Exciting news in the weather department this week with a bonafide cold front coming late Wednesday. The 0600 UTC GFS has the cold front draped over northern Utah at 0300 UTC 12 Sep (9 PM MDT Wednesday), so right now the frontal passage looks to happen in the evening in the Salt Lake Valley, although give that a +/- 3h window given the inherent uncertainties.
The pre-frontal airmass is fairly dry, so the models aren't going for much in the way of precipitation prior to or during the frontal passage, although there may be some high-based convective clouds and wind. Perhaps the biggest concern will be fire-weather conditions. The National Weather Service has issued a red flag warning for most of the lowland regions of Utah for 11 AM Wednesday to midnight Wednesday night given the wind and low humidity.
Thursday will seem fall like. High temperatures at KSLC are likely to be around 72F. Somewhat elevated areas like the east bench will struggle to hit 70. Think of it as a fast-forward to the wonderful temperatures of early October.
Mountain snow? It's a possibility, but the moisture-starved nature of this system means that nothing to a skiff is probably most likely and that will be on Thursday when the coldest air arrives rather than with the cold frontal passage. Below is our downscaled and combined EPS+GEFS ensemble forecast of the total snowfall through 0000 UTC 13 Sep (6 PM Thursday) showing a mean from the 82 ensemble members of about a half an inch and a maximum of 3.4 inches at at Alta-Collins.
I'm hoping to do a deeper dive into this newer ensemble product soon, but I'll share a few details here, with the caveat that we're still testing and tweaking:
- The ensemble will replace our current NAEFS product and is based on all 51 members of the ECMWF Ensemble Prediction System (EPS) and all 31 members of the NCEP (US) Global Ensemble Forecast System (GEFS).
- Model forecasts are downloaded on a 0.25 degree latitude-longitude grid and downscaled to 800 meter grid spacing.
- Snow-to-liquid ratio is based on a new algorithm that we have developed using data from 14 snow-study stations in the western United States, including at least 3 sites in northern Utah. This should be a big improvement over what was in the NAEFS.
- Due to the use of the EPS, which is not available (for free) until 8 hours and 40 minutes after the EPS initialization time, this product will be available a couple of hours later than the NAEFS, but that's a tradeoff I'm willing to make as the EPS is the best ensemble product out there.
- We are anticipating providing more and better graphics, although we're still trying to figure out what we can provide in a reasonable amount of time.
This is a very exciting development - big thanks to you and your team. a few questions:
ReplyDelete1) I assume you are creating a plume product similar to the old NAEFS one that was previously available?
2) If so, are you essentially adding new plume lines for the 51 EPS members alongside the existing NAEFS members?
3) Or are you changing the way that the downscale is happening for those lines previously included?
The NAEFS product will be sunsetted. The NAEFS consists of 31 members from the GEFS (US) and 21 members of the Canadian Ensemble. The former will be included in this new product, the latter not included, mainly because we are having a hard enough time dealing with the dataflows for 82 ensembles and I don't want to push it to 103.
DeleteThe downscaling will change some. It is the same method, but both the EPS and the GEFS now provide output on quarter degree grids (they used to be half degree, at least of the GEFS). So, we are tweaking the method some.
Most of what we are doing behind the scenes right now is trying to improve processing efficiency.