Being an automated station, I'm wondering if these numbers are accurate, but we have seen some impressive precipitation rates at this location in the past.
Saturday, December 18, 2010
Timpanogos update
The Sundance-Mid Mountain (SNM) MesoWest site is reporting a remarkable 24-h snow depth increase of about 15 inches with 3.56 inches of snow-water equivalent, a positively Cascadian snow density.
Being an automated station, I'm wondering if these numbers are accurate, but we have seen some impressive precipitation rates at this location in the past.
Being an automated station, I'm wondering if these numbers are accurate, but we have seen some impressive precipitation rates at this location in the past.
Mount Timpanogos lights up
Thusfar this year, the Cottonwoods have really cleaned up during most storms. This morning, however, Mount Timpangos is getting the action. Check out the latest radar loop, which shows an area of high radar reflectivity sitting over the Timpanogos ridge and environs.
A quick look at the morning sounding from KSLC shows a shallow cold pool below 775 mb. Above this layer, however, the flow is southwesterly to about 600 mb, which results in maximum upslope flow on the west face of Mount Timpanogos.
Of course, the question that remains is why is Timpanogos lighting up compared to areas favored in southwesterly flow, such as Ben Lomond? Something to ponder.
A quick look at the morning sounding from KSLC shows a shallow cold pool below 775 mb. Above this layer, however, the flow is southwesterly to about 600 mb, which results in maximum upslope flow on the west face of Mount Timpanogos.
Of course, the question that remains is why is Timpanogos lighting up compared to areas favored in southwesterly flow, such as Ben Lomond? Something to ponder.
Friday, December 17, 2010
Change is coming
After a couple of days of sublime snow conditions and ideal weather, major changes are now underway that will bring much warmer and wetter weather to the Wasatch.
A narrow filament of moisture, known as an atmospheric river, is being pulled from the subtropics and into California over the next couple of days, with the leftovers after passage over the Sierra Nevada and other western ranges penetrating into Utah. The evolution of this atmospheric river is well captured by the latest SSMI/AMSRE-derived total precipitable water imagery from the Space Science and Engineering Center at the University of Wisconsin-Madison.
In this loop, subtropical moisture is pulled northward just west of Hawaii where there is strong southerly and southeasterly flow between the subtropical high centered east of Hawaii and a subtropical low near the dateline (Long = -180). This moisture curves anticyclonically (clockwise) around the subtropical high and then spreads eastward toward California as the westerly flow is enhanced by the digging cyclone over the North Pacific.
A lot of large-scale weather features are coming together to bring you this weekend's weather. For example, without the extraction of moisture from the tropics west of Hawaii, the atmospheric river would likely be much less potent. It does pay to examine the entire Pacific Basin when forecasting for Utah.
Update 1:35 PM Dec 17
The subtropical low west of Hawaii is an example of a Kona Low. See Simpson (1952), Morrison and Businger (2001), and Otkin and Martin (2004).
A narrow filament of moisture, known as an atmospheric river, is being pulled from the subtropics and into California over the next couple of days, with the leftovers after passage over the Sierra Nevada and other western ranges penetrating into Utah. The evolution of this atmospheric river is well captured by the latest SSMI/AMSRE-derived total precipitable water imagery from the Space Science and Engineering Center at the University of Wisconsin-Madison.
In this loop, subtropical moisture is pulled northward just west of Hawaii where there is strong southerly and southeasterly flow between the subtropical high centered east of Hawaii and a subtropical low near the dateline (Long = -180). This moisture curves anticyclonically (clockwise) around the subtropical high and then spreads eastward toward California as the westerly flow is enhanced by the digging cyclone over the North Pacific.
A lot of large-scale weather features are coming together to bring you this weekend's weather. For example, without the extraction of moisture from the tropics west of Hawaii, the atmospheric river would likely be much less potent. It does pay to examine the entire Pacific Basin when forecasting for Utah.
Update 1:35 PM Dec 17
The subtropical low west of Hawaii is an example of a Kona Low. See Simpson (1952), Morrison and Businger (2001), and Otkin and Martin (2004).
