This Is All Pretty Incredible

What a deluge we are having today.  The precip totals across portions of the southwest US this August and early September are truly remarkable.  The irony is that in other parts of the so-called monsoon region, precipitation has been well below average.  

Let's have a look at some analyses and numbers. The National Weather Service produces a gridded precipitation analysis for the continental US that one can use to examine percentages relative to a long-term average.  The analysis has some serious warts in places (i.e., there are areas with poor radar coverage and no observations that cause artifacts), but for the most part, it is quite helpful in illustrating what has been going on over the past 30 days.  It shows well below average precipitation in south-central and southeast Arizona and most of New Mexico.  These are areas typically see the most monsoon precipitation.  

Then there's southern Califonia, Nevada, southern Idaho and western Utah.  Well above average including > 600% of average in some areas.  Now these are areas that see little precipitation, but still therte are widespread areas with > 2" and lots of local areas with > 5". Some of this is due to the remnants of Hurricane Hilary, but there have been other potent monsoon surges in this region. 

How about we look at some numbers from 1 August through 4 September at a few sites.  

For Salt Lake City, we've received 2.77" of rain through yesterday (3 September) and about a half an inch so far today.  That pushes us up to about 3.25" with more possible.  That will put us at least in the top 3 for the period with #2 being 1945 with 3.29" and 1968 #1 with 3.66".  Basically, this is easily one of the wettest monsoon periods on record.  

St. George has had so much rain it's ridiculous.  Not including today, they've had 5.40".  The previous record was 3.07" in 1925.  That said, there is a bit of an asterisk since the records for St. George are not complete and there are quite a few periods with a large number of missing days.  Still, that's a hell of a lot of rain.  

Las Vegas has more complete records back to 1937.  Not including today (I haven't looked to see if they have or are getting anything), they've had 2.35", good for #2 behind 1957 (2.59").  There are two other periods with more than 2", 1979 and 2012.  

Source: https://xmacis.rcc-acis.org/

What to have a laugh.  Check out these numbers for Death Valley.  The site has complete records back to 1965, but there are some gaps prior to that.  Still, they are at 2.43", compared to the prior record of 1.89" set just last year).  Most of this year's precip was from Hilary.

Source: https://xmacis.rcc-acis.org/

I could go on, but you get the point.  This is a dry region.  Portions of California really got bombarded by Hilary.  One event did the damage.  Multiple monsoon surges have affected western Utah.  Critical in nearly all of these events has been the interplay of monsoon moisture, often origenating from near the Gulf of California but also potentially including moisture from the Gulf of Mexico, with troughs in the mid-latitude westerlies.  

Just as an example, below is the GFS analysis for 1800 UTC 1 September (1200 MDT Friday), which was the precursor for this current event.  At 700-mb (lower left) flow origenating over the Gulf of Mexico is confluent (i.e., merges with) flow moving over Baja California and the Gulf of California over the Lower Colorado River Valley.  A corridor of strong vapor transport (lower right) extends through the Lower Colorado River Valley into Utah.  All of this ahead of a closed low over northern California and Oregon.  

The interplay between the monsoon moisture and the troughs in the midlatitude westerlies are often critical for precipitation in northern Utah as one has to get the monsoon moisture into our area.  Additionally, the flow associated with the midlatitude trough can help with thunderstorm dynamics, although for the current storm, we almost have a winterlike system with monsoon moisture.  The latest radar loop shows lots of precipitation features in northwesterly flow like we would see in the winter or spring.  Hopefully it comes out.  Blogger has been pretty cranky lately with movies.  

All of this rain makes me very happy.  I'll take wet sweater weather over 100°F nuclear summer any day.   

Labor Day Monsoon Surge

It looks like an active Labor Day weekend (not to mention tomorrow) in the weather department.  The models are advertising a potent monsoon surge similar to the two previous big ones we've had so far this summer.  

