria Island. Significant ridging is now pushing southeast from the Beaufort Sea. The intense low is now over central Quebec, heading northeast, with blowing snow reports appearing over northern Quebec. Ridging persists between these two features, with a 1030 hPa high centered over southern Nunavut.
Light southwesterly winds are being observed at Baker Lake at this time. We are now only 4-6 hours away from the predicted frontal passage. Will your forecast for the 00-24 hour time period include blizzard conditions for Baker Lake?
UMOS Winds - interactive map GEM Regional 48-hr fcst initialized 00 UTC 06 Feb 2003
CMC 4-Panels GEM Regional 48-hr fcst initialized 00 UTC 06 Feb 2003
CMC Snow on the Ground Analysis Global Model valid 06 UTC 05 Feb 2000
Surface Station Observations Surface station observations across Nunavut for 0400-0400 UTC 05-06 Feb 2003
Yes No
As predicted, by this time the trough has passed over Baker Lake. Observations for the past 12 hours show that there have been periods of snowfall and blowing snow conditions. And by 1100 UTC, winds passed the 22 knot threshold for blizzard conditions.
CYBK - Baker Lake 71926 BAKER LAKE ARPT & UA NT CN 64'18N 96'05W 18m CYBK 060000Z 18002KT 15SM SCT020 BKN210 M32/M36 A3016 RMK SC3CI2 SLP217 CYBK 060100Z 22006KT 15SM SCT020 BKN210 M31/M36 A3011 RMK SC3CI2 SLP199 CYBK 060200Z 21008KT 2 1/2SM IC OVC015 M29/M34 A3005 RMK SC8 SLP180 CYBK 060300Z 22012KT 1 1/2SM -SN OVC012 M27/M31 A2999 RMK SN3SC5 PRESFR SLP160 CYBK 060400Z 22010KT 1SM -SN OVC015 M25/M30 A2992 RMK SN3SC5 PRESFR SLP136 CYBK 060500Z 22014KT 3/4SM -SN OVC012 M21/M25 A2985 RMK SN3SC5 PRESFR SLP111 CYBK 060600Z 24018KT 3/4SM -SN OVC015 M20/M24 A2979 RMK SN3SC5 PRESFR /S07/ SLP092 CYBK 060700Z 25025KT 1/2SM -SN BLSN OVC015 M17/M20 A2969 RMK SN4SC4 PRESFR SLP059 CYBK 060800Z 26015KT 1/2SM -SN BLSN OVC015 M17/M20 A2969 RMK SN6SC2 SLP059 CYBK 060900Z 28017KT 3/4SM -SN BLSN OVC015 M16/M19 A2962 RMK SN6SC2 PRESFR SLP032 CYBK 061000Z 29013KT 1 1/2SM -SN BLSN OVC015 M15/M18 A2959 RMK SN3SC5 SLP024 CYBK 061100Z 33026G34KT 1/2SM -SN BLSN OVC015 M13/M16 CYBK 061200Z 35035G40KT 1/8SM IC +BLSN SKC M18/M22 A2958 RMK BLSN7 /S05/ SNW CVR/MUCH LOOSE SLP021
The surface trough over Nunavut is now intense. It extends from the low's center in the northeast, southwestward across the Territory. Behind the system, the high over the Arctic Ocean has strengthened slightly to 1039 hPa (just off the MSLP map). Northwest of the trough, strong northwesterly winds extend back across the Queen Elizabeth Islands, and the ice of the Arctic Basin to the North Pole.
At 500 hPa, the cutoff low is now at 4760 m and located just southwest of the western tip of Baffin Island. This is creating a strong northwesterly flow over Victoria Island and mainland Nunavut.
Looking at imagery from the polar-orbiting NOAA-12 satellite, the 11 micrometer IR shows the cold air mass making its way southeastward across Nunavut. At 1155 UTC, the leading edge of the air mass has not yet passed over Baker Lake. By 1335 UTC it is to the south of town. The surface observations at Baker Lake show a drop of 10ºC during that time.
At first glance it may be tempting to think the 11 micrometer IR is showing low-level cloud or blowing snow as the front heads southeast. But in this situation, the 11 micrometer IR is showing sensitivity to differences in temperatures between the very cold air moving in from the northwest and the air mass ahead of it.
