Wednesday, November 20, 2013

The upcoming cold pattern

The upcoming weather pattern will be rather conducive for some very chilly nighttime temperatures. In fact, our forecast temperatures will expose the region to the coldest readings since the third week of January. So the question is what's delivering this cold air, how long will it last, and how cold can it get?

Let's first examine what got us here. On Tuesday night, the deep trough which delivered widespread rains, and high mountain snows to much of the region shifted east of the Continental Divide and was quickly replaced by cold northwest flow which originated from the Yukon.

500 mb map for late Tuesday night. Green=moisture, white=dry
Shortly after the northwest flow moved into the region, winds began to stream down the Okanogan Valley, and fill all of eastern Washington and north Idaho with very dry and cooler air. The map below depicts this intrusion of the significantly drier airmass.  The coloring represents the surface dewpoint temperatures with purples and pinks showing depicting the much drier air. By 8pm on Tuesday this much drier (and eventually cooler) air started surging through the Okanogan Valleys and moved over the entire Inland Northwest by Wednesday morning.

Time Lapse of dewpoints between Tuesday night 11/19 and Wednesday Morning 11/20. 

So why are we looking at dewpoints in this image and not temperatures? Well since the surge of drier air was generally accompanied by gusty north to northeast winds, the temperatures remained relatively warm (although it may not have felt that way). Typically when we get these polar intrusions the coldest weather doesn't arrive with the initial air mass change rather it comes the following night and beyond. So in this case, the dewpoints are telling us we will likely see a very chilly night across the region. In other parts of the county, the afternoon dewpoints are a great indicator for how low the temperatures could get overnight, provided the air mass doesn't change, winds remain light and skies stay relatively cloud-free. Around here, that trick doesn't fare quite as well, however we can still use them to foretell overnight temperatures, once again provided the same assumptions. So at least for the next couple nights we are expected to see light winds, and clear skies, so conditions are prime for some very cool temperatures. However unlike many November events featuring a polar intrusion, we lack a key ingredient for really cold temperatures: snow cover.

Below is a map of the estimated snow depth over the region as of this afternoon. Note there weren't many valleys which had snow on the ground, most of it was found over the mountains.
Estimated or modeled snow depth map as of  Wednesday afternoon 11/20
Why is the snow important? For one it helps lower the heating during the day, as a pure white snow cover will reflect much of the daytime solar heating right back into the atmosphere rather than heat up the ground. Just as important is the snow blanket actually keeps the ground from heating the air at night. So if skies are clear this ground heating process is inhibited, and the afternoon dewpoints can be easily undercut by the rapid cooling. There have been many nights here on the West Plains of Spokane where we have hit a forecast low temperature within an hour or two after sunset provided clear skies, light winds, and fresh snow cover. As of 6pm, the dewpoint was hovering right around 0°F at our office. So if there was snow cover we would easily drop into the single digits. But instead we are forecasting an overnight low in the teens.

So right now we think the coldest night of the week will be tonight/Thursday morning, but why? This airmass really isn't going anywhere. One reason is the air mass transition was really just a glancing blow. Take a look below at the upper level jet stream chart for Thursday afternoon. For us to see a prolonged cold snap, ideally we should be on the cold side of the jet. This would reinforce the surge of cold air from the north or northwest. But in this case the jet will remain to our north and east keeping the coldest air east of the Continental Divide. What that means is each day we should slowly modify or heat up this polar airmass. Without a fresh snowpack in place, that should happen quite readily, the question is how quick?

Upper level jet stream (blue & purple shading shows fast wind speeds and location of jet)
Well much depends on how quickly we can recover from tonight's lows. If temperatures get into the single digits, we might be hard pressed to climb far into the 30s during the afternoon hours. The reason is the sun angle is so low this time of year. At the sun's peak elevation (or solar noon), the sun is only about 20° above the horizon. That means much of the energy is spread out over a large area, rather than concentrated over a small area. This is analogous to shining a flashlight at a table or desk. If the flashlight is pointed straight at a surface the light will be more intense and concentrated, whereas if its pointed at an angle, its much more diffuse and dimmer. So what does this mean? It means the sun only has so much potential to heat the ground this time of year. That's one of the reasons we commonly get persistent valley inversions and fog this time of year. While temperatures near the ground are often slow to warm, conditions above the ground are more apt to change and with the jet stream remaining to our northeast, those temperatures are going to warm.

Below are maps of the 850 mb or (4500' above sea level) temperatures for Thursday and Saturday.
Thursday 850 mb temperatures 

Saturday 850 mb temperatures 
Over Spokane, the Thursday temperatures will be around -5°C and possibly warm to 5°C by Saturday. So that translates to about 18°F or warming. Will warming of that magnitude translate to the valley bottoms? Unless there is some wind to mix that warmer air down to the ground, the odds are not good given the ever lowering sun angle. Since we don't see any wind events in the near futures, what's more likely is a few degrees of warming each day and a growing possibility of fog and stratus. We also will see little if any chance of precipitation for the next week.

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