DIY Forecast Tools, Part I

I know this blog is read by numerous DIY weather forecasters and others looking for the next big storm.  I thought I would take a few moments today to discuss some of the unique products that we have on the Utah Weather Center web site (http://weather.utah.edu) that might be useful for their efforts. 

1. NAEFS-Experimental Precipitation Forecasts

 NAEFS is an acronym for the North American Ensemble Forecast System, a combination of medium-range ensemble forecasts produced by the US National Weather Service and the Meteorological Service of Canada.  Ensembles are great for weather forecasting, but the NAEFS lacks sufficient resolution to capture the detailed effects of the mountains of western North America.  

The NAEFS-experimental precipitation forecasts takes forecasts from the NAEFS and downscales them to higher resolution.  The downscaling method is quite simple and based on monthly high-resolution precipitation climatologies for the western U.S.  The good news here is that the resulting forecasts are much better than those produced directly by the NAEFS.  The bad news is that this method does not account for other unresolved effects (e.g., lake effect) and when you downscale using climatological precipitation-altitude relationships, you don't account for situations that depart dramatically from those relationships.  For example, frontal passages feature weaker enhancement over the mountains compared to climatology, whereas some pre- and post-frontal periods can produce much larger enhancement.

Despite these warts, the NAEFS products we produce are quite useful.  We generate plots of 7-day downscaled quantitative precipitation forecasts (water equivalent) and snowfall for several regions.  Below is an example for northern Utah.  

We also do plumes for many locations including Alta-Collins, Canyons (mid mountain), Park City Base, Brighton, Sundance, Snowbasin, etc.  Below is the latest for the base of Park City. 

2. HRRR forecasts

The HRRR is the High-Resolution Rapid Refresh, a 3-km model run hourly at the National Centers for Environmental Prediction that is used for short-range (< 18 hour) forecasting.  It can be quite useful for anticipating the timing of frontal passages, whether or not snow will develop in a few hours, etc.  We provide plots of (1) surface wind and simulated radar reflectivity and (2) surface wind and 1-h accumulated precipitation.  Below is a forecast of the former for later this afternoon showing just a few showers along the UT-ID border.

Because I like to look at long loops of past data for context, the loops on the Utah Weather Center include a large number of analysis frames showing the past weather.  I suppose that chews up a lot of data on your mobile phone plan, but hey, you get what you pay for!

3. NAM-4km/GFS-13-km

For those of you who think resolution is everything, we provide forecasts from the high-resolution NAM nest, which has 4-km grid spacing compared to the regular 12-km NAM.  We also provide high resolution forecasts from the GFS at 13-km grid spacing, which is as close to native resolution as you can get. 

I don't use these much for precipitation because they have an overforecast problem over the mountains.  They always forecast large precipitation amounts and about 1 in 10 times they get it right.  Unfortunately, they give you a false alarm the other 9 out of 10 times and false alarms are extremely damaging for ones meteorological reputation!

4. NCAR 3-km Ensemble (Coming Soon)

The National Center for Atmospheric Research (NCAR) runs a 10 member high-resolution (3-km grid spacing) ensemble once a day.  It produces forecasts out to 48 hours.  We're working with it offline right now and hope to put it up on the Utah Weather Center sometime this winter.  I don't know how skillful the forecasts are, but the horizontal plots sure look awesome.  Here's an example from a couple of days ago.  

We've been working on plume and box-and-whisker plots to indicate accumulated precipitation and periods when heavier precipitation rates are likely.  

5. Acknowledgements

Special thanks to Trevor Alcott, Tom Gowan, NCEP, and NCAR for their coding expertise or models!  

I'll try and cover some more tools in a future post as I couldn't get to them all today.  

Lake Effect Reflections

Last night we got a bit of love from the Great Salt Lake, with the storm evolution exhibiting a number of key characteristics of lake-effect systems.  My apologies for color aliasing problems, but I couldn't correct them in the time I had available.

The action started over Tooele County and the Oquirrh Mountains and then spread eastward to the Salt Lake Valley and Wasatch range.  There are two major flavors of lake effect generated by the Great Salt Lake, banded and non-banded and you can see both in the loop above.  The band is a weak one, but affects the Oquirrhs and the western Salt Lake Valley.  The non-banded precipitation is further east.  Note the disorganized nature of the echo cells over the eastern Salt Lake Valley and the Wasatch Range.

The Oquirrhs actually receive quite a bit of lake effect, as much as upper Little Cottonwood Canyon and, because they receive less precipitation from other storms, lake-effect contributes a greater fraction of their cool-season snowfall (about 8%).

Forecasts of lake effect are notoriously tricky.  Although we've improved a lot in anticipating when lake-effect is possible, specifics on location and amount remain very challenging.  We have a paper that just came out in which we simulated 13 past events over the Great Salt Lake using a next generation, high-resolution forecast model with 1.33 km grid spacing and found the model skill to be about the equivalent of what we find for summertime thunderstorms.  In addition, the model frequently produced intense lake-effect bands during situations when only non-banded lake-effect was observed.

We still have a lot of work to do in this area!