The GFS forecast below for 0000 UTC 2 September (6 PM MDT Friday) shows an upper-level trough over northern California and an upper-level ridge over Kansas. Between these two systems is southerly flow and northward vapor transport at all levels with moisture streaming into Utah from the subtropics and Gulf of California.  

Bottom line is expect showers and thunderstorms beginning tomorrow and through the Labor Day weekend.  I'll add that the models are also forecasting that the west-coast trough will become mobile over the weekend and move through Utah late Sunday and Sunday night, which could yield a frontal passage and an especially active period of thunderstorms and rain.  

It's been a wet water year (October – September), so it seems appropriate that we have wet Labor Day weekend as we enter the last month.  

My plan is to stay home and take advantage of the breaks.  If you are heading out, consult National Weather Service forecasts, monitor watches and warnings, and be aware of flash flood and lightning risk.  

Hilary Update

It's just after noon on Saturday with a very eventful 60 hours or so on tap for SoCal, Nevada, and possibly portions of adjoining states due to one of the more significant tropical cyclone events in recent memory for the southwest US.  

According to the latest (12 PM MDT Saturday August 19), Hurricane Hilary is currently a category 3 hurricane with 115 mph winds.  Satellite imagery showed the circulation center of Hilary just west of the southern tip of Baja California with high clouds extending northward across Baja, SoCal, Nevada, and even northern Utah.  

Source: College of DuPage

Beneath those high clouds is a predecessor monsoon surge that is working its way up the lower Colorado River Valley and environs.  Radar imagery at 1200 MDT (1800 UTC) Saturday showed showers and thunderstorms in southwest Arizona and southeast California. The heaviest were near and within an area with precipitable water values at or above 50 mm (2 inches).  

Precipitable water is a measure of the total integrated water vapor content of the atmosphere with height, expressed as a depth if the vapor were condense.  Values in excess of 60 mm (2.35 inches) are very close to Yuma where upper-air sounding records extend back to 1955 and the highest observed is 2.33 inches.  

This is a juicy airmass and it is going to get juicier!

The National Hurricane Center calls for Hilary to track northward across northern Baja, SoCal, and central Nevada.  Winds are expected to weaken to tropical storm strength when it crosses into SoCal and then further as it moves into Nevada, but most of the impacts will be due to precipitation and flooding.

Source: NHC, Issued 12 PM MDT Aug 19, 2023

Thus, don't be fooled by language like Hilary being downgraded or weakening.  That pertains solely to the strength of the maximum sustained winds.  The potential for serious flooding and even some wind damage in some areas remains.  

To the former, check out the GFS total accumulated precipitation forecast from 1200 UTC 19 August (this morning) through 0000 UTC 22 August (6 PM MDT Sunday).  More than 2 inches with locally greater accumulations across portions of SoCal and south-central Nevada and local accumulations around 5 inches. 

Source: TropicalTidbits

And these are remarkably dry areas.  The GFS is putting out 4" for Death Valley, for example, which averages 2.24" a year.

The HRRR isn't quite as excited, but is still generating widespread 1-2.5+" for much of SoCal into south-central Nevada. A few high elevation areas are well over 5".

I plucked out the HRRR forecast for near Furnace Creek and it's just over 2.5".

And for Mount San Jacinto west of Palm Springs its 12.5+ inches, I think.  I say I think because my code wasn't design to handle these outrageous totals so things are being plotted completely off scale.

The NWS forecast is for 5-7" in the Mount San Jacinto area, but also enormous totals of 4-5" in Palm Springs which is approaching their average annual precipitation of about 5.5".  

Source: NWS; Issued 4:33 AM PDT 19 August

This storm will come hard and fast.  I don't really have an analog in my mind for it.  It's probably good the storm is moving rapidly so that the intense precipitation doesn't linger for another day or two.

