Coming Break in Poor Air Quality

University of Utah, William Browning Building webcam

Have you noticed the smog in Salt Lake county this Thanksgiving weekend? If you're familiar with winter time weather in Utah, you know the inversion is the culprit. It's pretty dirty, and the chart below proves it. In the last two days, one hour PM 2.5 concentrations have reached the "red" air quality category. (In case you didn't know, PM 2.5 is not good to breath. PM 2.5 is made of tiny particles, smaller than 2.5 micrometers, and get in your lungs causing respiratory problems.)

Salt Lake City

November 25-30, 2013

airquality.utah.gov

What we need is a big storm. Storms help improve air quality because they bring a new air mass with clean air (although, the air is usually cold becomes it comes from our north). Also, storms are generally windy and can mix out the inversion. Pollution can then be mixed out and diluted vertically.

Fortunately, we have a storm on it's way. Yea!! The November 30, 00z GFS model run shows a strong cold front will enter Utah Tuesday morning. Of course, a good snow storm will accompany this cold front. More good news for skiers!

GFS Forecast for Tuesday December 3, 6:00 pm

weather.utah.edu

American Geophysical Union

Next week is the AGU conference in San Francisco. At the conference I will present a poster on my work this summer with NASA's Student Airborne Research Program.

Click the image below to see my poster

You can read more about the work I did this summer here:

Final presentation
Student Airborne Research Program

It's Still Blowing!

It's still blowing outside. This wind event is pretty persistent. A snapshot from the MesoWest map interface shows how persistent this windstorm has been. Over the last 24 hours, the max gust has been 67 mph at the UDOT portable weather station. This location in Farmington has continually measured the strongest winds of this windstorm.

MesoWest.Utah.edu

It was a bit windy around noon in Spanish Fork. Winds were generally from the east, suggesting this is part of the downslope wind. However, the strength of these winds was not nearly the same as the winds north of Salt Lake. Below shows data from my weather station. It's hard to see an east wind from my station because it is blocked by a house. Still, we peaked at a gust of 19 mph with moderate sustained winds that aren't too unusual.

MesoWest.Utah.edu

Another CWOP station not far from mine isn't blocked by a house on the east side, so it measured some higher gusts, shown below. Again, the east, northeast wind was persistent most of the day, but stronger gusts were measured.

MesoWest.Utah.edu

Seeing more indicators of the downslope windstorm in Spanish Fork suggests the low pressure to our south is moving. (By the way, that low pressure to our south is causing some weather related problems for New Mexico--snow! See the news store here.)

I wish I had my camera, because I saw a beautiful example of what a rain shadow looks like from the lee side of a mountain. Driving towards Provo, I could see Mount Timpanogos. The wind was blowing from the east. There were lots of clouds on the backside of the mountain. Just above the mountain crest was a thick lenticular shaped cloud. As the winds blew over the mountain and spilled over the the west side, the clouds dissipated. It was beautiful!  I did find some pictures of Mt. Timp. from the BYU webcam. This isn't exactly what I saw, but here are two pictures that sandwiched the time I was looking at the mountain this afternoon. The first image was taken November 23, 2013 at 12:09 pm, and the second was taken at 1:49 pm on the same day.

Wind Chasers

Just as the forecast models from a few days ago suggested, a downslope windstorm occurred along the Wasatch Front between Salt Lake City and Ogden since Thursday afternoon. The National Weather Service extended the high wind warning until Saturday afternoon. 

Thursday night on the Bountiful bench, a citizen weather station reported gusts over 60 mph! Below is a time series plot of the winds for 9:00 pm Thursday to 9:00 am Friday morning. The orange dots indicate wind direction. For most of the time shown below, the wind is from the east and turns south, southeast in the early morning. The solid red lines indicates wind speed, usually averaged over 10 minutes. The dashed green line indicates wind gusts, which are those relatively short lived bursts of air that almost knock you over if it catches you off guard.

Other stations in Centerville and Farmington recorded winds speeds up to 80 mph! These winds are strong enough to cause quite a big of damage. Driving through Bountiful Friday morning we saw tipped over trash cans and fallen tree limbs, but nothing too big had fallen. However, the news had plenty of pictures showing much more damage. Thursday night, the Utah Department of Transportation closed the freeways to large semi trucks to prevent any tip overs or accidents.

This Friday Morning I participated in a some weather balloon launches. Students and staff at the University of Utah launched two weather balloons from the Bountiful bench to measure the wind and temperature profile of the atmosphere during this wind event. We set up our weather observation site on the mountain side just under the Bountiful "B".  It was sure windy up there. My friend Alex measured the wind gust at about 45 mph.

The instrument attached to a weather balloon is called a radiosonde. Below shows me holding one of the radiosondes before attaching it to the balloon. It looks like a small styrofoam cup with a wire sticking out because it that is just what it is. Inside is a small circuit board with components on it to make measurements. On the cup is a little note that reads, "If found, please call and return to the University of Utah" and a phone number and address are given.

Before attaching the radiosonde to the balloon you have to establish a GPS and radio connection with a receiver. We connect the receiver to a computer in the truck so we can receive and record the measurements the radiosonde makes when it rides through the atmosphere on a balloon. It takes a few minutes for the radiosonde to make a GPS connection, but when I heard the chirping sound, I was done walking aimlessly around the parking lot holding the. It was time to tie it to a balloon.

The hard part about filling weather balloons in the wind is, well, filling them in the wind. They want to flop and fly all over the place before we're ready to let them go. Using a big, blue tarp, we cleverly held the balloon in place as we filled the balloon with helium. Fill the balloon with helium, tie a good knot, and tie the radiosonde to the bottom by a string.

Quick countdown...5....4...3...2...1...and let her go!

Then you watch it float away until you can't see it anymore. If everything goes well, like it did for both these balloon launches, you can watch the data come into the computer. 

By radio, the radiosonde sends back measurements of pressure, temperature, and humidity. A GPS tracks its position, thus measuring wind speed. When all the data comes in, it looks like this...

(An explanation of would be rather long, so I won't go into it here. Just know that the solid black squiggly line on the right represents temperature, and the black squiggly on the left represents dew point temperature. If you so desire, more info on how to read a skew-T chart can be found here. Or stay tuned for a future blog post on skew-T/Log-P charts. For info on how I made this chart, refer to my article here.)

This video shows our adventures on the Bountiful Bench.

Well this downslope windstorm was exciting, but we don't tend to see them in Spanish Fork (location of my personal weather station). Why do areas north of Salt Lake have extreme wind events and not Utah. This questions hasn't been extensively studied, but the orientation of mountains is likely to have a most of the answers.

Downslope windstorms form when we have strong easterly winds blowing aloft. Easterly winds are can be associated with a strong cut-off low centered to the south and west of Utah, as this case shown in the weather analysis below. Winds blow around a low pressure counter-clockwise (shown by the yellow wind barbs).

These winds force cold, stable air over the mountain barrier which then plummets down the lee side of the mountain. This puts the Wasatch Front in a rain shadow, depressing clouds and precipitation.

But what is the difference between north of Salt Lake City and south of Salt Lake City? Consider the mountains upstream.

Upstream flow from Spanish Fork is intercepted by the Uintah Mountain range. In contrast, upstream of Bountiful and Farmington is much less rigid Wyoming. So, Spanish Fork residents, we may have the Uintah Mountains to thank for protecting us from these strong downslope windstorm events.

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.