January 30, 2011

Epic Storm to Bring Severe Weather to the South

Filed under: Mid-latitude cyclones,Severe — Erin @ 5:45 pm

Any “weather weenie” is well aware of the fact that a major storm is coming for the eastern United States (yes, it really is that big). Accuweather.com and the Weather Channel are beginning the hype, Twitter is abuzz with forecasts and speculation (at least in the weather-weenie corner of it), and even the stolid National Weather Service has gotten in on this, with the Storm Prediction Center calling for “Particularly Dangerous Situation” winter storm warnings. (A PDS ordinarily is issued for severe weather—typically, in fact, for the type of severe weather event that has a historic tendency to generate violent tornadoes.) Indeed, blizzard conditions will be widespread across the Midwest, probably affecting an area as large as the “Storm of the Century” of March 1993. The culprits are a low pressure center and a mass of arctic air coming from Canada. The unseasonably warm air that we have enjoyed in the Southeast is going to fuel the system, so to speak.

I am not going to touch on the snow and ice aspects of this. The area that will undergo winter storm or even blizzard conditions is simply too large for me to give anything other than a large-scale overview, for one. And two, this is primarily a Southeastern blog as far as actual forecasts are concerned, and we in the South are going to have to deal with the system in a different way: severe thunderstorms. That warm Gulf air that will feed the system is going to be generating what’s known as a “warm sector.”

Mid-latitude cyclones such as this one have three distinct sectors: one to the east of the cold front, one to the west, and one that remains to the north. The north sector is what will be producing the snow and ice; north sides of cyclones typically have a constant flow of precipitation, because the cold front associated with the low does not actually pass over these regions. The west side is the “dry” side, in general (though there are exceptions); this is also typically a cold part of the system. That’s what’s going to bring a return to winter temperatures for the South; it will follow the passage of the cold front. And the warm sector is the southeastern side—the side that will have southerly flow from the Gulf of Mexico. These 60s and 70s that we’ve been experiencing, in other words.

There will be several factors that will create severe weather for us. I am going to illustrate these with images from the North American Mesoscale (NAM) model, downloaded from TwisterData.com. First is the location and intensity of the jet stream:

That image shows that the jet at 300 mb (~9000 m) will have 140-knot winds over ArkLaTex on Tuesday evening. The jet typically does have strong winds on the order of 80 knots, which is why it is a death sentence (via shear) for hurricanes in the summer, but these same winds that rip warm-core tropical cyclones to shreds create conditions that enhance cold-core cyclones in the cool season, when the jet is much farther south. 140 is quite strong even for winter. Furthermore, the location of the jet indicates that air will be drawn to the northwest along the eastern end of the strongest points—the exit region, in meteorological parlance. The image doesn’t appear to depict this, but the southeastern flow I am referring to is taking place vertically, and this image only shows winds at the 300 mb level. This will create areas of divergence in Mississippi, Louisiana, and eastern Arkansas. Divergence leads to a “vacuum” of sorts—an area of lower pressure—and therefore to rising air.

The second factor is the extreme temperature gradient.

Again, take a good look at Arkansas. This is an image from the same time as the one depicting the jet stream, but much lower in the atmosphere (850 mb, or 1500 m). The 850-mb low is located around Fayetteville, and you can see the wind flow around it. But look at the colors, which represent temperature. That is a very strong cold front, and the reason for it is the extremely unseasonable warm weather across the Gulf states. You can see the southerly flow of warm, moist air, creating the warm sector. The wind flow will be moving warmer air into areas of cooler air (the wind barbs are crossing isotherms), a process called warm-air advection. Look at the strength of the winds at this level, too. There are 65-knot winds forecast at 850 mb! This is hurricane-force.

Finally, the pressure gradient itself:

I will direct your attention to the isobars over the Plains. The low hasn’t even fully intensified yet, but already there is a 40 mb gradient because the high is so darn strong. It is this pressure gradient that will be generating the intense winds with this system, producing blizzard conditions north of the freezing line, and severe weather south of it.

Now, the $64,000 question: What type of severe weather are we talking about? None of the above sounds like particularly good news, but fortunately (for north MS, at any rate), I do have some comparatively good news now. I do not think our tornado risk is going to be all that high, nor our large hail risk. Instead—you guessed it—I think we are in line for some very strong winds as the cold front approaches us late Tuesday night/early Wednesday morning, combined with a lot of rain and the chance of small hail. There will be some instability, but CAPE levels are not going to be much higher than about 750 J/kg across Mississippi. The helicity index, a parameter reasonably predictive of supercell formation, is not expected to be very high. I think there is a chance of supercells in south MS (let’s say south of I-20), but at the present time I am not seeing this type of severe weather event on a large scale.

However, as people in north MS can certainly attest after the repeat punches of severe weather over the past year, high winds can be damaging enough on their own, and this system has the potential to generate some very strong winds for us. All in all, though, I think we are dodging not one, but several bullets with it.

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!

October 26, 2010

Major Storm Unfolding in the Midwest

Filed under: Mid-latitude cyclones,Severe — Erin @ 10:56 am

The megastorm that weather people have been talking about for several days has materialized, and it is already bringing very high wind and long lines of severe squalls and tornadic supercells to the Midwest and upper Tennessee Valley. Rain and thunderstorms associated with the cyclone extend as far south as the Gulf states. Blizzard conditions are expected for areas on the cold side of the storm, which is expected to bottom out its pressure in the 960 mb range. The low, centered over Minnesota as of this writing, is already at 966 mb. The factors causing this powerful system are many: Very unseasonable heat in the Southeast has led to a powerful warm sector for the cyclone, and a surge of cool air to contrast with this has caused development of a strong jet stream (>100 knots) in the upper levels of the atmosphere. Had Hurricane Richard’s remnant low been drawn more to the east, as the models were suggesting a couple of days ago, the system would have become even more powerful. A mid-latitude cyclone of this magnitude is not common. It is a credit to modern technology that our computer models were able to accurately predict this rare of a system several days in advance.

The Storm Prediction Center issued a high risk for today, a rare occurrence, but with dozens of tornado warnings already called for the Midwest where the storm’s warm sector meets its area of highest vorticity advection, I expect that this will end up verifying.

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