Thursday, July 27, 2017

Another Look at the Deluge

I've had an opportunity to dig in a little deeper to the deluge that hit the Sugarhouse area early Wednesday morning.

Storm-total precipitation was greatest at a citizen weather station between Foothill Drive and Parley's Way near the mouth of Parley's Canyon.  Maximum precipitation rates at the site were 1.64 inches in 30 minutes, 2.28 inches in 60 minutes, 2.39 inches in 2 hours, and 2.41 inches in 3 hours.  If accurate, those all have recurrence intervals near 200 years, as also reported by Salt Lake City Public Utilities.

Soundings collected at the Salt Lake City airport at about 6 AM either during or in the wake of the storm (depending on when the sounding was actually released) showed a precipitable water value of 1.41 inches.  Precipitable water is the depth of water that you would have if you condensed all of the water vapor out of an atmospheric column.  Such values are high, as one would expect in a flash-flood scenario, but not unprecedented, as illustrated by the chart below (red line represents the daily highest value).  The all time record is 1.66 inches.

You might wonder how you can get more precipitation than would be produced if you condensed all of the water vapor out of an atmospheric column.  The answer is horizontal moisture transport, which allows storms to process water vapor from the surrounding area.  Thus, storm intensity has to do with many factors besides how juicy the airmass is.

Radar imagery shows very nicely how a well-defined band of precipitation developed early in the event and extended from WSW to ENE across the Sugarhouse area.

Below is a radar estimate of the 3-hour accumulated precipitation (inches) for the period ending at 1109 UTC (0509 MDT), which captures most of the precipitation.  Note the banded area of high precipitation with a maximum located along Foothill Drive just south of campus where values exceed 1.5 inches.

The southern diamond is the site that observed 2.41 inches, the northern diamond covers our sites near the mouth of Red Butte Canyon where 1.33 and 1.21 inches were observed from midnight to 6 AM.  The former is much higher than the radar estimates, whereas the latter are closer.  Further sleuthing is needed to reconcile the differences.

Any observations from the area along I-15 just north of the Point of the Mountain that also appeared to get pounded (with even more precipitation)?  I'm curious what happened there.

Wednesday, July 26, 2017

The Early Morning Deluge

Thunderstorms with heavy precipitation moved into and intensified over the Salt Lake Valley last night, leading to some urban and small stream flooding.  The radar loop below runs from 0800–1300 UTC (0200–0700 MDT) and shows some especially strong storms in and around Sugarhouse and near and around Draper.

Radar estimated precipitation amounts key in on both of those areas reaching over 1.5 inches in the Sugarhouse area and nearly 2 inches near Draper.  

Gauge-measured rainfall amounts since midnight as reported to the National Weather Service include 2.38" at Sugarhouse near Parleys Way, 1.35" at our mountain meteorology lab on the University of Utah campus, and 1.20 inches in Draper.  

Contrasts between radar estimated and gauge-measured precipitation are not uncommon and careful analysis will be needed to identify the magnitude of peak precipitation amounts.  Nevertheless, it is clear from both Sugarhouse and Draper received an impressive amount of rain for such a short period of time.  Data from the National Weather Service Precipitation Frequency Data Server ( suggests that in the Sugarhouse area, precipitation amounts of about 2 inches in 6 hours have a return interval of 100 years.

Here's another perspective from the same web site showing the magnitude of 6-hour precipitation with a recurrence interval of 100 years for Utah (click to enlarge).  In the Salt Lake Valley, values are generally around 2 inches, but are slightly larger on the east bench, reaching about 2.2 inches along I-215.

Additionally, precipitation fell in somewhat less than six hours and probably was especially intense in short bursts.  Local news media reports some flooded basements, road closures, power outages, and transit disruptions.  

A quick word on recurrence intervals.  Perhaps most importantly is that they are estimates of the recurrence of precipitation at a point, not in an area.  Hence, given the chaotic and localized nature of thunderstorms, we might expect a storm like this to strike somewhere in the Salt Lake Valley more frequently than every 100 years.  

Tuesday, July 25, 2017

Beautiful July Weather

Aided by a weak mid-latitude trough sitting along the coast of northern California, our first major monsoon surge of the season has finally brought relief to northern Utah this morning.

It's worth taking a look at a 1-month meteogram for the Salt Lake City International airport to put the weather this morning into perspective.  Currently, the temperature is 69ºF, the lowest temperature observed since July 2nd, bringing our record run of consecutive 70+ days to an end at 22.  Good riddance!

The dewpoint is currently 63ºF, and peaked at 65ºF around 2 am, which I suspect is the highest observed since at least last summer, although that needs to be confirmed.  While no good for swamp coolers, it's wonderful for the skin.