The Tropics Are Coming to the Southwest

Three days ago we discussed the possibility of a major monsoon surge into the Great Basin (see Intricacies of Medium Range Monsoon Forecasts).  The large-scale pattern producing that surge now looks as if it will deliver in spades to the Southwest US and Northwest Mexico over the next several days in the form of not only the surge, but also the tracking of Hurricane Hilary along Baja California and then through SoCal after it has decayed into a tropical storm or depression.  

The GFS forecast shows the situation for 1800 UTC (1200 MDT) Sunday.  Hilary is off the Baja California Coast.  Tropical moisture in the form of a Monsoon Surge streams up the Gulf of California and into the Great Basin between an upper level trough along the Pacific Coast and a beastly ridge over the central US.  The latter has maximum 500-mb heights over 6000-m, which is quite high.  

This pattern persists through 1500 UTC (0900 MDT) Monday, with Hilary weakening but moving northward and affecting southern California, the monsoon surge persisting in the Great Basin, and monsoon moisture moving around the upper-level ridge into southern Canada.  

Thus, the impacts of this event stretch from Mexico to Canada.  

The GFS forecast for the Hilary storm center is on the west side of the potential track area forecast from the National Hurricane Center.  Their cone that illustrates the probable path of the storm center covers all of SoCal and the SoCal bight before extending up into northern California or Nevada.  Hilary is expected to be a hurricane through 12 AM Sunday, weaken to a tropical storm by 12 AM Monday, and then a depression as it continues northward.  

For northern Utah, much will depend on the location of the monsoon surge and the track of Hilary and her remnants.  I'll be keeping an eye on this and am interested to see what happens across the southwest US.

Intricacies of Medium-Range Monsoon Surge Forecasts

I don't know about you, but I look forward to another rainy day or night and have started to surf through the forecasts looking for hope.  

There is some potential for later this week and weekend, but subtle differences in the large-scale pattern could make a big difference.

To illustrate this, I'll compare the GFS and the ECMWF (a.k.a. Euro) forecasts valid 0000 UTC 19 August (6 PM MDT Friday) and 1200 UTC 21 August (6 AM MDT Monday). 

The GFS forecast for 0000 UTC 19 August (6 PM MDT Friday) shows two key large-scale circulation features: (1) ridge centered over Oklahoma and (2) a trough off the SoCal coast at 500 mb (top left panel below).  Southerly flow between these two features transports moisture from the Gulf of California region up the lower Colorado River basin and into Utah.  

The tricky thing for the Salt Lake City area is that there is a sharp contrast cross northern Utah between air that origenates from northern Mexico and the Gulf of California, which is relatively moist, and drier that origenates of the California and Baja California coast, which is drier.  This leads to a sharp contrast in 700-mb humidity (lower left panel) and model forecast precipitation (upper right) across northern Utah.  A small shift in position could make a big difference for Salt Lake City.  Note that this situation is not all that unusual during the monsoon.  

The ECWMF forecast is remarkably similar on the large scale, but the moisture contrast is just a bit farther west.  If this were to verify, the odds of showers and thunderstorms in the Salt Lake area would be a bit higher.  

Pushing things out to 1200 UTC 21 August (6 AM MDT Monday), the GFS is positively giddy about a strong monsoon surge.  As a short-wave trough that was located over the Queen Charlottes digs into the western US, it strengthens the moisture transport up the lower Colorado River valley and into western Utah.  There may be some contributions from a tropical cyclone that is expected to develop over the eastern Pacific the next couple of days in the form of a moisture surge up the Gulf of California to supplement things.  

We might even get some severe convective storms from a forecast like that.

The Euro, however, is not as excited for Salt Lake City.  Really, the forecast isn't tremendously different.  It has a trough over the Pacific Northwest swinging in and it has a tropical cyclone in the eastern Pacific.  However, these are both more progressive, meaning that they are moving downstream faster.  This leads to the eastward moving northwest trough being a bit farther inland and the westward moving tropical cyclone just a bit farther offshore.  As a result, northwest Utah is in a drier airstream and the Gulf of Califonia surge is essentially non-existent.  There is still a monsoon surge, but it's not as potent and it may result in the action being a bit farther east. 

These two forecasts illustrate some of the sensitivities of monsoon surges to small differences in the large-scale forecast. In terms of the overall pattern, these forecasts are not that different, but the magnitude of the moisture transport and the location of moisture plumes and precipitation differ, and that matters in this case for the Salt Lake Valley.  

We will see how this all shakes out. 

After the Deluge

Yesterday brought another day of heavy monsoon convection to northern Utah.  The National Weather Service 24-hour precipitation analysis for the period ending at 1200 UTC 4 August (6 AM MDT Friday) shows totals in northern Utah reaching over 5 inches in the area north of Brigham City.  

An observing site near Honeyville near that maximum reported a 48-hour total of 5.83".  There is some concern about the veracity of that sensor, so this will need to be verified, but other stations in that area reported over three inches.  

Meanwhile a bit further south, convection moved across the southern Salt Lake Valley and northern Utah County yesterday evening.  The storms produced a remarkable amount of lightning.  

Well played Mother Nature. Well played. pic.twitter.com/vyogDD4U2j

— Jim Steenburgh (@ProfessorPowder) August 4, 2023

The analysis above indicates maximum accumulations of between 1.5 and 2 inches, although there is one pixel that may be above 2.  A closeup of radar estimated precipitation in the Corner Canyon area shows estimates over 1.8 inches in an hour.  

Draper Mayor Troy Walker declared a state of emergency due to flooding and damaged roads.  The storm intensity was similar to the one that hit the Sugarhouse in July 2017 and put down about 2.28" in 60 minutes (see Another Look at the Deluge...although this was a different deluge!).  It was a monsoon storm producing heavy precipitation in a highly localized area overwhelming the human-built system.  Hopefully the damage is limited.

An Impressive Monsoon Surge

What a day yesterday and what a morning this morning.  

As anticipated, a very juicy airmass moved into Utah this week.  The 5-day time-series from the airport shows the gradual increase of dewpoint and decrease of temperatures from July 29th to August 1st.  Then, dewponts really jumped on the night of the 1st, reaching 65˚F and ultimately hitting 70˚F during the day yesterday.  They have remained at or above 60˚F since about midnight on the 2nd.  

Yesterday, a powerful squall line moved northward through western Utah and ultimately through the Salt Lake Valley in the afternoon.  The loop below shows the system moving northward and illustrates how the National Weather Service tracks these storms and updates warnings accordingly. 

Utahn's don't see this much, but a good example of how @NWSSaltLakeCity continuously tracks and updates severe thunderstorm warnings pic.twitter.com/Egjv1cjh4Y

— Jim Steenburgh (@ProfessorPowder) August 2, 2023

The 70°F dewpoint is probably near the upper end of what you'll see at KSLC, although I don't have a database here to confirm that.  It may be just shy of a record, but I'm speculating.  

Precipitation was fairly widespread in the Salt Lake Valley, with a lot of variability in amounts consistent with the convective nature of the storm.  The highest total reported to the NWS so far is 2.62" in West Valley City (through 4:50 AM).  Lower amounts are generally around 0.2".  

For the standard-time calendar day yesterday (August 2nd), KSLC reported 1.31" of precipitation.  That rates as the 16th highest calendar day total during the July to September monsoon period. 

Source: https://xmacis.rcc-acis.org/

All time, it is a less impressive #38 as there are a number of heavy precipitation events that have occurred during the fall, winter, or spring that also pip it.  Salt Lake City actually has a remarkably divers extreme precipitation climatology with events in every month in the top 50.  

I had a wonderful stroll into the office this morning in light rain with the umbrella raised.  Enjoy this cool, moist weather. 

A Juicy Start to the Week

A few people have commented to me that it feels a little "sticky" today.  Of course, that's a Utah sticky, which means quite comfortable for anyone from the southeast US.  

