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| Mean monthly Intermountain cyclone frequency and genesis (Jeglum et al. 2010). |
The intensity (or amplitude) of Intermountain cyclones also tends to be greater in the spring.
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| Two-dimensional histogram of peak 850-mb Intermountain cyclone amplitude vs. month (Jeglum et al. 2010). |
Not coincidentally, Shafer and Steenburgh (2008) found that the frequency of strong Intermountain cold frontal passages is also highest in the spring, although the peak is sharper and in June.
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| Monthly frequency of strong cold frontal passages over the Intermountain West (solid) and western United States (dashed, Shafer and Steenburgh 2008) |
If the models are on track, we have a great Intermountain cyclone event on tap for Friday and Saturday. The 1800 UTC 23 Feb initialized NAM produces Intermountain cyclogenesis in the direct lee of the southern "High Sierra" for 0000 UTC 26 Feb (5 PM MST 25 Feb, Friday afternoon).
The low center then tracks into northeast Nevada, with Salt Lake City in the so-called "warm sector" and strong southerly flow ahead of the low center at 0600 UTC 26 Feb (11 PM 25 Feb).
As the low center moves into southwest Wyoming, the cold front rotates across Salt Lake City early Saturday morning.
The NAM cyclone is as close to a "classical" frontal cyclone as you can get in the Intermountain West. Although cyclogenesis occurs in the lee of the Sierra Nevada and orographic forcing is important, the low center forms along a pre-existing frontal boundary. The amplitude of the frontal wave then increases, resulting in a "open wave" cyclone as the low center moves across northern Utah. The Bergen School meteorologists responsible for the Norwegian Cyclone Model would be quite proud!
This is all fine and dandy, but a look at the GFS illustrates that there is great uncertainty in this case with regards to cyclone track. In particular, the GFS forecast for 0600 UTC 26 Feb puts the low center in southern Nevada, with a surface trough and frontal zone draped across central Utah, well to the south of the NAM trough and frontal zone.
In contrast to the southerly warm-sector flow predicted by the NAM, the GFS forecast puts Salt Lake in cold, post-frontal northerly flow for Friday night bar hopping! Further, the GFS puts a band of what would be heavy snow across Utah Country, whereas the NAM frontal band is near the Utah-Idaho border.
This case provides a great example of how you can have confidence in a synoptic event, but differences in positioning make weather forecasts for specific locations very difficult.
Does it follow that DRE events increase in frequency and "severity" during spring as well? I suppose it gets hard to define, count, and catagorize DRE events to begin with ....
ReplyDeleteI read your 2008 paper and found it very interesting. I have noticed one factor (perhaps this was mentioned somewhere) that may contribute to this increase in cold front intensity. Working with some weather-related projects in California over the last several years, I have observed that many of their coldest storm events tend to take place in early spring. In fact, analysis of the snow:rain (or snow:total monthly precip) ratio shows that this ratio tends to peak in March at many sites. A good example with a long-term climatology is Blue Canyon, where this ratio is highest in March and second highest in April (http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?ca0897). A spring storm temperature minimum, if this is indeed the case, might be due primarily to the sea surface temperature minimum near early spring, or perhaps more to seasonal changes in storm track and jet stream patterns. What is your impression of this?
ReplyDeleteI'm not familiar enough with the Pacific SST climatology to comment on that portion of your hypothesis. I suspect, however, that the frequency of deep mid-tropospheric closed lows may be highest along the California coast in the spring. See Bell and Bosart (1989, MWR). I can't access this from where I'm sitting, so I'll leave it to you as a research exercise :-).
ReplyDeleteI suspect it may have a lot to do with storm track, and the Bell paper does show a strong maximum in closed low activity near California in the spring. However, I also looked at some buoy data, and SST's near CA appear to reach a minimum around April. This late SST minimum did not seem to be the rule further north (WA/OR). In any case, I think that the apparent spring temperature minimum of these west coast (or at least California) storm events may be a significant factor in the intensity of intermountain frontal systems in the spring.
ReplyDeleteIt seems that most of the strong frontal systems occurring in the Great Basin during the spring are driven by the deep, mid-tropospheric closed lows Jim mentioned - possibly many of the same systems that bring California some of its higher snow/rain ratio precipitation events. If experience serves me right (not always!), it seems that the typical path for these closed lows to take into this area is southward near the U.S. west coast...
ReplyDeleteIs it possible that the minimum in regional SST over the EPAC during this time of year lends itself to a much smaller amount of heat release into the closed lows moving overhead? This might result in less airmass modification, increasing the snow/rain ratio, and aiding in driving strong cold fronts once these upper-level systems come ashore.
It seems like the SST minimum is part of it. I think most of the NH reaches this minimum around the first of March, while the CA coast SST obs seem to suggest that April is the coldest there (some of the buoy sites on http://www.ndbc.noaa.gov/ have monthly average SST plots). There also seems to be much more of a tendency for deep trough development over the southwestern U.S. (including California) in the spring. This may have a strong relationship to the severe weather maximum in the plains states in the spring also, which they really don't see in the fall to anywhere near the same extent.
ReplyDelete