Wednesday, February 26, 2014

Sunsets and Storms

Much to talk about today.  First, have you seen the sunsets the past couple of days?  Unbelievable.  The photo below, taken Monday doesn't do them justice, but does capture a nice sun pillar if you look close.

The presence of altostratus clouds (upper right of photo above) and lenticular (mountain wave) clouds has certainly helped paint the scene, but I'm wondering if we might have some aerosols, possibly Asian dust or pollution, moving over us in the upper atmosphere to give us a bit more orange tint than usual.  Perhaps someone can dig in and have a look as I have other important things to keep me busy today.


After a sunny day today, things are looking pretty active for Thursday through Saturday and possibly Sunday as two major troughs embedded in the southwesterly to westerly large-scale flow rumble through the state.  There is a lot going on, so I'll summarize quickly with two graphics from the 1200 UTC NAM.  The first is the total precipitation (snow water equivalent) produced by the NAM through Saturday afternoon showing more than 5 inches over portions of southern California, more than 1.5 inches in portions of southwest Utah, and up to 2 inches in the northern mountains of Utah.
Source: NCEP
Timing and intensity will vary geographically and there's still some uncertainty with regards to the specifics.  For Alta and the upper Cottonwoods the 1200 UTC NAM generates precipitation in two pulses on Thursday and Thursday evening, and then again late Friday and Saturday, yielding a total through 5 PM Saturday of nearly 2.5 inches of water.  Being that these are generally warm storms (there are some fluctuations in temperature as each goes through), our snow algorithm is converting that to about 25 inches of snow with a mean water content of about 10%.

The first storm might be a bigger producer in the northern Wasatch and Timpanogos area given the prevailing southwesterly flow.  Uncertainty in the track of the second storm, and it's interactions with another system digging into the Pacific Northwest is such that I'm not going to speculate on the details of that event.  Through Saturday, the events look to be rain producers in the valley, with snow tickling the benches at times.

Atmospheric Rivers

Both storms contain atmospheric rivers, filaments of strong moisture flux with a subtropical tap.  In contrast to the previous atmospheric river events we've seen the past few weeks, the atmospheric rivers in these events interact with the high Sierra.  During the first event, for example, low-level flow deflection upstream of the Sierra combined with the depletion of water vapor by heavy precipitation over the Sierra and upstream ranges of southern California, means that the strong atmospheric river conditions do not penetrate directly into northern Utah.  You can see this to some degree in the image below.
Source: NWS
No worries, the remnants in this case should suffice.  Note, however, that as the atmospheric river slides southeastward, it eventually moves past the high Sierra and a nice filament of moisture is able to penetrate with less topographic modification into southwest Utah.

Whatever the northern Wasatch get out of this storm, it would have been more if it wasn't for the high Sierra.

Explosive Cyclogenesis

The formation of a cyclone, a large-scale area of low pressure, is known as cyclogenesis.  Explosive cyclogenesis is the rapid formation of a cyclone, generally defined as having a deepening rate of either 24 mb in 24 hours or 18 mb in 12 hours at 60ºN latitude.  Because it is more difficult for a cyclone to deepen at lower latitudes, equivalent deepening rates at 35ºN are 16 mb in 24 hours and 10 mb in 12 hours.

The second in this series of storms produces an explosively deepening cyclone off the coast of California that is forecast by the NAM to have a central pressure of about 975 mb at 0300 UTC Friday (2000 MST Thursday).

In the NAM, the cyclone deepens about 18 mb in 12 hours, which is fairly impressive for this part of the world.  It won't be a good time to be sailing from California to Hawaii, and, although the storm weakens some, I suspect it will produce some strong winds on the coast as it makes landfall.

1 comment:

  1. Dr. Jim! This post was loaded! This is why I read your blog:
    atmospheric rivers,
    low-level flow deflections,
    topographic modifications,
    explosive cyclogenesis, the list just goes on and on…