How could this be? Especially since the 11 micrometer IR is commonly referred to as an atmospheric "window" channel, supposedly only seeing energy from cloud top and surface? It is true that the 11 micrometer is relatively insensitive to atmospheric gases, but there is just enough sensitivity to low-level water vapor to bias the observed brightness temperatures toward the temperature of the low-level air mass. The larger the temperature contrast between air masses and the more moist the air masses, the greater the effect on the 11 micrometer IR brightness temperatures.
How widespread are the blowing snow conditions at this point? Take a look at the MODIS satellite products (MODIS VIS/IR, Baker Lake). Be sure to switch between the two available times, 1730 and 1740 UTC. Evaluate and take note of the motion of the features in this imagery. Then drag and drop the five correct features onto the MODIS 2.1 micrometer image here:
MODIS VIS/IR, Baker Lake MODIS 0.84 µm, 2.1 µm, and false-color composite 1730 UTC 06 Feb 2003
The deep surface low continues to develop into 07 Feb, with a center at less than 972 hPa. To the northwest, the high pressure over the Arctic Ocean builds to 1046 hPa (just off the map). The gradient between the low and the high is strengthening, as are the northerly winds.
The main surface trough sweeps south to southwest, across central Hudson Bay, and continues west across northern Manitoba and northern Saskatchewan. This elongated trough is evidence of the extensive nature of the conditions favorable for strong winds. With snow on the ground in most parts of the region, blizzard conditions persist for most communities behind the trough.
At 500 hPa, a strong 4630 m cutoff is over the extreme northern portion of Hudson Bay, with continued strong northwesterly flow over the eastern Northwest Territories, southern Nunavut, and the northern Hudson Bay region. To the east, the western portion of the long-wave trough is weakening rapidly—“shearing off”—and becoming more positively-tilted. The new, digging cutoff is new reinforcement for the old long-wave trough.
And things just continue to build. Strong northwesterly surface flow now reaches well into the Hudson Bay as the tight surface gradient moves southward and southeastward. The surface low is just northeast of Southampton Island and nearly stationary. The strong high has built from the Arctic Basin southeastward to eastern Great Bear Lake, and continues south-southeastward into northern Manitoba. The intense pressure gradient, with isobars oriented pretty much north-south, is creating high winds consistently above 22 knots across the region. Baker Lake reports blizzard conditions with gusts over 40 knots.
CYBK - Baker Lake 71926 BAKER LAKE ARPT & UA NT CN 64'18N 96'05W 18m CYBK 070000Z 32030G36KT 1/4SM +BLSN BKN200 M33/M38 A2969 RMK BLSN2CI3 PRESFR SLP057 CYBK 070100Z 31028G34KT 1/4SM IC +BLSN SCT200 M33/M37 A2968 RMK BLSN2CI1 SLP054 CYBK 070200Z 32031G38KT 1/8SM IC +BLSN BKN200 M33/M37 A2966 RMK BLSN3CI1 SLP045 CYBK 070300Z 32035G43KT 1/8SM IC +BLSN BKN200 M33/M38 A2961 RMK BLSN3CI2 PRESFR SLP030 CYBK 070400Z 33037G43KT 1/8SM IC +BLSN SKC M34/M38 A2964 RMK BLSN4 SLP040 CYBK 070500Z 34022G28KT 1/4SM IC +BLSN SCT200 M34/M39 A2971 RMK BLSN2CI0 SLP064 CYBK 070600Z 32033G39KT 1/8SM IC +BLSN SKC M35/M39 A2966 RMK BLSN5 SLP047 (breaks in obs from 0700 to 1200)
To the south, in central Manitoba, a secondary low pressure system is evident along the trough and is moving southward fairly rapidly. This secondary low is located on the arctic front.
At 500 hPa, the still-strengthening cutoff low, now 4580 m, is nearly stationary. The strong northwesterly flow is entrenching itself west and southwest of the cutoff, with strong flow sagging into northern Alberta and Saskatchewan.
Looking at the surface observations, Baker Lake reported winds in the 25-35 knot range with intermittent ice crystals falling early on 07 Feb. Then the data feed went out sometime after 0400 UTC. In general, only isolated light snow showers have been observed in the region over the past 24 hours. Previous to that, the Arctic Weather Center's 24-hr Precipitation Analysis for 06 Feb does show an 11 mm accumulation of precipitation at Baker Lake.
