Snow, Rain, Wind, (and Thunder?)

Yesterday's weather system brought a nice 1 to 3" of snow to many locations.  You can see an extensive list of snow reports on the National Weather Service website.  Now we have Storm #2 arriving 24 hours later.  This one will be a little different in a couple of ways.  First, this storm is a little wetter and more extensive than the first.  This means more people will see precipitation.  Here's the radar forecast from the HRRR model valid at 5pm this evening:

5pm radar forecast from the HRRR model

This is a forecast from a computer, not an actual radar image.  But it gives you an idea what the radar will look like around 5pm.  And just about everyone in the Inland NW will be seeing some sort of precipitation at this time.  Which leads us to the other difference.  While Storm #1 was all snow, this one will be more of a mixed bag of rain and snow.  The HRRR forecast for precipitation type at 5pm is shown below.

5pm precipitation type forecast from HRRR

This graph is a little hard to read, but the horizontal blue lines represent snow, while the vertical green lines are for rain.  The HRRR expects the precipitation by 5pm to be rain south of a line from Deer Park to Grand Coulee.  This includes Spokane and the Palouse.  The precipitation may initially start in the metro area as wet snow, but it won't be able to accumulate much and will change to rain later this afternoon. Also notice there are some small pockets of red scattered around the region. This represents a chance of freezing rain. If freezing rain really does occur it will be rather isolated and likely quite brief. 

Here's the storm total snow forecast from the National Weather Service.

Storm total snow ending 4pm Thursday

Most of the valleys across the north (Republic, Colville, Sandpoint) will see all snow and pick up 2-6" of snow, while the mountains will receive up to a foot.

Storm #3 will follow on Thursday afternoon/evening this will be a weaker system.  It will once again bring rain or snow to Spokane and the Palouse while the northern valleys will pick up a little more snow.

Storm #4 is still taking shape but it consistently looks like it will be the strongest of the bunch.   Today's GFS model forecast backed off a little from it's previous forecast, while some of the other models came out a little stronger today.  As a result, they're all in very good agreement at this time.  We often take this as a sign to increase our confidence in the forecast.  Even though they all agree, could they all be wrong?  Yes, but the odds are growing increasingly slim.  

Here's a breakdown of how this storm will unfold:

• Precipitation will spread over the area on Friday around or shortly after sunset.
• Spokane and the Palouse will see rain from this storm, with a possible mix of snow at the onset.
• Saturday morning will be rainy and breezy.  Spokane will be in the lower 40s by sunrise and top out in the mid 40s during the day.
• The northern valleys will start as snow and will accumulate a few inches.  They will gradually change to rain overnight.  Colville will probably change to rain before midnight, while Bonners Ferry could hold on to snow until Saturday morning.
• The cold front will move through the area during the middle of the day.  Behind the front winds will really blow.  Below is our forecast peak winds for Saturday.  Areas in white can expect gusts in excess of 50 mph.

Peak wind gusts expected on Satuday

We will likely be issuing Wind Advisories for much of the area and possibly High Wind Warnings.  You'll want to stay tuned to this forecast.

Here's a couple of other things to consider with this storm.  The rain along with the melted snow won't be able to soak into the ground since the ground is solidly frozen (from our early December cold snap).  So expect some water  ponding, even on your lawn.

The other interesting tidbit is the outside chance of a thunderstorm.  Thunder?  In early January?  Yes, it's happened before, and this is pretty much the classic set up for it:  warm moist air ahead of a strong cold front.  Below is the SREF model probability of thunderstorms. 

24 hr thunder chances ending 10pm Saturday. 

You can see a small area of 10% over southeast Washington.  If thunder does occur, we don't expect to see hundreds of lightning strikes. However a few claps of thunder are a possibility especially over southeast Washington and the southern Idaho Panhandle, including Pullman and Lewiston. 

Weird Temperatures

  In previous blogs, we've noted how there are a few ingredients that affect temperatures. We're all familiar with the affects of sunshine and its resultant heating. In meteorological terms we like to call this the "diurnal cycle", since it corresponds to the daily sunrise and sunset cycle. Here's a graph of what we're talking about:

Hourly temperature graph for Deer Park, WA. 

This shows the temperature at Deer Park, Washington for an "average" winter day, one that wasn't affected by clouds, precipitation, wind, etc.  With time starting on the left, the temperature warms during the day shortly after sunrise. The temperature then reaches it peak around 3pm after continuous solar heating. The temperature then rapidly plummets after sunset, continuing to cool through the remainder of the night until the sun rises again and the cycle repeats itself. Sunrise and sunset for this time of year are around 730 am and 400 pm. 

On the past few nights, we've seen some temperatures that haven't "behaved" like the average diurnal cycle.  The image below shows Deer Park on the 7th through the 9th.  

Hourly temperatures for Deer Park, WA 12/7-12/9 2013

On the morning of the 7th (left part of the graph), temperatures started off around -5°F, and then gradually warmed to 13°F in the afternoon under sunny skies.  And then the temperature quickly plummeted back to 0°F by 6pm.  But then a strange thing happened; the temperature warmed overnight, and actually reached 13°F again at sunrise on the 9th.  In other words, the temperature at 8 am on the 9th was the same as during the previous afternoon.  How could this happen?

Let's take a step back and discuss how the sun actually warms us.  The suns rays don't warm the air directly.  Instead, the suns rays largely pass through our atmosphere and reach the earth's surface, where it is absorbed by the ground, roads, buildings, etc. This energy then warms the air above it.  

Earth-Atmosphere Energy Balance

After the sun sets, there is no incoming solar energy.  But the earth's surface still radiates it's energy (i.e. it cools).  If the skies are clear and the air is dry, this process is very efficient, and the cooling is rapid.  But if the skies are cloudy, the warmth that is radiated by the earth is trapped, and this acts to warm the air, even at night.

Nighttime heat radiation 

So in the case on the 8th/9th of December, under sunny skies, the incoming solar radiation warmed Deer Park up to 13°F.  But as soon as the sun set, the earth quickly cooled under the clear skies.  At 6pm, clouds moved into the area.  Not only did this stop the cooling process, it actually warmed the air.  The clouds "catch" the energy emitted from the earth and re-emit it back to the surface, just like a blanket on your bed.

A similar event took place last night.  Here's the temperature trace at Deer Park again, this time for the 10th and 11th.

As before, temperatures reached their high around 3pm (25°F on Tuesday), cooled quickly to 10°F by 7:30pm, and then warmed overnight.  All of this warming could be solely attributed to the cloud cover.  There is no "warm wind", warm front,  or anything like that.

All of this leads to the question:  In the winter,  at our latitude, what's better, a sunny day and clear night, or a cloudy day and night?  If you're not a fan of cold temperatures, then as nice as a sunny day is, it's probably better to hope for the clouds.  The prevalent low clouds and fog typically found in the Inland Northwest during the winter may seem dreary at times.  But it keeps our temperatures on average much milder than other locations at our latitude (e.g. Great Falls, Duluth, and Caribou, ME). The clouds have the most apparent impact on our low temperatures. See the chart below for details.