Weather Knowledge - 56
Why do Atmospheric Rivers Occur?
A significant atmospheric river event will occur tomorrow (Sunday) and Monday, with heavy precipitation in the mountains and South Sound.
The latest UW WRF model forecast predicts up to 10 inches in the Cascades, with the southern WA Cascades being hit the hardest (see below). In contrast, a profound rain shadow will extend from the NE Olympics to Bellingham.
Atmospheric rivers in our region are associated with plumes of warm, moist air from the south, southwest, and west.
NOAA weather satellites can sense the water vapor, which is very evident in the water vapor image from this morning (below). A massive plume of water vapor is moving our way.
Numerical weather prediction models, like the UW WRF model, can skillfully forecast such water vapor plumes, which release massive amounts of water as the air is forced to rise by our region's substantial terrain.
Meteorologists favorite diagnostic for atmospheric rivers is integrated water vapor transport (IVT), which is the amount of water vapor times the wind speed, summed up in the vertical (that is where the integrated comes from). This quantity is more closely related to the potential for regional rainfall.
The prediction of IVT for Sunday morning is shown below, with the arrows showing both the direction and magnitude of the moisture transport Large values, but not record-breaking by any means.
Such moisture plumes are generally warm as well. In fact, to get large values they HAVE to be warm since the amount of moisture air can hold depends on temperature (warm air can potentially hold more water vapor than cold air).
Atmospheric rivers form in our region when strong southwesterly atmospheric wind currents occur, currents that move large values of water vapor out of the subtropics.
The interesting thing about atmospheric rivers is that they are like regular rivers in another way...not only does water come into the river at the beginning, but there is a convergence of water vapor from the sides along the way, not unlike the small streams that drain into a river over its entire path.
Weather Knowledge - 55
Weather Knowledge - 54
'Frost Flowers' And 5 Other Interesting Phenomena Our Meteorologists Watch For In Winter
1. Frost Flowers
Frost flowers are fairly rare to see, but conditions over the Thanksgiving 2024 holiday near Ridgetop, Tennessee, allowed Morning Brief newsletter reader Leslie R. Ratliff to pass along the great example below.
They develop where there is moist, unfrozen soil during below-freezing temperatures (32 degrees or lower).
Water pulled up from the soil into a plant's stem freezes, expands and splits the stem. As more water is drawn up through the stem split, more freezing occurs, eventually forming a fragile petal of the frost flower.
These amazing formations often melt or sublimate (transition from ice to water vapor) quickly after sunrise.
2. Pancake Ice
Resembling frozen lily pads, pancake ice forms when there is some wave action on a body of water and temperatures are just below freezing.
Pancake ice can begin as a thin ice layer (known as grease ice) or slush on the water surface, which accumulates into quasi-circular disks. The "lily pad," or raised-edge appearance of pancake ice, can form when each disk bumps up against one another, or when slush splashes onto and then freezes on the slab's edge.
The floating disks of ice can be up to 10 feet in diameter and four inches thick.
3. Snow Rollers
This winter phenomenon occurs when strong winds pick up moist snow and blow it along the ground, eventually building a cylinder of snow that is often hollow in the middle. When the snow roller grows too large for the wind to propel it farther, it stops
Here are the optimal conditions for snow roller generation, according to the National Weather Service:
- Existing icy or crusty snow cover, so additional snow will not stick to it.
- Additional wet, loose snow on top of the icy, crusty snow cover.
- Wind strong enough to scoop out balls of snow and propel snow roller forward.
- At least some slope to the ground.
4. Hoarfrost
Frost is not an uncommon sight during the cold months of the year, but hoarfrost is in a league of its own.
One or more days in a row of freezing fog (fog with air temperatures of 32 degrees or colder) is a perfect scenario for the formation of hoarfrost. This is because there is more moisture in the air compared to a day that might produce a typical frost.
With the extra moisture in the air, the interlocking crystal patterns of frost become more intricate and much larger, building up to a greater depth on tree branches, signs and fences, as seen below.
5. Boiling Water Freezing In The Air
An experiment we see sometimes during frigid arctic air outbreaks is tossing boiling water into the air.
Once airborne, the boiling water changes state into steam/ice as it enters the cold, dry air mass, which is based on the Mpemba effect.
It works best when the air is very dry in subzero temperatures.
6. Thundersnow
For many meteorologists and weather enthusiasts, hearing the crack of thunder during a snowstorm triggers excitement.
When thundersnow occurs, snowfall rates tend to be very heavy, many times reaching an inch or more per hour.
A pocket of unstable air aloft is needed to generate this weather phenomenon.
Typically, these pockets of instability develop as upward bumps on top of the flat layer of snow clouds already in place. This development occurs in response to strong lifting from an upper-level disturbance in the atmosphere.
If temperatures are between 14 degrees and -4 degrees Fahrenheit inside the upward bump where the elevated pocket of unstable air resides, snowflakes and small hailstones can form. As the snowflakes and hail collide, electrical charges will build up, making lightning and thunder possible.
Most often, the lightning flash associated with thundersnow illuminates the cloud cover with no distinct bolt visible. However, on rare occasions we see cloud-to-ground strikes like this one from Idaho in March 2024.
Chris Dolce has been a senior meteorologist with weather.com for over 10 years after beginning his career with The Weather Channel in the early 2000s.
Weather Knowledge -53
3rd September 2024-
Relationship between CO2 and Global Temperature:
