Dan's Wild Wild Science Journal

Severe weather threat area for Saturday from the Storm Prediction Center (NOAA)

A deadly weekend tornado outbreak is becoming more likely this evening. Conditions are coming together for potentially violent long track tornadoes across much of Mississippi and Alabama on Saturday. Tennessee, Arkansas and Louisiana are also threatened.

It is quite possible conditions may reach the threshold for a rare High Risk outlook from the Storm Prediction Center (NOAA) in Norman.

Several of the tornado watches tomorrow will be PDS watches. (Particularly Dangerous Situation).

Significant Tornado Parameter forecast from a NOAA NWP Model. This is for 4PM CDT Saturday. Courtesy Earl Barker.

As I mentioned in my previous post last night, the parameters for an outbreak of strong and long track tornadoes are quite high. The numbers have actually increased some. One of these parameters that forecasters look at is the significant tornado algorithm.

This index looks at instability and wind shear along with low level moisture to indicate areas where the highest threat is. Based on this index, a high risk may be needed for NW Alabama and nearby areas on Saturday.

There are already severe thunderstorms with possible tornadoes developing in Mississippi.

The greatest threat for tornadoes will be during the day and early evening tomorrow. It is very possible that more tornadic storms could develop across Arkansas and Louisiana before sunrise.

Don’t get in the “fraidy” hole yet, but you might want to vacuum the carpet in it…

Severe Storm Forecast from the Storm Prediction Center (NOAA/NWS)

It may sound crass and I do not mean it to be that way, but there are likely to be deaths from tornadoes in the next 48 hours.

If only we could only say exactly where!

What we can say is that conditions are coming together to make the development of a few strong tornadoes and they will likely be in Mississippi and Alabama on Saturday. Here in Alabama there is special concern because nearly 100,000 people will be at a major stock car race in Talladega. Here in Huntsville is a major art festival Panoply.

The odds of a tornado touching down at any one spot are minuscule, but the odds of a tornado in the area are rather high. So all we can do is to warn people to be alert and to have a NOAA Alert Weather Radio.

One other thing. If you live in a mobile home, make arrangements to go somewhere else during a tornado watch on Saturday. Believe me when i say that several will be issued. Your odds of being killed or injured from a tornado are many times higher in a mobile home. Leave the windows closed (Opening them can increase damage) and consider a walk in closet or a bathroom for safety.

You might wonder just how we know tornadoes are possible?

Tornadoes require two basic ingredients. Wind shear and instability.

A tornado can develop with a lot of one parameter and a little of another. The big outbreaks happen when you have a LOT of both.

CAPE is a measure of instability. Above 1000 is very unstable. from UIUC.

This weekend looks to have a lot of  both.

Instability can be measured in different ways but one preferred way is to use numerical weather prediction models to estimate the CAPE.

CAPE stand for Convective Available Potential Energy. Remember potential energy from high school physics? Let me just simplify it and say that numbers over 1000 joules/kg are bad.

Wind shear can also be measured (and is) in different ways. The direction of the wind shear that a storm sees as it moves along is most important and meteorologists look at what is called the storm relative helicity as an indicator. This gets even more complicated (can you say calculus?), but the basics are this- over 200 is bad.

Helicity is one measure of wind shear. It's one of the two ingredients for tornadoes.

Here are some actual NWP charts for those parameters on Saturday. I will let you check out the numbers.

There are a lot of meteorologists at NOAA and at TV stations (like me) who will be working together for very long hours to give as much warning as possible.

Take them seriously this weekend, even if you never have before.

Mother nature may be in a rotten mood.

Image from NASA Modis Satellite 2 pm UK time. The thinning ash cloud is being absorbeb by a low pressure SE of Iceland.

Great news for air travelers this evening. The eruption in Iceland is now putting out much less ash and the cloud is much lower. The low level ash is not headed toward the UK as the image above shows.

The snow-cap that covered the volcano has melted away. This has reduced the steam/ash combination that was climbing to over 3,000 meters.

Even more good news tonight. The upper level and lower level winds will push any new ash cloud to the North of the UK and France. It looks likely that airports will be opening across Western Europe tomorrow and this is now being confirmed by the BBC.

Click image for the full resolution! The ash layer is at 10-16,000 feet now instead of the higher altitudes over the weekend. From NASA. This new cloud is headed toward the UK.

There is actually some good news tonight. The ash cloud over Europe has diminished and begun to sink south of Paris and London. Here is the outlook for tomorrow regarding the no go areas.

Ash extent forecast for Tuesday 7 am London time. From UK Met Office.

Most of England is in the clear by 1 PM! From UK Met Office.

Late word tonight that the eruption has picked back up. The current wind flow will steer the new ash cloud toward the UK, Central England in particular. So the openings of the airports tomorrow may be brief. See below.

The ash will be at a lower altitude but it may become more dense and put an end to the planned reopening of the airspace over the UK tomorrow. Further east there is more hope for flights.

This happened last night in Dekalb County, Alabama.

Hail Damage in Rainsville Alabama. Sent in by a viewer.

The storms kept me up until 5am. It was a real dark and stormy night!

The hail punched through to the insulation! It was one 25cm deep in some places!

Notice the grill on the window A/C unit!

The size was not that big- there was just a LOT of it.

Later,
Dan