Looking Back on 4/27...

I can't believe it's been 2 years since the fateful day of April 27, 2011. Really the entire month of April was very active weather-wise, as it seemed we were under a tornado watch about every third or fourth day. But something about 4/27/11 was different...

The day began with a very intense QLCS (quasi-linear convective complex) plowing through Mississippi into Alabama during the predawn hours of 4/27. While widespread wind damage occurred, an EF-3 tornado was dropped in eastern Tuscaloosa County. Unfortunately, 5 people lost their lives from the morning round of storms that wound up leaving over 250,000 people in Alabama without power.

Unfortunately, some people thought that was the main round of storms and that they just came through several hours earlier than expected. That was not the case, however. I distinctly remember that by 8 AM in Tuscaloosa, the sun was shining brightly. This was the atmosphere "recharging" from the morning storms. Temperatures quickly soared through the 70s and got into the lower 80s by early afternoon.

The increased instability values led the SPC to issue this extremely rare 45% circle on the tornado probabilities, centered from Columbus over to Tuscaloosa/Birmingham to the Huntsville area. The stage was set for a massive tornado outbreak...

By 2 PM, the SPC had put most of west and north-central Alabama under a tornado watch, with the Particularly Dangerous Situation (PDS) designation. The probabilities (tornadoes, hail, wind) in that watch were maxed out by SPC standards, meaning each category had a greater than 95% of occurring.

By 3 PM though, the first damaging tornado was already being captured on camera in Cullman, AL. But other storms were developing and quickly intensifying and organizing over east-central Mississippi as well. And they were moving very quickly to the northeast at 50+ mph.

By late afternoon, the parameters over west-central Alabama were seemingly off the chart. The significant tornado parameter (STP) was an astounding 12 units, when units of 1-3 are common with tornadoes and values of 4-6 associated with strong tornadoes. So with a  12, we knew that the chances of long-tracked, damaging tornadoes were extremely high.

A long-tracked supercell originated just south of Philadelphia, MS and moved quickly NE and was in downtown Tuscaloosa by 515 PM. The tornado crossed I-359 very near the 35th St bridge overpass. It moved northeast, causing catastrophic damage to the Forest Lake community before crossing McFarland Blvd near the 15th street intersection. It continued northeast crossing into Holt and Alberta City causing more catastrophic damage. The tornado continued northeast into western Jefferson County, bee-lining downtown Birmingham. All along the tornado's path, it caused extensive damage. While the tornado itself wasn't continuous, the parent supercell traveled over 300 miles from east-central MS to western North Carolina before dissipating.

The images I seen the night of and seeing it firsthand the next day is something I will never forget. I remember the sky being brilliant blue the day after, after being so vicious and black the day before.  Sitting in the middle of the intersection of 15th and McFarland, with damage visible in all directions was probably the most surreal of moments. But, the blue sky made for a very ironic scene but was a signal that we could and would recover.

God Bless..
IW

Severe Weather Discussion - 4:15 PM 4/18/13

Here's the updated tornado probabilities from SPC on their latest day 1 outlook. While the tornado probabilities remain the same from earlier, they are not as expansive as earlier. Also, SPC dropped the moderate risk and now just their standard "slight" risk remains, and can be delineated by the brown circle shown above.

Here's the Jackson, MS, radar as of 4 PM this afternoon. Notice a line of storms basically paralleling the MS River, and their associated severe thunderstorm/tornado warnings as seen by the yellow and red polygons, respectively.

The atmosphere has had a little trouble destabilizing, as seen in a sounding earlier this afternoon launched by MSU in Greenville, MS. Still, continued southerly flow is maintaining sufficient moisture (dew point temperatures at least >63 F) across central MS, with Td's closer to 70 right near the front. This is likely due to moisture pooling.

But, it appears the primary risk of severe weather is now shifting/focusing across central MS, basically between highway 82 and I-20. These storms appear to be moving into a slightly more unstable environment as shown by SPC mesoanalysis page. Mid-level lapse rates are a little steeper, towards 7 C/km, as well as CAPE values over 1000 J/kg. So it is possible to see a continued up-tick in severe weather activity at least over the next several hours west of I-55 in MS, or at least a current maintenance of intensity.

