Wasatch Weather Weenies

Strong southwest winds have predominated the last 24 hours or so and much of Saturday night and Sunday, although as I type this at 3:30 PM MDT, we're in a bit of a lull.  Observations from the University of Utah, for example, show the persistent nature of the southwesterly flow, with only a brief period late yesterday (around 2000 MDT) when winds were not southerly to westerly.  The view from the upper avenues this morning showed something obscuring the visibility in the southwest corner of the valley.  Observations from the University of Utah show several period of moderate PM2.5 concentrations during the night and this morning, indicating that there was something in the air. Dust or smoke is hard to say for sure based solely on PM2.5 concentrations, but my hypothesis is that it was dust and not smoke due to a lack of any noticeable campfire smell despite significant visibility obscuration in the plume.  I wonder if the active dust emissions source we iden...

There isn't much change in the Rosa forecast from yesterday to today, other than increased confidence that the remnants will track across the southwest.  The ECMWF ensemble favors tracks across western Arizona or southern California, although there are some where it moves further west and stays over the offshore waters of the southern California bight.  The mean of this ensemble is a track that moves directly up the lower Colorado River Valley.  The GEFS ensemble is now fairly similar, with perhaps a slight tendency to bring the system a bit further to the east. The 0600 UTC 28 Sep GFS forecast of sea-level pressure and 3-hour accumulated precipitation is below.  I am toying with some new software and have no idea how well the loop will display, but thought I would give it a shot.  Apologies for the lack of time stamps and a scale.  Note that while Rosa will get lots of attention, the surge of monsoon moisture ahead of the decaying tropical cyclone is sub...

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...

Yesterday brought a rare flirtation with precipitation to the Salt Lake Valley, with a few showers evident in the area late in the day.  Although precipitation was reported in some areas of the Salt Lake Valley, and measurable rain fell in some areas further south including around the Pole Canyon fire, no rain was recorded at the Salt Lake City International Airport.  As a result, after 24 days, September precipitation still sits at a whopping "trace."  Will we make it through the month with no measurable precipitation?  Well, the odds are probably better than not, but it is not a lock.  There's still about 6 days left in the month, and there are some ensemble members that bring a trough through and generate some precipitation late in the month, as illustrated by the ECMWF ensemble forecast below with 51 members.  This ensemble was initialized at 6 PM last night and 5 of the 51 members produced precipitation at the airport yesterday evening (grey lin...

We have reached the time of year during which the mid-latitude westerlies are strengthening and becoming more active yet the so-called "monsoon" remains active.  Knowledge of the interactions between these two circulation regimes is often important for weather forecasting not only over the southwest U.S., but other parts of the continental U.S., including the midwest. An excellent example is provided by today's forecast.  The situation at 1200 UTC (6 AM MDT) this morning shows an upper-level short-wave trough in the mid-latitude westerlies just off the Pacific Coast.  At the same time, a tongue of monsoon moisture extends northward over the Gulf of California, with relatively high precipitable water values (color contours) across much of the surrounding region.  I have drawn an arrow below to indicate the flow that has led to enhanced precipitable water values along this tongue.  The latest GFS forecast shows how this moisture is drawn northward ahead of ...

As we have discussed in two previous posts and covered on local news, the Pole Creek and Bald Mountain Fires exploded last Wednesday (12 September) and have forced widespread evacuations.  The map below shows the situation as of yesterday morning, with the Bald Mountain fire to the west and the fire perimeter near Elk Ridge and Woodland Hill and the Pole Creek Fire having done damage on both sides of US-89. There is a paucity of wind data in the region surrounding the fire, so I need to get creative here.  We'll take a look at two sites.  The first is the Birds Eye "IRAWS" (incident remote automated weather station) at 6544 ft elevation in the mountains immediately east of Birdseye along US-89 (top graph below).  At this location, winds through the 10th of September exhibited a clear diurnal cycle, increasing during the day and decreasing at night (red line sustained, green pluses gusts, blue dots wind direction).  Sustained winds during the day, however, w...

My mom is in town, so we spent the day on the Alpine Loop Highway with a visit to Sundance.  The colors in the upper elevations are starting to change and probably next weekend it's going to be quite nice.  Even this weekend was fine. Less beautiful were views of the Pole Canyon and Bald Mountain fires, although I'm not sure we could see the latter directly.  Note the cumulus sitting on top of the smoke plume, a likely reflection of heat and moisture from the fire. Some of you have asked about how dry it is.  I will focus first on precipitation.  For the first half of September, we've had a trace of precipitation at the Salt Lake City Airport.  Such dryness is not exceptionally uncommon.  Going back to 1875, there have been 13 years with no precipitation, 16 years with trace, and another 18 with .05" or less of precipitation in the first half of september.  Basically, you have 47 out of 145 years (31%) in which the first half of Se...

