Arctic Cold without Snow

Although a few valley locations are getting some snow from this weekend system, the vast majority of our area will not have any snow on the ground this week.  Why does this matter?  With the arrival of the cold arctic air mass tonight, our temperatures are going to take a nose-dive.  But just how cold depends on a few factors.  One factor is clouds.  As we all know by now, clouds at night act as a blanket and keep us warmer than clear nights.  

But another factor is snow cover.  Snow on the ground does 2 things.  During the day, it reflects the suns rays rather than allowing the ground to absorb them, which keeps us colder.  At night, it acts as a blanket on the ground, keeping any warmth in the ground from warming us at night.  So, in short, if we have snow on the ground, we should expect colder temperatures this week. But aside from some snow showers Monday evening, what we have on the ground now is what we're going to have for the week.

So do we have any instances of arctic air masses without snow on the ground?  Indeed we do. Lets take a look at a couple of those to get an idea of what has happened in the past.

In December of 1998, the Inland Northwest had a surge of arctic air without any snow on the ground.  Below is a table showing the high and low temperatures at Spokane Airport leading up to Christmas of that year, as well as the amounts of precipitation, snowfall, and snow on the ground.  Also included are the temperatures (in Celsius) at 850mb and 700mb, which are about 5000' and 10,000' above ground.  These temperatures provide a good reference for comparison to other years.


1998

Date	Max	Min	Precip	Snow	Snow on Ground	850mb Temp °C	700mb Temp °C
17	44	25	Trace	0.0	0	-0.9	-12.5
18	29	14	Trace	Trace	Trace	-12.3	-21.1
19	15	3	0.00	0.0	0	-19.1	-25.1
20	8	-4	0.00	0.0	0	-22.1	-24.9
21	10	-3	0.00	0.0	0	-20.3	-21.5
22	14	2	0	0	0	-17.7	-21.3
23	17	6	0.00	0.0	0	-13.3	-17.7
24	27	10	0.19	2.3	2	-7.1	-9.1
25	37	24	0.52	5.2	3	-2.1	-8.3

As you can see, without any snow on the ground, the temperature was able to fall below zero on the 20th and 21st, with a single-digit high on the 20th.  Spokane was rewarded for this short burst of cold with snow on Christmas Eve and Christmas Day.

 A weather pattern almost identical to our current situation occurred in early December of 1972.  Below is the temperatures from that prolonged event.


1972

Date	Max	Min	Precip	Snow	Snow on Ground	850mb Temp °C	700mb Temp °C
1	50	35	0.02	0.0	0	7.0	-4.5
2	40	17	0.00	0.0	0	-5.7	-17.7
3	20	9	0.01	0.2	0	-14.3	-18.5
4	18	4	0.00	0.0	0	-15.9	-22.5
5	18	4	0.00	0.0	0	-14.9	-25.5
6	13	8	Trace	Trace	0	-17.9	-23.7
7	10	-5	0.00	0.0	0	-20.5	-24.5
8	10	-11	0.00	0.0	0	-21.5	-21.7
9	11	-7	0.00	0.0	0	-17.9	-20.5
10	12	-8	0.00	0.0	0	-17.5	-13.7
11	13	4	0.12	1.5	0	-14.9	-14.5
12	15	6	0.07	1.0	3	-14.9	-18.1
13	13	-5	Trace	Trace	3	-14.5	-14.9
14	19	5	Trace	Trace	3	-8.1	-12.9

Compared with 1998, this event was longer, and the lowest temperature reached -11°F, again, with no snow on the ground.  The 850mb temperatures generally bottomed out around -22°C and -21°C respectively, and the 700mb temperatures were around -25°C and -24°C.  So, similar events in the magnitude of the cold.

Below is a table of the forecast temperatures for our current event (as of Monday afternoon)

2013

Date	Max	Min	Precip	Snow	Snow on Ground	850mb Temp °C	700mb Temp °C
30	35	28	0.01	0.2	Trace	1.4	-6.5
1	44	34	0.22	0	0	3.4	-6.3
2						-6.4	-17.0
3	26	17				-8.6	-17.6
4	18	12				-12.2	-21.0
5	14	6				-13.4	-19.8
6	14	6				-14.6	-21.8
7	13	5				-16.3	-18.5
8	17	6				-13.0	-16.7
9	18	9				-11.6	-14.8
10						-15.0	-15.0

The 850mb and 700mb forecast temperatures come from the GFS model.  And while they are cold, they're not as cold as the 1998 and 1972 events.  So our current forecast Max/Min temperatures make sense: cold, but not quite as cold as those other 2 events.  

It's worth noting that there are other computer models such as the Canadian and ECMWF that have forecast 850mb and 700mb temperatures slightly colder than the GFS, but still not as cold as the 1998 and 1972 events.

But we started this blog saying that there are other factors to temperature forecasting; cloud cover may ultimately determine just how cold we get.  For those folks that have snow on the ground (such as Porthill and Bonners Ferry, Idaho), the nighttime lows will almost assuredly be 10 to 15 degrees colder than our current forecast for Spokane.