A Forecast Eisenhower Would Have Liked

Plans are nothing; planning is everything

- Dwight David Eisenhower

I've always liked that quote and, although the analogy isn't perfect, I thought of it when I woke up this morning and looked back on the forecasts from yesterday.

Those forecasts (see previous posts), called for a frontal band to move into the Wasatch overnight and give them 3-6" of snow or so, followed by a break late this morning and afternoon.  

Instead, the front stalled to our west.  It gave some decent snow to the northern Wasatch, but only an inch overnight at Alta.  Instead of the break, however, the precip shield that was expected to intensify over eastern Utah pushed far enough north to give us snow this morning and it was heavy at times in upper Little Cottonwooed.  Note the interval stake at Alta-Collins went from 1-7 inches in 2 hours.  Nice!

Even now, although showers are light, we're still under the back edge of the precip shield. 

Source: NCAR/RAL

I thought of Eisenhower's quote primarily because when I looked at things this morning, the pattern actually made perfect sense, even if the results were different than expected.  Nevertheless, forecast amounts, errors in timing and intensity are quite significant for highway and avalanche forecasting and this morning is a reminder that we have more work to do to get the gory details right.  

Thursday talk:

FYI that I'll be giving a talk at Gore Auditorium at Westminster College at 7 PM on Thursday (12 Nov) on the Secrets of the Greatest Snow on Earth.  The talk is free and open to the public with a book signing before and after if you are interested.  Click here for more info. 

A Three-Part Storm

A three-part storm will affect northern Utah through Wednesday morning.

The system really has three parts.  In the first, a band of frontal precipitation drags across late today and tonight.

Snow levels will initially be near 6500-7000 feet, but drop to bench level overnight.  The NAM is bearish on this phase of the storm, generating about 0.17" of water equivalent at Alta-Collins through 11 AM tomorrow, whereas the GFS is more bullish going for 0.53".  Something in the 3-6" range seems most likely.

During the second phase, we could be caught between two lovers as the frontal system intensifies to our south and east as a post-frontal area of instability showers remains upstream.

Thus, as things stand now, it appears late tomorrow morning (or even mid morning) through afternoon we'll be in a bit of a break.

Tomorrow night everything depends on the post-frontal crap shoot (see The Post-Frontal Crapshoot for more information).  The models indicate northwesterly flow with orographic precipitation, as shown in the GFS below.

The latest NAM time-height section just came in and you can see the moist, northwesterly flow from about 3Z Wednesday (8 PM MST Tuesday) through Wednesday morning.

This is a pattern where lake effect is also possible, although anticipating amounts and locations is pretty much impossible at this time.

I see a wide range of possibilities for the Cottonwoods for tomorrow night given the fickle nature of these post-frontal periods.  Probably the heart of the range of possibilities is something in the 3-6 inch range tomorrow night, but I've seen situations where patterns like this have produced more, and I've seen situations where they've produced less.  Keep your fingers crossed.

The Times They Are a Changing

November is here and it can be quite remarkable how different the climates and microclimates are in northern Utah compared to October as the sun appears lower and lower in the sky each afternoon.

Yesterday I rode up City Creek Canyon.  Temperatures in the Avenues were in the upper 40s when I left, which isn't too bad, but I fortuitously wore pair of insulated full-finger gloves and stuffed a soft-shell in my camelbak.  Most of lower City Creek was fine, but the upper mile and a half is now almost fully shaded and quite cold on a day without wind.  In addition, the road has some patches of snow on it now, with a complete coating at the top.  Passable on a mountain bike and a road bike up to the very top, but very cold!  What a microclimate.

Meanwhile, even though it was sunny and comfortable in the Salt Lake Valley, we're not "mixing out" the valley like we do in summer and early fall when a ridge builds in.  Yesterday afternoon's sounding shows a pronounced capping stable layer based about 1000 m over the valley floor, which explains some of the hazy gunk that was lingering around the valley.  

Source: NOAA/SPC

Today, however, should be warmer and quite splendid for November.  The south winds are picking up and highs will likely reach the mid and upper 50s.  Temperatures in the morning sounding were quite warm and near freezing all the way to 3000 meters.  At the top of Mount Baldy (11,000 ft), it's currently 27ºF, so it will be a nice day to soak up some rays on sunny aspects at all elevations.

20 Years at the U

This week marks the 20th anniversary of my arrival in Salt Lake City to start as a professor of meteorology (we've since changed the name of the department from meteorology to atmospheric sciences).

To say I'm fortunate to be here is an understatement.  In 1995, the meteorology department at the U was quite small with only 7 faculty members.  They wished to expand in the area of mountain meteorology, but they also had a pressing need for someone who could teach synoptic meteorology and weather analysis and forecasting.  I finished my Ph.D. in spring 1995 and was about as raw and inexperienced as a young professor could be, but could fill both needs and was offered the job.  I was also fortunate to have Professors John Horel and Jan Paegle here as mentors as they were incredibly helpful in helping me navigate many of the challenges one faces as a new faculty member.

Over 20 years it has been my privilege to teach and advise countless undergraduate and graduate students.  I'd like to thank all my former students out there for making this job so satisfying and rewarding.  Special thanks for those students, like Neil below, who have broken trail for me.  With great physical exertion comes the clarity of mind needed for deep learning.