December 10, 2010

A Big Storm, Cold, Thunder, and Perhaps Winter Precip?

Filed under: Forecasting,Mid-latitude cyclones — Erin @ 5:55 pm

Think it was pretty cold this past week? Well, wait till Sunday and Monday. But before we get to that point, we’ll have had quite a system to pass through the area, featuring thunderstorms, strong winds (and the accompanying frigid wind chills), and—though the National Weather Service isn’t officially forecasting it—I think an outside chance of frozen precipitation. Areas outside the Southeast are in line for much more frozen stuff.

Yup, it’s meteorological winter (December through February), all right.

The low that will quickly become a powerful cyclone is analyzed right now as 1002 mb. In a short period of time, it should be located in the Plains states and closing on the Midwest. By the time it reaches Iowa (approximately early tomorrow morning), it will be generating rain and snow to its north as the warm sector (that’s where we are) forms.

Look at this image from the North American Model (NAM). Classic cyclonic shape.

This is a forecast for late Saturday night or the wee hours of Sunday morning, which may be a bit slow. Our storms—generated by uplift along the cold front—may arrive earlier than this. The timing is going to be critical, especially in consideration of the outside chance of wintry precipitation. If the moist sector of the cyclone pushes through faster (forced along by cold, dry air on the other side of the cold front), then we’re not going to get anything of that kind. Jackson NWS doesn’t want to forecast any such precipitation, either; I freely acknowledge that the odds aren’t great and it makes sense that they would not want to go out on a relatively unsupported limb.

However, there is some model support for my thinking on this. Here’s the NAM forecast for 6 hours later.

Observe that precipitation is occurring north of the 0-degree line. Observe where the 0-degree line is.

Looking at imagery for the lower levels of the atmosphere, we can see that it will be a very, very close call for northeast Mississippi, and the type of frozen precipitation (if any) cannot be guessed at with any accuracy. This is because when a cold front passes, there is not a straight vertical line dividing the area of retreating warm air from the advancing cold air. Instead, it is an angle. Since cold air sinks, we are far more likely to have cold air at the surface and some warm air still lingering aloft in the first couple of hours after the “edge of the front” (a very fuzzy demarcation, but you get my point) moves through.

If we do get any winter weather as a parting gift from this cold front, it’s likely to be mixed with rain, and the ground will already be wet from the rain we already would have had (and warm from having been in the warm sector prior to the frontal passage). Accumulation is not even on the table.

What about those thunderstorms? Are we in line for another severe weather outbreak like the one from the end of last month? Probably not. That event had CAPE values that would raise eyebrows in the spring, let alone the fall. These potential energies just will not be present for this front. However, thunderstorms are expected to occur Saturday evening, likely bringing a lot of rain and lightning, and an outside chance of small hail.

After the front has passed, we are going to get a glancing blow from the arctic air mass that is behind the system. Highs are not likely to reach 40 in too many areas north of I-20 on Sunday and Monday. The wind chill on Monday morning is going to be dangerously cold, approaching 10 degrees and getting close to zero around highway 82 and points north. Take note of this if you have to go to work or school.

In the wake of the previous cold spell and the one that is coming up soon, I’m hearing some grumbling about the forecast that NOAA made for a “warmer and drier winter than average” for the Southeast. That forecast is still on tap. However, “warmer and drier than average” typically does not mean that it will be so much warmer and drier that we will be able to notice it every day! We are talking about a couple or three degrees on average for the entire winter season. That allows for plenty of below-average events, such as this upcoming one. And indeed, if you look at the long-term GFS, it shows a rather significant warmup to temperatures approaching 70 degrees until the next system pushes through just before Christmas and drives temperatures down to the 40s again. Such long-term forecasts cannot be trusted in the winter season, because the weather is so volatile, but it’s definitely food for thought!

December 6, 2010

Snow in Dixie Watch: Tuesday-Wednesday, Dec. 7-8?

Filed under: Forecasting — Erin @ 11:19 pm

I’ve been keeping an eye on this for several days, but the models have been cagey about it, and I have been reluctant to make a blog post about it. For the past two years, predicting snow for central Mississippi in December has been, let’s just say, a fiasco, even for the National Weather Service office, and I’m not overly inclined to get burned a third time running. As a general rule, snow in the South tends to occur in late winter. That said, this is looking promising enough to warrant a blog post.

It’s not a big system. In fact, it’s not easy to identify just what is going on with a surface chart, because the snow (if any) will be driven primarily by upper-atmospheric troughs that do not extend to the surface. What we have got here is a low pressure system off Atlantic Canada and a very strong jet streak that has dived as far south as the Gulf of Mexico.

Here is one level of the troposphere, 500 mb. The upper-level trough is evident enough here, and would you look at that—its base is right over us:

However, there is no obvious trough over the South on a surface map:

What you see in the surface map that appears to match up with the trough at 500 mb is actually a temperature gradient. The thin black lines are the pressure lines, and the thicker lines that correspond with the thick lines at 500 mb are actually temperature lines. There is a reason why this temperature gradient shows up as a trough at the upper levels. When the air is cold, it contracts. When it is warm, it expands. When you consider that the temperature change occurs over time, it can be seen as cold or warm air moving in at a given layer. This process is called thermal (or thickness) advection. Here are simple diagrams indicating how thermal advection happens at the mid levels:


These two diagrams are by no means representative of every advection scenario that can occur. Thermal advection can happen at any level of the atmosphere. Whether the advection results in forced ascent from the surface or forced descent to the surface depends on exactly at what level of the atmosphere the strongest contraction/expansion is occurring and what is happening below and above it.

We are going to be in a forced ascent situation, as shown in the second diagram. This results in pressure levels that are expected at a given altitude in the atmosphere (in this example, 18,000 feet) actually occurring at a lower altitude, thus the “trough.” But look: Relative to the lower levels of the atmosphere, there is a high. This is shown on the surface map. What’s important is that there will be forced ascent, not that there will be relatively high pressure at the surface.

This feature is what is going to generate precipitation. By creating a relative low aloft, it causes air at the surface to rise (nature abhors a vacuum), generating cloudiness. Precipitation is expected to fall as snow because there is no warm air getting between the surface and the layer of clouds, owing to that extremely southerly and extremely strong jet.

Will there be snowfall? Probably. The big question seems to be whether any of it will stick. It has been cold enough lately to freeze the ground, but a big question mark is a layer of dry air at the middle to lower levels of the atmosphere. The forced uplift may take a while to fill this layer of air with moisture. We’ll have to see. In any case, keep an eye to the sky tomorrow night!

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