Category Archives: Precipitation

Lessons in Cold-Frontal Precipitation

If you have studied introductory meteorological text books, you've probably seen schematics of frontal precipitation based on the one below, which is from a seminar paper published in 1922 that synthesized existing knowledge on frontal cyclone evolution into a coherent depiction known as the Norwegian Cyclone Model due to it's development in Bergen, Norway.

Source: Bjerknes and Solberg (1922)
The cold front is depicted on the left, with a narrow band of precipitation forming where the leading edge of colder air is intruding into the warm airmass.  This band of precipitation is known today as a narrow cold-frontal rainband (or alternatively, snowband if cold enough).  

Although, conceptually simple, not all cold fronts behave in this manner.  Sometimes a wide cold-frontal rainband exists upstream of the surface front, as depicted below. 
Source: Matejka et al. (1980)
In some instances, the wide cold-frontal rainband exists in isolation, with no narrow cold-frontal rainband present.

Which brings us to today's frontal passage.  At 1400 UTC (7 AM MST) there was a clear separation between the surface trough, which was draped across central Nevada and northwest Utah and a wide cold-frontal rainband (really a snowband) that was further upstream and northwest.  


The 1300 UTC initialized HRRR forecast valid 1900 UTC (1200 MST) very clearly shows the surface cold front, as defined by the wind shift, pushing into the Salt Lake Valley, well in advance of the trailing precipitation band.  


So, this is an instance where we will expect a mainly dry frontal passage, with precipitation moving in later (let's hope things fill in better than in the HRRR forecast above!).  

The visualization below depicts the structure of a similar front on the 27th of November when the surface front was moving across the Great Salt Lake Desert and into the Skull, Tooele, and Salt Lake Valleys.  The cold air behind the surface front was quite shallow and during this period the surface front was dry.  Deeper cold air was further upstream and accompanied by a wide cold-frontal rainband (rainband not shown).  I suspect todays frontal passage will be similar in structure.  


What controls the precipitation characteristics of cold fronts in our part of the world is not fully understood.  The topography appears in some instances to form a new surface trough ahead of the approaching Pacific cold front, and this trough sometimes becomes a new cold front ahead of the precipitation system.  On the other hand, we also see events that feature bonafide narrow cold-frontal rainbands.  These differences can, however, often be anticipated by the HRRR.

Which brings us to the forecast.  I'm sticking with a 3-6" storm total at Alta through noon tomorrow (Thursday), most with the wide cold-frontal rainband (or more correctly, snowband) trailing the surface front, with a few snow showers from the wrap around late tonight and tomorrow.  The snow will be of the low-density variety, which is a shame, as we really need a pasting with high-density snow right now.  

If Alta gets more than 8 inches, consider it a Christmas miracle.  If they get less than 2 inches, you had better take some time to reflect on your behavior this past year, because you are clearly on Santa's naughty list.

A Great Month of Outreach, Education, and Radar Meteorology with DOW7

Professor Powder attempts to keep DOW7 from leaving
After more than a month in Salt Lake City, DOW7 departed the University of Utah campus and began its trip home to Boulder this morning.

It had a great run in northern Utah, despite an uncooperative Mother Nature who was quite stingy providing storms.  We exhibited the DOW for 1500 visitors at the Natural History Museum of Utah and 300 at the University of Utah.

Meteorological outreach at the Natural History Museum of Utah.  DOW in the background. 
In the DOW during sidewalk exhibit at the University of Utah

Several graduate students are now fully trained DOW operators.  Students in our cloud microphysics, synoptic meteorology, mountain meteorology and a radar special topics class were able to participate in operations either on campus or in the field.  My mountain meteorology class is presenting results this afternoon from their initial analysis of a precipitation event in the Ogden Valley.

Seven people stuffed in the DOW.  A common scene during field operations. 
Officially, we did eight "intensive observing periods", or IOPs:

IOP0- Practice IOP scanning some weak snow showers over the northern Wasatch (Location: Antelope Island Marina)

IOP1- Leeside precipitation in the Ogden Valley (Location: Huntsville)

IOP2- Frontal precipitation over the Salt Lake Valley and mountain-induced precipitation over the northern Wasatch (Location: Fielding Garr Ranch, Antelope Island)

IOP3- Exploratory effort to examine precipitation over Ben Lomond (Location: Just south of Willard Bay)

IOP4- Exploratory effort for eared grebe migration (Location: Lakepoint)

DOW near Lakepoint in the Tooele Valley scanning for eared grebes on November 25th.  National Weather Service radar imagery showed the first signs of migration last night, so the truck returned to Boulder just a little too soon. 
IOP5- Cold front with topographic interactions over Tooele Valley (Location: Stansbury Island Causeway)

IOP6- Cold front, influence of Oquirrh and Wasatch range on precipitation, small-scale precipitation structure in and around Cottonwoods (Location: South Jordan Trax Station)

