Monday, September 1, 2014

August Precipitation in Context

August was a great month in Salt Lake City.  I haven't run my sprinklers in quite a while.  If only every summer could by like this.  How unusual was it?  Let's have a look.

Although we often present historical weather data in chronological order (i.e., ordered by year), it can be helpful to instead present the data in ascending order, as I have done below for the August precipitation in Salt Lake City from 1928–2014.

As highlighted above, the median and average August precipitation in Salt Lake City for the period of record are 0.54" and 0.81", respectively.  The median represents the value at which 50% of the years are drier and 50% of the years are wetter and it is lower than the average, but the average is skewed higher by a small number of wet Augusts.   Thus, a curious aspect of the August precipitation climatology in Salt Lake City is that there is a 64% chance of precipitation at or below the average and only a 36% chance of precipitation above average (it is not uncommon for precipitation distributions to be skewed in this way, especially in arid regions).  So, if you want to make some money off your friends, bet them each year that the August precipitation will be below average.  You'll win about 64% of the time and slowly but surely build up a war chest of cash that you can blow in Vegas where the odds are certainly not in your favor.

So now let's look at 2014.  It seemed really wet and it was with 1.77" falling at the Salt Lake City airport.  That's more than double the average and more than three times the median.  It was also the 11th wettest August on record.

However, although it was a relatively wet August, the historical record shows that it can be far wetter.  In 1968, 3.66" of rain fell at the Salt Lake City international airport, more than double what fell this year (compare blue August 1968 accumulation with green August 2014 graph below).  In fact, 1968 was a very wet month across much of the region.

Given the spotty nature of monsoon rainfall, ideally one needs to look at a larger number of stations to gain a better context for the entire Wasatch Front.  Perhaps someone out there can dig through the data and add comments about accumulations at other sites.  Concentrate on sites with good data back to 1968 since that seems to be a critical year.

Sunday, August 31, 2014

Smoke from a Distant Fire

It's been a while since we've had an airmass push into Utah from the northwest, but it happened last night and it brought the smoke from a distant fire, partially obscuring the Wasatch Mountains this morning.

The smoke is from fires in Oregon and possibly northern California.

Source: NIFC
The 2-day loop below shows the push of high pressure and northwesterly flow into Utah overnight.  

And with the smoke came a bump in PM2.5 concentrations overnight, as if we don't get enough crappy air in the winter.

Source: Utah DAQ
It will be nice when those fires are out and the northwest returns to a source of pristine air!

Saturday, August 30, 2014

Signs of a Wet August and the Start of Fall

Thanks to an active series of monsoon surges the Wasatch are remarkably lush right now, with firm, moist trails (at least on north aspects) and a jungle-like feel (by Utah standards).

Adding to the ambiance was a light, steady shower in the early afternoon.  That shower was produced by an approaching upper-level trough in the westerly midlatitude flow – a harbinger of the return of the storm track as fall approaches (hooray!).

Indeed signs of fall are beginning to appear, with a few trees already beginning to pack it in, even at these lower (7000–8000 ft) elevations.

Boy scouts are working to realign the trail in the area we were in today.  Although it's great they are out working on the trails, we wondered why the realignment was needed and the existing trail was in pretty good shape.  All we could figure was that there's a desire to move the trail away from the stream that runs down the canyon.  However, it appeared this was being done at the expense of mature aspens.

Hopefully those trees were diseased or dead as otherwise it's a damn shame to lose them.  

Friday, August 29, 2014

The Long Life of "Tropical" Cyclone Cristobal

Living a long life typically requires hitting the genetic jackpot, living in a relatively safe and healthy environment, taking care of yourself, and then perhaps just being plain lucky.  Swiss mountain guide Ulrich Inderbinen provides such an example.  He climbed the Materhorn 390 times, summited it a final time when he was 90, climbed 13,000 foot peaks in his 90s, worked until he was 95, and passed away this summer at 103.  Not too shabby.

Although not as far out there on the longevity limb as Ulrich Inderbinen, "Tropical" cyclone Cristobal, which moved poleward off the west east coast of North America this week, lived a pretty good life, benefiting from a caterpillar-to-butterfly-like metamorphosis from a tropical cyclone into a midlatitude cyclone as it moved into the North Atlantic.  

