El Niño Phenomenon Now Entrenched in Atmospheric Pattern

After over a year of waiting and watching for the infamous El Nino to form, it now appears that the atmosphere is finally responding to the presence of abnormally warm waters off the coast of Ecuador. These warm waters, the textbook signal of the El Nino phenomenon, have been in place for several months now, and look to persist in coming months.

Climate Prediction Center

The above image shows observed sea surface temperatures across the Equatorial Pacific on the top panel, with observed sea surface temperature anomalies on the bottom panel. On that bottom panel, we see a large swath of oranges (warmer than normal water temperatures) extending from the coast of Ecuador, all the way into Micronesia, relatively close to northeast Australia. It is that swath of warm waters that tells us the El Nino is present in the oceans.

However, things get messy quickly. In order for an El Nino to 'function' properly, we need it to appear in the oceans and in the atmosphere. For the last year-plus, we haven't gotten the atmosphere to cooperate; if you recall, typical El Nino conditions include wetter than normal conditions in the South, warmer weather in the North, and some drier weather in the Ohio Valley, as the graphic below indicates.

CPC

CPC

Suddenly, in these last couple weeks, we've gotten some pretty noticeable indications that the El Nino is now present in both the oceans and the atmosphere. The most significant indicator, with respect to the United States (since the El Nino affects the world as a whole), is most certainly the heavy rains in the South.

CPC

This chart shows accumulated precipitation in millimeters from April 26th to May 25th, 2015. Notice the significant precipitation stretching from the Central Plains down to the Gulf Coast, most prevalent in Oklahoma and Texas, both of which are dealing with deadly floods as I type this. Looking at the average precipitation anomaly chart for April-May-June in the composite graphics earlier in this post, we can see that the Southern Plains are typically very wet during El Nino summers. In addition, we've also seen some relative drying-out in the Southeast and Ohio Valley regions, both of which tend to see below-normal precipitation during the April-May-June period of a 'typical' El Nino.

CPC

Another signal of the El Nino is the presence of enhanced convection over the Equatorial Pacific, the same region we analyzed in the observed sea surface temperature charts. Notice the swath of blue on the OLR (Outgoing Longwave Radiation) anomaly panel (bottom panel) over the Equatorial Pacific. That blue swath indicates above-average thunderstorm activity, in the same sense that oranges and reds signify below-average thunderstorm activity. The warm waters of the El Nino in the Equatorial Pacific help create enhanced thunderstorm activity in the area, which is showing up on thunderstorm monitoring charts above.


What does this all mean going forward?


We're solidly entrenched in an El Nino pattern at this juncture. I'm planning additional research later this summer, but it's rather likely that this very wet pattern will continue in the Southern Plains throughout the summer, as El Nino's tend to allow such wet patterns to continue across the South and the West. It's also expected that the East and Central US will see a higher threat of a cooler summer, enhanced by the sea surface temperature configuration off the west coast of North America.
As far as precipitation, outside of the South Plains, some continued drying may be anticipated in the Southeast and Ohio Valley. Don't be surprised to see portions of the East US slip into a drought as we move ahead in the year, while drought conditions may very well ease in the West.

Andrew

Sunday, May 17th Severe Weather Discussion

There is a threat for severe weather over portions of the Midwest and Great Lakes on Sunday, May 17th.

SPC

The Storm Prediction Center has issued an Enhanced Risk of severe storms for southeast Minnesota, much of Wisconsin, and northern Illinois in response to the expected severe weather today. The primary threats for those regions are large hail, damaging winds, and potentially more than a few tornadoes, particularly in northwest Illinois and west Wisconsin. A Slight Risk of severe weather extends from the Minnesota/Canada border into Michigan, Iowa, Indiana, Minnesota, Arkansas, and extreme southeast Oklahoma. These areas can expect thunderstorms with potentially large hail, damaging winds, and a threat of a tornado or two.

NOAA

The latest visible satellite imagery over the Midwest shows an area of significant clearing from extreme eastern Iowa to southern Illinois. Continued cloud cover persists over the eastern side of Illinois, into Wisconsin and Missouri. It is expected that the areas with more clearing/sunshine will achieve the best threat at severe weather before storms start to fire later this afternoon.

COD

The latest 14z HRRR depicts thunderstorm formation in the first few hours of the afternoon, with those storms attaining super cellular characteristics shortly thereafter. The above image depicts updraft helicity (spinning motion in the updraft, which can help identify the tornado threat). While storms will likely not track in these exact motions, you get the idea that some severe storms are favored in Wisconsin and Illinois this afternoon.

To Summarize:

- Severe thunderstorms are expected in Minnesota, Wisconsin and Illinois this afternoon.
- The primary threats are hail, damaging winds, and a few tornadoes.

