Inland Northwest Weather Blog: February 2015

Saturday, February 28, 2015

Update on Snow Chances

In our last blog, we talked about the chances for snow in our area on Monday and/or Tuesday. Our conclusions were:

Any precipitation would fall as snow.
The overall weather pattern was not a good one for snow in the Inland Northwest.
Not all of the computer guidance agreed on exactly what was going to happen.

Two days later, those conclusions still hold true. Usually at this point we have a pretty good idea of what's going to happen. But in this situation, we're still no more confident than we were two days ago. Here's why.

Here's the GFS forecast for Sunday evening. The low is way up in northern Canada with a trailing cold front (blue line).

GFS forecast of precipitation (green shading) and sea-level pressure (thin lines) for Sunday evening.

Here's the forecast for Monday afternoon.

GFS forecast of precipitation (green shading) and sea-level pressure (thin lines) for Monday afternoon.

As you can see, the front splits. Half of it goes into the upper plains while the other half drops down the West coast. This leaves the Inland Northwest dry, which is pretty common for this kind of storm moving down from the northwest.

Let's look at the Canadian GEM model:

Canadian forecast of precipitation (blue green shading) and sea-level pressure (thin lines) for Monday afternoon

This looks very similar to the GFS model. The GEM does bring a bit more snow into the Panhandle, but keeps eastern Washington dry.

So why isn't our confidence higher? Because the ECMWF has a slightly different scenario. It thinks the cold front will hold together as it passes over our area. So much so, that it gives about 1-2" of snow to the Panhandle and eastern Washington, east of Moses Lake.

What's more is that each model has been consistent with itself. Often, we'll see forecasts from the computer models that will change 12 hours later. In those situations, our confidence is lower. But in this case, each forecast from each model is very similar to its previous forecast. So the GEM and GFS insist that the Inland Northwest won't see much from this storm, while the ECMWF continues to insist that we will see 1-2" of snow.

So, are there any other computer forecast models? Yes, there are. Here's the UKMET forecast:

UKMET forecast of precipitation (blue and green shading), sea-level pressure (thin black lines) and 500mb heights (red lines) for Monday afternoon.

You can see that the UKMET has light precipitation over all of Washington. This is similar to the ECMWF, just lighter on the precipitation. If this were to verify, eastern Washington might see a dusting of snow.

The SREF model is an ensemble, which means that it's actually a group of 23 similar models. Here's a display of the SREF forecast snowfall for Spokane:

SREF forecast of snow for Spokane

Of the 23 SREF models, only 6 of them have any snow for Spokane, and only one has nearly an inch of snow.

So given all of this, the odds are slim that eastern Washington will see any snow on Monday. Not impossible, just not likely. Thus, the forecast of "a 40% chance of snow showers" for Spokane. The northern Panhandle has a bit better odds, but still not a slam dunk.

After that, we don't see any significant precipitation for at least another week.

Thursday, February 26, 2015

Is There Snow in our Future?

It's been so long since we've seen snow around here, that most of us have put away the boots and snow shovels for the season. Is there any chance we'll have to get them back out of storage? It's not likely, but there is a chance. Let's take a look.

First, the stage will be set in the next couple of days. A somewhat weak storm is moving down from the north today (Thursday), but this will largely miss our area to the west. Some folks are getting some rain and high elevation snow, but mostly it's just clouds. As this storm moves south of the region on Friday, it will pull down some cold and dry air from Canada.

GFS forecast precipitation (green shading) and surface pressure (thin black lines) for Saturday morning

Saturday will be a rather raw day, with temperatures in the upper 30s to lower 40s and a brisk north wind. This new air mass will mean that any precipitation from the next weather system will fall as snow instead of rain. That next system will arrive Monday and Monday night. There are three computer models that we look at for the weather 5 days out: the GFS (US model), the GEM (Canadian model) and the ECMWF (European model).

Here's the GFS forecast for Monday night:

GFS forecast precipitation (green shading) and surface pressure (thin black lines) for Monday night

The GFS shows the majority of the next system sliding down east of the Rockies, which is what most of these storms do. But you can see that the GFS does indicate some snow making it into the Panhandle and extreme eastern Washington.

Now take a look at the GEM model for this same time:

GEM forecast precipitation (shaded) and surface pressure (solid lines) for Monday evening

Not identical to the GFS, but similar. The ECMWF (not shown) actually has a bit more precipitation for eastern Washington than the GEM or GFS. Earlier runs of the ECMWF were showing a more significant storm for the Inland Northwest, but the more recent model forecasts have backed off on that idea.