Through 8 AM, the airport has only picked up .03" of precipitation.  That's not much, but it's the most rain we've had on a calendar day since June 13.  Again, relief!  Other locations have surely had more.
Enjoy this weather while it lasts.  Today will probably be the coolest day of July.  We'll see a chance of thunderstorms for the next few days, but by the weekend, the high 90s look to be back.

Saturday, July 22, 2017

AltaBird Ski Report

It's a holiday weekend in the Wasatch, which means it's best to start early to avoid the crowds.  I believe this weekend and the next are also considered the "Wasatch Wildflower Festival," although in my view the mountains need no promotion this time of year.  

My hike this morning took me to the top of Mt. Baldy and Hidden Peak, enabling a quick assessment of ski conditions (sans skis).  Near as I could tell, Main Chute doesn't go any more, although there are some turns to be had a the top.

Pipeline?  A careful look also shows it no longer goes and is bisected by a couple of snow free strips.

TAYers will need to hit the Timp "glacier" or head to better options in the Cascades and Sierra.  I'll stick to the trails.

In case you are wondering, the overnight minimum at the Salt Lake City airport was only 76ºF, making it virtually certain that this will be the 20th consecutive day with a minimum of 70 or higher (technically 71 since that's the lowest temperature observed in the past 20 days).

The good news is that the models are still advertising a major monsoon surge for northern Utah early next week.  Keep your fingers crossed.

Friday, July 21, 2017

July Is the New Hades

I'm beating a dead horse, but here's an updated look at July now that we're 2/3 of the way through the month from hell.

Perhaps most remarkably, every day this month has an above average minimum and maximum temperature (normal range in plot below indicated by green background).

Source: NWS
Making that even more remarkable is that the National Weather Service uses the 1981–2010 period for their average.  That's a period that is relatively warm compared to the rest of the 20th century.

And the first 2/3 of July, not surprisingly, is the hottest on record, 2.3ºF warmer than 2007.

Source: NOAA Regional Climate Centers
I have been looking at the models the entire month and waiting for evidence of a bonafide monsoon surge into northern Utah.  We've been tickled by the monsoon moisture in places like Utah County and over the central Wasatch, but for the far north, we've mainly been on the edge of the action with just some light or trace accumulations.  

There is some hope of something happening next week.  The GFS loop below, which runs from 0000 UTC 21 July [1800 MDT Thursday (yesterday)] through 0000 UTC 26 July (1800 MDT Tuesday) shows the drier air moving into northern Utah for this weekend and then a surge of monsoon moisture through eastern Nevada and Utah Monday night and Tuesday.  

GFS 500-mb height (black contours) and precipitable water (color contours)
That's the best surge I've seen the GFS advertise so far this summer.  However, the intricacies of monsoon moisture transport are such that I'm keeping my hopes temperated and will simply be keeping an eye on things over the next few days.

Thursday, July 20, 2017

One Streak Ends, Another Continues

At just before 1 AM MDT on 19 July, a weak thunderstorm moved over the Salt Lake City International Airport, producing .01" of rain.  The thunderstorm was isolated in nature, but was enough to produce measurable precipitation at the airport for the first time in 34 days.

So that streak, although not even close to record breaking, is over.  If my notes are right, the record at the Salt Lake City airport for consecutive days without measurable precipitation is 63 days and there have been 9 instances of 40 or more.  Of course, .01" isn't really much in July when it evaporates off in a few minutes.

Getting to record breaking streaks, our consecutive day streak with minimum temperatures above 70 continued through yesterday and now sits at 17.  The overnight low this morning was only 77 or 78 degrees, so that streak will go to 18 unless we can get some thunderstorm cooled air to drop things to below 70 this afternoon or evening.

Speaking of thunderstorms, we noted on Tuesday that a monsoon surge and associated thunderstorm activity would push right to the doorstep of Salt Lake County tomorrow afternoon and that's pretty much what happened.  Below is the precipitable water analysis (contours, a measure of how deep the water would be if you condensed out all of the water vapor in the atmosphere) for 0000 UTC (6 PM MDT) yesterday afternoon.  Note the sharp dropoff from central Utah to the northwest Utah border.

Some strong storms popped over Utah, Summit, and Wasatch Counties and did try to sneak into Salt Lake County, but for the most part, we missed out on the action.  Pity, although the cloud cover and cooler outflow was appreciated.

Some of that monsoon moisture remains in the area today, so there's still the hope of something popping and giving us some rain this afternoon or evening.  Keep your fingers crossed.

After that, we see a shift in the flow and drier air moving into northern Utah.  By Saturday afternoon, precipitable water values over northern Utah are less than 15 mm, greatly reducing the thunderstorm threat, which will likely consists of just isolated storms over the highest terrain (Uinta Mountains, you know who you are).

With drier air moving in, there is a chance that we might actually see a minimum temperature below 70 if not with a storm this afternoon (low chances) then sometime over the next few days.  Another reason to keep your fingers crossed!