The severe risk east of I-55 appears a little more uncertain, but if storms can maintain current intensity, wind damage and an isolated tornado threat will be possible across east MS into west Alabama. A tornado watch remains in place until 11 PM this evening for central and western Mississippi.

Continue to remain weather aware...
IW

Severe Weather Discussion - Thursday 4/18 930 AM

Here's the latest day 1 convective outlook from the SPC. It features a large moderate risk, that stretches from the northern half of Mississippi into the southern parts of Lower Michigan. This large risk is primarily for damaging winds, some significant; however, there's also a relatively high threat (10%) of tornadoes inside this moderate risk.

Surface observations show a frontal boundary across east-central Texas, west of Texarkana, stretching NNE into western Arkansas up to just east of St. Louis, then stretching NE to the IL/IN border. Ahead of this feature, warm, moist southerly to south-southeasterly flow is prevalent. Locally here in north MS, temperatures are already into the lower 70s with dew point temperatures ranging from 61-64 F. Visible satellite imagery shows many breaks in the clouds, and I expect temperatures to soar into at least the low 80s, if not higher. Dew point temperatures may hover right where they are now, but I expect an increase in those values toward sunset. Also interesting to note, you can see low level clouds streaming in from the southwest on the visible imagery, while mid to high level clouds are streaming in from the west...very neat, yet slightly troubling...

Already in Arkansas, the SPC just issued a tornado watch across north-central Arkansas into southeast MO due to storms that could potentially be increase in coverage and intensity. Shallow, pre-frontal convection has already fired up across south-central AR and just west of Little Rock. As the boundary layer continues heating throughout the day, it is expected these storms will continue to deepen and acquire severe thresholds over the next several hours. SPC mesoanalysis forecasts SBCAPE values exceeding 2000 J/KG within the next 6 hours in advance of these storms over east Arkansas down into the MS Delta.

The wind profile will remain highly supportive of organized convection throughout the day today, particularly the upper-jet structure. Very diffluent patterns are noted at 300mb, which favors enhanced uplift at the surface. Also, the area over central AR, where the shallow convection has already developed, looks to continue to be in a region of PVA at the mid-levels as well...meaning continued uplift potential.

These atmospheric characteristics should carry over into the northern half of MS later this evening. CAPE will likely not be as high as what is being forecasted across the NW Delta into east Arkansas, however it will be sufficient to maintain a potentially intense squall line that should have already coalesced. As the low-level jet increase nocturnally later this evening, the wind/tornado threat may pulse up once more during the late evening across north MS into southern Middle TN. Any supercellular structure either ahead of the line or that can establish itself within a squall line will have enhanced tornado potential.

Please remain weather alert today...
IW

More Severe Weather Chances...

Another Thursday, another chance of severe weather. Currently, as of 5:50 PM Wednesday, there are twin-tornado-warned supercells in southwestern Oklahoma, as seen below...

Here locally in the GTR, we've seen a mixture of sun and clouds the past couple of days, with temperatures in the 80s with dew point temperatures already into the middle 60s. This has essentially "primed" us up for severe weather chances for tomorrow late afternoon into tomorrow evening.

This is the convective outlook from SPC valid for 12z tomorrow thru 12z Friday - 7 AM to 7 AM. Notice an enhanced moderate risk in place from Vicksburg up to near Elkhart, IN. Inside this moderate circle, there's a 45% chance of severe weather within 25 miles of a point, which is very significant probabilities. 

This a surface temperature/pressure map valid for 21z / 4pm tomorrow. Notice a very strong frontal zone across central parts of Arkansas, with backed winds ahead of it out of the SSE. This front will be one of many factors that produce lift to get thunderstorms going. The backing wind profile will allow for enhanced helicity in the lowest 1km of the atmosphere, which favors tornado development. Of course other ingredients are needed for tornadogenesis, but extra shear is certainly aiding in tornado potential.

