Saturday, April 4, 2015

April 9th, 2015 Potentially Significant Severe Weather Outlook

There is an anomalously high risk for severe weather on Thursday, April 9th.

Storm Prediction Center
(click all images to enlarge)
The above image shows the Storm Prediction Center's long range outlook chart over the Day 4 to Day 8 (3 to 7 days from today) period. This particular map is valid for Thursday, April 9th, and it shows a very ominous forecast.
The SPC has outlined the following regions for a 15% chance of severe weather within 25 miles of any given point:

- extreme southern Wisconsin
- eastern Iowa
- northern and southern Illinois
- western Indiana
- northwest and extreme southeast Missouri
- east-central and southern Arkansas
- southeast Kansas
- central Oklahoma
- northeast Texas

The 15% chance of severe weather, put into words, generally suggests that there is a noticeable risk for severe weather in the areas mentioned. Residents in those areas should keep an eye on the forecast as the date approaches, and watch for inclement weather when the event arrives.

The SPC has outlined the following regions for a 30% chance of severe weather within 25 miles of any given point:

- central and southern Illinois
- much of Missouri
- all of northwest Arkansas
- extreme southeast Kansas
- eastern Oklahoma

The 30% chance of severe weather, put into words, indicates that there is a relatively good chance that severe weather will strike the areas mentioned above. In addition, there is a risk for more significant severe weather. Residents in those areas should continue to keep an eye on the forecast for this date, and closely monitor all watches and warnings when the event date arrives.

Weather Prediction Center
Let's now identify the factors playing into this severe weather threat. Looking at a surface map for the morning of Thursday, April 9th, we see an elongated stationary front which has drifted northward, now displaced over Nebraska into southern Minnesota and southern Wisconsin, diving into central Iowa and Kentucky. We also see a broad area of low pressure centered in Kansas, with a warm front draped across Missouri, a trough along the Oklahoma/Arkansas border, and a dry line in western Oklahoma and central Texas. All of these items will play a role in the severe weather risk for this day. It is expected that the broad area of low pressure will meander its way northward as the day progresses, bringing the warm sector further north as well. Going off of this map alone, any tornado threat would likely be centered along the warm front, which could move as far north as central Illinois and central Iowa.

Now that we know what will be causing this severe weather risk, let's now look at forecasted severe weather parameters.

The first item that must be looked at is Convective Available Potential Energy, or CAPE. This parameter defines the amount of instability in the atmosphere. In short, higher instability means that the air is more buoyant- it is able to rise easier, and thus create thunderstorms. On the evening of April 9th, we see a broad sector of elevated instability on the order of 2000 to 3000 joules per kilogram extending from Iowa to southern Texas. Higher instability values are found in Texas and Oklahoma, where the dry line is expected to produce more instability-based storms than those further to the north.

The next chart that must be looked at is the upper air flow, shown for the same timeframe as the instability chart above. The jet stream forecast shows a merged subtropical jet stream entering along Baja California, along with the jet streak produced rounding the base of the trough in the Rockies. This will likely produce an environment favorable for severe weather just east of the combined jet stream, particularly in the 30% risk areas described above. If you look closely, you'll also see an area of divergence, where the contour lines seem to bulge away from southern Wisconsin and northern Illinois. This indicates rising air, favorable for thunderstorm formation.

To summarize:

- A severe weather threat exists across the Midwest and Southern Plains on Thursday, April 9th.
- A more enhanced zone of severe weather is present in Illinois, Missouri, Arkansas, and Oklahoma.
- This could be a large-scale severe weather event, and must be monitored in coming days.
- Appreciable uncertainty still exists with the spatial coverage and extent of this event.


April 8th, 2015 Severe Weather Outlook

There is an elevated risk for severe weather on Wednesday, April 8th.

Storm Prediction Center
(click all images to enlarge)
The Storm Prediction Center has posted a 15% risk for severe weather within 25 miles of any given point for Oklahoma, eastern Kansas, and much of Missouri. If you're familiar with the Storm Prediction Center's Day 1 through Day 3 outlooks, you might be wondering why you don't see the 'Slight' or 'Enhanced' demarcations on this chart. In the long range (beyond Day 3), confidence can be high enough to outline an area for 15% or 30% risk of severe weather, but because it is so long-range, that's about as confident as these outlooks can get.

Weather Prediction Center
The first thing we want to do is look at the surface map to identify what features will be inciting this severe weather event over the aforementioned states. On the morning of Wednesday, April 8th, we see an elongated stationary front, stretching from western Kansas across southern Missouri, meeting up with a low pressure system in southern Indiana. We also see another storm system in western Oklahoma with a small warm front and associated trough in Texas. If you were to click through different days to get a context for where these boundaries are moving, you would find that the stationary front will be drifting north to cover Kansas and Missouri by evening on Wednesday. This means that the stationary front should cause the severe weather over Kansas and Missouri, while that smaller system in western Oklahoma combines forces with another system dropping out of Colorado to produce a severe weather threat in Oklahoma.

Now that we know what we're looking at, let's start checking out model forecasts for our severe weather parameters.

The first thing to look at is the forecasted instability, since instability (the action of warm air rising as cool air surrounds it) is necessary for all thunderstorm formation. This chart above shows the Convective Available Potential Energy (CAPE, the meteorological definition of instability) forecast for the evening of April 8th. We notice a corridor of high instability on the order of 3000+ joules per kilogram extending across eastern Kansas and central Oklahoma. It is expected that a boundary, potentially a dry line, will be available to conjure up such instability. We see lower, but still notable instability values over Missouri. Here, the stationary front should be able to fire up thunderstorms as well, some of them potentially being severe.

Also helpful is looking at the wind pattern aloft; here, the 300 millibar jet-stream wind forecast is displayed for the same timeframe as our graphic above. For this chart, we see a large trough dipping in the Western US, and allowing a sharp jet streak to form around the base of this trough, in Arizona and New Mexico. Additionally, we see the subtropical jet stream poking in to Baja California, and even providing a jet streak (a band of stronger wind speeds than the surrounding air) in western Texas and Oklahoma. The jet streak nosing into Kansas and Oklahoma will aid in severe weather development, especially with the positioning of the jet streak just west of the axis of highest instability in those states. In some research I've done this past winter, tornado formation can be favored in the areas directly east of a jet streak; in this case, that favors the Oklahoma, Missouri, and Kansas regions.

To summarize:

- A severe weather threat is unfolding on Wednesday, April 8th for eastern Kansas, Oklahoma, and much of Missouri.
- As of now, model forecasts are showing a potentially tornadic environment.
- Very large hail could be a prominent threat in Kansas and Oklahoma.
- All of this is subject to change.