Mesoscale Discussion 1686
   NWS Storm Prediction Center Norman OK
   0921 AM CDT Tue Sep 08 2020

   Areas affected...Northern Colorado...southeast Wyoming...and western
   Nebraska

   Concerning...Heavy snow 

   Valid 081421Z - 081815Z

   SUMMARY...Moderate, to at times heavy, snow rates will continue
   through the mid to late morning hours before conditions gradually
   improve across northern Colorado, southeastern Wyoming, and western
   Nebraska.

   DISCUSSION...Mid-morning regional radar loops continue to show a
   broad region of banded precipitation across the central Rockies.
   Sub-freezing temperatures continue to advect southward along the lee
   of the Rockies, and wet-bulb cooling within the Rockies has allowed
   for several locations to fall below freezing. The 12 UTC RIW
   sounding shows a saturated, sub-freezing profile that supports
   snowfall, and the DNR sounding revealed a similar thermodynamic
   profile above ~2 kft. As a result, snowfall continues for northern
   CO into southeast WY and western NE, with a few locations briefly
   reporting heavy snow and visibility down to one-half mile. 

   These conditions are expected to continue over the next couple of
   hours as lift ahead of the main synoptic low continues to be
   augmented by frontogenetical lift in proximity to a strong
   baroclinic zone between 850-700 mb. Heavy precipitation should shift
   into western CO over the next few hours as main zone of ascent
   translates southwestward with the upper-level low. Although profiles
   will continue to evaporatively cool across western CO, the snowfall
   potential is uncertain below higher elevations due to warm
   near-surface temperatures. 

   The baroclinic zone is expected to undergo some degree of
   frontolysis during the early to mid afternoon hours, and the main
   low should begin to stall near the Four Corners region. This should
   lead to a gradual reduction in lift and in the potential for
   precipitation banding and heavy snow rates. There are some
   indications in morning guidance that a small region of mid-level
   frontogenesis may develop across southern CO/northern NM and support
   moderate to heavy precipitation, but surface wet-bulb temperatures
   in the upper 30s and low 40s across this region cast uncertainty
   into the snowfall potential. Trends will be monitored heading into
   the afternoon hours.

   ..Moore/Thompson.. 09/08/2020

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...UNR...BOU...CYS...GJT...

   LAT...LON   39810725 40040758 40530736 41140696 42000568 42630446
               43030346 42800300 42260301 41220406 40420498 39930552
               39520614 39620663 39810725