DOW at the South Jordan Trax Station with frontal/orographic cloud over Wasatch
IOP7- Mountain and lake-effect precipitation (Location: Baccus Highway near 7000 South)

Mother Nature's stinginess forced us to take what we could get and do a couple of all-night operations.  IOP1 and IOP2 covered the same storm.  We just moved the DOW from Huntsville to Antelope Island as the storm slid south, changing the IOP number.  IOP6 and IOP7 were also the same storm and we just moved the radar from the South Jordan Trax Station to the Baccus Highway as the winds veered and orographic and lake-effect precipitation evolved.  Knowledge of meteorology, terrain, and potential site characteristics are a real key to making such efforts successful.  Not to mention some motivated graduate students willing to work graveyard shifts.  During such operations, we rotate crews and bring in a fresh driver for moving the DOW in the morning.

Special thanks goes to our sponsor, the National Science Foundation, and the operators of the DOW, the Center for Severe Weather Research, for making the visit possible.  The Center for Severe Weather Research extended the DOW visit a few days to let us capture our most recent storm and for that we are grateful.  I'm fairly certain that storm will make it into at least one master's thesis and maybe more.

OREO IOP1 Is ON!

It's great to finally have some weather again in northern Utah.  I was woken last night by strong southerly winds, and they appear to have transported in a pretty good plume of dust this morning. 

We have a team leaving the University of Utah today at noon to begin operations for the Outreach and Radar Education in Orography (OREO) Intensive Observing Period 1 (IOP1).  Every meteorological field program needs a good acronym (hence OREO) and we usually name each observing period as an "IOP."  IOP0 is typically used for a practice IOP, which we did about 10 days ago on Antelope Island.  Since then we've been waiting on weather.

We plan to operate this evening and tonight from a site just east of Huntsville to examine the spillover of precipitation across the northern Wasatch and into the Ogden Valley.   Already, there's some interesting things happening there.  Radar imagery very clearly shows echoes developing not on the windward side of the Wasatch, but downstream.  Much of this is just sprinkles or virga, but it is a hint that perhaps there is some sort of lee wave present at the moment. 


The HRRR forecast for 1Z (6 PM MST) is optimistic, with band of precipitaiton extending across central Nevada to the northern Wasatch.  The location and movement of that area of precipitation will partly dictate the success or failure of our mission tonight.  We're hoping it is in the right place at the right time. 


Tomorrow, we may be working on post-frontal snowshowers in northwesterly flow.  We have a couple of sites selected to operate out of, but will make final decisions in the morning with updated forecasts in hand.

Keep your fingers crossed!

About Last Night

April showers bring May flowers.  The combination of abundant moisture and good large-scale forcing yielded a solid frontal precipitation band that swept through northern Utah last night.  Below is the KMTX radar image from about 0100 UTC (1900 MDT) yesterday evening.  Something for everyone.
Source: NCAR/RAL
About all we missed out on was severe thunderstorms.  There were some lightning strikes in the area, as indicated below, but I didn't see any strong wind or hail reports on the SPC web site this morning.  That's probably for the best.  It is only in the warped mind of a meteorologist that one is disappointed when severe weather doesn't materialize.

Source: lightningmaps.org
Rainfall reports reported to the National Weather Service show accumulations over .9 inches at several sites along the east bench.  The airport came in with 0.65 inches.  Those are good totals for a relatively brief storm.  

As of 7 am, Alta-Collins has observed precisely 1.00" of water and 7 inches of snow.  I suspect that the first tenth of an inch or so of water fell as rain as temperatures at that location (9662 ft) were in the 40s until 6 PM.  After that, cream on crust.  The snow depth is back up to the 125" US-unit psyche point.  Nice, but for those of you attending the March for Science this weekend, that's 317.5 cm.

Looking Back at Yesterday’s Epic Deluge

Yesterday we were in clear outlier mode in Salt Lake City as we broke the all-time record for precipitation on a calendar day in March and experienced one of the wettest days ever.  I mangled some of the numbers in the previous post, so we'll take a little bit of time today to look back on the event and set the record straight.

The primary large-scale feature driving the event was a mid-level trough that moved very slowly across Salt Lake City and stalled very near the Wasatch Crest for most of the day.  This trough was a large-scale feature extending northeastward to the Canadian border, and served as the locus for precipitation development along much of its lengths.  The image below shows the scene at 2000 UTC (2 PM MDT).  Wind barbs are at 700 mb (10,000 ft).  Gaps in the radar echo coverage in portions of Wyoming reflect both poor radar coverage and some orographic effects.


Cloud and precipitation bands of this type are sometimes called "wrap-around" because the wrap around the backside of the surface or low-level trough.  Indeed that was the case yesterday, as shown below with the sea level pressure analysis, although I'm not entirely satisfied with that classification of this system for a variety of reasons I won't get into here for lack of time.


