Wednesday, October 1, 2014

October Ridges Are Good

If there is one month of the year when a high-amplitude upper-level ridge is good in northern Utah, its October.

Ridges in the winter (November–March) are bad.  They bring drought to the mountains and cause a buildup of pollution in the valley.

Ridges in the summer (June–August) are bad.  They are obscenely hot and contribute to ozone generation and poor air quality in the valley.

I suppose that a ridge in April and May is OK, but nothing beats a ridge in October.

In October, a ridge may bring unseasonably warm temperatures, but at a time of year when unseasonably warm means ideal temperatures.

In October, a ridge also means no precipitation and for skiers, that's actually a good thing.  Everyone may get riled up for snow as the days shorten, but October storms rarely produce a deep snowpack, and thin snow just rots when a ridge finally build in, leading to avalanche problems later in the ski season (see the classic Wasatch Weather Weenies post Patience Young Jedi Knight).  It's better if the storms hold off for most if not all of October, and then hit us in succession in November (Exception: October 2004, which provided more than 100" at Alta).

Right now, the forecast for the several days of the month looks perfect.  A broad ridge is expected to build in over the west over the next few days.  Other than a shower or two today, it looks pretty dry.

Source: NWS
The extended is always more of a crap shoot.  The GFS 500-mb height forecast for next Friday, October 10, shows a Rex Block (a.k.a. high-over-low) block along the west coast (upper left hand panel).  For the most part, the various members of the Global Ensemble Forecast System (GEFS) agree with this solution (lower left).

The ECMWF ensemble has a somewhat different solution, but still hints at the jet being to our north, although one can't rule out the possibility of something sneaking in here.

So, it looks pretty dry through this coming weekend, after which the models suggest it may remain dry with a greater chance of below average precipitation than near or above average precipitation.  That's just what we want this time of year.  Let the snow hold off for another month, after which the firehose can open, preferably at maximum intensity.

Monday, September 29, 2014

Positively Pixelated

You don't see this every day, but for the 24-hour period ending at 1200 UTC (0600 MDT) Sunday, all of Utah received measurable precipitation, at least as estimated by the National Weather Service's Advanced Hydrologic Prediction Service.

The Salt Lake City forecast office put together the graphic below summarizing the remarkable increase in river flows during the event.

Source: NWS
How about that Beaver Dam Wash in Arizona, which went from no flow to 11,800 cubic feet per second!

Source: NWS
In case you are wondering where Beaver Dam Wash is, it runs along the Utah Nevada border and eventually empties into the Virgin River in NW Arizona (see the BEA in the map below).

Source: NWS

Sunday, September 28, 2014

Late September Teaser

A sprinkling of snow in upper Cardiff Fork this morning serves as a teaser for the approaching ski season.   Some aspens are pre-peak, others past peak, so enjoy the fall colors while they last.

Saturday, September 27, 2014

Washout Game on Tap

It's been impressively wet overnight and thus far today, especially in western Utah.  Twenty-four hour accumulations for the period ending within 1 hour of 1841 UTC (1241 MDT) show several locations in the Dugway area with more than an inch of rain (ignore sites with 0 or very light accumulations as these are almost surely bad).  The 1.22" maximum observed at a couple of sites represents about 15% of their 7.75" annual average.  Accumulations in the Salt Lake Valley and surrounding area are generally around 0.6", with more to come.

Source: MesoWest
A south–north oriented precipitation band is slowly moving across the Utah and is likely to be bringing periods of rain, possibly heavy at times, this afternoon and during the Ute football game this evening.  

The forecast from the High Resolution Rapid Refresh (HRRR, courtesy NOAA's Earth Systems Research Lab), which will become operational in the near future and provide short-range forecasts at 3-km grid spacing for the continental United States, shows bands of heavy showers moving through the Salt Lake Valley prior to and during the game, with just a few short-lived breaks.  Here are the hourly radar reflectivity forecasts for 2300 UTC (1700 MDT) through 0400 UTC (2200 MDT).

The HRRR lightning threat product shows the potential for some lightning during this period (one time selected for brevity).

The bottom line is that a Washout Game is on tap for tonight, with the potential (but not a guarantee) for a lightning delay. 

Friday, September 26, 2014

Potential Ute Football Deluge

There's not much change in the forecast from yesterday except that the storm is moving in just a bit slower.  The net result of this slight delay is that the 1200 UTC model runs generally put the heaviest precipitation directly over Rice–Eccles Stadium for tomorrow's football game.  The NAM, for example, is putting out more than 0.5" of rain from 6–9 PM tomorrow.  Kickoff: 6 PM.  Bring the galoshes.

If you are interested in tracking the moisture surge, see the NOAA Earth Systems Research Lab's GPS meteorology web site.  They use delays in GPS satellite radio signals to estimate the water vapor content of the atmosphere (sometimes called "precipitable water").  Observations from Needles, CA, for example, show a dramatic increase in precipitable water over the past 24-hours.  

Meanwhile in Salt Lake City, we're starting to see things moistening up from the dry air that was in place yesterday (hence the developing cumulus cloud cover this afternoon).  Values should exceed a full inch by tomorrow and, with cooler air moving in, that equates to a "washout" game instead of a "blackout" or "whiteout" game.

Thursday, September 25, 2014

Outlier Mode Begins

Although there remain some differences in timing and details between the models, we remain on track for what could be an extremely wet period in northern Utah and other portions of the state.  Given the heat today, I'm officially declaring the start of "Outlier Mode."

The record high for today is 92ºF, which is also the latest 92ºF day in the record books at the Salt Lake City International Airport.  As of 1:35 PM, it was 89ºF at the airport.  I'm guessing a 91ºF max is probably most likely, but maybe we can squeeze out a record.

Tomorrow there's a chance of showers and thunderstorms as the upper-level trough approaches, afterwhich your ark-construction needs must be complete as the system taps into juicy air over the eastern Pacific and Gulf of California.  The forecasts below show the forecast integrated water vapor transport produced by the GFS (bottom) and the average of all members of the Global Ensemble Forecast System (GEFS) on Saturday morning.  There is a narrow plume of strong water vapor transport extending from Baja into Utah.

Source: NWS
Moisture transport of the intensity forecast is fairly unusual in the Intermountain West this time of year, with return intervals of about once every 10 years in late September and early October.  

Source: NWS
Not surprisingly, given the moisture and strong forcing associated with the approaching upper-level trough and frontal system, the models are going berserk for precipitation.The graph below shows the accumulated precipitation produced by the 21 members of the Short Range Ensemble Forecast (SREF) system.  The mean (black line) calls for nearly an inch of precipitation at the Salt Lake city airport from about 0000 UTC (6 PM) Friday afternoon to 0000 UTC (6 PM) Saturday afternoon.

There is, however, quite a bit of spread and much is going to depend on timing and location.  By and large, Saturday looks like a fairly wet day.  Those of you heading to the Utah–Wazzu football game should keep an eye on the forecast and hope for a break.  Coach Wittingham might want to consider having his players practice with wet footballs today and tomorrow.  There's even a possibility of thunderstorms, so hopefully the University has an appropriate lightning safety plan in place as we have discussed previously.  Kudos to the University of Michigan for taking proper precautions last week.

In case you are wondering, it appears that there have only been five calendar days during which more than 2 inches of rain was recorded at the Salt Lake City Airport (or probably downtown for really old observations), but two of those days are in September.

May 3, 1901
June 5, 1885
July 13, 1962
September 5, 1970
September 26, 1982

September is a good month for a deluge because of the combination of monsoon moisture with cool-season storm systems.  I won't be surprised if we see some locations with 2+ inches on Saturday.