Sunday, March 30, 2014

April 3rd Potentially Major Severe Weather Outlook

Confidence is increasing in a potentially major severe weather event on April 3rd.

This is an update to yesterday's post on this severe threat. A full update will come tomorrow.

The Storm Prediction Center has outlined the areas of northeast Texas, eastern Oklahoma, Arkansas, southwest Missouri, and southeast Kansas for a severe weather risk on Thursday, April 3rd. 

An upper level low will push eastward into the Southwestern states at the beginning of the workweek, reaching far northern Baja California by Wednesday evening, as the 500mb relative vorticity and height contour map shows above. We see that the maximum vorticity is not 'tilting' in any particular way, meaning it isn't pointing towards the southwest or southeast. Instead, we see the contour lines rather symmetrical around the upper level low, signifying a neutrally-tilted storm system.

USA Today
As the graphic shows, there are two types of tilts to a trough- positive and negative. The positive tilt trough sees the jet stream bending towards the southwest, as the Energy Pocket, also known here as the 500mb maximum vorticity, pushes in that direction. The negatively tilted trough indicates the system has reached maturity, as the vorticity maximum now pulls the system to the southeast. The mature storm now produces more vigorous storms, hence why we are concerned more when a negatively tilted trough comes around, compared to a positively-tilted trough.

When looking back to the GFS model forecast at the top of this post, we can now confirm that the storm is at a neutral tilt. This means it is stronger than the positively-tilted storm, but not at the mature level that is defined as a negative tilt. Because the upper level trough is of a neutral tilt, we still have to be concerned about the formation of a capping inversion that must be eroded before the storms can commence.

Example of a capping inversion

This graph might look complicated, but we're only going to look at the circled portion. The red line represents temperatures, and the green line depicts dewpoint. The lines bend and move as you move up the graph, as they show temperatures and dewpoints with height- the height legend is located on the left side of this chart, in hPa (interchangeable with millibars). If we start at the surface, at the bottom of the graph, we see how temperatures begin to decrease with height. This indicates instability, as warm air rises and cool air sinks. However, we suddenly arrive at a point in the atmosphere, around 850 millibars, when the temperature warms quickly and the dewpoint drops. This means the air from the surface that had been rising can no longer rise, because temperatures have now warmed above what they were a bit lower below this circled portion. Thus, thunderstorms cannot form. This phenomenon is the capping inversion. The capping inversion can be broken, as the warm temperatures in the circled portion are cooled down. When they cool down enough, the capping inversion is broken and thunderstorms can now freely form. A 'broken cap' is shown well in this example graph below.

Example of no capping inversion
In this graph, we no longer see the sudden warming of temperatures. All we see is a steady cooling in temperatures throughout the troposphere, which tells us the cap has broken, and thunderstorms are free to form.

There actually is a very slight capping inversion in this graph, too. Can you find it?

If you guessed it was located at the very bottom of the red line, you're correct. There is a slight inversion at the surface as temperatures warm a bit before rapidly cooling. The red line breaking to warmer temperatures only slightly signifies a weak cap that should be easily broken.

With our neutrally-tilted storm system, we're going to need to break the cap, but when that does happen (which it will), the fireworks will begin.

Note: Two images will be displayed below. There will be the jet stream forecast on top, with an example graphic on bottom. The top graphic (forecast jet stream) will be discussed first, before we shift to a discussion about the different regions of the jet stream, which is when the example graphic will be used. Bear in mind the example graphic is NOT a current forecast.

Forecasted jet stream for April 3rd
Divergence circled, more on divergence will be discussed below.
EXAMPLE GRAPHIC for the topic discussed below.
The top image of the two above shows the GFS jet stream forecast for the evening of April 3rd, when we expect this severe weather event to occur. There is an area of divergence that I circled, on the right exit area of the jet stream (continue reading for explanations on those terms). In the bottom example image, I separated the jet stream into four sections. We have the 'Left Entrance' region, on the bottom left part of this diagram; the 'Right Entrance' region in the bottom right; the 'Left Exit' region in the top left, and the 'Right Exit' region in the top right area. The severe weather event looks to be located over the Right Exit region, and being in the Right Exit region is a big deal.

In the Right Exit region, we see divergence aloft, meaning air is being pushed up and outward, as the diagram above shows. This divergence acts as a helper for the formation of convection, including (but not limited to) thunderstorms. Even in winter, being in an area of divergence allows for the formation of snow. If the divergence is strong enough, thundersnow can occur as well (with other atmospheric conditions cooperating, of course). For our severe weather event here, with the divergence centered in the Right Exit region, I'm closely watching Oklahoma, Arkansas, and portions of Missouri and Texas for severe weather this Thursday, April 3rd.


April 9-15 Multiple Potentially Significant Storm Systems

I'm seeing the threat arise for not one, but two potentially significant storm systems.

Tropical Tidbits
The GFS model has been consistently bringing in a strong upper level low into Japan around April 4th, beginning to attain a negative tilt on the image above, valid for the afternoon of April 4th (for more information on negatively-tilted storms, please click on this link to see the post published yesterday on this topic). There is a rule, well explained by Joe Renken, that states a weather phenomenon in East Asia will be reciprocated in the United States 6-10 days later. This means that if there is a storm system in Japan on a certain day, we can expect a storm in the US 6-10 days after that. The same goes for high pressure and warm weather. If we take the April 4th day and extrapolate it out 6-10 days, we arrive at the April 10-14 timeframe for what could be a hefty storm. I say it could be strong, as the strength of these East Asian systems has been reflected in the resultant United States storm . For instance, a strong storm over Japan does usually result in a strong storm in the US 6-10 days later, and that's what we're looking to see in this April 10-15 timeframe.

But we're not just looking for one system. This time, there are indications we could see two systems.

Tropical Tidbits
About a full day after the original system moves out from Japan, we see another swath of significantly below-normal heights enter Japan. The GFS image above, now valid for April 5th, reflects this, and we can see our first storm system that was discussed above now located just west of the ridge in the Bering Sea. This second storm system is kind of a tricky one. I'm watching closely here, as it could end up being one storm with residual cold weather just hanging behind. However, this forecast says we are in for two storm systems, and since we're entering spring, these strong storm systems can create nasty severe weather. For that reason, I'll err on the side of caution and highlight two storms in this post, but do realize that this may switch back to one significant storm.

The pattern I had highlighted earlier last week, which showed how the Northeast was at the most risk, is now a bit more hazy than when we last analyzed this timeframe. Model guidance is no longer as favorable for an East Coast impact, but rather than drop that region from a potential impact zone, I'll still tentatively keep the Central and East US in line for this storm. We should know much more about what this storm(s) could do in about 4 or 5 days from today.

As you can tell, there's a lot of uncertainty. Let's sum up what we do know.

- There is the potential for at least one significant storm system around the April 9-15 period.
- Severe weather does look to be a potential factor in this timeframe.
- Cooler and unsettled weather can be anticipated for this timeframe.