Lake effect and river effect clouds in northeastern Montana

GOES-16 "Red" Visible (0.64 µm, top) and Near-Infrared "Snow/Ice" (1.61 µm, bottom) images, with hourly plots of surface observations [click to play MP4 animation]

GOES-16 “Red”Visible (0.64 µm, top) and Near-Infrared “Snow/Ice” (1.61 µm, bottom) images, with hourly plots of surface observations [click to play MP4 animation]

* GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing *

As arctic air began to spread eastward across Montana — where the coldest temperature in the US was -12ºF — behind an inverted trough (surface analyses) on 04 November 2017, GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (above) revealed bands of “lake effect” (from Fort Peck Lake) and “river effect” (slightly upstream, from the Missouri River) clouds. On the Snow/Ice images, sow cover (and cold ice crystal clouds) appear as darker shades of gray, in contrast to supercooled water droplet clouds which are brighter white. Note that surface air temperatures at Glasgow (KGGW) and Jordan (KJDN) were generally in the 15 to 20ºF range.

A 1-km resolution Aqua (overpass times) MODIS Sea Surface Temperature product (below) indicated that SST values were still 50ºF and warmer (darker shades of green) in parts of Fort Peck Lake. Farther to the west, a deeper portion of the Missouri River exhibited SST values in the mid-40s F (cyan) — this area  was likely the source of the river-effect cloud features. The temperature difference between the surface air and the warmer lake/river water was therefore in the 30-35ºF range.

Aqua MODIS Sea Surface Temperature product [click to enlarge]

Aqua MODIS Sea Surface Temperature product [click to enlarge]

In a toggle between 250-meter resolution Terra (overpass times) MODIS true-color (Bands 1/4/3) and false-color (Bands 7/2/1)  Red-Green-Blue (RGB) images from the MODIS Today site (below), the false-color image helps to highlight the bands of supercooled water droplet river effect and lake effect clouds (brighter white) — snow cover (and high-altitude ice crystal clouds) appear as shades of cyan.

Terra MODIS true-color and false-color RGB images [click to enlarge]

Terra MODIS true-color and false-color RGB images [click to enlarge]

A 30-meter resolution Lnndsat-8 false-color image (below) captured the dissipating remnants of the Missouri River cloud plume at 1800 UTC; a few cumulus cloud streets could also be seen over Fort Peck Lake, along the far eastern edge of the image swath.

Landsat-8 false-color image [click to enlarge]

Landsat-8 false-color image [click to enlarge]

Will it Snow Tonight and Tomorrow Morning in Western Washington?

The answer is yes, but the amounts and distribution will be complex, elevation/location dependent, with considerable uncertainty.

The issue, as it almost always is with western WA lowland snow, is temperature.  Near sea level, the temperatures will be marginal, something compounded by the fact we are early in the winter season.  If this was January, I would calling Seattle's DOT folks to rev up the plows and sand trucks.

The general set up is nearly ideal.  An upper level trough will be coming in from the northwest (see map at 4 AM tomorrow), bringing cool air and precipitation.

Associated with the upper trough, a tight low pressure center will move to the SW tip of Washington, producing a pressure pattern that draws cold air into Washington, particularly through the Fraser River Valley.  Perfect.  The trouble is that the air in BC is not super cold and the temperatures (shown by the color shading) are on the warm side from Seattle southward.  Marginal for snow below 500 ft.  The only way to overcome this is heavy precipitation, with lots of cooling due to melting from snow falling in from above.

The precipitation will occur mainly between midnight and 9 AM Sunday. 

Here is the UW WRF snow forecast for the 24 hours ending 4 PM Sunday.   Huge variations.  The central and southern Cascades get hammered, with over a foot in many locations.  With northeasterly flow from the Fraser River Valley headed for the Olympics, the northern slopes will get snowed in, with Port Angeles and Sequim getting a piece of that.    There will be snow over central Puget Sound, but it will be very light near the Sound and will increase substantially for elevations about 500 ft (maybe 1-3 inches, higher amounts above 1000 ft).  Upslope conditions will provide more snow around the Hood Canal.

What are the ensembles (running many forecasts) say?  Here is the output from the National Weather Service SREF (short-range ensemble forecast) system that shows a plot of three-hour snow amounts at Sea Tac for the various members of the ensembles.   A lot of variability (therefore uncertainty), ranging from a trace to a few inches.  But most are going for something. 

The other NWS ensemble (GEFS) also has snow at Sea Tac  for most of its members (see below), with an ensemble mean (average) of about .7 inch.
My take from all this is the following.  The mountains will get plenty of snow (feet), solidifying a good start to the base yesterday.   Thanksgiving skiing beckons.   Near the Sound and sea level (below 500 ft), there will be some very light snow (trace to .5 inches).  But go higher (above 500 ft)  and away from warm water and amounts will get to 1-3 inches.   Like on the hills above Bellevue.   Several inches near the Hood Canal and around Sequim/Port Angeles.

As shown by the ensembles, there is considerable uncertainty.  Modest changes in precipitation intensity will make the difference between rain and snow (rain for lighter intensities).   

And keep in mind it will be hard for the snow to stick to roadways in contact with the ground, since we are so early in the season.  The latest road temperatures shown in the Seattle SNOWWATCH web site (boxes, see below), indicate that road temperatures are now in the low to mid-40sF.
Anyway, enjoy the unusually early snow over the lowlands.  And remember to change the clocks tomorrow as well.  Another hour to enjoy a hot drink and watch the flakes fall.