Andrew

May 8th and 9th, 2015 Severe Weather Outlook

The Storm Prediction Center has outlined a severe weather threat on Saturday for a large swath of the Central and Southern Plains, including cities such as Dallas/Fort Worth and Oklahoma City, among other metropolitan centers.

The SPC outlook on the left denotes the risk of severe weather in one of two categories. The first category is a 15% chance of severe weather within 25 miles of any given point on the map. This risk is equivalent to a Slight risk in shorter-term outlooks. The second category, which would be shown in red, denotes a 30% chance for severe weather, and is usually issued in the long-term outlooks when forecasters feel there is a good threat for a severe weather event. As you might expect, the 30% risk area is not issued all that often.

The mid-level prognosis on Saturday includes a negatively-tilted shortwave pushing into the Central Plains, amidst a large trough encompassing the Rockies. This set-up is great for those who have been waiting for summer-like weather in the Central and East US, as we see how far north the contour lines are forced with the ridge in the eastern half of the country. With this set-up, however, comes the severe weather threat for the Plains. The Storm Prediction Center has indicated that all modes of severe weather are likely for the area outlined in the top image, including damaging winds, large hail, and even the threat of tornadoes. Looking over model guidance right now, it looks as if the tornado threat is rather low. Despite this, and perhaps it's just me being more optimistic with severe weather forecasts lately, I wouldn't be surprised if we saw more than a handful of tornado reports on the Storm Prediction Center database when this is all set and done. An outbreak isn't expected, but the threat is there; it's just the severity that's in question.

The upper air pattern is rather indicative of where the severe weather may be most probable. In the jet stream forecast on the right, we see the subtropical jet stream in Baja California and Mexico, as well as a jet streak rounding the base of the trough from eastern California to New Mexico. These two ribbons of wind combine somewhere along the US/Mexico border near west Texas, and make a move into the Southern Plains. This sequence is interrupted by an area of divergence poking through in Kansas, where the two streams diverge again. This diverging of jet streams generally puts the area between the streams under the gun for severe weather, since rising air is disrupting the jet stream and forcing it to split.

The instability forecast, namely the Lifted Index outlook in the contour lines, shows a very ripe environment for thunderstorm development across Kansas, Oklahoma, and Texas, where Lifted Index values could dip below the -8 mark.
When the forecasted Lifted Index drops below zero, the atmosphere is considered to be unstable. When it drops below -4, the atmosphere is considered to be very unstable. Numbers below -8 can indicate significant instability, while any positive numbers indicate a stable environment, and a low thunderstorm threat.


Summary
- Severe weather is possible on both Friday and Saturday, May 8th and 9th.
- The Central and Southern Plains are at risk of all modes of severe weather on both days.

Andrew

Saturday, April 25th 2015 Severe Weather Discussion

Headline
High resolution model guidance is in agreement concerning a severe weather event unfolding on the afternoon of Saturday, April 25th.

Timing 
Consensus is for thunderstorm development to commence in a 1PM to 4PM window, with severe weather expected to become a threat beyond the 4PM period into about 10PM.

Areas affected 
Southern Illinois, southern Indiana, much of Kentucky, and eastern Tennessee are most at risk for this severe weather threat. A lesser, but still present threat exists into the Southeast states.

Discussion
High-resolution model guidance capable of simulating more mesoscale convection features is in agreement of thunderstorm development between 1PM and 4PM, with additional development a possibility until about the 6PM hour for the initialization region of eastern Missouri into southern Illinois. Thunderstorms will be forming in the midst of a shortwave being shunted eastward as a result of a powerful upper level low stationed near the Canadian Maritimes and Northeast US.

Model guidance sees thunderstorm formation initially as discrete, with rotating updrafts becoming a common theme across southern Illinois in the late afternoon and early evening hours. Environment appears favorable for severe weather on the order of large hail and possible tornadoes as the event begins in southern Illinois, transitioning to southern Indiana and western Kentucky. Model guidance then sees a transition to a more linear storm mode, maximizing the damaging wind potential in Kentucky and Tennessee, reducing the tornado threat in the process.

Summary
Severe thunderstorms, some with the threat for large hail and tornadoes, will form in eastern Missouri, southern Illinois, southern Indiana, and western Kentucky in the late afternoon and early evening hours.
Severe thunderstorms, with the threat of large hail and damaging winds, will persist into central Kentucky and eastern Tennessee in the evening hours.

Andrew

An Introduction

This blog will be the log / database for experimental indices relating to tornado formation and strength, among other ventures. As of early April 2015, two experimental indexes are undergoing real-time testing and application. Occasionally, forecasts may be posted, either as an article or under the page tabs at the top of this site.