An Examination of HRRR and HRRRX Performance and Utility For Two Similar and Challenging Colorado Snowstorms

One of the lessons we all learned in our Synoptic Meteorology class was that Colorado was a “birthplace” of cyclones.  Systems crossing the Rockies reform east of the Front Range and then go on to become the storms that may become significant across the eastern two thirds of the nation.  As might be expected, being in a formation zone for storms can make the Front Range a very tough area to predict wintertime snowfall.  Small changes in where a storm forms east of the mountains can have profound effects, with the problem amplified by the large sensible weather difference that can result from small changes in the lower level wind field that make the difference between upslope and downslope on the east side of the mountains.  The forecast for the position of leeside cyclone development has also long been a challenging issue for numerical weather prediction (NWP) models.  The old NGM and AVN models were notorious for predicting the surface low to develop too far to the north, resulting in poor precipitation forecasts for the highly populated Front Range of Colorado.  Obviously, models have come a long way since the NGM days, but two snowstorms in northeastern Colorado from this past winter remind us that the problem is not yet totally solved! 

In this presentation we examine two major snowstorms along the Front Range and in northeastern Colorado, one in mid-December 2015 and the other in mid-March 2016, which produced significant (>12” in each event) snowfalls.  For both storms NWP guidance from operational models indicated that downslope flow would dominate the Front Range, even within 24-h of the onset of the event.  Both storms verified farther south than these forecasts indicated, resulting in limited lead-time for warnings.  The parent systems were similar, a progressive, fairly compact upper-level wave traveling eastward across Utah and then Colorado, and even the timing was similar, with heavy snows developing along the Front Range in time for the morning rush hour. 

We will review the synoptic setting for the two events, but the main focus will be to examine the performance of the then operational version of the HRRR (High Resolution Rapid Refresh) model and contrast it with a then experimental HRRR that is scheduled to become the new operational version in late August 2016.  The HRRR is a 3-km horizontal grid resolution convection-allowing model that is currently run on the CONUS scale at NCEP once per hour out to 15 hours.  It is initialized with the RAP (Rapid Refresh) model, which is run at NCEP at a resolution of 13-km once per hour out to 18 hours, but the HRRR also  assimilates radar and other data on the 3-km scale.  The HRRR first became an operational NCEP model on 30 September 2014, after being available to forecasters for several years previously on an experimental basis from the Global Systems Division (GSD) of ESRL/NOAA in Boulder, where it has and continues to be developed.  In addition to examining and comparing the performance of both HRRR models, we will also look at it’s utility in helping to update the forecasts for both storms.  A key issue that we will examine is if there were consistent trends in the new HRRR runs that gave confidence to change the ongoing forecast, and if so how did these trends compare in the two HRRR versions and compared to other NWP guidance.