RWIS

Applications of the Utah Mesonet to Winter Road Maintenance

John Horel, Mike Splitt, Bryan White.
Department of Meteorology
University of Utah

Utah Mesonet: Monitoring Weather Conditions in the Intermountain West

Project began during 1994 in cooperation with National Weather Service

Surface observations obtained from local, state, and federal agencies and private firms

Data retrieved, processed, and displayed every 15 minutes

Goals:

provide access to weather information in a timely fashion for operational decision makers
archive and utilize weather data for research related to understanding weather phenomena in the intermountain region

On-line information: UDOT U. Utah, SLC NWSFO

Orographic Precipitation Along the Wasatch Front:

Background Information on the Factors that Control When It Will Start Snowing, For How Long, and When It Will Stop

What has happened in the past?

Characteristics of Snow Storms along Wasatch Front

Clouds Contain Mixture of Water Vapor, Ice Crystals, and Cloud Droplets

Ice crystals grow at expense of cloud water
Types of ice crystals (for example, plates, columns, needles, graupel, ice pellet) depend on relative humidity and temperature

Characteristics of Snowfall

Density of snow (relationship between snow depth and its water equivalent) varies between .40 (mush) to .01 (powder); typical value is .10 (10 inches of snow equivalent to 1 inch of water)
Rate of snowfall:
- Denver: 524 hours of snowfall per year. Out of the hours of snowfall:
- 65% of time only trace of water equivalent
- 17% of time only .01 inches
- 12% of time .02-.04 inches
- 6% of time greater than .05 inches (.5 inches of snow per hour)

Primary Factors that Control Amount and Duration of Snow during a Winter Storm:

Is it cold enough? (For example, -8 to -12C at mountain-top height, 3km MSL or 700 mb, to snow near the valley floor?)
Is there a means to lift the air? Front, jet aloft, flow across mountains?
Is there sufficient moisture available? type of air mass flowing into Utah?
Winter Forecasting - Utah Style

Monitoring Moisture Content in the Atmosphere

Relative humidity varies between 0% (dry) and 100% (cloud) and is a:
- measure of degree of saturation with respect to water. Air is saturated and can't hold any more water when RH is 100%; air is unsaturated below 100%.
- poor measure of actual water content in atmosphere.
- measure that depends on both temperature and moisture content (RH is typically high in morning (cool) and low during afternoon (hot))
Dew and frost point temperatures are measures of the amount of water vapor in atmosphere.
- Dew point Temperature is temperature to which air must be cooled (at constant pressure) until dew forms
- Frost point Temperature is temperature to which air must be cooled (at constant pressure) until frost forms
- Dew and frost point temperature are always less than or equal to air temperature.
  - If dew/frost temperature close to actual temperature then RH close to 100%
  - If dew/frost temperature much colder than acutal temperature then RH close to 0%
- Frost point temperature is higher than dew point at temperatures below 32F. It is easier to form frost at temperatures below 32F than to form dew
- Difference between dew and frost point temperatures
- Dew point temperature readily available; frost point may be estimated by adding 1F for every 5F below freezing
- Dew/frost point temperature defined at 5ft or so above ground level; air temperature adjacent to ground may be considerably lower, so easier to reach dew or frost point.
- Given uncertainties in measurement and difference between dew and frost point, adjust dew point temperature up a couple of degrees to identify temperature at which ice likely to form on roadways.
- Adjust dew point temperature downwards a few degrees to factor in effects of road treatments (pre-storm liquids, post-storm salt).

Example: February 24-28 1998

Parley's Summit: Feb 23-24 Feb 24-25 Feb 25-26 Feb 26-27 Feb 27-28
Bluffdale: Feb 23-24 Feb 24-25 Feb 25-26 Feb 26-27 Feb 27-28
Lake Point: Feb 23-24 Feb 24-25 Feb 25-26 Feb 26-27 Feb 27-28
Local Temperature Analyses: Temperature Salt Lake Valley Temp
Local Dewpoint Temperature Analyses: Dewpoint Temp Salt Lake Valley Dewpoint Temp

Monitoring the Current Weather Using the Utah Mesonet

Weather conditions at UDOT sites

Weather conditions at other sites of interest

Mesonet images

Local Weather Conditions: What Makes Weather Near the Wasatch Front Unique?

