Tuesday, September 27, 2016

Degraded Air Quality in Late September

Dude, I can barely see the Oquirrhs!
I hope this isn't a harbinger of things to come, but there is actually an elevated air quality event underway over Salt Lake with some characteristics similar to what we see during the winter.

As everyone is aware, late last week and last weekend and deep upper-level trough and associated cold-air intrusion rumbled through Utah.  The maximum temperature on Friday was only 51ºF.

As the trough moved downstream, a high-amplitude upper-level ridge built rapidly over the western United States.  This led to significant warming in the mid- and upper-levels of the atmosphere at a rate faster than solar heating could warm the colder air near the surface.

As a result, yesterday afternoon, the atmosphere over the Salt Lake Valley was capped by a series of stable layers.  The lowest of these stable layers was only a few hundred meters above the valley floor.

Source: Storm Prediction Center
Weather camera images yesterday showed a clear layer of gunk over the Salt Lake Valley with a pronounced top.  Look toward Lone Peak in the image below.

Source: MesoWest
And the buildup of pollution is evident in the time series below, which shows that we are now in the moderate air quality category for PM2.5.

Such conditions are a bit unusual for September, but there are probably three issues at play.  The first is the depth of the weekend trough and strength of the cold air, followed by the building of a strong ridge and associated rapid warming aloft.  Second, we received a great deal of rain Friday and Saturday, leading to high soil moistures.  As a result, a portion of the sun's energy that often would be partitioned into heating the ground and atmosphere is being used for evaporation and transpiration.  Finally, we simply have to realize that we have more people living here and driving farther than every before.  I suppose there might also be some smoke sources out there, although I'm unaware of any major incidents in the immediate area.


Monday, September 26, 2016

Looking Back at Summer

White capped mountains signal that fall is finally here
Meteorological summer ended earlier this month.  Astronomical summer ended on Friday.  Although temperatures will rebound some this week, summer is finally over.  Good riddance.  Don't let the door hit you in the butt on the way out.

The climate numbers for summer (June–August) were released last week by the National Centers for Environmental Information.  You already know that this summer was the 2nd hottest on record at the Salt Lake City airport, but how did things go statewide?

Well, it was a hot summer by old-timers standards, but not for the young.  With a statewide average temperature of 71.6ºF, this summer was tied for only the 5th warmest on record.  However, the four ahead of this year, as well as the other years tied with it, have all occurred since 1994.  

Source: NCEI
So, compared to the climate of the latter 19th century and most of the 20th century, this was a very hot summer statewide.  Compared to the climate since 1990, it was hot, but not exceptionally so.

Globally, Boreal summer (June–August, summer in the northern hemisphere) was the hottest on record, although from a statistical perspective, it's probably in a dead heat with last summer.

Source: NCEI
For the year to date, however, 2016 is easily the front runner.  It's a near virtual lock for hottest year on record unless something truly surprising or catastrophic happens in the last 4 months of the year.  

Source: NCEI
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Friday, September 23, 2016

Severe Convective Storms Yesterday, Mountain Snow Today and Tonight

Wow, what insanity.  After a meteorologically boring summer, fall comes in with a bang.  I'll go into rapid-fire mode for this one:

1. Ogden Supercell?: Yesterday afternoon, a long-lived thunderstorm with supercell-like characteristics developed near Dugway Proving Ground and tracked to the Ogden Area over about a 2.5-hour period.  It was a right-moving storm, in that it was moving somewhat to the right of the steering-layer flow and the tracks of other convective cells.  This is common characteristic of supercell thunderstorms, which are long-lived thunderstorms with rotating updrafts (note: there are also left-moving storms, but they are less common).  The Doppler velocity signature of the storm, however, wasn't especially pronounced, so I'll leave it to the severe convective storms experts to ascertain whether or not the storm qualifies for Supercell status.

2. Severe straight-line winds.  The storm brought strong straight-line winds to the Ogden area and the northern Wasatch Front.  Here's a remarkable video of the straight-line winds from Antelope Island:

Colleagues here in our mountain meteorology group installed a sensor on the playa just east of Antelope Island earlier this summer that collects data every minute.  The passage of the storm's gust front was accompanied by a nearly instantaneous drop in temperature of more than 20ºF, wind shift from SSE to W, and a wind gust of 75 mph.  

3. Ogden tornado. The storm also spawned a tornado south of Ogden, which was captured in a video posted by Neil Essig on YouTube.

3. Tornado Damage Scale.  I have been asked by a few people how strong the tornado was.  Tornadoes today are classified using the Enhanced Fujita (EF) Scale, which is an updated version of the Fujita Scale originally developed by Ted Fujita, a meteorological pioneer in many areas, including forensic investigations of tornadoes and severe storms.

Source: NWS
Other than to say it was probably an EF-2 or lower tornado, I hesitate to speculate on the tornadoes EF rating based on videos and photos, especially given the wide-spread straight-line winds observed in the area.  Careful site surveys are needed to ascertain the characteristics of the damage path, the degree of damage, and to separate damage from the tornado and the straight-line winds.

5. Try to avoid saying it was "only" and EF-0, EF-1, or EF-2 tornado.  I used to tell people that I really wanted to see a tornado, and then I saw the August 11, 1999 tornado move through the Avenues.  I was completely naive about the damage that a "small" tornado could do.  Like a surgical knife, it cut through the neighborhood, doing considerable damage along a path one or two houses wide, with nearby homes mostly unscathed.  For those impacted, "only" and EF-0, EF-1, or EF-2 makes little sense.  Their homes and lives have been upended.

6. Overnight snow.   Yup, the white stuff has arrived in the upper elevations of the Wasatch Range.

Source: Alta
The Alta-Collins automated sensor shows a total snow depth of 7", but don't get too excited about that as it was at 4" when there was no snow on the ground prior to the storm.   You can see this in the 5-day time series below.  Accumulations so far at that elevation (9700 feet) probably are around 3 inches.  Not quite enough to break out the new skis...

7. Future snow.  The circulation center for the system is currently moving over us and precipitation is occurring mainly to the north and west of the Salt Lake Valley.

If you feel disappointed, don't despair, we will see more snow today and tonight.  I'm going to go for another 5-10 inches through 6 am tomorrow morning at Alta-Collins.  There's a chance for more, and indeed there were 2 members of last night's NCAR ensemble that pumped out more than 1.5" of water today and tonight for that area (which would probably give us something like 12-18" given the high densities we are dealing with), but 5-10" is the most likely range.  

Thursday, September 22, 2016

Today's Thunderstorm Potential

Editors Note: Post has been updated to include correct SPC categorization of thunderstorm risk, which was erroneously called "severe" in the original.

Yesterday evening, the northern portion of the state won the thunderstorm lottery with strong storms developing over the West Desert and Great Salt Lake and moving across the Northern Wasatch Front, Box Elder County, and Cache County.

Strong wind gusts were reported at a remote observing site east of Snowville (78 mph) and the Logan airport (61 mph).  Lightning data shows numerous cloud-to-ground strikes across that area, but none south of Ogden or over the Salt Lake Valley.  

Source: lightningmaps.org
The National Weather Service watches, warnings, and advisories map is lit up statewide today.

Source: NWS
Issues of concern for today include flooding and flash flooding, high winds, and severe thunderstorms.  The Storm Prediction Center has upped our severe thunderstorm risk from severe marginal to slight.

Source: SPC
The weather situation this morning shows that we are currently sandwiched between two precipitation bands, one to our west that is near a surface trough (not shown) and the other to our east.

During the course of the day today, the surface trough is expected to progress slowly into northwest Utah, while the precipitation band to our east remains roughly between Salt Lake and Vernal.  The big question mark for our weather is whether or not something will bubble up in the intermediate dry slot.  The HRRR shows this as a possibility.  Note the strong simulated reflectivity cells between the two precipitation bands early this afternoon.  

However, these types of storms are very chaotic and we're just going to have to see how this plays out.  

This evening the surface trough will approach Salt Lake, and should be accompanied by showers and thunderstorms.  It's not the usual rapidly moving cold front.  It sort of stalls over northern Utah as the upper-level trough moves through and eventually the so-called "wrap around precipitation" moves across the state.  Tonight through tomorrow night looks quite unsettled and the mountains will get snow.  I need to run to class, however, so discussion of that will need to wait until tomorrow. 

Wednesday, September 21, 2016

Severe Convective Storms Possible

Forecasts haven't changed much overnight, except that moisture is streaming into northern Utah a bit earlier than expected today, increasing the potential for scattered thunderstorms this afternoon.

Over the years, I've learned to pay careful attention to weather situations that involve the interaction of upper-level troughs in the midlatitude westerlies with monsoon moisture as this often leads to the development of severe convective storms.  The pattern through Thursday night fits this description and demands this careful attention. 

Patterns of this type have the three key ingredients for convective storms: Instability, Moisture, and Lift.  With regards to instability and moisture, I've cherry-picked a couple of soundings from the 0600 UTC NAM forecast for Salt Lake City (below).  In these soundings, plotted on a thermodynamic diagram known as a Skew-T, the forecast environmental temperature and dewpoint are displayed as red lines (with temperature > dewpoint and the right-most line).  The grey line is the temperature of a hypothetical surface parcel if lifted "dry" to the point of saturation or cloud base (grey circle) and then "moist" above that with water vapor condensing and releasing heat.  The hatched area shows how much warmer this parcel would be than the environment and equates to the Convective Available Potential Energy (CAPE) of the parcel, a measure of the amount of energy the parcel would acquire if it were lifted in this fashion.  Although there are problems with this theoretical approach, the magnitude of the CAPE provides a rough guess at the strength of the instability if convective clouds can form.  
The plots above show CAPE values of 709 and 957 Joules.  For our part of the world, those are pretty high forecast values.  In addition, the precipitable water, a measure of how deep the water would be if you condensed all the water vapor out of the atmosphere, is 27.5 and 24.1 mm (roughly an inch).  Again, those are fairly high values for our part of the world, especially in the latter half of September.  

So, we have instability and moisture, but we also have lift.  First, we will be downstream of the upper-level trough axis, an area typically associated with large-scale lifting.  We also have a nearby cold front and in all likelihood a variety of smaller-scale surface boundaries as the upper-level trough moves in to initiate convective storms.

So, three boxes checked.  However, we also have some wind shear, which helps storm organization. 

Based on these ingredients, the Storm Prediction Center has placed much of central and eastern Utah in a Marginal Thunderstorm Risk category from tomorrow morning through Friday morning.   Salt Lake and Provo are on the edge of the Marginal Threat Area.  

A marginal risk means isolated severe thunderstorms (Note: showers and non-severe thunderstorms will be greater in coverage) that are limited in duration and/or coverage and/or intensity.  The main threats from these storms are strong winds and small hail.  

With regards to rainfall, through tomorrow afternoon, the NCAR ensemble is generating the greatest average precipitation, maximum precipitation, and highest probabilities of precipitation greater than 1" or 2" between Salt Lake and Vernal.  
That doesn't mean, however, that other areas won't see some showers or thunderstorms.  Let's see how this comes together and keep an eye on official forecasts, watches, and warnings from the National Weather Service

Tuesday, September 20, 2016

Summary of the Action over the Next Few Days

Source: e-trailer.com
Some weather excitement is on the schedule for later this week as the first significant mid-latitude trough of the fall taps into monsoon moisture to give us a wild ride.  

Phase 1 covers today and tonight, although northern Utah isn't really in on the action.  An upper-level trough dropping into the western U.S. helps to coax a monsoon surge and the remnants of Hurricane Paine (doesn't that just roll of the tongue) into the southeast US, with showers and thunderstorms affecting a broad area from SoCal and BaCal through southeast Utah and California.  

Although for us its "No Paine No Gain" today and tonight, the monsoon surge establishes a reservoir of moisture over the southwest that is pulled into northern Utah as the upper-level trough subsequently develops along the west coast and the surface cold front impinges on northern Utah.

Thus, tomorrow (Wednesday) will be at transition day as moisture streams northward.  After that, I'll describe the weather as "highly unsettled" as showers, thunderstorms, and a slow moving front impact northern Utah Wednesday night, and Thursday.  I hesitate to attempt to forecast gory details as there is a complex stew of ingredients coming together.  I do know that the weather often gets interesting around here (heavy rain, strong thunderstorms, etc.) when midlatitude troughs toy with monsoon moisture in the fall, so I look forward to seeing how things play out.

Amongst the new tools for us to examine on weather.utah.edu this winter are downscaled forecasts from the Short-Range Ensemble Forecast System (SREF).  Forecasts from the 26-member SREF for Salt Lake City show the activity picking up Wednesday evening and night and continuing into Friday.  Variations in the slopes of the accumulated precipitation lines show uncertainties in the timing of precipitation and the wide range through the period (from about 0.3 inches to 1.7 inches) shows uncertainty in the total accumulated precipitation for this portion of the storm.
Nevertheless, if my quick math is correct, we've had only 0.43" of rain in the past 3 months and it looks like we have a good shot of exceeding that, perhaps by a lot, by Friday afternoon.  For what it is worth, I moved my automated sprinkler system to the "off" position this morning.

The most recent model forecasts call for snow levels to drop Thursday night.  Right now it's looking like snow at times in the upper Cottonwoods through Friday and possibly Saturday.

Yesterday's GFS was calling for something like 28" of snow for Alta.  Although it's easy to be seduced by a forecast like that, remember it's a long-range forecast and the GFS has been prone to overforecast since it was upgraded to an effective grid spacing of 13 km.  In fact, I find the precipitation produced by the GFS to be of limited value (in the summer, it is nearly unusable!).  The NAEFS ensemble from last night shows most members in the 5-10" range for Alta, with a few larger-accumulation outliers.

Further fueling anticipation is that this is the first possible accumulating storm of the year.  Let's keep those emotions in check and see how things look in a couple of days.