Tuesday, December 13, 2016

A Stronger, Juicier, and Warmer Atmospheric River

I've been busy writing a proposal and not paying attention to the weather today, so when I finally got around to looking at the maps, the first word that came to mind was wow.

In particular, what caught my attention was the forecast for Thursday night and Friday.  Yes, there is a warm front moving through tomorrow, but that's small potatoes compared to the event being advertised for Thursday night and Friday.

The 1200 UTC GFS forecast shows an extremely broad atmospheric river extending into the Intermountain West at 0600 UTC Friday (11 PM Thursday) with very large values of integrated water vapor transport (over 400 kg/m/s) sneaking around the southern and northern ends of the Sierra Nevada and high values even downstream of the southern high Sierra.

Those values are quite a bit higher than observed during the previous atmospheric river event and are unusually high for December.

Compared to the previous atmospheric river event, the forecast event also features higher peak temperatures, larger 24-hour precipitation totals at Alta (and probably other sites), a stronger upper-level trough, and flow trajectories from the southwest.

As a result, the 12-km NAM, which is typically a relatively conservative forecast model, is pumping out some big numbers with 2.82 inches at the Alta grid point from 5 PM Thursday to 5 PM Friday (with a bit more thereafter).  That is a very robust 24-hour precipitation total for Alta.

Unfortunately, this looks to be a very warm storm until cooler air moves in on Friday.  From 5 PM Thursday to 5 PM Friday, NAM forecast 700-mb temperatures are between -3.3 and -0.6 C, with the highest temperatures Thursday from 8 to 11 PM.  Although one can't pin point snow levels at such lead times, these numbers are consistent with snow levels reaching about 8000 feet during that period.  Basically, this period of the storm looks like one where the rich (i.e., high elevations) will get richer (i.e., more snow) and the poor (lower elevations) will get poorer (snow loss) until temperatures and snow levels fall on Friday.

Of course, it can be wise to temper one's meteorological excitement with a does of ensemble reality.  The NAM is relatively wet (in the upper third) of precipitation forecasts for Alta produced by

All of that is impressive, but compared to the various members of the downscaled short-range ensemble forecast system (SREF), the NAM is probably in the upper 20% of precip forecasts for that period and this is still a fairly long-lead forecast.  Nevertheless, it does appear atmospheric river #2 is on its way and it will be bringing low-elevation rain and high-elevation Cascade Concrete before temperatures and snow levels fall on Friday.


  1. I looked for "Pineapple Express" beer for you but DABC does not stock it. I thought about asking them to special order, but they have declined in the past.

    So I gave and continue to give you Pineapple Express beer in spirit.

    Any thoughts on how climate change affects these sub-tropical (near Hawaii) originating atmospheric rivers. Judging by the wetbulbzero from the NAM12, rain/snow line looks to be higher in this week's event than in last week's. Being out last Saturday it was raining hard up to about 6500 feet in the morning, and the rain/snow line looked like it got up to 7500 feet during the day, but I was at 9000 feet until 5pm. Wetbulbzero maxes at 9600 feet 8pm Thursday, which translates to a rain/snow line of about 8500 feet, maybe 1000 feet above last week.

    Your basic response on climate change has been it loads the dice for warmer wetter storms. In this case can we say anything about how climate change affects the temperature of the source airmass, Hawaii, and how that airmass cools as it travels northeast to the Wasatch. The forecast is for a rapid cool down as the cold front arrives following the atomospheric river, from Mt Baldy 27F 8pm Thurs to -7F 8am Sat.

    Could it be climate change has nothing to do w how warm this event is, it's just that the cold front arrives late. Or maybe climate change is stalling the cold front?

  2. right side up storm as I see it.