The National Weather Service reports that the daily precipitation was 1.97".  Note that this is for the midnight to midnight calendar day based on Mountain Standard Time.  We are currently on Mountain Daylight Time.  If the calendar day was defined based on daylight time, the daily total would have been an even larger 2.07 inches, based on surface airways observations provided by the airport.  This highlights an important aspect of meteorological records.  Calendar day records are not the same as 24-hour records, since the latter can use arbitrary start and end times.  Often, major precipitation events straddle calendar days and are broken into two smaller pieces.  If you are using statistics of 24-hour precipitation based on calendar day records to design your storm-runoff system, this is an issue to consider!  Nevertheless, many meteorological records are based on calendar days, I suppose for historical and practical reasons.  For example, many of our meteorological records are collected by volunteers who provide daily observations of maximum temperature, minimum temperature and precipitation amount.

Below is a summary of the 50 largest calendar day precipitation events at the Salt Lake City airport and, prior to the creation of the airport, downtown.  1.97" makes yesterday the 6th wettest calendar day on record and the wettest March day on record.

Source: NOAA Regional climate Centers
The precipitation, however, was not evenly distributed during the calendar day.  It was heaviest prior to about 6 AM MDT, when over an inch fell in the prior 6-hour period, and ceased at 5:40 PM MDT (at the airport, showers lingered on the east bench).  Radar imagery at about 4 AM MDT (0957 UTC) shows that the area around the Airport and portions of the I-15 corridor to the north experience very heavy precipitation with areas of radar reflectivity greater than 35 dBZ (yellow) and localized pockets above 40 dBZ.  Although I'm not showing a loop, these areas of heavy precipitation were quasistationary for much of the night and very important in pushing the precipitation amounts at Salt Lake City to record levels.  

Source: NCAR/RAL
As wet as it was at the airport, there were locations that were wetter.  Tooele recorded 2.49 inches of precipitation and the Rocky Basin Settlement SNOTEL in the Oquirrh Mountains recorded 2 inches of precipitation water equivalent and 22 inches of snow from midnight to midnight MST.  

The central Wasatch didn't do as well because the strongest precipitation along the band was just to the west, along with the northwesterly flow behind the mid-level trough.  As a result, the large-scale precipitation dynamics in the central Wasatch yesterday were weaker.  If one looks at the wind time series on top of Alta's Mt. Baldy, one can see the slow trough passage from 0000 MDT on the 23rd to 1800 MDT on the 23rd with the wind shifting very gradually from south to north-northwest.  Note, however, how weak the winds were for much of the period, especially from 0000-1200 MDT.  It wasn't until the trough had passed late in the day that the northwesterly flow intensified, but by then, the larger-scale precipitation dynamics were dying.  

Source: MesoWest
It wasn't a total disaster for the central Wasatch.  For the midnight to midnight MST period, Alta-Collins picked up about 9 inches of snow and 0.82" of water.  However, this is less than observed in the northern Wasatch and far less than observed in the Oquirrh Mountains.  

The spring pattern continues tonight and tomorrow with a cold front moving in and bringing mountain snow to the central Wasatch starting early tomorrow morning.  The NAM meteogram below tells the story pretty well.  This looks to be a quick hitting event, dumping several inches of snow in the morning, tapering off quickly to snow showers afternoon.  

Source: weather.utah.edu
Last night's NCAR ensemble produced anywhere from 0.6 to 1.6 inches of water for Alta-Collins.  Most members are between 0.6 and 1.1 inches of water.  

Probably 4-8 inch totals lie in the most likely range, but lets keep our fingers crossed we can do a little better.  

Juicy Air for Late October, But Will We Get Much Rain?

A major slug of tropical and subtropical moisture reached California late yesterday in advance of a land-falling upper-level trough and began to push inland overnight.  As of 1200 UTC (6 AM MDT) this morning. a narrow band of heavy precipitation was pushing through the Central Valley with a broader band of precipitation extending inland from northern California to southwest Idaho.  Some scattered showers were found over northern Utah, although only a few drops were reaching the ground.


Precipitable water, a measure of the depth of water you would have if you condensed all of the water vapor out of the atmosphere, is very high with this event, reaching 40 mm along the southern California coast.  Although losses to precipitation and other factors favor a decline in precipitable water as airmasses move inland, we're still looking at relatively juicy air over northern Utah by late October standards.  Precipitable water in this morning's sounding at the Salt Lake Airport was 0.76" (19 mm) and the 0600 UTC NAM calls for values that high to remain over the northern portion of the state as a filament of high precipitable water air pushes up the lower Colorado River Basin through this afternoon.


