Wednesday, September 17, 2014

An Interesting Situation Tomorrow and Tomorrow Night

As a forecaster this time of year, one needs to be alert when midlatitude troughs interact with monsoon moisture.  For much of the summer, monsoon thunderstorms in Utah form in an environment of weak to moderate instability and weak flow.  The primary forecast concerns are flash flooding and, in some instances, strong winds from microbursts.  

Add a midlatitude trough to the mix, and you have stronger synoptic forcing, stronger instability, and stronger flow.  If the instability, moisture, and shear are right, strong or severe thunderstorms can develop.

I'm not sure if we have the necessary mix of instability, moisture and shear for strong or severe thunderstorms tomorrow in parts of Nevada and Utah tomorrow (and tomorrow night), but the pattern warrants attention.  The forecast for tomorrow afternoon (6 PM MDT) shows a midlatitude trough extending across northern California with large-scale southwesterly flow over Nevada and Utah.  

Although much of the moisture associated with the remnants of Hurricane Odile are in Arizona and New Mexico, some does sneak into Nevada and Utah through tomorrow afternoon (green color fill at upper left below).  In the NAM forecast for tomorrow afternoon, one can also see a tongue of Convective Available Potential Energy (CAPE, yellow to orange color fill at upper right) along the Nevada–Utah border.  

CAPE is a measure of instability.  The maximum CAPE in that tongue exceeds 1000 J/kg, which is high for the Intermountain West, but on the margins for generating severe convection.  Further, although we have some monsoon moisture to play with, it's not exactly a juicy airmass.  So, as things stand now, portions of Nevada and Utah may see some showers and thunderstorms tomorrow, but the risk of strong or severe storms seems low. That being said, this is a pattern that I typically watch carefully as surprises sometimes happen in the Intermountain West and things could get interesting if the conditions prove a bit juicier than currently forecast. 

1 comment:

  1. I wonder if terrain channeling (mostly oriented north-south in the Great Basin) of lower-level moisture may be underestimated by many forecast models such as the GFS due to limited terrain resolution. This morning you can see that 60+ dew points have reached Las Vegas, and when this happens in a pre-trough situation there could be a good deal of low-level transport northward through eastern NV and western UT. In this case the low-level flow may be especially important because the mid and upper level remnants of the hurricane are going elsewhere, but a lot of low-level moisture is pooled to the south in the lower Colorado River basin.