Here's a chart of surface-based CAPE (convective available potential energy) valid at 4pm tomorrow. Notice values around 2000 J/KG over a large part of central Mississippi. At least during the late afternoon into the early evening hours, there won't be a shortage of instability/buoyancy in the atmosphere...this is another critical ingredient to thunderstorms and severe storms.

Here's a chart depicting surface dew point temperature and surface lifted index at 4pm tomorrow. According to weatheronline.co.uk: The Lifted Index (LI) is defined as a rising parcel's temperature when it reaches the 500 millibars level (at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 mbar. Greater negative numbers are associated with greater instability. Values across Mississippi range from -4 to -7 C which is fairly substantial. Moisture also will also be copious; dew point temperatures will at least be in the mid 60s F throughout the day and night tomorrow.

Given all these ingredients, some supercells would be possible ahead of the frontal zone tomorrow afternoon across west-central into north-central Mississippi. However, it is expected that we will have some warming aloft, in the low/mid levels that will act to "cap" off any storms from developing. Therefore, we'll have to wait on the front and an approaching upper-level jet streak to provide the necessary forcing to remove the capping inversion and allow storms to grow/develop.

***Bottom line***

All this being said, it appears many ingredients are coming together for a fairly stout severe weather episode across the Mid-South region into north Mississippi tomorrow afternoon and tomorrow evening. This is why SPC has went moderate risk on their day 2 convective outlook.

Expect an intense squall line to be ahead of the frontal boundary, moving east into northwest/west Mississippi late tomorrow afternoon between 5-8 PM. It is likely that this line will be capable of producing significant wind gusts as well as embedded tornadoes. Any breaks in the line will favor a little higher tornado threat. Any bowing segments will favor enhanced damaging wind gusts. Like I talked about earlier though, the threat for pre-frontal supercells is fairly low...however...if any are able to form, they would have the highest chance at producing a tornado and perhaps be capable of producing a strong tornado. Any severe weather threat for Mississippi should be over by 3 AM Friday morning.

If you live anywhere in the red-circled regions above (SPC moderate risk) as well as the yellow shaded areas surrounding it, please be weather aware tomorrow all day as changes can be rapid and short-noticed.

Stay safe out there, and God Bless!
IW

Severe Weather Discussion - 4/11/13

*This is for central Alabama...the overall severe threat in Mississippi (except potentially areas in southeast Mississippi) looks relatively low.*

A very murky weather situation unfolding across the area this morning. Radar imagery shows an expansive area of showers and embedded thunderstorms along the Mississippi River. Most of this activity is actually occurring behind the front, while isolated activity is occurring ahead of the front across central and southern Mississippi.

Ahead of this front, the air is already warm and muggy, characterized by surface temperatures in the low 70s and dewpoints in the low and mid 60s. Continued moisture advection today will only enhance moisture content across the area. Behind this front, temperatures are dropping off rapidly; for example, Jackson, MS has a temperature of 73 and Monroe, Louisiana has a temp of 48!

Currently, mesoanalysis shows the most unstable air located over central and southeastern pats of central MS back into southern LA. Consequently, the SPC has a tornado watch in effect for these areas until 3 PM this afternoon. However, that axis of instability will begin to shift northeast into Alabama throughout the day today as the cold front moves only very slowly to the east. Surface winds also are backed around to the south-southeast across much of the state, creating some directional shear needed for potential tornadogensis if any storm can mature (more on this later).

By 1 PM, the front should be near the MS/AL state border, with showers and thunderstorms along and behind this feature. The heaviest activity should be right along the front. Since instability values should have increased by this timeframe, some severe storms with damaging winds and perhaps a tornado are possible with these storms over west Alabama (mainly west of US highway 43).

