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.


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