The Pole Creek and Bald Mountain Fires in Juab and Utah counties blew up this week, have burned 68,000 acres, are threatening homes and structures, and are currently only 2 percent contained.  I first saw the smoke billow or pyrocumulus cloud (tough to tell if there was a pyrocu or not) associated with one or both of these fires on Wednesday when it poked up above Lone Peak. Yesterday's MODIS image from NASA's Terra satellite showed smoke from the fires traversing the Uinta Mountains and extending all the way to the Black Hills and beyond.  The remarkable linear nature of the plume likely reflects the strong southwesterly flow that has predominated over the western U.S. the last couple of days.  The Salt Lake Tribune reported this morning that on Sunday the Pole Creek fire was a half acre in size.  It grew to 50 acres by the end of Monday and then grew "exponentially Wednesday and Thursday.   Collectively, the two fires have burned 68,000 acres...

So many thoughts have been racing through my head over the past 24 hours or so since Florence began to have a major impact on the Carolinas and environs.  I will start with a quick look at the rainfall. Radar estimated precipitation from 0343 EDT Wednesday 12 September through 0834 EDT Friday 14 September from the Morehead City radar shows a broad region of more than 3 inches extending from southwest of Wilmington to near Cape Hatteras along the North Carolina coast.   The heaviest precipitation has been in the region surrounding Morehead City, with some localized estimates of more than ten inches (magenta shading).  Source: NWS Gauge observations from a Remote Automated Weather Station (RAWS) site in Croatan National Forest northwest of Morehead City shows and accumulation of almost 11 inches, which is consistent with the radar estimates.  This precipitation falls over a roughly 12 hour period.  Source: MesoWest Precipitation frequency estima...

Hurricanes provide an incredible perspective on what Demuth and Morss (2012) called the creation and communication of risk information.  Most people today get risk information from a nebulous web of sources that can include radio and television media (local and national), social media (e.g., twitter and facebook), and friends and family.  This web of information is highly complex and strongly affects impressions of the risk information and ultimately individual decision making.  During hazardous weather events such as hurricanes, I often share information with family and friends via social media.  Over time, I have streamlined those posts to emphasize the forecasts produced by the National Weather Service, which includes the National Hurricane Center, and the importance of following information provided by local officials.  I realized that in many cases, adding my $0.02 about the evolving weather situation was just adding to the noise and obscuring the messa...

Although Hurricane Florence is getting a vast majority of the media attention, there are actually two current hurricane threats to the U.S. at present as Hurricane Olivia is expected to weaken but still i mpact Hawaii.  Each of these systems is indicated by arrows in the analysis below.  GFS analysis and observed satellite imagery at 1200 UTC 10 September 2018 We will begin with Florence.  According to the National Hurricane Center public advisory issued at 11 AM AST Monday September 10, Florence is currently a category 3 hurricane with maximum sustained winds of 115 mph, with higher gusts.  It is expected to remain a major hurricane through Thursday when it approaches the eastern seaboard.   It is a blessing and a curse that modern science can track and predict the movement of these hurricanes days in advance.  The challenge is that while we have a decent idea that Florence will be a significant threat to the eastern seaboard, we can't pinpoi...

Another shallow layer of smoke pushed into northern Utah last night and is obscuring views and torturing lungs today. The view from Snowbird around 11 AM showed conditions were better in the upper elevations than the mid-to-low elevations.  The smoke seemed fairly layered visually, and was densest perhaps below about 8,000 or 8,500 feet.  The view toward the Oquirrhs suggested smoke may have extended to about 10,500 feet or perhaps a bit higher, but wasn't as dense at those altitudes. The morning sounding shows a series of stable layers extending to about 775 mb (7500 feet).  Perhaps this explains the apparent denser smoke in the lower levels.  Northwesterly flow, however, extended to just over 700 mb (10,000 feet), which might explain why some of the smoke extended to the higher elevations.  The view driving down the canyon was reminiscent of wintertime inversions.  I've taken many photos of gunk from this pull off. I keep reminding myself...