Thursday, December 16, 2010
Mountain cold pockets
There are a few days a year when the air is so clear, dry, and still that localized pockets of cold air can remain entrenched in the Wasatch anywhere the sun doesn't shine. Today was such a day.
Just have a look at some of the MesoWest observations this afternoon. At Alta, it's 20F at the top of the Collins chair, but a frigid 8F in Collins Gulch and 12F at the base.
The conditions are ideal for these cold pockets to persist through the day. We had a calm, clear, dry night last night, so strong cold pools developed. We are near the winter solstice, so the sun angle is very low. Today the winds were light and the dewpoint very low, which enables the snow surface to remain very cold anywhere it is shady. Without much of a breeze to stir things up, we're just not going to mix out these cold pockets.
Just to highlight how dry it is, check out the dewpoints on Mount Baldy. They dropped below 0F at around 10 PM last night, and were near or below -20F after midnight.
Mountain weather observers report limited surface hoar formation last night, despite the clear cold conditions, and perhaps these very dry dewpoints offer a partial explanation. There simply wasn't much vapor in the atmosphere for deposition onto the snowpack.
Just have a look at some of the MesoWest observations this afternoon. At Alta, it's 20F at the top of the Collins chair, but a frigid 8F in Collins Gulch and 12F at the base.
The conditions are ideal for these cold pockets to persist through the day. We had a calm, clear, dry night last night, so strong cold pools developed. We are near the winter solstice, so the sun angle is very low. Today the winds were light and the dewpoint very low, which enables the snow surface to remain very cold anywhere it is shady. Without much of a breeze to stir things up, we're just not going to mix out these cold pockets.
Just to highlight how dry it is, check out the dewpoints on Mount Baldy. They dropped below 0F at around 10 PM last night, and were near or below -20F after midnight.
Mountain weather observers report limited surface hoar formation last night, despite the clear cold conditions, and perhaps these very dry dewpoints offer a partial explanation. There simply wasn't much vapor in the atmosphere for deposition onto the snowpack.
Wednesday, December 15, 2010
Subtropics to pay a visit
A remarkable week is on tap as a series of storms will pummel the western United States and bring the warmth and wet to Utah. Winter should persist until Friday, when the first surge of warm air and moisture moves into Utah late in the day.
We will have it easy compared to California, which will be seeing heavy rain associated with two atmospheric rivers that are expected to impinge upon the Sierra Nevada the next few days. An atmospheric river is a narrow plume of moisture that is typically connected to the subtropics or tropics and extends into the mid latitudes. The GFS precipitable water and sea level pressure forecast for 1800 UTC 18 Dec (Saturday) produces a marvelous atmospheric river (black arrow) that extends from a region of subtropical moisture just north of Hawaii into California.
The atmospheric rivers are especially dramatic if you examine the GFS forecast loop. You can see how they are extracted from the reservoir of moisture over the tropics and subtropics.
What I find remarkable about these rivers is how much moisture they lose over the Sierra Nevada. Note how the PW decreases markedly across that barrier, which reflects water vapor loss to precipitation. If we could just remove the Sierra, Utah would be a much wetter place!
We will have it easy compared to California, which will be seeing heavy rain associated with two atmospheric rivers that are expected to impinge upon the Sierra Nevada the next few days. An atmospheric river is a narrow plume of moisture that is typically connected to the subtropics or tropics and extends into the mid latitudes. The GFS precipitable water and sea level pressure forecast for 1800 UTC 18 Dec (Saturday) produces a marvelous atmospheric river (black arrow) that extends from a region of subtropical moisture just north of Hawaii into California.
 |
| 72-h GFS forecast of precipitable water (color fill) and sea level pressure valid 1800 UTC 18 Dec. |
What I find remarkable about these rivers is how much moisture they lose over the Sierra Nevada. Note how the PW decreases markedly across that barrier, which reflects water vapor loss to precipitation. If we could just remove the Sierra, Utah would be a much wetter place!
Goldilocks doesn't quite deliver
Only 7-8 inches last night in the Cottonwoods. Not quite enough for a true Goldilocks day. It will, however, freshen things up even if the porridge is a little too cold.
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