Wednesday, November 13, 2013

So what comprises an accurate snowfall forecast?

So what comprises an accurate snowfall forecast?

Confidence is growing that portions of the Inland Northwest are going to see a moderate snow event beginning Friday and ending sometime on Saturday. Temperatures will assuredly be sufficiently cold over the mountains and should likely be cool enough over some of the valleys. The weather pattern looks very consistent between model solutions. Looking at the spaghetti charts below, there is good consistency between different model perturbations with all runs showing a deep trough located over the northwest US by Friday afternoon. Recall, the closer the lines are to one another, the better the model agreement and thus the confidence in a model solution.  For a more thorough explanation of ensemble forecasting and spaghetti charts, please refer to our posts about a persistent ridge in October.

Spaghetti charts for Friday afternoon
If this pattern materializes as shown, there will likely be a good precipitation event focused over parts of the region. The question is where will the greatest threat occur.  Below are the latest forecast chances for receiving 4" and 8" or more of snow between 4am Friday and 4am Saturday. 

4" or greater snowfall chances 4am Friday 11/15- 4am Saturday 11/16

8" or greater snowfall chances 4am Friday- 4am Saturday
We are confident that the mid-level wind direction will be out of the west to southwest, which should focus the heaviest precipitation amounts near the Cascade Crest and over the Idaho Panhandle due to orographic ascent (think of the wind forcing the moisture over the mountains). There is also little doubt that temperatures over the mountains will cold enough to support snow.This confidence can be can be expressed utilizing a probablistic forecast which displays the chance of meeting or exceeding a certain weather parameter. Typical weather forecasts you read on our webpage or see on TV do not typically show these chances. Rather we produce what's termed a deterministic forecast. 

Deterministic forecasts show a specific value (or range of values) for any given location, whether it be a temperature, wind speed, or snowfall amount. The map below is what we are deterministically  forecasting (as of 1pm Wednesday) between Friday morning and Saturday afternoon. For Spokane, the latest deterministic forecast shows values ranging from 1-3", a far cry from the probabilistic forecasts seen above. 

Snowfall totals from Friday morning-Saturday afternoon---issued 1pm Wednesday
We do produce a widely used probabilistic forecast and that refers to the chance of seeing measurable precipitation (anything .01" or greater during the 12hr period). Here's what our written forecast says for this period for Spokane. Notice through the entire period we mention a chance of precipitation of 70% or greater. Nowhere though do we really mention specific snow amounts, rather we broadly characterize snowfall accumulations. 

So why the difference between the forecasts? Much of revolves around the terrain and the impact it can have on temperatures and precipitation amounts. Valley temperatures for this event could remain at or above least through much of Friday and early Saturday across the Spokane Area, Palouse, and Lewiston area. Confidence is higher that temperatures will remain cold enough for snow over the valleys of northeast Washington and the Idaho Panhandle. Colder air will likely arrive later on Saturday. But temperatures and precipitation aren't the only factors that go into a snowfall forecast. There are issues to consider. Here's a list of parameters used for a snowfall forecast? 
  • QPF or precipitation amounts
  • Snow levels
  • ground temperatures
  • ratio of water to snow equivalent
  • time of day and expected road temperatures
So lets look at some of these parameters, like we do at our office this time of year and calculate a snowfall total for Spokane.

Precipitation forecast early Saturday morning
The image above is fairly straightforward. We are forecasting precipitation over most of the region aside from western portions of the Columbia Basin. The forecast for Spokane calls for 0.13" of precipitation with significantly heavier amounts expected to the north and east of Spokane, especially over the mountains. If temperatures were cold enough, it would all fall as snow, but we can't determine that just by looking at this parameter.  

So how about temperatures? Below is the forecast for early Saturday morning. Locations with temperatures below freezing are blue and purple. Locations above freezing are green and yellow. Notice most of the Columbia Basin is green, but locations such as Spokane show a forecast right around freezing. 

Temperatures for early Saturday morning
So can snow fall at these forecast temperatures? Absolutely, but not necessarily. There is more that factors into the equation. What are the temperatures above the ground and what would they support for snow levels?

A snow level  is the elevation at which the precipitation will fall as all snow. Our forecast for Saturday morning calls for a snow level at 2300 feet. That means locations such as the Spokane Airport, South Hill and our office would be high enough for snow, while locations such as the Spokane Valley (at 2000' and lower) are more likely to see rain (provided the snow level forecast is accurate).

Snow level forecast for early Saturday morning in feet above sea level
So if most of what falls early Saturday morning is snow over the West Plains how easily will it accumulate? That depends on what the temperatures of the surface its falling on are. The fact that its falling early in the morning will negate the impact of daytime heating on the road surfaces or the ground. But if temperatures don't cool much overnight due to significant cloud cover that notion isn't as clear cut.  Clearing skies during the evening would lead to a much better chance of accumulating snows by morning. Timing a snow event during the shoulder seasons (before the cold of winter sets in or ends) is critical. A significant snow accumulation during the afternoon hours is much less likely than one during the late night or early morning hours given near freezing temperatures.

So the only other factor to consider is what will the ratio of snow to water be. Recall, we were forecasting about 0.13" of rain in Spokane. A crude rule of thumb is take that rainfall and multiply by 10 to come up with a snowfall total (1.3"). But really its not that easy, especially with complex temperatures and complex terrain. Across eastern Washington and north Idaho the typical snow:rain ratio is 13:1 or 13" of snow to every 1" of rainfall or liquid. But that also can be misleading since much depends on the temperatures. A cold air mass yields higher ratios than a warm one and that more or less explains our snow ratio graphic below. Other factors can come into play are how strong is the lifting in the atmosphere. Strong ascent leads to higher snow ratios. An unstable air mass can also lead to very high snow ratios (like we see in the spring with heavy snow showers and huge snowflakes or dendrites). 
Snow to water ratios for early Saturday morning

So we put all those pieces together and we come up with the following snowfall forecast for early Saturday morning. Light snow is forecast over the West Plains and South Hill, but less than 1" in expected with no accumulations in the Spokane Valley.
Snowfall forecast for early Saturday morning

Obviously there is a lot that goes into making a snowfall forecast. It's unquestionably one of the most difficult facets we forecast in the Inland Northwest. Even with a perfect forecast of the precipitation amount, the resultant snowfall can vary significantly from our prognostications. To add another layer of complexity, what would happen if the moisture from the system decided to remain bottled up near the Canadian border, rather than slowly sag south toward Oregon? Would that lead to a good forecast or would it present a huge forecast bust? We always hope for a perfect forecast, but there are so many variables at play, that fallibility potential can be very high, even after the onset of the event. 

1 comment:

  1. I admit it -- I love reading about weather. It's amazing to see all the technology and knowledge that goes into what looks like a simple forecast. All the microclimates in my area (Stevens County) certainly add their own layer of complexity.

    That said, I noticed in graphics two and three that the colors in the legend don't translate into the colors in the map. I realize I can just count the "rings" to find my area, but thought it was a glitch worth noting.

    Bert in Rice, WA