Unlike storms in December or January, this system will be more spring-like. What does that mean? In the heart of winter, our atmosphere is typically very stable. Snow tends to be more widespread and light. But in the spring, the surface is warmer while the upper-atmosphere is colder. This causes a more unstable atmosphere. The result is precipitation which is more showery but can be heavier. But it also can be more difficult to accumulate snow on the ground during the afternoon hours.

Temperatures on Tuesday might be rather chilly, with some locations not making it above the freezing mark. If this happens, it would be the coldest day since January 3rd.

GFS forecast high temperatures for Tuesday March 3rd

Thus, at this point it doesn't look like a widespread heavy snow event. Rather, some locations in the Panhandle and extreme eastern Washington could see a few inches of snow. It's too early to have much confidence in this scenario. In fact, it's more likely that this storm will stay to the east of our area, as they typically do.

Stay tuned and we'll update this blog in a day or two as the forecast becomes clearer.

Monday, February 23, 2015

Mountain snow at record lows? Any relief in sight?

Many across the Inland Northwest may be wondering if our mountain snow pack is at near record levels and if there is any relief in sight. We'll answer those questions in this blog.

First, let's look at precipitation in the mountains. Given the lack of snow pack in the mountains, the assumption might be we haven't had much moisture. As the image below shows, this is clearly not the case. All of the Pacific Northwest mountains have received their normal precipitation since the start of what we call the Water Year (October 1st).

So, how does our snow pack look - not good!

Clearly, a good portion of what has fallen in the mountains has been in the form of rain. This is especially true across the central and southern Washington Cascades, and Oregon. Much of this rain fell early in the season (October and November) before a snow pack had been established. There have been more mountain-rain events during the winter as well.

So, now let's answer the question, is this a record for low snow pack? Let's break it down by region, starting with Northeast Washington and North Idaho. Each dot is a SNOTEL site showing where we are at this year compared to normal.

More simply put...

red = awful snow pack

orange = not good

yellow = a little low on snow

green = doing well

As you can see, the snow pack is not good, and downright awful in spots. Now, to see if this is a record we need to look at individual sites. Let's look at Mount Spokane (Quartz Peak) and Lookout Pass.

Blue Line = highest recorded snow pack

Green Line = where we should be

Black Line = this season's trace (2014-2015)

Red Line = lowest recorded snow pack

So looking at the black line (this year's trace), both sites are close to a record low, but not quite there. It's important to note that SNOTEL data started in the late 1970s and early 1980s. Thus, the traces for "lowest" and "highest" snow pack ever only go back about 35 years at most.

How about Southeast Washington, it looks very bad. See for yourself...

Let's look in the Blue Mountains at Touchet...

Also well below normal, just above the record low value.

And finally, the Cascades, how do they look? The West Slopes are in bad shape. Mission Ridge area also in bad shape. Further north however, the mountains surrounding the Methow Valley are doing much better.

Let's see the traces from Stevens and Harts Pass

Stevens Pass isn't looking good, but is above record low value. Harts Pass in the North Cascades is actually running right where they should be for this time of year.

So, while the snow pack is low this year, most areas are not at record levels. You may be asking yourself "what's the problem?" since the mountains have received their normal precipitation, just in the form of rain instead of snow. The mountain snow pack acts like summer precipitation in the western US. By that we mean, in the West, we don't get a lot of rain in the summer. But the mountain snow pack acts like rain in the mountains, and keeps our rivers running through the dry summer/fall months. With a low snow pack, it will likely melt earlier than normal. This will likely result in low stream flows in the late summer and fall months. Whether this has an impact (e.g. irrigation, fish, etc) remains to be seen.

The Northwest snow pack typically peaks around April 1st, so we still have about a month to at least ease the bleeding. Looking ahead, are changes in store? Yes, but not in a big way. Here's the GFS precipitation forecast for Thursday night and Friday.

GFS 24-hour Precipitation forecast ending Friday morning

The light green shading represents 0.01" to 0.10" of liquid, which isn't much. The low pressure system (red L near Portland) is actually moving from north-to-south, meaning that most of the precipitation will stay west of our area. For the mountains, all of this will fall in the form of snow. In the lower elevations, the precipitation could be in the form of rain or snow. But as is typical with spring weather here, snow that falls during the afternoon/evening hours probably won't accumulate.

