Potential for Downslope Windstorm

We are still a few days from Friday, but current models from the NAM suggest a cut-off low will slide south of Utah, causing easterly winds Friday into Saturday. This is a recipe for a downslope windstorm event.

Downslope windstorms are caused when stable air is forced over a mountain range. The air then descends rapidly down the lee side. We often observe winds powerful enough to knock over trees and power lines, transport trampolines, and get rid of your kid's plastic swimming pool you wish you could throw away (or maybe you will get your neighbor's plastic pool from down the road). A strong downslope windstorm occurred on December 1, 2011--an event Davis County residents remember all too well.

This map from the Storm Prediction Center shows the probability of strong wind events in the United States. The highest probabilities are in the mid-west where tornadoes and severe thunderstorms are common, but there is a highlighted area over northern Utah related to these downslope wind events.

More on downslope windstorms:

• Glossary of Meteorology
• Wasatch Weather Weenies
• KSL: Windstorm Dec 1, 2011

Forecasting Saves Lives--If You Listen

An interesting news article about the tornadoes in the mid-west earlier this week. Thanks to accurate forecasts, many lives were saved. But it wasn't the forecast itself that saved lives, it was people who decided to seek shelter when they heard the warnings.

Forecasts, warnings spared lives from tornadoes

http://www.ksl.com/?sid=27107156&nid=157&title=forecasts-warnings-spared-lives-from-tornadoes&fm=home_page&s_cid=queue-9

Path of a EF-4 tornado through Washington, Illinois (photo Charles Rex Arbogast).

Storm reports from the National Weather Service Storm Prediction Center indicates 85 tornado reports and hundreds of other damaging weather reports. Click Here for more info. (Note: these reports are preliminary)

Snow for some

Several days ago, skier's got excited about the "potential" for lots of snow in the mountains. As weather models recalculated the weather, this excitement dried up a bit. There will still be a winter storm, but the snow will mostly stay in the mountains in Northern Utah. Temperatures at upper levels look fairly warm (-8 C at 700 mb) which is a little too warm for it to snow in the valley's.

Below is an ensemble of forecasts for Saturday afternoon (the time I'll be at a football game). The dark red indicates 90% of the models agree there will be over a 0.05 inches of precipitation. Again, this storm looks like it will mostly impact the mountains because of orographic effects. Since the models don't really resolve terrain well, this thin band of precipitation probably should look a little thinner.

From this we will still expect some precipitation during mid-day and afternoon tomorrow. But what kind of precipitation. The left picture below shows relative humidity in the atmospheric column and winds and temperatures at 700 mb (about the top of the mountains). Temperatures are about -8 C at the top of mountains, which is cold enough for snow (because it is below freezing), but not cold enough for us to get much snow at the surface. We can expect rain and some flakes. The right panel shows 6-hour precipitation amounts from the NAM model. 

In Spanish Fork we could see a little snow, but I think it will be mostly rain. Usually it needs to be at least -12 C to get snow in the valley. I like living on the bench in Spanish Fork because it seems winter storms from the northwest get pushed into our little corner of mountains and channeled up the canyon. I like to believe this gives us more snow, but I'll try to keep an eye on it this winter to see if that is true.

The Eye of Haiyan

Below is a sequence of visible satellite images from Super Typhoon Haiyan.
External link here.

First Snow!

 The first snow flakes of the season arrived in Spanish Fork on November 3 at about 9:30 AM.

The flakes are a conglomerate of broken dendrites (your classic looking flakes with six arms).

Below are IDV radar images. I tried downloading a loop, but my computer is too old and IDV crashes all too often.

(IDV radar image with 9-point smoothing)

Before the snow started the air temperature was 38 F. As the snow started the air temperature dropped to 32 F. As soon as the band of snow passed the snow melted quickly, but air temperatures stayed cool and we didn't get any warmer than 35 F. I imagine this was because we had some cold air advected behind the front, but I wonder how much of the temperature rise was depressed due to the take up of latent heat by the melting snow. Below is my brother and friend. You can see our mailbox in the bottom left. Someone smashed it with a pumpkin last night.