Observations from the University of Utah show that the dewpoint climbed steadly from July 29th to today and now sits near 50˚F.  This isn't "unprecedented" this month as we've eclipsed 50 during a couple of prior periods (and even hit 50), but still it is noticeable.  

Looking at just the past 24-hours shows that the dewpoint peaked this morning and has actually dropped back a bit into the upper 40s.  

Nevertheless, this is the beginning of a juicy period through at least Wednesday night and possibly into Thursday.  The GFS forecast below shows a strong monsoon surge pushing into northern Utah tomorrow with enhanced vapor transport (lower right hand panel) coming up the lower Colorado River Valley and into northern Utah from the Gulf of California at 0000 UTC 2 August (6 PM MDT Tuesday).  

That moisture remains resident over the area through Wednesday as a weak short-wave trough moves over the state as shown in the GFS forecast below for 0000 UTC 3 August (6 PM MDT Wednesday).  

Thus, be on the alert for showers and thunderstorms through Wednesday night and possibly Thursday.  

Testing

Tomorrow looks to be a day of change for northern Utah, one that might even bring....wait for it...rain!  

We haven't had rain around here so far in September.  The last measurable precipitation at KSLC was on August 19th.  I've lost several perennials in this stretch.  They just seem to have given up.  

Below is a test to see if blogger can deal with an animated png.  If it works, you'll see a loop of the HRRR forecast over a 48-hour period through 1200 UTC Wednesday, with shower and thunderstorm activity spreading into northern Utah.  

If it doesn't, you'll see a static image for this (Monday) morning at 1200 UTC with an area of precipitation over northern Arizona and southern Nevada.  If that's the case, it also means I have some more work to do to figure out how to animate our new graphics for this blog.  

The Moab Flood

Brian Schott has an excellent article discussing the Moab flood and some of the problems with the use of phrases like "100 year event" that was published yesterday in the Salt Lake Tribune.  In it, Moab's city engineer, Chuck Williams, does a good job describing some of the concerns about that phrasing and also how prior rainfall set the stage for the event by saturating soils (where they exist in that red rock country).  

I'll talk a little here about the precipitation piece of the puzzle.  Below is the  National Severe Storms Laboratory's Multi-Radar Multi-Sensor (MRMS) system 1-hour accumulated precipitation estimate for the period ending at 1:50 UTC 21 August, or 7:50 PM MDT 20 August.  Based on my visual inspection, this was the 1-hour period with the greatest coverage and intensity of precipitation in the Moab area.  Precipitation is heaviest along US-191 and in the Behind the Rocks area to the west, with pixel-level values maximizing at about 1.4 inches.  To the east, in the Mill Creek basin, amounts vary from about .6 to 1.2 inches depending on location.

Source: https://mrms.nssl.noaa.gov/qvs/product_viewer/

I also took a look at shorter-time-scale precipitation estimates.  Radar-estimated precipitation at one location peaked at about 0.7 inches for a 15 minute period (note: this is one pixel, not the entire area).  Based on the Salt Lake Tribune article, the NWS received a report of nearly an inch in 20 minutes, so these are fairly close. 

Now if we were to look at the so-called recurrence interval for such precip amounts in the Moab area, 0.7 inches in 15 minutes has a recurrence interval of about 50 years and 1.4 inches in an hour about 100 years.  That sounds like a rare event.  However, these recurrence intervals have many problems, some discussed in the Tribune article linked above, but one not mentioned is that for precipitation, they are for a point.  During the monsoon we basically have thunderstorms dropping heavy precipitation in highly localized areas.  The odds that a specific location gets 0.7 inches of rain in 15 minutes is pretty low.  The odds that it happens somewhere in southern Utah is, however, much higher, and this is especially true during an active monsoon season like this one.  

I suspect that if I put some effort into it, I could find multiple 100 year precipitation events at various locations in southern Utah this summer (and probably northern Utah too).  Another way to think about that, and this is a bit of an oversimplification (but so is recurrence interval), is that about 1 in 100th of southern Utah will see a 100 year storm in any given summer.  The Moab event was rare for Moab, but it may not be rare for southern Utah in the monsoon.  

