Wasatch Weather Weenies

Yesterday (Tuesday) morning, the Salt Lake City International Airport observing site, which is located at the south boundary of the airport just north of I-80, observed as rogue of a rogue shower as you will ever see.  The storm produced rain at the observing site from about 0725–0755 MDT, generating 0.16" according to the rain gauge.   KSLC Time Series 0725–0755 MDT 6 Oct 2026. Source: NWS. The radar at 1317 UTC (0717 MDT) showed the rogue cell moving over the airport as it intensified.  The cell was perhaps 5 km across and developed pretty much over or just upstream of the southeast corner of the airport.     The weather cam image at the top of this post, taken looking west from the University of Utah, shows just the narrow area of falling precipitation associated with the cell.  What conspired to make such an isolated precipitation shaft is known only to Mother Nature.   Radar-estimated precipitation from the National Severe Storms La...

Two very powerful cyclones, one from the tropics and one from the mid to high latitudes, are going to conspire over the next several days to break the jet stream.  The tropical cyclone is Typhoon Choi-Wan, currently southeast of Japan with maximum sustained winds of 110 knots.  Source: Joint Typhoon Warning Center The mid to high latitude cyclone hasn't formed yet, but is expected to develop over the Sea of Okhotsk on Wednesday.  The ECMWF HRES forecast below from Tropical Tidbits is valid 1800 UTC Wed Oct 7 and includes sea level pressure contours (black lines) and 500-mb height (color fill).  The big circle identifies the mid to high latitude system, the little circle Choi-Wan, which at this point is transitioning into an extratropical system, and the black line the upper level ridge downstream of these two systems.   Source: Tropical Tidbits By 1800 UTC Saturday 10 October, those two cyclones have merged into a massive 976 mb low over the Bering Sea....

Yesterday was one of those days when if you were skiing at Alta in the afternoon, consider yourself blessed or, alternatively, it's better to be lucky than good.  From 1100 to 1600 MDT, Alta-Collins picked up 9" of fresh, including 3" in an hour from 1200 to 1300 and then again from 1400 to 1500.  Water equivalent was .46", so this was 5% water content.  Winds on Mt. Baldy during that period never guested over 10 mph.   I took a look at forecasts from the 12Z models on the prior day (2 April) and the GFS was going for nothing.  The HRRR .15" water and 2.7" of low-density snow.  Even yesterday morning, expectations were low.  The Utah Avalanche Center Forecast that morning called for 0.5 to 1" of snow. This isn't to throw them under the bus as they do a great job, but just to illustrate that yesterday's snowfall was pretty unexpected.  It wasn't handled well by the models or the forecasters.  So what happened? Well, morning broke with not...

Great Salt Lake Effect forecasts from computer models have historically exhibited poor reliability for a few reasons.   One is that many computer models have had insufficient resolution (or grid spacing) to adequately resolve the Great Salt Lake and surrounding mountain effects.   Another is that the Great Salt Lake is simply very sensitive to small changes in the ambient flow and surface conditions, so when you look at a handful of model runs, they don't always capture the full range of possibilities. Let's take a look at how these issues are manifest in current model runs, starting with the GFS. The effective grid spacing of the GFS is 13 km.  At it's current level, the Great Salt Lake is about 25-km wide, meaning that in northwesterly flow, the GFS at best has two or three grid points that include lake characteristics.  In addition, the GFS is unable to distingish between the Stansbury, Oquirrh, and Wasatch Ranges, so it cannot separate mountain effects ...

You may have seen some guesses about what will happen this coming ski season.  Such predictions are often issued with great fanfare and overconfidence.  I'll pick on AccuWeather here because they are an easy target.  The outlook they issued in late September 2022 included the following. "Unfortunately, we have bad news as far as the drought goes in parts of California, Nevada and the Southwest," Pastelok said. "The main storm track will be even farther north than it was the first half of the winter season last year including the late fall." Then there was the accompanying snow outlook.  What a spectacular crash and burn! The reality is that these seasonal forecasts, based largely on the loading of the dice due to the anticipated presence of El Nino or La Nina, have somewhat limited practical utility for most applications.  If you didn't have any knowledge of meteorology, you might expect there to be a 33% chance of below average, 33% chance of near average,...

