10020601 - Baltimore Snow

Baltimore is receiving a large amount of snow today as well as yesterday. It looks like some of the current totals are in the 20" to 30" range. I believe a general rule for converting snowfall levels to rainfall levels is to divide by 10 so this would be the equivalent to 2" to 3" of rain, but this number varies depending on what type of snow is falling. So, in terms of the mass of the precipitation, it's pretty moderate, but the fact that it's snow means it's a big mess.

It looks like one of the reasons for the snow is due to some interesting upper level dynamics. There are areas of fast winds that appear within jet streams called jet streaks. A simple model of a jet streak says that it can be divided into 4 quadrants. Two of these quadrants produce areas of divergent air in the atmosphere. Generally, you can think of this as air being stretched or pulled apart horizontally. This produces lift in the atmosphere. So, if you've got moist air to lift and freezing temperatures all the way to the surface, you can get snow. This is probably way oversimplified as there are a lot of other things going on, but for this post, I'll stick to that.

If you look at a 300mb chart, you can see two areas in the jet stream that I've circled. Divergence occurs in the right-entry region and the left-exit region of these jet streaks. When you get two jet streams coming together or a split in the jet stream, you can have two jet streaks positioned so that the left-exit region of one streak overlaps with the right-entry region of another streak. This is what appears to be occurring today in the upper levels over the mid-Atlantic states which is no doubt contributing to the snow. I've circled these areas in the image below...

09120301 - College Station Snow

So, I spoke too soon yesterday in saying Dallas was getting snow. It looks like Dallas may not get any snow now. We will see.

As for College Station and much of south east Texas, we'll be right in a bulls eye, strangely. It seems like it would be intuitive that is CS receives snow, that points north would receive snow. Not in this instance. One of the limiting factors (maybe THE limiting factor) is going to be the lack of moisture in the atmosphere. Here in CS, we're getting our moisture from the Gulf in a bit of strange dynamics. Anyhow, tomorrow should be an exciting weather day in CS. we could get 1-4" in the area.

09120201 - Dallas Snow III

I've removed the question mark from the title. It's pretty clear now well have some winter weather in Texas on Friday which should be exciting. This event is pretty strange in that the winter weather is so deep into the state, but this is part of the reason the whole scenario is setting up.

We have a strong cold front coming in on Friday with the 850mb freezing line extending south, deep into Texas and almost to the coast by sometime Friday. This is important as the atmosphere will be mostly at or below the freezing point all the way from about the 925 level up. The surface will probably be slightly above freezing until the snow starts and then you'll see the temps drop due to evaporational cooling.

We've got a trough swinging in at upper levels along with two jet streaks to our east. Anyhow, SE Texas will be in the proper quadrants of the jet streaks to support upper level lift and we should have some moisture swinging around from the north, down over the Gulf, and then twisting around and heading back to the northeast. This will provide the necessary moisture for the precip. So, the timing of the trough, the front, the moisture, the amount of moisture, etc all will contribute to how this event pans out. Right now, it looks likely that we'll get at least 1/2 inch here in CS and possibly more. It should be an interesting scenario to watch.



current >1" probability map above for 0300UTC on Saturday (9pm Friday).

More updates to come.

09120101 - Dallas Snow II?

So the latest NAM model is now in somewhat of an agreement with the GFS run yesterday and this morning. A good shot of snow associated with a shortwave trough and a shot of moisture behind the exiting upper level low. This could all go away sometime this week as we get closer to Friday, but it certainly makes things interesting. It's pretty rare that any kind of winter precipitation makes it to the Gulf coast of Texas. In the image following the one below, the big blue area in the panhandle moves southwest and makes it all the way to the coast.

09113001 - Dallas Snow?

Looks like the current run of the GFS model has a lot of snow in the forecast for Friday for much of Texas. Crazy! The other models don't agree. We'll see how it turns out.

09110201 - Dallas Climatology

It looks like Dallas is on a precipitous (<-- thank you) decline from a rainfall perspective (climatologically speaking). The average rainfall amount for DFW in October is 4.11 inches which is the highest monthly average for the year except for May at 5.15 inches. The rest of the the monthly averages are near 3 inches or less. I would guess that these peaks in May and October are just due to the seasonal transition in which DFW sees a lot of frontal passages. That October total of 4.11 inches is book ended by average totals of 2.42 inches and 2.52 inches in September and November respectively which is quite a difference. Compare these numbers with the amount of rainfall DFW officially received this October of 8.05 inches of rainfall. That is good enough for a #6 position on the list of wettest Octobers with 14.18 inches in 1981 at #1.

The weather will begin to be a little cooler and drier now with the air masses behind the fronts staying around longer and occasionally reinforced by additional cold fronts. This makes for boring forecasts unfortunately, but snow averages will be on the increase with a monthly average high of 1.2 inches in February.

See the rest of the data at the link below...

http://www.srh.noaa.gov/fwd/?n=dfwann

09092601 - Cirrocumulus?/GOES Imagery

I took some photos of the sky today near sunset as the cloud pattern was striking. I've tried to determine, since taking the photos, how high the clouds were, but I couldn't find and ceiling data for that hour from any of the reports. Anyhow, I believe that these are cirrocumulus, but they could be low enough to qualify as altocumulus. The prefix alto, in this case, would mean a 2000m-6000m height. Any clouds over the 6000m height would carry the cirro prefix.

One characteristic of cirrocumulus that these had was that in some areas they appeared in distinct rows. This is due to wind shear. In this case, the sub-tropical jet stream was somewhere above these clouds and moving very quickly in a different direction. Wind shear is the change in speed and/or direction of the wind with height. The jet in this area, at this time, was moving at about 70 knots out of the SW at around 9000m high.

Below is a picture of the same clouds from the GOES weather satellite parked over the eastern US. This is 1km resolution imagery and so far is the most detailed I've seen. Click on the image for some additional detail. This should cover most of the state of Texas and was taken around 6pm today.