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).


  1. Its interesting to watch what happens to that subtropical system west of Hawaii in the GFS forecast. Looks like it gradually migrates to the northwest and eventually gets absorbed into a large trough in the western Pacific about a week from now. At any rate, it sure has a nice tropical tap into the ITCZ. These sutropical systems are very interesting to me... they are clearly cold-core systems but distinctly different from the mid-latitude type.

  2. I believe this is an example of a Kona Low. I'll update the post and include a few references.

  3. Thanks for the references. I noticed that this one has developed a lot of deep convection right around the center ( and which suggests it might be developing some warm-core characteristics. I think the GFS analysis hints at this also. The ocean temperature there is in the mid to upper 70s F (

  4. Just thought I would post an update comment for those who might be interested... it looks like the primary core of the Kona low west of Hawaii is developing into some sort of tropical cyclone. It has a well-defined eye and everything. See discussion and images from central Pacific Hurricane Center at I'm not sure how common this type of thing is.

  5. Thanks for the info on the Kona low. Very interesting.