At the beginning of the loop below, Cristobal is barely a glint in his parent's eye and just a weak cluster of thunderstorms associated with a tropical easterly wave between Africa and the Lesser Antilles, islands that form the eastern edge of the Caribbean Sea.  He moves eastward and eventually experiences a quick birth and adolescence, growing into Hurricane Cristobal near the Bahamas.

The poleward movement of a hurricane into the cool waters of the higher latitudes often results in cyclone death, but Cristobal pulled off the necessary metamorphosis to extend his life, transitioning into an extratropical cyclone off the coast of the northeast United States and southeast Canada.  Such a transition requires a favorable large-scale environment, and Cristobal was fortunate to find himself in that environment as he crested the hill of middle age.  

Reborn as an extratropical cyclone, Cristobal is enjoying a productive maturity and is expected to survive until he reaches Iceland (see lower panel below).  

That's a lifetime from easterly wave through tropical cyclone to extratropical cyclone that spans nearly 2 weeks and almost 60ยบ of latitude, and a remarkable example of one type of tropical–extratropical interaction that occurs during late summer and fall in the Northern Hemisphere.  

Thursday, August 28, 2014

Regionally, Mother Nature Can Still Bring It

There is a nice article in the latest EOS, a publication of the American Geophysical Union, discussing the remarkable ice cover on the upper Great Lakes this past spring [See Cold Water and High Ice Cover on the Great Lakes in Spring 2014 by Clites et al. (2014), unfortunately paywalled unless you are an AGU member and login with your membership account].
Source: Clites et al. (2014)
Some tidbits from the paper:
  • Lake Superior wasn't completely ice free until June 6th.  Yeah, June!
  • At the end of April, 51% of Lake Superior, 23% of Lake Huron, and 10% of Lake Michigan were still ice covered. 
  • In the 40-year period of record at the end of April, the previous record on Lake Superior was 30%, in only a few years was significant ice observed on Lake Huron, and at no time was their significant ice on Lake Michigan.
A quick look at data from NCDC shows that this past Dec–Apr was the coldest in Michigan (which flanks Lakes Superior, Michigan and Huron) since 1920 and the 4th coldest on record.  The period is a dramatic outlier during the 40-year period of good ice-cover records.

Source: NCDC
During the same period, globally averaged temperatures were the 6th warmest on record, illustrating that Mother Nature can still bring it, but only regionally.

Source: NCDC

Wednesday, August 27, 2014

Wet, But How Wet?

What an August it has been.  Lots of rain and relatively cool days.  Nobody can complain this year.

Although it has been wet, a quick look at the weather records suggests that the area has seen wetter.  Let's start with the Salt Lake City International Airport.  Through yesterday, the accumulation for August 1–27 was 1.77 inches (green line), well above average (brown line).  That ties it for 8th amongst the wettest August 1–27 periods.  However, there have been similar periods in past years that were much wetter, including 1968 when 3.66 inches fell (blue line).

Source: NWS
Given the fickle nature of monsoon convection, however, observations at a point can sometimes be misleading.  Let's look at a few other stations.

Alta has been deluged this August with an accumulation of 5.33" so far, which puts it just behind 1983 (5.97").  There is some missing data for the period though (black dots), including yesterday, which will add to the total.  On the other hand, the data at this location is very spotty.  For instance, there is no data for August 1–27, 1968, which was the wettest such period at some nearby stations.  Thus, we can't make any definitive conclusions here, but we can say it's certainly been very wet.

Source: NWS
If we head a bit west, the situation at Tooele looks pretty similar to Salt Lake City.  This year's August 1–27 period is well above average with 1.99" of rain, but still behind the 1968 deluge year.

Finally, if we go down to Utah County, records from the school that shall not be named show August 1–27 to be wet with 2.54", but not record setting.  Here, long-term records are also spotty (1968 is missing, for example).

I suspect if we had a dense and complete precipitation observing network covering 50 years we would find that August 1–27,  2014 ranks in the top 10 such periods at most locations in the Salt Lake area and perhaps near record levels at a few spots that have really gotten walloped by the thunderstorms.  During the past 50 years, August 1–27, 1968 may be the wettest such period on record, although I've only looked at a few stations and there could be some exceptions to this given the hit-and-miss nature of monsoon thunderstorms.

If you are interested in records for the month, we still have 4 days left.  Although there are no big monsoon surges in the offing, we have a chance of some showers and thunderstorms later today and perhaps we'll see something pop up later in the period.  Some sites could add to their totals.