Looking farther into the future, we get another chance of some light precipitation early next week. Here's the same GFS forecast for next Monday and Monday night.

GFS forecast of 24 hour precipitation ending Tuesday morning, March 3rd

Again, not a strong storm by any means. Other models (Canadian, ECMWF) are even less excited about this system. So there's nothing coming in the next week that will bring much mountain snow.

How about farther into the future? The 6-10 day outlook isn't encouraging. Here's the expected temperature pattern:

Just about the entire lower-48 states will be below normal next week. So that's encouraging. But here's the precipitation outlook:

The dry weather continues for the western US. That doesn't mean we won't see any more snow. The GFS does show some other potential for snow into March. But suffice to say, the weather pattern doesn't look cold and wet.

Wednesday, February 18, 2015

Rare and Beautiful Clouds Over the Inland Northwest

If you live in Eastern Washington or Northern Idaho and looked up to the skies the morning of February 18, 2015 between 6:30 and 8am, you may have observed a rare and spectacular phenomenon! This morning, many folks (including us at the NWS Spokane office) couldn't help but be in awe of these amazing clouds.

Photo courtesy of Jon Fox

But what are they and why do they form?

This phenomenon is often referred to as fallstreaks or cloud holes. Much research has been done on the subject dating back to the late 1950s. Since this phenomenon is relatively rare, it's easy to conclude that the atmospheric conditions in which it forms are also very specific. Research has shown that the holes or fallstreaks mostly commonly form in altocumulus or stratocumulus clouds.

After several days of mostly clear skies over the Inland Northwest, moisture was approaching from the west bringing the region a layer of altocomulus stratiformis. As we do every morning at 3am, we launched a radiosonde that sampled the atmospheric conditions just a few hours prior to the sunrise and cloud punch holes. This image below shows that sounding.

The area on the sounding circled shows increased moisture where the cloud layer most likely formed a few hours later. The approximate height of this cloud is at about 400 mb, 7 kilometers, or 23,000 feet. According to the sounding, the temperature at that layer is approximately -30° Celsius.

On visible satellite imagery, we captured a few images as the sun came up and illuminated the feature from above.

So what's the explanation?? I'm going to quote one of many articles and papers that have been written on the subject. This excerpt comes from an article that appeared in Weather - December 2008, Vol. 63, No.12 by David E. Pedgely.

"These circular (or sometimes cylindrical) holes are formed when aircraft penetrate supercooled liquid cloud layers such as altocumulus. The passage of the aircraft is believed to produce large numbers of ice crystals, which then grow quickly by the Bergeron-Findeison mechanism in the vapor-rich conditions (water-saturated air is supersaturated relative to ice). This growth depletes the cloud of water vapor, causing the liquid droplets to evaporate and the formation of a hole in the liquid cloud layer with a visible trail of ice crystals falling out beneath."

"It seems that their most likely explanation is the penetration of shallow, highly supercooled altocumulus clouds by aircraft whose wakes freeze some droplets over a radius of about 100 m. These ice particles are then spread by wake turbulence over a radius of 1 km or so in about 10 minutes within the cloud, where they grow at the expense of the droplets. The resulting crystals fall into, and continue to grow in, clear but ice-suspersaturated air, and gravity sorts them into a fallstreak, where a weak downdraft develops through drag and evaporation."

In short, an aircraft traveled directly above or through the cloud layer. It has been proven that it is not a result of anything the aircraft is emitting, instead it's the sudden and drastic decrease in pressure over the wing or propeller tip that initiates the process of depleting the cloud and causing the liquid droplets to evaporate. The result is the appearance of a hole that has been created with trailing ice crystals falling out beneath.

A quick search of aircraft routes this morning show how many flights follow a similar path departing out of Seattle and head east-southeast near the origin of the fallstreaks.

Many folks also noticed a sun dog (or parhelion) as shown in the picture below to the right of the sun. This also has to do with atmospheric optics. The plate-like hexagonal ice crystals falling through the hole refract the light from the sun. This halo is fairly common with cirrostratus clouds present.

While the cloud hole punch or fallstreak phonemena are rare because of the very specific atmospheric conditions necessary, there are numerous examples available online from around the world. Many of the meteorologists (including myself) here at the National Weather Service have never seen this in person so we certainly understand the curiosity that this ignited.

The combination of cloud physics and optics can result in stunning scenery. Here are a few more photos from NWS employees.