A mid November Rocky Mountain low pressure system and series of associated disturbances brought rain and snow to parts of the Rockies and adjacent plains 20-22 Nov 2019.

By the morning of 21 Nov, analysis of GOES-16 upper-level water vapor imagery revealed the broad upper low centered over southern Nevada, with a more potent shortwave trough rounding the base of the broad low, lifting northeast across southeast Arizona (Fig 1). An associated jet was also rounding the base of the low with it’s entrance region poking northeast into New Mexico. A separate, localized jet was analyzed across eastern Utah. A cold front associated with the previous day’s shortwave trough was diagnosed in the imagery pushing south across E New Mexico, Texas, and Oklahoma through the morning. Finally, trailing west-east oriented energy associated with the northern plains shortwave trough is apparent in water vapor imagery slowly sagging south across the northern Rockies/High Plains, marking the transition from moist atmosphere (south) to very dry atmosphere (north).

Of particular interest was the apparent “fanning out” of cold (high-level) clouds over Utah/Colorado/Wyoming in the northeast quadrant of the broad low and interface with the northern energy. Overlaying the upper-level (above 350 mb) GOES-16 Derived Motion Winds (DMWs) in Fig 1, a GOES-R baseline product, this “diffluence” pattern becomes quite obvious in the wind field, with apparent divergence in wind direction and convergence in wind speed northward (>60 knots to <30 knots from south to north across Colorado). Areas of upper level diffluence such as that in this example don’t necessarily represent vertical motion in either direction in the atmosphere below it given competing convergence/divergence aloft considering geostrophic balance. This particular region was experiencing a relative lull in precipitation coverage and intensity during this period between stronger large scale forcing mechanisms. Isolated light to moderate precipitation was still observed over the higher terrain given favorable moisture and orographics, along with possible larger scale lift near the Utah jet steak.

Visible imagery during the morning across the region was quite difficult to interpret given recent snowfall and various cloud layers all presenting similar reflectance.

By combing the 0.64 um, 1.6 um, and 10.3 um bands into the Day Cloud Phase Distinction RGB, we can easily distinguish the bare surface (dark blue), snow (green), low clouds (cyan), and high clouds (red). Widespread low cloud cover is apparent over the high plains in the present of easterly upslope surface flow in the wake of the cold front.

Bill Line, NESDIS