The parade of warm and windy storms continues this week. Today and tonight we may see a mountain snowshower at times, but it won't add up to much.
Tomorrow afternoon is when things begin to get interesting. The GFS forecast for 0000 UTC 15 March (6 PM MDT Tuesday) shows a Pacific cyclone moving across northern Nevada with the trailing frontal trough extending across northern California and the Bay Area and the accompanying atmospheric river producing moderate vapor transport across southern and central California.
The lower right-hand panel in the GFS forecast above is an analysis of integrated vapor transport (IVT) magnitude (color fill) and vectors. Note that the strength of the atmospheric river, as indicated by the magnitude of IVT, declines with inland extent, as is often the case. This occurs due to the loss of water vapor to precipitation. These losses are especially large over the high Sierra south of Lake Tahoe, which is why there is often minimum downstream of that high barrier over Nevada, as is the case in the GFS forecast above.
During this period, Utah is in strong southerly flow. This pattern persists through 1200 UTC 15 March (6 AM Wednesday). The IVT in Utah is generally near or just below low-end atmospheric river magnitude (250 kg/m/s; yellow shading) through this period.
Thus, we would expect periods of valley rain and mountain snow late tomorrow and tomorrow night. The models do differ quite a bit on amounts during this period. For example, from 12 PM MDT Tuesday to 6 AM MDT Wednesday, the 6Z GFS produces 0.41" of water equivalent and 2.9" of of high-density snow for Alta Collins. In contrast, the 12Z HRRR produces 1.32" of water and 7.9" of high-density snow.
I suspect the fundamental difference in this case is topography, with the finer-scale HRRR topography yielding more precipitation than the GFS. That said, we have seen the HRRR overproduce sometimes in situations like this in which the synoptic forcing (e.g., associated with fronts) is weak. That's not a guarantee, however. I'm inclined to go with something like 5-10" of high density snow for Tuesday afternoon through 6 AM Wednesday, using the HRRR as near-upper-end guidance.
But wait, that's not the end of the storm. During the day on Wednesday, the main upper-level trough and cold front move through. The GFS currently puts the trough at crest level right over northern Utah at 2100 UTC 15 March (3 PM Wednesday). The ECMWF HRES has similar timing (not shown).
Overall, this looks to be a higher-density storm. We might end with some low-density snow on Wednesday if we don't shift to graupel (although I like graupel, so either way its a win).
Bottom line is that more snow is coming. Enjoy.
Your GFS derived appears to be the only forecast that got it right for this weekend. Id love to see 30" but seems like a Steenburgh grain of salt is in order again.
ReplyDeleteThat said I had forgotten just how good of a day the right 6" can make for.
that’s what she said 😊
DeleteThe GFS and the HRRR have very similar performance characteristics at Alta over many cases. It's not really possible to pick one over the other consistently. Over many many cases, the house is going to win.
DeleteI was more speaking to both of the LCC products conservatism last storm relative to NOAA and the other local snow forecasting personality, which appears to be happening again. In fact, some of them are currently upping their forecast totals (thinking temps will drop faster) but also weighing 3" of water as more likely than before.
DeleteInteresting comparison today through Wednesday afternoon between the 12Z run of the GFS and HRRR model guidance. The GFS has 1.84" water and 18.4" snow, the HRRR has 1.98" water (very similar to the GFS) but only 10.4" snow. It will be fun to see how the snow density plays out with this storm.
ReplyDeleteThe wet-bulb zero in the HRRR is nearly 1000 ft higher than in the GFS and the HRRR also generates almost all of the precipitation before 6 AM when it is warmer. In contrast, the GFS produces almost 3/4 of an inch of water after 5AM when the storm is colder.
DeleteAdditionally, the HRRR SLRs tend to be a bit lower than the GFS. This is because the HRRR has consistently higher winds over Alta than the GFS.
Thanks for pointing out the key differences in the two models, hopefully the GFS wins out.
DeleteIt looks like Alta Collins ended up with 1.74" water and 16" of snow, the GFS model guidance did a nice job.
ReplyDeleteAs you know, this title "The Next Storm" is now dated and the actual "Next Storm" begins, according to LCC guidance GFS Monday morning continuing through Wednesday on a slightly less warm SW flow than this just now past storm with wetbulbzero generally below 6000 feet except Tuesday midnight when it is 7400 feet. Accum between Monday and Wednesday is over 20 inches.
ReplyDeleteI know you don't care why it is snowing so much in the Wasatch and that you are just enjoying BDOY after BDOY, but have you seen any scientific work on why it is snowing so much this year in the Wasatch.
Holy shsssst what an unbelievable year, even if a lot of the storms have been warm with high wetbulbzero.
NOW!!! is the time to PRAY for MORE SNOW, LESS WIND, and LOWER WETBULZERO with VERY MANY ICE NUCLEATING PARTICLES
Sure, this post is dated, but you get what you pay for :-).
Deletewe have had a lot of snow because the storms have hit us this year
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