Saturday, September 21, 2013

Signs of an Approaching Cold Front

There are many signs of an approaching cold front. The first sign is a happy meteorologist. This sign is especially noticeable in early fall when the first front of the season pushes by.

In the last 24 hours we've seen quite a drop in pressure in Spanish Fork. Decreased pressure is another sign some kind of weather pattern change.

We've also seen a consistent and strong southwest flow. It'll be breezy the rest of the day with winds picking up as the front gets closer.

You might not expect it, but another sign of a cold front is rising temperatures. Even this morning we've see temperatures rise quickly. I expect we'll be in the mid 80's by the afternoon.

Now we can look at a weather model and see when the cold front is expected to come through Utah and Spanish Fork. The below figure is the GFS (Global Forecast System) model run at 12z. The forecast shown is for Sunday at 18z (12:00 MT). This model shows the front will pass through Utah between midnight and 3:00 Sunday morning. There is a lot of information here, so I'll only point out some of the most interesting parts.

The upper-left figure shows the height of the dynamic tropopause. All that means is the blue color indicates low heights and orange colors indicate high heights. The blue, low heights indicates a trough or approaching cold front.
The bottom-right figure shows 6 hour precipitation. The model doesn't show much rain will accumulate in northern Utah, but the mountains have a good change. If it's cold enough, the mountain tops may see some snowflakes. Yep, winter is on it's way again. It seems like it comes around every year.

So enjoy this warm September day, but keep you jacket in a place you can find it. You'll need it tomorrow morning.

Thursday, September 12, 2013

Crazy Monsoon Flow

The last few weeks have been pretty active. Lots of cloud development in the afternoon through evening. All of this exciting weather comes from our south and east. An upper level trough now moving up the west side of Utah has pulled a lot of the moisture from the south. With breaks in the clouds we get enough solar heating to cause some big thunderstorms. Southern Utah has had quite a bit of rain and flooding. Colorado was hit hard, as is shown later.

These are satellite images of the western states on September 12, in around 5:00 MT. These images come from the GOES-West satellite. Not all satellite images show the same things, so let me explain the difference between the two images.

This image is a Visible image. If you were an astronaut and looked down at earth, this is what you would see (except it would be in color). Notice the two areas I have circled. The blue region off the coast of California consists of widespread stratiform clouds. These are typical off the coast. The red area shows more patchy clouds. These are cumulus and cumulonimbus clouds, also known as thunderstorms. They are produced by convection, or rising motion, when the sun heats the ground.

This image is from approximately the same time, but has a little different features. This is an Infrared or IR image. Instead of looking at the visible light the instrument detects infrared radiation. Essentially it measures temperature. Dark parts of the map are warm and lighter areas are cold. Looking at a cloud free area (as indicated from the visible image) such as Southern Arizona we see a darker area, so we know the temperature is higher. No look at an area with clouds. Since the satellite can't see through the clouds, the white blotches are the cloud tops, or the temperature of the cloud top. The patches of clouds over the states are very cold. Take a look again at the blue circled off the coast of California. The clouds aren't as cold. That is because the clouds tops are lower than the tall cloud tops on the thunderstorms. Thus, this image can tell us a little more about the clouds. We can see where thunderstorms cells. But be careful! Cirris clouds are high level, cold clouds that appear white, but are not thunderstorms. Also notice the comma shape of the clouds. A low pressure is centered near the southwest boarder of Utah and winds are blowing counter-clockwise. 

This was taken from the University of Utah campus on September 11, 2013 about 5:30 PM

This is a pretty anvil topped cumulonimbus cloud. Picture taken from Provo looking north on September 12, 2013 at 7:00 PM.

The most extreme weather was dumped on our neighbors in Colorado. Some places reported up to 10 inches of rain in 24 hours! (Needless to say, there widespread flooding.) Below is the image is provided by the Advanced Hydrologic Prediction Service of the National Weather Service.

This is from MesoWest showing reported 24 hour precipitation totals for the same period.


