Thursday PM Update: Weekend System Slams California, Models Back Down on Snow

There is not a lot of change from our update last night, but in effort to keep you all abreast as to what we’re watching, here’s another update.

Our storm system is currently slamming California with heavy rain, wind, and eventually snow. Many California ski resorts could see up to 2 feet of snow before all is said and done Friday. With rainfall totals in the Bay area already at 1 - 3”, with locally higher amounts, the NWS continues numerous Flash Flood Warnings, Flood Warnings, Coastal Flood Warnings, and Flood Watches for north central California until 9:30pm this evening, as well as wind, blizzard, and winter storm warnings.

GOES 16km Water Vapor - IR3, RAMMB

By Friday the system digs into the southwestern US and begins its trek east. This is where we start to watch it carefully for its eventual impacts on Colorado.

Rather than reiterate everything we’ve been saying for days now, here’s a rundown of what concerns we have for snowfall potential along the Front Range, and what we continue to like about this setup. With the system coming onshore today and tonight, we expect the models to slowly get a better handle on things going forward as well.

The good, the bad, the ugly
The models, nearly unanimously, have backed off snowfall across eastern Colorado with this system since yesterday. Ironically, as of the 12z suite, the upgraded GFS was leading the way with highest totals for Denver, while the EURO and GEM had backed off snowfall totals considerably from yesterday’s 12z run. The NAM has been fastest, but also pretty wet at 18z, and the operational GFS continues to suggest basically no snow for the city.

Here's a look at the current model spread for snowfall this system (at 10:1):


Now, there could be number of reasons for the models backing off on QPF. The first, and simplest, is that the models are on to something and that this trend may well turn out being correct. If this were the case, we would not expect much snow for most of the I-25 urban corridor. Another possibility is that the models are struggling with this system, and we’ll see them come back around in the next few runs.

One potential issue is that the models are putting too much stock in convection out in front of this the system, which can be detrimental to our precipitation chances along the Front Range historically. Convection can rob the incoming system of energy and available moisture, leaving the I-25 corridor lacking in both. We’re putting less weight on this for now, which means if this is indeed what the models are trying to do, we should see them correct wetter for the FR going forward. The other issue we’re seeing, especially in today’s runs, is a trailing low to the northwest of Colorado Sunday. If this low trails too close to the primary feature, its winds can also act to shut off precipitation along the Front Range more quickly, and effectively split the energy of our main system. Lastly, models with the lowest QPF for Denver are trying to introduce the dreaded downslope off the Cheyenne Ridge, with lots of lee side troughing; this is certainly a concern, and would hinder moisture transport to the Front Range.


Of course, then there’s the eventual track. Despite being all in the same ballpark, there understandably is quite a bit of fluctuation run-to-run with the exact positioning of the low. While it may seem frivolous to worry about a few miles here, a few miles there, the fact of the matter is those few miles can shift the heaviest precipitation dramatically, from a bullseye over Denver, to a bullseye across the northeast Plains, to bullseye over southern Colorado. The greatest risk for Denver with respect to current projections is that this system ends up digging too far south for a good snowfall event in the city.

All this considered, there’s still a lot to like about this system. It’s got almost everything you’d look for in a good upslope storm for Denver. Almost. Overall we still like many aspects of this storm: the strength, the track, while varying, is pretty good for this far out, and tilt as it moves across the region Saturday night through Sunday. We believe this leaves the door open for models to flip once again in how much snow they produce for Denver.

It’s been a long wait for snow across eastern Colorado, and it looks like we’ll need to be patient yet before calling this one a “snow globe” or “snow go” for Denver.

There are several more days to go, so we’ll continue to send along updates as this system progresses. For what it’s worth, models continue to suggest there will be another system on this one’s heels by the middle to later part of next week, so buckle up!

Brendan Heberton

Brendan is founder of Weather5280. He is co-founder of FreshyMap, and develops software for geospatial data analysis and visualization.

Denver, Colorado
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