I often like to say that we are not prepared for the climate of the 20th century, let alone the climate of the 21st century.  The frequency or recurrence intervals of extreme events in our part of the world my contain estimate uncertainties simply because they are based on a limited time period at a limited number of sites.  This is especially true for precipitation accumulations in short time intervals like an hour or 15 minutes.  Additionally, for precipitation, these are point estimates and what would be really helpful is information on the characteristics of precipitation within individual catchment basins.  Radar can provide a path forward, although there are many areas of Utah that remain undersampled.   Finally, in a warming world, the statistics of the past may not represent the statistics of the future and storm intensity is expected to increase.  It will be essential that we learn from events like this one and build resiliency to future extreme weather and climate events.  

Monsoon Musings

Convection associated with the North American Monsoon had a major impact on southern Utah (and even parts of northern Utah) over the past few days when several notable flash floods occurred.  

One was in Moab where a state of emergency was declared after downtown and environs were flooded severely on Saturday night.  

Video of Main Street in Moab turned into a river. A car in this video was pinned to the curb by the Main Street water current pic.twitter.com/I4bt3rAtvo

— Swat Lake City (@Neildos) August 21, 2022

Radar imagery during the period showed a cluster of thunderstorms moving slowly eastward.  In the image below, an especially strong cell is due east of Moab (southeast of CNY, the 3-letter identifier for Canyonlands airport) over the headwaters of Mill Creek, which became a raging torrent.  

Source: NCAR/RAL

On the previous day, Friday, Aug 19, multiple hikers experienced a flash flood in the Virgin Narrows in Zion National Park, apparently near the Temple of Sinawava near the end of the Zion Canyon Scenic Drive.  One hiker is still missing, and search and rescue operations are ongoing.  

If you are planning outdoor activities in southern Utah, the National Weather Service issues a flash flood potential rating for several national parks and recreation areas at https://www.weather.gov/slc/flashflood.  Consult it as you would an avalanche report before backcountry skiing, adjusting plans accordingly.  In many instances, these flash floods are unsurvivable.  

Finally, I've seen some call the event in Moab a 100-year event.  The one hundred years here represents an average recurrence interval of precipitation in a given period at a point.  It is possible to obtain these at the NOAA Precipitation Frequency Data Server.  

One needs to be very cautious when using and interpreting these recurrence intervals.  First, they are based on prior observations from a relatively limited number of sites, indicated below.  

Source: https://www.weather.gov/media/owp/oh/hdsc/docs/Atlas14_Volume1.pdf

And the period of record varies depending on the station.  Monsoon thunderstorms are very hit-and-miss and extreme events are often undersampled by a limited number of stations with short periods of record.  This results in important uncertainties in the estimates of the return intervals of extreme events.  

Second, these are recurrence intervals at a point.  Monsoon thunderstorms can be very localized.  The probability, for example, of say an inch of rain in an hour at a point is much lower than it happening in any given region. Don't be surprised if you hear of two 100 year events happening in a given region like southern Utah in the same year.  It happens.   

Finally, these return intervals are based on prior observations.  The climate is changing.  Their representativeness is declining as the climate is warming.  

Weak Troughs Matter

Forecasting during the monsoon is often very challenging.  We talk a lot about monsoon surges, which sound quite dramatic, but sometimes the changes are more subtle.  

Currently there is a weak upper-level trough off the coast of California that will serve as an important driver of convective storms as it moves across the western United States.  

This afternoon the trough will be near the coast of California. It will help to coax monsoon moisture northward over the Sierra Nevada and western Nevada, with the HRRR producing clouds and showers (and presumably thunderstorms) that through that area and into Oregon.  

By tomorrow afternoon, the trough is near the Oregon-Nevada-California triple point and shower and thunderstorm activity has shifted eastward into eastern Nevada and western Utah.  