After this timeframe is when the details get blurred with regard to this storm system. The HRRR model (rapid refresh) actually shows a low-level jet max/core moving into central Alabama, centered from Demopolis to Pell City, early this afternoon just ahead of the aforementioned line of storms.  This will create the necessary low-level shear (coupled with backed sfc winds) to create 1km helicities of upwards of 300 m^2/s^2. Also, an enhanced area of 250mb diffluence will overspread the area, which will greatly enhance surface uplift. What this means is this: IF surface-based CAPE values can achieve 2000 J/KG as the latest HRRR shows, then some isolated supercell storms could develop across central Alabama, mainly anywhere east of US 43 and southeast of I-20. If these CAPE values remain AOB 1000 J/kg, then a supercell or two is still possible, but this threat would be reduced. Regardless, if any supercell forms, it would have the best potential to produce a tornado.

Otherwise, expect some severe storms area-wide in association with a slow-moving line of thunderstorms. Damaging wind gusts will be the primary threat with this feature, but a spinup tornado or two is possible. If a cold-pool becomes well established and this line can accelerate any, a more enhanced wind damage threat could evolve across eastern sections of Alabama (and the SPC has a 30% wind probability to at least loosely account for this possibility). Any supercells that form will have the best potential at producing tornadoes.

One last note: given the slow movement of this overall system, some areas of flash flooding could develop with training showers and storms, both along and behind this frontal boundary.

Stay close to a source of weather information today, and God Bless!

IW

Severe Weather Next Week?

We've been dealing with some rainy days across Mississippi the last couple of days, but some sunshine and warmer temperatures are on the way for this weekend and into next week. An area of high pressure will influence our weather at least for Saturday, which will give us a cool start to the day, but temperatures will rebound nicely into the lower 70s. But this feature will begin to slide eastward during the day Saturday, and we'll see a return to some southeasterly to southerly flow across the area for Sunday.

This is a surface map featuring temperatures and pressure values for Sunday afternoon. Weak high pressure has shifted into the western Atlantic surrounded by a low just to the north of New York, and another just east of Bermuda (almost off the page). This high will reassert itself and be anchored over the west-central Atlantic for the next several days.

As typical for most mid-latitude cyclones, here's a surface image for Monday afternoon. You can see strong surface low pressure developing to the lee of the Rockies in southeast Colorado. Notice the high pressure system over the western Atlantic as well. These two systems will create a fairly strong pressure gradient over the central and eastern part of the country during the week and therefore we'll see windy conditions across the area. Also, we'll see warming temperatures into the week, with temperatures likely cracking the 80 degree mark starting on Monday but more likely to occur during the day Tuesday.

Here's a look at moisture and its advection. Given the aforementioned two strong pressure systems, moisture transport will likely be enhanced off the Gulf of Mexico into the Lower Mississippi Valley and eastern Plains. Dewpoint temperatures will already be into the 50s for Monday, and these numbers will only go up through the week in advance of a powerful storm system ejecting eastward.

Here is the Day 4-8 convective outlook from the Storm Prediction Center. Notice the progression and expansion of the severe weather risk from Monday (red circle, day 4) to Wednesday (green circle, day 6). This is an indication to the potential severity of this storm system, as it will be ejecting eastward and strengthening throughout the week.

Here's the upper level jet structure / jet stream / for Wednesday. Notice a large and deep upper-level trough across the center of the country, with the parts of the Southeast, including parts of MS/AL/TN located in the favorable right entrance region of a strong upper jet streak. Additionally, there is almost a 90 degree difference in the winds at this level / 250 mb / across MS/AL particularly. This indicates that we have enhanced diffluence, or the spreading apart of the wind vectors, across this area. What this means is that there could be some enhanced surface lift as a result of this diffluence across this particular area.

It should also be mentioned that this area will also be collocated with at least moderate instability during the Wednesday-Thursday timeframe. As such, it appears that a relatively high threat of severe weather exists across parts of the Lower MS Valley. Now, models are still having a fairly difficult time in resolving the specifics of this storm system. But as it stands now, parts of the central and eastern Great Plains  (Monday/Tuesday) into the Lower MS Valley (Wednesday/Thursday) stand to have a risk of severe weather, including all modes of severe weather.

If you live in any of the areas featured in the SPC outlook shown above, you should remain abreast of the weather forecast throughout the weekend into next week.

Enjoy the weekend, and God Bless!

IW