The highest observed in the sounding record for Salt Lake City after mid October is .91 inches (23.1 mm).
Source: Storm Prediction Center
Looking through the NAM data, there are two peaks in precipitable water today and this evening, one at 22.8 mm, the other at 20 mm, so we're unusually high for this time of year.

Of course, if you want rain, you need to convert that moisture to precipitation.  To our north, that will happen today as the large-scale forcing favors ascent there and the band will spread inland across southern Idaho.

The challenge we have in the Salt Lake Valley is that although the total water vapor in the atmosphere is very high, that reflects moisture at mid and upper levels rather than near the surface.  As can be seen in this morning's sounding, dewpoint depressions are more than about 10ºC below 700 mb.

Source: NCAR/RAL
As a result, the sky is overcast and grey, but it's hard to anything other than a few drops to the valley floor.


As the upper-level trough swings in, however, we should start to see more showers get going.  Still most members of the NCAR ensemble call for .05 to .15 inches at varying times during the afternoon and evening.  Those are relatively modest amounts.  However, there is one member that goes for 0.3 inches, so there is a slight chance that someone will do better than that.

I'm scheduled to have my roof partially replaced today, so the odds are good for that .3 inches to fall in the avenues just after they strip off the old roof and just before they put on the new moisture barrier...

Most Exciting Week in Months on Tap

Yesterday evening served up the quintessential "Spectacular September Sunset."  Golden hues everywhere.  My old cell phone doesn't do it justice, but if you were out, you know what I'm talking about.  




The computer models are suggesting that this coming week will be the most exciting meteorologically in months.  The main culprit is a midlatitude upper-level trough that will develop along the west coast over the next couple of days.  First it will toy with the remnants of Hurricane Paine and bring a pronounced monsoon surge into the southwest (missing us though). 


Then, as the trough amplifies and migrates eastward, a major precipitation event will develop over much of the interior west with showers, thunderstorms, frontal precipitation, and the like.  Very interesting!


The pattern above suggests a very active period later in the week with the bottom falling out temperature wise.  We haven't seen anything like this in months and will take a closer look as the system approaches.  

Frustrating Forecasting

Developing thunderstorm over the Oquirrh Mountains yesterday morning
It's been a tough go finding something to blog about in the current pattern.  With weak troughing and instability over the region for many many days, we've had nothing but hit-and-miss showers and thunderstorms.  Such patterns are the ultimate in forecasting frustration for me. There's no magic pill you can take to forecast this stuff.  You can go for modest changes in coverage and frequency (e.g., today probably won't be as active as yesterday in the Salt Lake Valley), but even that's fleeting.  A couple of days of this I can live with, but it's been going on for so long I feel like Bill Murray in Groundhog Day!

It does look like we'll be getting a drying out and a warming up early next week.  Probably by next Wednesday I'll be wishing for a return of the unsettled weather!

Overnight Deluge

A slow moving precipitation band brought significant rain to the Wasatch Front late last night and early this morning, especially north of Salt Lake City.  Radar imagery below shows the strong returns over the northern Wasatch and lowlands upstream (northern most red rectangle).


As of 9:30, Snowbasin-Middle Bowl recorded 0.81" of water equivalent since 4 AM.  And yes, some of that precipitation has fallen as snow on the upper mountain.


Ogden airport picked up a half inch, which is a good dousing for the lawns and gardens.

Further south, the central Wasatch (southern rectangle) were late to get in on the action.  Alta-Collins has had only .12" of water equivalent since 3 AM, although temperatures are now down to 30ºF, putting that site above the melting layer for now, and they may get a couple more hours of snow.  A little something for the diehards.

Meanwhile, in the Salt Lake Valley, the U is up to .27", which should be enough to keep my sprinklers off for another week, which is good because I just discovered that I have some work to do to get some of mine working.


Efficient Spillover of Precipitation across Wasatch Range

A quick look at the radar loop this morning shows that the Salt Lake Valley will be seeing steady precipitation for most of the morning.


A closer look reveals that the broader-scale precipitation shield is moving southward as the echoes move from the northeast.  This is a classic "wrap around" situation in which we receive precipitation being generated in the northeasterly flow behind the upper level low.  Those cells are being generated well above crest level as the flow at mountaintop level shifted to southwesterly overnight.


Of one goes up a bit higher, however, to 500 mb (about 18,000 feet above sea level), one finds the magic northeasterlies and the layer where those cells are being generated.


One might wonder why with northeasterly flow we're not seeing downsloping and drying on the west side of the Wasatch Range.  The answer is wind shear.  The upper-level flow is northeasterly, but as can be seen in the Ogden Peak time series, the flow at crest level has a westerly component.  Surface observations in the valleys show generally light, disorganized flow.  Thus, there really is no downslope and we're seeing efficient spillover of precipitation across the Wasatch Range.

The rain will help keep the foothills green going and the sprinklers off for a while longer and is certainly appreciated.