Today's 0600 UTC initialized GFS forecast, covering the period from 1800 UTC (1200 MDT) Thursday 27 September through 1200 UTC (0600 MDT) Tuesday 2 October is below and calls for a mid-latitude trough to interact with monsoon moisture and the remnants of Hurricane Rosa to bring precipitation into the southwest U.S., including Utah.  It is a very believable forecast.  You will almost surely see tweets of forecasts from this and other models with statements about what is likely to happen, but we need to remember that we are still 4 days from the initial arrival of forecast precipitation into Utah.  Further, uncertainty remains concerning the interaction of the mid-latitude trough and the hurricane, something that we discussed in the previous post.  The likelihood that the remnants of Rosa move across the southwest U.S. is now high, but not 100%.  Most tracks produced by the GEFS ensemble, for instance, bring the low center across Arizona, but there are two ...

Rosa is currently a category 1 hurricane located over the eastern Pacific south of the Baja Peninsula. Source: National Hurricane Center Over the next several days, she is expected to drift toward the WNW and interact with a complex a complex sequence of mid-latitude troughs.  The latest (0600 UTC) GFS calls for Rosa to curve toward the northeast, with her remnants migrating through the interior U.S. southwest and eventually contributing to a very strong mid-latitude cyclogenesis event along the U.S. Canadian border.   The concerns associated with such a scenario are numerous, including the possibility of heavy rainfall on new burn scars (with related debris flows) and what would probably be some exciting weather for early October in the high plains of the U.S. and Canada.  The problem is this.  These sorts of interactions between tropical cyclones and mid-latitude troughs are notoriously tricky to forecast.  Yesterday, the European center mode...

Christmas is still a full week away, but that isn't preventing people from making specific forecasts for the big day.  This is a common issue when a major day or holiday approaches.  The desire to provide specifics far in advance.  10-day forecasts for trick-or-treating weather on halloween.  Guarantees of a white Christmas, etc.  Unfortunately, the existence of a holiday does not magically clear up the meteorological crystal ball or alter the fundamental chaotic nature of the atmosphere.  I saw one forecast this weekend for a brutally cold Christmas day with a high of 18ºF.  Presumably this was based on forecasts like the one below from yesterday morning's GFS which did indeed have some exceptionally cold air over northern Utah with 700-mb temperatures below -20ºC. However, last night's GFS paints a totally different picture with a ridge over the west and 700-mb temperatures of -4ºC over northern Utah.  For the ski areas, that's a swing ...

Changes are afoot that will result in a shift in the weather for Utah and a significant rainfall event in the eastern United States. The pattern currently developing over North America will give weather forecasters headaches from coast to coast.  It features multiple interactions between upper-level closed lows and high-amplitude ridges, with a tropical cyclone thrown in just for good measure.  The interactions between these features are notoriously difficult to nail down and this leads to considerable forecast uncertainty. The low hanging fruit is in Utah.  Our run of well-above-average temperatures will come to an end on Friday as an upper-level trough from the Pacific swings into the Intermountain West, bringing cooler air and showery weather. After that, it's tough to say.  The models have been all over the place dealing with the interaction of Friday's trough with another upper-level trough dropping down from British Columbia over the weekend (such an i...

