Wednesday, November 12, 2014

Detailed Look at the Cold Surge

The new High Resolution Rapid Refresh (HRRR) modeling system is now providing remarkably detailed analyses and short-range (0-15 hour) forecasts operationally at 3-km grid spacing.  This is a major advance for weather analysis and forecasting in the United States and I'm hoping it should prove useful for ski forecasting this winter.

Below is the HRRR analysis for 1300 UTC (0600 MST) this morning showing the remarkable structure of the current cold surge interaction with the high topography near and along the Continental Divide.  Analyzed surface temperatures are below -12ºF across much of central Wyoming and southwest Montana.  The strong pressure gradient associated with this cold airmass is driving easterly and northeasterly flow into the Columbia Basin, Snake River Plain, and northern Utah.  In the case of the latter, one can see how the Uinta Mountains splits the cold flow, with easterlies pushing across Evanston to the Wasatch Mountains and northeasterlies pushing across Dinosaur National Monument into the Uinta Basin.

HRRR 1300 UTC 12 November 2014 surface temperature (ºF, color fill), wind vectors, and sea-level pressure (contours)
A close up of winds over northern Utah shows remarkable structure and detail including an area of strong northerly over the northwest Great Salt Lake, a remarkable anticyclonic (clockwise) turn of flow around Promontory Point, outflow from Weber Canyon extending over the Great Salt Lake just north of Antelope Island, downslope winds near Farmington and Centerville, downslope winds near the University of Utah, and strong northerlies on the Wasatch Back extending into the Heber Valley.  I've identified each of these with red boxes below.

RRR 1300 UTC 12 November 2014 surface wind speed (miles per hour, color fill) and vectors
The analysis above is nice, but it does have some shortcomings.  It has overdone to some degree the easterly component of the flow over the Great Salt Lake.  Note how the observed winds at 1300 UTC over the northern Great Salt Lake were strong but northwesterly rather than northeasterly.  In addition, the flow on the northern end of Antelope Island is northerly rather than easterly.

Source: MesoWest
Near the Wasatch, the HRRR is unable to capture the fine-scale details of the flow, even at 3-km grid spacing, but one can find areas of easterly flow along most of the Wasatch Front include the areas noted above, as well as northerlies penetrating into the Heber Valley (latter not shown).

Source: MesoWest
Even with these shortcomings, the HRRR is a major improvement over previous lower resolution analysis and forecast systems.


  1. The -20 F air spilling across the Continental Divide near Yellowstone National Park raced down the Snake River Plain, making their temperatures about 20 degrees F colder at the same elevation compared to Salt Lake City. The town of Twin Falls (approx 4,000 feet elevation) was down to the upper single digits this morning, while SLC was in the upper 20s. Another striking contrast that caught my eye in the late morning obs showing the sharp gradient in air mass... Phoenix is reporting 73 F while Denver is 2 F.

    1. If those Rockies weren't there Texas wouldn't get as cold either (cold air dams against the eastern side and coriolis causes it to hug mountains all the way to Texas). Nice write up on the canyon winds, was just looking at the utah mesonet---impressive winds in Parley Canyon, yet none at the airport. Check out Colorado Springs 5F, Alamosa 52F which is 88 miles SW over the mountains.