Do you think you can back off the forecast of blizzard conditions within the next 48 hours?
UMOS Winds - interactive map GEM Regional 48-hr fcst initialized 00 UTC 08 Feb 2003
Arctic Weather Center 24-hr Precipitation Analysis valid 00 UTC 07 Feb 3003
CMC Snow on the Ground Analysis Global Model valid 06 UTC 07 Feb 2000
Surface Station Observations Surface station observations across Nunavut for 0400-0400 UTC 07-08 Feb 2003
By 1200 UTC on 08 Feb, the surface systems have weakened somewhat. But enough gradient has moved west to give blowing snow to Ekati and Lupin, two communities in western mainland Nunavut.
CYOA 00000 EKATI (PWS) NT CN 64'42N 110'37W 469m CYOA 081200Z 32018KT 3SM BLSN SKC M30/M33 A2999 RMK BLSN2 -30.0/-33.0/TR/43/MUCH LOOSESLP236
CYWO YWO 00000 LUPIN ARPT (CON) T CN 65'46N 111'15W 490m CYWO 081200Z 29018G24KT 3SM BLSN SKC M31/M34 A2993 RMK SLP218
The surface low is now 978 hPa, moving slowly east, and is over Baffin Island. The surface high to the northwest has essentially split, with a portion remaining over the Arctic Ocean and a second part over western Canada. Weak troughing is occurring between these highs.
At 500 hPa, the strong cutoff has also drifted a bit to the northeast, but strength is maintained. The most energetic portion of the western region of this large trough is moving into the northern plains of the U.S.
But there is still blowing snow observed at Arviat and Churchill. 15-25 knot north-northwesterly flow is still common from northern Saskatchewan and Alberta, north-northwestward. In the southern portion of this region, significant cyclonic flow persists and extends into the Hudson Bay.
Aloft, persistence is again noted, with even some reinforcing flow enhancement, slightly cyclonic, over north-central Nunavut.
Is there an end in sight? Does it look like the blowing snow conditions will let up in the next 48 hours? Which of the following factors will play a major role on wind conditions in the Baker Lake area over the next 48 hours?
UMOS Winds - interactive map GEM Regional 48-hr fcst initialized 12 UTC 09 Feb 2003
Arctic Weather Center 24-hr Pricipitation Analysis valid 12 UTC 09 Feb 3003
CMC Snow on the Ground Analysis Global Model valid 00 UTC 09 Feb 2000
(Choose all that apply, then click Done.)
Rapidly increasing pressure to the north and west of Baker Lake Rapidly decreasing pressure to the south and east of Baker Lake Increase in isallobaric component of winds Rapid increase in 10 m winds General persistence
Between 0000 and 1200 UTC on 10 Feb, the winds start letting up. Most stations are no longer reporting blizzard conditions, though winds are still above 15 knots in Baker Lake with periods of blowing snow.
CYBK - Baker Lake 71926 BAKER LAKE ARPT & UA NT CN 64'18N 96'05W 18m CYBK 100100Z 30019KT 5SM -SN DRSN OVC020 M29/M33 A2948 RMK SC8 SLP986 CYBK 100200Z 31025KT 3SM -SN BLSN SCT020 M31/M35 A2945 RMK SC3 SLP975 CYBK 100300Z 31023KT 4SM -SN BLSN FEW020 OVC065 M31/M36 A2941 RMK SC2SC6 SLP963 CYBK 100400Z 31015KT 4SM -SN BLSN SCT020 M31/M35 A2939 RMK SC3 SLP955 CYBK 100500Z 31024KT 3SM -SN BLSN BKN020 M30/M35 A2934 RMK SC6 SLP940 CYBK 100600Z 32023KT 3SM -SN BLSN OVC020 M29/M33 A2933 RMK SC7 SLP936 CYBK 100618Z 32024KT 1/2SM -SN BLSN OVC020 RMK BLSN2SC6 CYBK 100700Z 33018KT 2SM -SN BLSN BKN020 M30/M34 A2932 REBLSN RMK SC5 SLP932 CYBK 100800Z 32014KT 5SM -SN BLSN BKN020 M30/M35 A2931 RMK SC5 SLP928 CYBK 100900Z 32015KT 5SM -SN DRSN BKN020 M30/M35 A2930 RMK SC5 SLP924 CYBK 101000Z 34016KT 4SM -SN DRSN SCT020 M29/M34 A2929 RMK SC4 SLP921 CYBK 101100Z 35012KT 10SM -SN DRSN SCT020 M30/M35 A2929 RMK SC4 SLP922 CYBK 101200Z 33016KT 4SM -SN DRSN BKN020 M30/M35 A2929 RMK SC5 SNW CVR/TRACE LOOSE SLP922
Is it over? Not really. Note the gradient that persists to the northwest through central Nunavut. There is also an embedded wave in this flow. The blowing snow threat in the region won't be over anytime soon.