Astronomical spring arrives today, but no matter what my daffodils say, meteorological reality will beg to differ in the coming days. This morning we have some snow falling in the mountains and rain in the Salt Lake Valley.  The precipitation began with your classic "cloud storm" scenario.  An incredibly dry airmass was in place over the Salt Lake Valley this morning, so much of the early precipitation evaporated before reaching the ground.  In this morning's sounding, taken about 5 am MDT, there was a 34ºF dewpoint depression at the surface (which equates to a relative humidity of 25%) and even drier air just above the surface layer. Source: NOAA/NWS/SPC A cold front moves in tonight, followed by a prolonged period of northwesterly flow that will gradually bring in even colder air on Thursday and Friday.  By Saturday morning, our 700-mb temperatures are forecast to be about -16ºC, which is below average even for January. The overall change in ...

Public forecasts issued by the National Weather Service and most news broadcasts are usually fairly vague with regards to snow accumulations.  They usually provide a range (e.g., 4-8 inches) for a broad period of time (e.g., tomorrow). There are, of course, many users who need far greater precision than that.  Determining how much salt to apply to highway for the morning rush hour, or whether or not to close a canyon to traffic to perform avalanche control work requires knowing very precisely how much snow is going to fall and when.  The temperature and the strength of the wind might also be important for these applications too.   With those examples in mind, let's think about the ramifications of the forecasts being produced for tomorrow (Wednesday) afternoon through Thursday morning by the 12-km NAM and its 4-km high resolution nest, which are remarkably different in terms of the timing and amount of precipitation. For the three-hour period ending at 3 P...

The surface cold front is through and we're starting to see precipitation fill in behind it over northwest Utah. This is a gut-check forecast if ever there was one.  The models are producing a narrow band of heavy precipitation behind the surface front.  The 1800 UTC NAM dumps out more than 0.75" of liquid precipitation in areas along the band by 1200 UTC (0500 am) tomorrow.  Snow levels will be flirting with the benches soon. 1800 UTC NAM accumulated precipitation (inches following scale at top) from 1800 UTC 6 Mar – 1200 UTC 7 Mar 2012 The precipitation band, however, is quite narrow and any error in position or movement has major ramifications.  One way to measure the forecast uncertainty is to run lots of different forecast models and see how many produce large precipitation accumulations.  The Short Range Ensemble Forecast (SREF) system run by the National Centers for Environmental Prediction includes 21 model forecasts which, when added to...

Source: http://www.consumertraveler.com Some forecast situations are fairly predictable with the tools we have today, some aren't.  Today's computer forecast models do a fairly good job simulating cold-frontal passages over northern Utah.  Thus, while we have a beautiful day today in the Salt Lake Valley and Wasatch Mountains, we know that change is coming tonight and that the Cottonwoods will see a pretty good burst of snow tonight .  There are uncertainties with regards to the distribution, intensity, and amount of snow, but the basic idea that there will be a snowstorm tonight is fairly predictable.  That wasn't the case 10 years ago when forecast models were nowhere near as good as they are today. In contrast, the post-frontal environment tomorrow is more of a crapshoot when it comes to snowfall.  There are a number of reasons for this: Post-frontal precipitation is produced by shallow convective clouds that are small in scale and typically initiated...

Earlier posts this week have discussed the uncertainty in forecasts of front trough and snowband position for this evening.  Here's a nice loop showing a sequence of forecasts from the NAM beginning with the 84 hour forecast from 1800 UTC 22 Feb and ending with the 12 hour forecast from 1800 UTC 25 Feb (today), all valid for 0600 UTC 26 Feb (11 PM tonight).  Meteorologists sometimes refer to this as "DModel/Dt".  Notice how the position of frontal trough and precip band jumps back and forth, from as far south as central Utah to near the Utah-Idaho border. Now, if you think that now we are dealing with a shorter lead time (24 hours or less), so presumably the forecast is more certain, think again.  The target area has narrowed, but for 1200 UTC 26 Feb (5 AM Saturday morning) the NAM puts the front right on top of Salt Lake City with heavy precipitation to the north, whereas the GFS puts the front roughly over Provo. NAM Forecast Valid 1200 UTC (0500 MST) 26 Feb...