Here are a few blog posts about this event:
The CoCoRaHS blog shows pictures of hail in Denver.
The Wasatch Weather Weenies post talks about how Utah was affected by the storm.
The Wasatch Weather Weenies also talk about the Colorado side of the storm.
Map Overlay

Tuesday, September 10, 2013

East Wind, Rain Shadow

Below is the 700 mb pressure surface (about as high as the mountains). We have a cut-off low pressure centered near near the Utah-Arizona boarder. The green indicates the relative humidity is pretty high, above 80%. This can be associated with clouds and rain. Notice the winds in Utah bringing the moisture are blowing from the east. The mountains will cause some topographic lifting, cause some rain on the east side of the mountains, and the west side--Salt Lake and Utah counties--will be stuck in a rain shadow. The west side is expecting to get less rain than the east side, all because the wind is blowing from the east. Let me explain...

When air flows up a mountain-side it cools. Clouds are created. When air flows down the backside of the mountain the air compresses and warms, dissipating the clouds. Kind of a cool concept. Below is a picture I took today of what I thought illustrated the rain shadow effect. This is looking south towards Mt. Loafer. The winds were blowing from the east, from the left side of the picture to the right (as the model suggested). Air moving up the mountain cooled, allowing clouds to form up to the crest. On the west side of the mountain air moved down the mountain. The air compressed and was warmed. There where no clouds on the west side of the mountain.
(c) bkb
This same principle is the same reason why Seattle is generally wet and Spokane is generally dry. Weather patterns tend to blow from the west to the east in the Northwest. The rain shadow in this case is on the east side of the mountain while the wet side (the windward side) is on the west side of the Cascade range.

Sunday, September 8, 2013

Gust Front

I was walking with some friends to the BYU vs Texa football game when my friend pointed to the sky and asked, "What is that?" I looked and responded calmly, "That is a thundercloud." Seconds later we were blown over by a strong gust front. If you haven't noticed yet by the color of the sky, a gust front is usually a good indicator that a big storm is headed your way. Below is a picture of a typical mature thunderstorm. The gust front is located at the front of the approaching storm.
Everyone walking to Lavell Edwards Stadium for the BYU vs Texas football game experienced this very strong wind. Leaves were blowing off trees, we were being hit by pine cones, and I even saw the roofing shingles of an apartment building blow off the top in one piece. What a mess that made. Looking at the vector wind data from the weather station on BYU's campus, can you pick out the time the leading edge of the gust front passed through Provo? Yep, it was around 4:00 PM (16:00).

Here is a view of the storm from the stadium. A friend saving us seat took this picture right as the gust front was blowing.

Picture by A. Reed

As the gust front blew I mentioned to my friend, "Oh, and it'll start raining really hard in the next five minutes." Sure enough, the rain down poured. We ran to the nearest building, a Jamba Juice, for shelter. The rain rained for a long time.   Anxious to get to the football game, the Jamba Juice workers kindly gave us some trash bags. We cut face holes in the bottom and wore them like wedding veils.

Making out way towards the stadium it seemed the game was already over. Everyone dripping wet, and walking back to their cars and apartments. The game was delayed for lightning.

Provo measured 0.76 inches of rain during this period. Doesn't sound like a ton, but considering the short time it caused some minor flooding. The storm drains couldn't handle that much water in that short a period.

Eventually we the rain stopped and people filled the stadium. BYU played very well and pulled out with a win against the Texas Longhorns: 40-21.
Below is the ESPN time-lapse video of rainstorm that caused a two-hour rain delay.

Sunday, September 1, 2013

Playground Fire

Last month I had another chance to visit and do maintenance on a weather station maintained by the University of Utah. This station is located in an area known as The Devils Playground in Box Elder County. A month earlier, in July, lightning started a wildfire that scorched over 2,500 acres of junipers, pinion pine, and sagebrush.

The slideshow of photos shows what the fire left behind--lots of black trees and ash. One of the pictures shows green bushes saved by the red fire retardant dropped by an airplane.

Our weather station was caught in the fire and suffered the same fate as these trees. It was left black and in ashes. As expected, nothing worked and we had to replace all the instruments. We didn't, however, expect to find the instruments totally melted away.

An old RM Young wind monitor
The Temperature and Relative Humidity Sensor (and shield)

Ground Pyronometer (and melted wires)
Precipitation gauge, untouched by the fire, but the intense
heat melted the tipping mechanism.

                         After the fire                                                           With new instruments

Weather data from this station can be found on the Mesowest website.

On the way home, we passed a grass fire on the side of the freeway. It was sure putting out a lot of smoke.