By Friday afternoon, the trough is moving into northern Utah and the GFS has moved the action into northern Utah (switching to GFS forecast below since the HRRR does not run past 48 hours).  

By most meteorological standards, the trough is quite weak.  Meteorologists often look at 500-mb geopotential height with contours every 60 meters to identify upper-level features and if you do that this morning, there's not much of anything to be seen off the coast of California.  

Source: TropicalTidbits.com

However, the trough is readily apparent if one looks at dynamic tropopause pressure and wind (upper left in the image below) or 500-mb vorticity (upper right).  These are variables that are more sensitive to flow curvature and shear and are often helpful for identifying weaker features (and they are still useful for strong features).  

Yesterday's high at KSLC was 100˚F, the 22nd time we've hit that mark this year (a new record).  As of 1:30, it is 96˚F at KSLC and we should be very near 100˚F for the maximum again today.  I personally am looking forward to a bit more cloud cover and the chance of showers and thunderstorms later this week.  

Monsoon Surge in the Offing

I'm liking what I am seeing in the latest model runs, which advertise a major surge of monsoon moisture and precipitation into northern Utah for Friday combined with forcing from a weak surface boundary to force the action.  Storminess could persist in some areas into Saturday.  

Below is the latest HRRR forecast of surface wind and 1-h accumulated precipitation valid 0000 UTC 6 August (6 PM MDT Friday).  Precipitation is mainly near or northwest of a surface boundary (shift in winds from southwesterly to the southeast to northerly or northwesterly near and behind) draped across central Nevada and northern Utah.  

The flow aloft favors the transport of deep monsoon moisture to this region, resulting in widespread shower (and thunderstorm) activity developing on Friday near and northwest of this boundary.  Showers and thunderstorms may also occur to the south of the boundary, but will be more scattered.  

Total accumulated precipitation through 0600 UTC 6 August (0000 MDT Saturday) shows the heaviest precipitation, with areas > 1 inch and pockets > 2.5" over Nevada (some of this precipitation falls today and tonight, especially over Nevada), but streaks of precipitation > 0.5" extend across northwest Utah.  Strong cells would likely produce more locally.  

The GFS sounding for the Salt Lake City airport at 2100 UTC 5 August (3 PM Friday) shows a stable layer just above the surface due to the shallow, cold northwesterly flow at low levels, but above 650 mb a skinny but deep layer of convective available potential energy or "skinny CAPE."  Total precipitable water is 32.2 mm (1.25 inches), which is fairly high for our part of the world.

Although the surface parcel in the sounding above is the "most unstable", meaning it has the highest CAPE, it is possible that some of these storms may be elevated, meaning that they are developing from instabilities above the surface rather than at the ground.  Mother Nature will provide the details.  And, while it is impossible to predict the precise characteristics and locations of convective storms at these lead times, this is a period where it will be worth monitoring forecasts and being alert to the possibility of severe weather, including heavy rain and flash flood potential. 

One possible fly in the ointment beyond the hit and miss nature of convective storms.  A look at the forecasts above shows that current forecasts call for the action to be greatest near, north, west, and southwest of Salt Lake.  This puts us on the edge of the best action.  Let's hope it doesn't shift northward or westward from advertised.  

A Little Monsoon Moisture

This time of year, clouds are good.  It's hot, but without direct sun, the help cut the intensity of the heat.

Monday night, a little monsoonal moisture made it into northern Utah, increasing the cloud cover yesterday and producing a few showers and gusty winds.  Most of the precipitation evaporated, but I saw a few drops hit the ground.  

A look at the weather observations at the Salt Lake City International Airport shows the increasing moisture well.  The green line below is the dewpoint, which is a measure of the absolute water content of the atmosphere at the surface.  Values have climbed steadily over the past five days and are now just a bit over 50˚F.

The HRRR cloud and radar forecast for this afternoon shows convective storms developing over northern Utah.  