This is the end of the "Blowing Snow: Baker Lake Case Exercise." This case exercise revisited an extended blowing snow event in northern Canada.
The Case Setting presented the climatology of the region of interest and provided various blowing snow and blizzard condition thresholds.
The Case Exercise addressed the synoptic patterns and existing snow conditions leading up to the event. It tracked the synoptics and low-level wind conditions as the event evolved through the use of model analysis, observations and satellite data. Forecast questions focused on:
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The correct answer is:
A B is correct C D
The B pattern is the most likely to set up in the next 24-48 hour time frame. Here we compare it to the GEM Regional 42-hour forecast.
That forecast is predicting the following:
MSLP A surface trough develops over north central Canada and extends from western Baffin Island southwestward into northern Saskatchewan. Behind the system, a 1038 hPa high is centered over the Arctic Ocean, and strong northwesterly gradient flow develops over northern Nunavut.
500 hPa Heights A cutoff low (4715 m) is located over western portions of Baffin Island, with very intense northwesterly flow over mainland Nunavut.
For 05 Feb, the Arctic Weather Center Snow Analysis indicates 21 cm at Baker Lake. It categorizes the snow in the Baker Lake area as C4: uneven layer of compact snow.
The UMOS wind forecast for YBK (Baker Lake) shows 8-10 knots from the southwest for this time period. Looking at stations upstream, to the northwest, UMOS winds are significantly higher behind the developing front. The GEM Regional 24-hr forecast MSLP shows the cold front approaching but definitely not reaching the Baker Lake area in this time period. Prefrontal geostrophic west-southwesterly flow appears to dominate during the period.
Referring to the windrose, strong southwest winds are climatologically rare to nonexistent in Baker Lake during the winter months.
According to the UMOS winds and the GEM Regional MSLP forecast, a front passes through Baker Lake between 0900 and 1200 UTC 06 February. The GEM Regional model shows a strong pressure rise and an intense northwesterly gradient flow developing behind the front. From 1200 to1800 UTC the UMOS winds are 25-35 kt from the northwest.
Given the predicted winds, snow on the ground, and the potential for snowfall, you can expect significant blowing snow to develop post-frontally at Baker Lake.
Yes, it makes sense to include blizzard conditions in the forecast for the Baker Lake region beginning approximately 0900 UTC on 06 Feb.
The trough is forecast to pass over Baker Lake around 0900 UTC on 06 Feb. At 0000 UTC, the surface front is located about 300 km to the north and west of Baker Lake, but it is approaching rapidly.
The surface winds at Baker Lake in the 0000 UTC model run are forecast to be from the northwest and consistently above 23 knots after frontal passage. The 24-hour forecast for 0000 UTC on 07 Feb shows an intense northwesterly gradient flow over Baker Lake, and the upstream reinforcement of this flow is evident.
As previously forecast, the winds, together with the existing snow on the ground and potential light snow that will fall as the front passes, point towards blizzard conditions.
The MODIS imagery shows widespread areas of blowing snow. Using the fader and time selectors to switch between the images, cloud, ground, and blowing snow features become identifiable.
The 2.1 micrometer shortwave IR is helpful in distinguishing the blowing snow from the snow covered ground and ice. In the visible image, the shadows cast by the clouds helps to set them apart from the snow-covered ground. The false-color composite allows these features to stand out even more sharply. By switching between the 1730 and 1740 time points, the motion of the blowing snow becomes apparent.
