Tuesday, June 12, 2018

For the Southwest, It's Not Just Bud

As of 9 AM this morning, tropical cyclone Bud was a Category 3 hurricane located about 525 km south southeast of Cabo San Lucas and the southern tip of Baja California.

Bud is getting a lot of attention for a number of reasons.  First, it is likely it will weaken, but remain a tropical storm as it approaches Baja California on Thursday.  Second, it will bring of surge of moisture that will cause heavy rainfall in parts of Mexico, with the remnants moving downstream, perhaps bringing some rain to parched areas of the southwest U.S.

It is important, however, not to fixate solely on Bud, as there's important stuff going on ahead of the main circulation center.  In particular, note in the loop below the surge of moisture pushing northward through the Gulf of California and northwest Mexico ahead of Bud.

This will lead to an increase in thunderstorm potential on Friday in Arizona before the remnant core of Bud reaches the state.  Such an evolution is common for tropical cyclones moving through this part of the world.

Meanwhile, in Utah, the forecast is quite interesting.  We discussed in the previous post that this forecast is characterized by a three-body problem, namely the interaction between Bud, a short-wave trough off the coast of California, and a short-wave trough digging southward over the Pacific Northwest.  In the 84-hour GFS forecast valid 0000 UTC 16 June (5 PM Friday) and plotted below, I've also highlighted three airstreams.  The first is northerly flow up the lower Colorado River Valley associated with the moisture surge.  The second is a much drier southwesterly flow originating over the eastern Pacific and California.  The third is a slightly more humid westerly flow over Nevada and Oregon of North Pacific origin.

Ultimately, all these ingredients create a complex situation on Saturday in which eastern Utah is in the tropical moisture with showers and thunderstorms, portions of west-central and northern Utah including Salt Lake City may be in the "dry slot" created by the dry flow originating over the eastern Pacific and California, and far northern Utah, southern Idaho, and far northern Nevada have just enough north Pacific moisture and forcing from the digging upper-level trough to kick off some showers.

By 1200 UTC (0600 MDT) Sunday, the tropical moisture is declining, but lingering just a touch over eastern Utah, but western Utah and Salt Lake City is in the heart of the southwesterly flow and the associated dry slot.  Showers have associated with the upper level trough have pushed northward.

If you have outdoor plans this weekend, this is a forecast worth monitoring.  There will certainly be times to get in activities, but others during which you could be dealing with thunderstorms, showers, or strong winds depending on time, location, and how all the ingredients come together.  Geographic precision is not possible yet at this lead time.  Clearly, this is a situation that is not well summarized by the icon-based forecasts issued by the local news.

1 comment:

  1. How much credence to you give to GFS precip estimates. It must be a fair amount since you are posting it. It has the advantage it goes out 10 days, but last winter you indicated it was upward biased in precip estimates for the Wasatch.

    NAM12 (Jun 12 18z) goes to 6z Sat.

    Casual comparison of GFS v NAM12 suggests GFS may be over-doing Wasatch precip.

    It will be interesting to see what FV3-13 has to say when it comes into range.

    The SREF seems to be the gold standard on precip. Mean at Collins is 0.1 inches cumulative by Sat 6z; spread from 0 to 0.5 inches. Mean at Salt Lake is 0.05 inches; spread from 0 to 0.25. (Oddly, in the 2018061215 run, some of the ARW members for Salt Lake have cumulative declining ... processing error?)

    Curious how this shakes out, plan is to be riding the Wasatch this weekend. A 0.05 inch rain burst is fine. A 0.25 inch burst is not fun.