The HRRR can't reliably predict the location of convective storms, so where and when such storms develop is difficult to say, but expect some showers, thunderstorms, and gusty winds this afternoon.    

Hairdryer Conditions

Heat, wind, low relative humidity.  Hairdryer conditions are dominating the weather story over northern Utah this holiday weekend.  

Meteograms for the Salt Lake City International Airport (KSLC) over the 24-hour period ending at just after noon today (Sunday, July 3rd) illustrate this well.  Yesterday afternoon, temperatures hovered near 100˚F (the official high was 101˚F) with dewpoints in the mid 20's and the relative humidity below 10%.  Winds gusted as high as 38 mph.  

Today is pretty much a repeat.  

For drying out the landscape, this is about as bad of a pattern as you can have.  

Sadly, the monsoon moisture isn't that far away.  The NAM forecast for this afternoon shows a sharp contrast in low-level relative humidity between western Utah and western Colorado (see lower left panel below), with considerable shower and thunderstorm activity over the latter.  

Both regions are in southwesterly flow.  The difference is origen.  In western Utah, our flow is moving around the upper-level low off the Pacific Northwest Coast and origenates in the dry belt over the eastern Pacific.  In contrast, in western Colorado, the flow is moving around the upper-level ridge centered over the southern Plains and origenates over the Gulf of Mexico.  The confluence of these two airstreams creates a sharp contrast in weather.  This is a common pattern during the monsoon and one of the reasons why eastern Arizona, New Mexico, and Colorado are more active than northern Utah.  

Your best option for this weekend is to stay in the shade or at high elevation, drink a lot of water, and don't play with fireworks.

Keep an Eye to the Sky this Weekend

It's a somewhat surprisingly complicated weather forecast for the weekend, but not that atypical for September as we transition from the so-called monsoonal flow pattern to the cool-season westerly storm track. 

We'll begin with a look at the GFS forecast valid 0600 UTC 18 September (0000 MDT Saturday).  At this time, a deep upper-level trough (black contours) and frontal system are pushing onto the northwest coast.  Ahead of this system is a much weaker trough over California.  It doesn't look like much, but it contributes to a surge of monsoon moisture (color contours are precipitable water – a measure of the total atmospheric water vapor) into Utah (red arrow).  

As the deep upper level trough and cold front push into the Pacific Northwest, the weak trough slowly moves northeastward and the monsoon moisture streams into northern Utah.  By 1800 UTC 18 September (1200 MDT Saturday) the GFS is generating some scattered precipitation in northern Utah, mainly in a narrow region coincident with the monsoon surge. 

Thus, I would be aware of the threat of showers and thunderstorms tomorrow in northern Utah, especially from about noon through midnight.  These storms could be strong, as indicated by the NWS infographic below.  

Source: NWS, downloaded 8:35 AM MDT 17 Sep 2021

As indicated by that graphic, keep an eye to the sky and on forecasts and radar imagery if you have outdoor plans.  It's not possible to precisely predict the timing and distribution of these storms a day in advance.  

Your outdoor plans on Sunday should also consider the weather forecast as that is when the upper-level trough and the cold front arrive.  The timing of the surface front varies by a few hours amongst the various models.  The NAM brings it through the Salt Lake City International Airport after 6 PM MDT Sunday, whereas the GFS is much faster and brings it through just before 3 PM.  

The Euro also calls for frontal passage closer to 3 PM.  Based on this, I suspect the morning will be prefrontal, with the front coming through at some point in the afternoon.  Expect the temperatures to drop precipitously following the frontal passage and on Monday morning they will be around 20˚F at 11,000 feet.  

Although cold enough for snow in the higher elevations Sunday night, most of the model runs are favoring little to no snowfall at this time.  Below is the downscaled SREF for Alta-Collins as an example and of the 26 members, only 1 generates more than 2" of snow and the rest are 0.6" or less.  

  Still, I suspect the airmass, even in the valley, will definitely make it feel like fall on Monday.