Lake-effect snow storms

Most of moisture that falls as snow during winter originates from oceans; very small component is likely from the Great Salt Lake
Lake affects weather near Wasatch Front as a result of:
- horizontal contrast in temperature between lake and surrounding terrain
- vertical contrast in temperature between relatively warm lake and cold air aloft
- slight increase in moisture availability
- reduced resistance to winds at surface
Impacts of Great Salt Lake on local weather:
- enhances local diurnal (daily) wind circulations:
  - morning breeze towards lake
  - afternoon breeze away from lake
- enhances convergence of surface winds over lake during night/morning hours (nocturnal peak in precipitation)
- enhances convergence of surface winds around lake during afternoon/evening (afternoon/evening peak in precipitation)
- destabilizes cold air masses flowing over lake
- net result is tendency for snow bands to form downwind of lake in post-frontal situations
  - winds from west: bands likely to form near Ogden-Bountiful
  - winds from northwest: bands likely to form in Salt Lake Valley
  - winds from north: bands likely to form in Tooele Valley
Lake-effect storms
Feb 26-28 1998

Downslope wind storms-canyon winds

Strong easterly winds along Wasatch Front result from:
- Enhancement of typical mountain-valley drainage flows during night/early morning
- Upper level closed-low to west-southwest of Utah that leads to easterly cross-barrier flow along the Wasatch crest
Major wind events result from flow across Wasatch barrier descending along west slopes of Wasatch
Winds tend to be strongest near canyon mouths as a result of superposition of enhanced drainage flow down canyons and flow over ridge
Strong winds are possible near slopes away from canyon mouths
Case studies:
- January 1997
- February 24 1997

Access to Other Weather Information

Forecasts, Advisories, Watches, and Warnings issued by the National Weather Service

Winter Weather Advisory: issued when several different types of hazards may cause significant inconveniences, including: snow, blowing snow, wind-chill, freezing rain or sleet. (Although uncommon in Utah, freezing rain is forecast when rain is likely to freeze as soon as it strikes the ground. Sleet is small particles of ice mixed with rain, which can also make the roads slippery.)
Winter Storm Watch: issued when severe winter conditions are possible during the next day or two.
Winter Storm Warning: issued when combination of heavy snow and other severewinter weather conditions are expected.
Heavy Snow Warning: issued for valley locations when 4 or more inches of snow is expected to fall within 12 hours, or when 6 or more inches is expected in 24 hours. Above 7,000 feet, issued when 6 inches in 12 hours or 10 inches in 24 hours.
Blizzard Warning: snow combined with winds in excess of 35 mph with near zero visibility.

Satellite Imagery

Radar imagery

Other Info

How Good Are Weather Forecasts in the Intermountain West?

Considerable effort underway to evaluate forecast skill of operational and research mesoscale models in regions of complex terrain

Focus on validation of precipitation and 3-D circulation

On-line information:

Tools Under Development: Utah ARPS Data Analysis System (ADAS)

Local analysis at high temporal (1 h) and spatial resolution (2 km)

Provide near real-time high resolution data over the complex terrain of northwes t Utah

Based on the Oklahoma ADAS (ARPS Data Analysis System) developed by the Center for Analysis and Prediction of Storms (CAPS)

Incorporation of large-scale and local data:

Rapid Update Cycle Version 2 (RUC2) analysis used to initialize ADAS
Utah Mesonet available at 15 minute intervals
NWS rawinsonde provides upper air data at 0 and 12 GMT
NWS WSR-88D velocity and reflectivity data obtained at 5-10 minute intervals (NIDS products)
Visible and IR Satellite imagery used to specifiy cloud water

Fall 1998: Use ADAS to initialize the Advanced Regional Prediction System (ARPS) to provide high resolution forecasts of mesoscale events

On-line Information: ADAS

Tools Under Development: MM5

Real-time forecasts and research simulations are conducted by Jim Steenburgh at the University of Utah.

Forecasts for the western United States and Utah out to 36 h

Forecasts