Wednesday, October 29, 2014

November Cold Front and Precipitation

Shown are three weather models. The NAM, GFS, and SREF. Each show potential for precipitation this Saturday afternoon with the approaching trough. Freezing levels are getting lower as we move closer to winter. Expect some snow in the mountains :)





Five Things a TV Meteorologist Won't Tell You

1. The United State is not the world leader in weather forecasting. Europe, the UK, and Canada have some of the best supercomputers and weather models to date. In fact, we pay them for a their weather forecasts. Korea and China are also competitors. Next time you want to blame someone for getting the forecast wrong, you should blame your politicians. (Capital Weather Gang article, Cliff Mass Blog)

2. What does 20% chance of rain really mean? (Blog Post)

3. Crucial weather satellites are being decommissioned and are not being replaced. This will have a negative impact on our weather forecasting abilities in the near future. Some replacement satellites are still being built and others are not even on the drawing board. Again, you can blame your politicians. (article)

4. A TV "meteorologist" isn't required to have a degree in meteorology. Some on-air meteorologist just have broadcasting or journalism degrees. The American Meteorological Society awards a Certified Broadcast Meteorologist (CBM) designation to those who graduate with a meteorology degree, complete a written test, and undergo a review process. (AMS requirements)

5. Forecasts for the next two or three days are pretty good. Forecasts beyond seven days are very uncertain. Forecasts beyond two weeks are impossible. Can't blame anyone for this. That's just the chaotic nature of the earth system. (NOAA Document)

Monday, October 27, 2014

% Chance of Rain

One common question people ask meteorologist is, "what does a 20% chance of rain even mean?" Why are meteorologist so uncertain about a weather forecast. To answer this question, you need to first understand that the atmosphere is in a state of chaos. Small perturbations or changes in the conditions can have large effects in the weather at a later time.

http://untamedskies.com/scientific-research/
Since before the invention of the computer, meteorologist have tried to explain and predict the weather with complicated partial differential equations. These equations, when applied to the entire world, can only be solved with supercomputers. Improvements in computers in recent decades is one reason weather forecasts are improving. Another reason is that these equations are also being improved. But the math equations used to predict the weather are complicated, imperfect representations of the actual atmosphere, and some of them can't even be solved. Instead, these math equations estimate the state of the atmosphere as best we can.

There are lots of different equations to predict the weather. Some work better than others in certain situations. In recent years, ensemble forecasting has become popular. In ensemble forecasts we run several different weather models with slightly different initial conditions or different equations and compare the results. A 20% change of rain essentially means that 20% of the model results suggest precipitation for a particular area. A 20% chance could be interpreted as "most of the models say there won't be rain, but there is enough instability in the atmosphere that if the conditions are actually like model X suggest, we could get rain." Obviously, if all the models in agreement there is higher confidence in the forecast and we can say there is a 100% chance of rain.

All the below images illustrate ensemble forecasting. The goal is to generate many possible solutions to the weather. This bigger picture helps in many ways determine what possible weather to expect. As you can see, every version of the weather model produces a slightly different result. We try to express these possibilities in terms of a confidence, or a percentage. It's common practice to average the results, but it is not always best to base forecasts solely on an average of the possibilities. A better way to approach weather forecasting is to keep in mind all the possible circumstances. They way I interpret "20% of of rain today" is this: We have the ingredients for rain today, but based on the collection of weather simulations, we are only 20% confident those ingredients will mix just right for the rain to occur.

http://mp1.met.psu.edu/~fxg1/ENSHGT_0z/ensloopmref.html
http://www.spc.noaa.gov/exper/sref/fplumes/index2.php?YMD=20141027&RT=09&PRM=Total-QPF&SID=PDT&INC=ALL&NNC=&max=&min=&mZOOM=4&mLAT=37.77478874678&mLON=-96.042896875&mTYP=roadmap
http://mag.ncep.noaa.gov/model-guidance-model-parameter.php?group=Model%20Guidance&model=gefs-spag&area=namer&cycle=20141027%2006%20UTC&param=500_540_582_ht&fourpan=no&imageSize=&ps=area
http://www.spc.noaa.gov/exper/sref/matrix.php?fhr=f036&startdate=latest&field=SREF_prob_totpcpn_0.05_3hr__

http://www.ecmwf.int/en/forecasts/charts/medium/ensemble-mean-and-spread-four-standard-parameters?area=North%20America

Sunday, October 26, 2014

cold front

source: weather.utah.edu
The above model image is a little complicated because you have to read it backwards--Today (Sunday) is on the right, tomorrow is in the middle, and Tuesday is on the left. Pay attention to the thick blue line. This is the freezing level and it is decreasing with time. This is the cold air behind the cold front that is passing through Utah right now.

Looking at Mesowest observations we can see the timing of this front. It appears to have passed Salt Lake City around 4:00 this morning. Frontal passages are evident from a change in rapid changes in temperature and humidity or rate of change, changes in pressure rate of change, and/or a change in wind direction. Looking at the station at the Salt Lake airport we can see all of these signals:

Temperature doesn't seem to drop suddenly, be can can see evidence of a change in air mass with the rapid drop in dew point temperatures. As the day progresses the temperature doesn't rise as rapidly even though it is sunny outside. This slow heating is due to the cold air that came behind the front.
source: mesowest.utah.edu

Changes in pressure also indicate frontal passages. Here we see the pressure holding steady until 4:00 this morning (the same time the front is identified in the temperature/humidity plot shown above). This is known as a pressure check, where fronts are identified as a time when there is an increase in the rate of pressure rise. Fronts can also be accompanied by a decrease in the rate of falling pressures.
source: mesowest.utah.edu

And finally, we frontal passages are also distinguished by a shift in wind direction.
source: mesowest.utah.edu

Other stations seem to agree with the timing of the front and show these same indicators of the frontal passage. Utah County began feeling the affects of the front between 6:00 and 7:00 this morning. BYU's weather station measured a wind shift at 7:00 AM.

 Finally, it's starting to feel like Fall. Time for some hot chocolate!

Thursday, October 16, 2014

Crepuscular rays

Crepuscular rays shine down on the Wasatch. According to Wikipediacrepusculum is latin for twilight, and you usually see this optical phenomenon near sunrise or sunset. There is an optical illusion here. Though the sun's rays look like they converge at a point the rays are actually parallel, much like the train tracks shown below. The rails look like they converge at some distant point, but they are really parallel (otherwise a train would have a difficult time going anywhere).