Saturday January 8th, 2011

Greetings and welcome!

I saw that both Iron Snowshoe and Paul Bunyan clubs groomed last night or yesterday. Both showed better than expected results.  The trails along F between here and Lakewood looked nice in most spots, and like ice with a little snow on top in others. The sub-inch of snow that we had was not enough to heal bare roads, fields, and driveways, though it covered them a bit. As Slug mentioned in his comment today, 1″ down, 5 more would do it.

The storm for next weekend is still on track and still showing up as snow. I see it for about a 3-6 storm. There is another one showing up the Tuesday after that with another 3-6+, and yet another behind that. It is a nice cartoon, let’s hope that it plays out.

Nathan left an interesting comment this morning. Since I’d rather do this than the work in front of me, let’s address it.

Nathan Says:
January 7th, 2011 at 4:45 pm Ray, Could you make a quick link for us to use for the GFS model you use. The NCEP page is very confusing. Also your link for the Unisys Forecast Models is broken or they have changed their page. I can get to the Prec/Pres 10 day loop here..

First of all, thanks for the comment.

I use the NCEP GFS for a couple of reasons. One is that it updates the fastest that isn’t behind a pay wall. Apparently the TV weather guys get theirs an hour or two faster.

The other reason is that it goes 16 days out rather than the seven or ten that most GFS sites use. Certainly the 16 day version is lower resolution at the longer time frames, but sometimes the weather has us looking that far out for a glimmer of hope.

I looked around quickly and didn’t find a better one, so let’s take a walk through this one.

When you go to the NCEP page there are a series of models offered (#2). I use mostly the NAM, the GFS, and the RUC. The NAM works pretty well for the 4-5 day range. The GFS goes out 16 days. Those two update 4 times a day. The RUC is a very short term model updated hourly, that only reaches out about 6 hours. It is useful for when the storms are here.

In #1 below it shows the last 4 model run times. Keep in mind that we are GMT -6, and that 18 UTC is noon CST. #3 gives you the date and the resolution that you want the model displayed  in, in my case usually medium.

So you click through the latest GFS, which is the 18UTC (noon) version, in medium, the next column left of the #3. Up comes this page..

My first stop is the 850mb Temp, MSLP, and 6 hr Precip box, #1.

The 850mb Temp is useful for telling where the rain/snow line is. It is the temp about 3,500-5,000 feet up in the atmosphere. MSLP is Mean Surface Level Pressure, showing the various high and low pressure surface systems. The 6 hour precip is just that, but expressed in rainfall equivalent. Usually a 10 or 12x multiplier works around here to convert it to snow.

So I’m at the 850mb Temp, MSLP, and 6 hr Precip box. If I have a certain time I am checking on I pick that, otherwise I run the loop, #3.

I can check the expected precip (QPF) of the storm by going to #2 and looking back at the storm’s collective rainfall. In other words if the storm will clear us by 72 hours out, I can go to hour 72 and look at the previous 24, 48 or 60 hours to see what the total precip for that storm or time frame will be.

So I am looking at next Saturday’s 6am frame 168 hours out. What does this tell me?

1) It is a 1000mb storm (MSLP). Not big, not tiny. The isobars, the circular lines, show me the pressure gradient. If they are close together, expect a strong storm and a lot of wind. This one is so so. There will be some wind, but not the 60mph stuff.

2) The low is tracking over far NW WI, usually not good for us. We want the L between about Green Bay and Chicago. If it is too far west, we get rain.

3) The color shading tells me that those 6 hours could bring 0.1-0.25″ of rainfall equivalent, or 1-3″ of snow.

4) The 850mb temp lines, outlined so subtly in pink, show the upper air temperatures. For theprecip to be  snow we want to see about -4C or colder on the 850mb temp. In this case we are closer to the -10C line than the 0C line, putting us probably in the snow side. Based on this map, I’d expect the rain/snow line to be between Sheboygan and Green Bay.

Got all of that? Good, there is a quiz later, and the grade goes on your permanent record..

Again this isn’t a definitive thesis on forecast models, but rather a little guidance. I encourage you to study up on it on your own if it interests you. Personally I find weather pretty fascinating. Eventually there is some physics involved, but when learning about stuff like CAPEs it isn’t boring.

OK so I promised a quiz later and here we are. Looking at this map, how much rain or snow will we see here, and will it be rain or snow??

If you answered 1-3″ of snow, you got it right. The -10C line right over us makes snow a good bet, and being in the 0.1-0.25″ band gives us 1-3″ of snow. In this case the 3-6 band is pretty close, so it may merit an upgrade to a 2-4 or 3-6 storm for those 6 hours. I guess that we will find out for sure next Saturday.

Have a good weekend and thank you for visiting!