Case Study 30"Pseudo-Null
Event: Eastern Colorado Snowstorm"
19 March 2000
Case Summary
This case is the first in a series of Null-Event cases. It studies a developing winter storm in the Desert Southwest, and it's effect on Eastern Colorado's weather, especially northeast Colorado, on 20 March 2000. Forecasts of over a foot of snowfall were issued as early as 3:22 p.m., Sunday, 19 March, for the Front Range cities from Denver to Fort Collins, valid for late evening Monday, 20 March. By 10 p.m. Monday, reports from most areas along the Front Range were from 3 to 5 inches of snow. Examining this data set will assist in understanding why the heavy snowfall didn't occur.
Overview
Looking at the Eta's 48-hour forecast valid 00Z 21 March, it is seen that ETA predicted well the 500 mb heights and the position of the closed low, when compared with the RUC analysis at 00Z 21 March. ETA also handled the position and strength of the surface low well with it's 48-hour forecast, valid 00Z 21 March, when compared with the RUC analysis at that time.
The ETA forecasts of strength and direction of upslope, as well as lapse rates, were not as accurate. The 48-hour forecast valid at 00Z 21 March shows northeast upslope up to 600 Mb The actual sounding from that time shows weak, more shallow flow, from a more northerly direction. Actually, the flow at all lower levels was more northerly and weaker than ETA suggested. Also, the strong inversion that existed just above the upslope was missed completely by the 00Z 19 March ETA run, until 24 hours later, when it is hinted at by the 00Z 20 March run, valid 00Z 21 March. The lapse rate was forecast to be conditionally unstable, and a March 19 weather discussion from NWS Denver mentions this, but the actual sounding showed stable low levels, up to a strong inversion. This may have hurt snowmaking potential.
The synoptic features needed to enhance the upslope were too far south and west, and the flow aloft over eastern Colorado was only weakly diffluent. The strong PVA that would bring needed divergence was occurring in Arizona and New Mexico on 20 March.
It can be seen that this was a complex forecast, with details that were handled well by the Eta, and others not well at all; and by studying this case, one may understand and learn not what caused this weather event, but what features prevented the heavy snow from occurring.