Monday, September 26, 2011

Here Comes Hilary?

No, not her, this Hilary,

a category three hurricane with sustained winds of about 125 miles per hour and one powerful eye.  

The latest (0600 UTC) GFS forecasts Hillary to make a hard right turn, saunter up the Baja Peninsula, and then move into the southwest United States.

Even if this track verifies, Hilary will weaken substantially before reaching the southwest United States, but that doesn't mean she won't pack a punch.  Of primary concern is the potential for heavy precipitation and related flash flooding.  The GFS brings abundant moisture into northern Baja by 132 hours, with the remnants of Hilary moving into southern Arizona by 150 hours.  This occurs as a mid-latitude trough moves onto the Pacific Coast, serving as the so-called "kicker trough" that initiates the eastward movement of Hillary.

Whether Hilary has any impact on Utah will depend greatly on how the large-scale pattern plays out over the next several days, as well as how quickly she moves northward.  The GEFS ensemble shows a relatively small standard deviation of 500-mb height in the forecast area by 1 October, which might suggest we can have some confidence in Hilary's track.

I think, however, that might simply reflect the fact that the 500-mb height gradient over the region is relatively small, so the standard deviations are not large.  I believe this remains a difficult forecast that is quite sensitive to small changes in the flow in mid-latitudes and the subtropics.  Keep an eye on things as even if Hilary's moisture doesn't make it to Utah, the midlatitude trough has the potential to finally bring some changes in the weather.


  1. This could also turn out a be a good case of the tropics wrecking havoc on the mid-latitudes. Even in the GFS loop you can see a large ridge build to our east as Hilary comes north. Looking way out in time, that ridge remains over the center of of the US putting Utah under a decent longwave trough. Pretty good agreement among the emsemble members:

    This will be fun to track and "wish-cast"

  2. It looks like the GFS is bringing us primarily the lower and mid-level moisture remnants, rather than just the upper-level moisture outflow. Winds look primarily westerly at upper levels (as opposed to southerly at lower/mid levels) which implies a good amount of directional shear. That could be interesting in terms of severe weather potential if things play out the way it is currently advertising.

  3. Indeed, the ensembles have remarkably little spread, which seems odd. Time will tell. We'll also have to see if Hilary will get into Utah. I agree that for now we probably mis out with much of her circulation and moisture, but the forecast is interesting anyway. Given how dry it has been, a soaker would be nice!