Friday, August 1, 2014

Was July 2014 Cooler Than July 2013?

There's been some discussion in the media about July 2014 being cooler than July 2013, which as you may recall, was the hottest July on record at the Salt Lake City International Airport (see July's Blistering Numbers from August 1st, 2013).  The National Weather Service also released this graphic yesterday showing a dramatic decline in temperature from last July to this July.

Source: National Weather Service
It turns out, however, that the story is a bit more complicated and I think it is questionable that this July was significantly cooler than last July along the Wasatch Front.  

The Salt Lake City airport is a single observation location and therefore it can be strongly influenced by the local climate at the airport or any problems with the temperature sensor.  If we take the difference in average temperature between July 2014 and July 2013 for all available observing sites along the Wasatch Front, we find that the difference at the airport is very anomalous and, in fact, most stations are within 1ºF of last year and some are warmer.

Now there are some warts in this data.  Nephi, City Creek Canyon, and the Salt Lake City Airport have complete records for July 2014, but the other stations are missing 1–3 days.  For instance, Alta is missing July 30, which was very cold.  If this day was available, the average for July 2014 would be cooler and the difference would be smaller but still positive.  Nevertheless, a quick look suggests if this missing data were properly accounted for, all the non Salt Lake City Airport sites would still be within 1ºF of last year and some would be warmer.   My take is that the two July's, at least in terms of average temperature, are largely indistinguishable for the Wasatch Front as a whole.  

So, what gives at the Salt Lake City Airport?  Well, there are times of year when the airport does have a very unique microclimate, such as during the heart of winter when temperatures during inversions can be much lower than found on the east bench.  During summer, however, that's usually less of a player.  

Instead, I suspect the issue is the anomalously high temperatures reported at the Salt Lake City airport during July 2013.  This is a topic we discussed last summer (see What's Up @ KSLC) when it was clear the temperatures being reported at the airport were unusually high compared to surrounding stations.   The instrument has subsequently been replaced and temperatures at the airport appear more in line with those from the surrounding area.  The bottom line is that temperatures last July were "jacked." 

One of the advantages of having a dense network of observing stations is that you can identify and account for these sorts of sensor problems and biases.  All sensors drift and there are times when adjustments need to be made to the observations when examining trends.  People sometimes ask me why we don't use the "raw" unadjusted temperature observations when constructing long-term climate trends and this is an example of why.  Quality control and adjustments need to be made to account for these sorts of sensor problems (here's a discussion of what the National Climatic Data Center does).  

In summary, the average temperatures of the two July's were pretty similar.  In other words, they were both too hot for my blood!

Thursday, July 31, 2014

Something Unusual...

It's not uncommon to see cumulus clouds popping up over northern Utah in July, but there's something unusual about these, at least for our summertime climate.  If you know or would like to guess, leave a comment.  

Let's Do It Again?

There's some potential that we will see another major monsoon surge impacting the region on Sunday and Monday.

As presently forecast, the event would feature a nice interaction between systems in both the tropical easterlies and the midlatitude westerlies, specifically a tropical wave that is currently located over southern Baja (red line below) and the westerly flow and broad trough currently located off the coast of California (red arrow).

The upper panel below is what is known as a dynamic tropopause analysis.  It is basically a map of the jet-stream level flow.  If you look carefully, you can see how the tropical wave near Baja slowly moves northward and eventually induces an amplification of the broad trough off the coast of California.  The two features then merge and begin to move northward toward Utah.

This leads to another surge of moisture into the state on Sunday (currently forecast to be a late day arrival into the far northern portion of the state.

If this comes together, we'll have another round of rain and moist, cool weather.

Wednesday, July 30, 2014

The Juicy Aftermath

A blanket of stratus obscures the Wasatch in the aftermath of two days of monsoonal rains
What a great couple of days.  Cooler temperatures and high humidities have done wonders for my skin and my disposition.  Let's have a look at some of the more remarkable aspects of the event.

Integrated precipitable water reached nearly 1.4 inches.  Although not a record, this is beyond the 99th percentile for the month and more than 3 standard deviations above the mean.  

Source: NWS
Given that we live in a region that is typically fairly dry, but punctuated by occasional monsoon surges with high moisture contents, one needs to be cautious in interpreting these moisture statistics.  That being said, I think it is safe to conclude this was a fairly juicy event, but there's been juicier.  

Dewpoints at the Salt Lake City airport have been at or above 58ºF now for 36 hours, with peaks at 65ºF yesterday morning and this morning.  A couple more mornings like this and I'll save a fortune on skin cream. 

Source: MesoWest
Preliminary maximum storm accumulation reports by region include 0.71" in Logan (Cache Valley), 1.73" in Corinne (northern Wasatch Front), 1.37"in Sandy (Salt Lake Valley), 1.32" in Levan (southern Wasatch Front), .83" in Cedar Mountain (Western Deserts), 1.64" near Morgan (Wasatch Mountain Valleys), 1.81" at Red Spur (Wasatch Mountains north of I-80), .65" at Alta–Collins (Wasatch Mountains south of I-80), 1.23" at Norway (Uinta Mountains), .35" in Duschesne (Uinta Basin), .18" at the Carbon County Airport (Castle Country), 1.12" at San Pete Reservoir (San Pete/Sevier Valleys), 1.31" in Fillmore (west-central Utah), and Ok, that's enough!

Basically, this wasn't a hit or miss event.  There were wide-spread variations in accumulations, but nearly everyone got something and there weren't any outrageous accumulations, although it is possible that the gauges missed something.  For example, there was a strong, stationary cell near Eureka yesterday afternoon, as well as another northwest of Eagle Mountain in Utah County, that may have put down more than indicated in the observations above. There was some localized flooding in places, including Eagle Mountain, but for the most part, we were spared the really nasty stuff.  

Still a chance of some showers and thunderstorms today, although they should be more scattered.  

Tuesday, July 29, 2014

Let's Build an Ark!

The University of Utah @ 12:40 PM
The University of Utah is currently experiencing a nice soaking rain, but stronger downpours are occurring elsewhere and could affect us later in the day.

Rdar imagery shows a nice cyclonic (counterclockwise rotating) circulation center over the West Desert.  Although there is widespread shower activity across much of northern Utah, some very strong cells can be found in the Salt Lake Valley and the area around Eureka, not to mention coming off the high terrain in far northwest Utah.

The Oak Springs weather station northwest of Eagle Mountain in Utah County recorded 1.94 inches in the past three hours.  Quite a deluge.  Guess we can turn off the sprinklers for a few days.

Very Refreshing!

Some of yesterday's storm activity in the Wasatch Mountains
The juicy tap delivered a refreshing evening last night with widespread rain and thunderstorms along the Wasatch Front and across much of Utah.  Integrated precipitable water, the depth of water in an atmospheric column if it were all precipitated as rain, reached nearly 4 cm yesterday afternoon, which is fairly high for northern Utah.  Values remain high this morning.  
Observations from the Salt Lake City Airport show how temperatures dropped from a high of 92ºF early yesterday afternoon to about 68ºF with the onset of rain in the afternoon.

Source: MesoWest
Since then, temperatures remained nearly flatlined at 68ºF.  Further, dewpoints are currently sitting around 64ºF.  That's very high for northern Utah.  I don't know what the record dewpoint is for the Salt Lake City Airport, but I think the highest dewpoint I can remember seeing there since moving here in late 1995 is 68ºF.  Maybe an adventurous student out there can rifle through the records.

It looks like a cool, humid day (by Utah standards) is on tap today with some showers and thunderstorms.  How refreshing!