Click to open new browser window with interactive image fader (167 kb)
Note that in the false-color composite image, certain areas, such as the open water along the Hudson Bay coast and the clouds in the lower left, have a yellowish tinge. Due to the characteristcs of the 2.1 micrometer shortwave IR and the 0.84 micrometer visible channels, this shading indicates the presence of some liquid water in addition to ice crystals. It is for this reason that the clouds in the lower left are likely mid-level rather than cirrus.
For more details on the use of satellite imagery in this situation, see Applications of Multi-Spectral Satellite Imagery for the Detection of Blowing Snow.
To this point, the GEM Regional model has accurately portrayed surface winds in the Baker Lake area. Based on the model predictions, the strong northwesterly gradient flow persists right on through 0000 UTC on 10 Feb at Baker Lake. During the 42-48 hour time period UMOS winds do decrease to 20-25 knots, but it would be hard to justify forecasting moderating conditions two days ahead.
It looks like conditions will be persistence for the next 48 hours.
Observed and forecast winds are not pointing to widespread blizzard conditions, but blowing snow is still very much a threat. The model and the UMOS winds do hint at some reintensification 36 hours out, and with it, short periods of blizzard conditions are possible.
High Resolution EOS-Terra and -Aqua MODIS Imagery
Using satellite imagery to monitor blowing snow events is a difficult task. Conventional visible, shortwave (e.g. 3.9 micrometer) and longwave infrared satellite imagery are fairly adept at detecting clouds and surface features, but do poorly with blowing snow. In the visible, there is little if any brightness contrast between the relatively diffuse patterns of blowing snow and underlying snow and ice to help distinguish between the two. In some cases however, the blowing snow may organize into distinct street-like patterns whose shadows may be seen in high resolution (1km or better) imagery. Image animation can offer additional help for these cases. Most of the blowing snow in the visible imagery shown here however, did not organize into obvious distinct structures.
Longwave Infrared: 11 micrometers
In the longwave infrared (e.g. 11 micrometer), since blowing snow is typically confined to low levels, there is little thermal contrast between the suspended ice crystals and already cold surface below to help separate out any patterns associated with blowing snow.
Shortwave Infrared: 3.7-3.9 micrometers
The shortwave infrared region between 3.7 and 3.9 micrometers, commonly used for discriminating cloud phase and low level stratus, also has its problems when trying to isolate blowing snow. At nighttime, snow and ice whether suspended or on the ground, all have similar thermal emission properties and appear relatively indistinguishable. During the daytime, the same snow and ice reflect similar amounts of solar energy making blowing snow again difficult to isolate from a snow and ice covered surface. Similar to the visible imagery, if the blowing snow does organize into distinct street-like patterns, shadows cast by those patterns may become apparent in the imagery. This was unfortunately not the case for this blowing snow event.
Shortwave Infrared: 1.6-2.1 micrometers
Imagery within the 1.6-2.1 micrometer shortwave infrared region offers the best hope in the detection of blowing snow. Note that since most of the energy in this spectral region is reflected solar, the application is for daytime detection only.
Clouds and suspended particles generally reflect more incoming solar at 1.6/2.1 micrometers than features on the ground, particularly for those surfaces covered by snow and ice. This makes 1.6/2.1 micrometer imagery a popular tool for the detection of clouds and especially thin low level stratus which are so common at higher latitudes.
The high-resolution 2.1 micrometer image shown here is from the polar orbiting Terra-MODIS imager. The 2.1 micrometer image was selected over the 1.6 micrometers since 2.1 tends to be even more sensitive to differences between suspended particles and the underlying surface.
Low level clouds associated with the passing frontal band are readily apparent in lower portions of the image, south of Baker Lake. Compare this image to the visible and notice now the appearance of lighter more diffuse looking banded and blob-like structures. And to the north of Baker Lake we even see with more clarity now some transverse wave-like cloud features that are likely associated with the strong inversion atop the advancing arctic air mass.
Since it is unlikely that we have blowing dust or smoke at this time of year, the diffuse features noted are most likely associated with blowing snow. Animation can help confirm this as we would expect these areas to be moving along with the strong northwesterly surface flow.
If we look at a short animation of MODIS imagery from Terra and Aqua between 17:30 and 19:15 UTC, we indeed see these suspected pockets of blowing snow moving southeastward.
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