Severe Weather Update - 715 PM 1/29

Here's an update on the severe weather situation unfolding across the Mid-South and Lower Mississippi Valley:

The above radar image shows prefrontal thunderstorms along the MS River, moving quickly NE. A tornado warning is in effect for parts of Bolivar County until 730 PM this evening with a severe thunderstorm warning in effect for parts of Washington County until 745 PM. These storms will certainly need to be watched vigilantly through the evening as they move into north Mississippi.

1km SRH (Storm relative helicity), or the degree to which the winds are veering in the lowest 1km of the atmosphere, are sufficiently high across northwest Mississippi to favor a few isolated tornadoes. The values are much higher back to the west where the better upper support is located at this time. However, that support and subsequent higher shear will be moving gradually eastward throughout the night, and additional prefrontal storms/possible supercells are expected across parts of central/eastern Arkansas into north/central MS and western TN through the night. Given the strong shear and increasingly moist/unstable airmass, tornadoes are possible in addition to damaging winds.

A squall line is still expected to sweep across the Lower MS Valley during the overnight hours and will be capable of producing widespread, and potentially significant, damaging winds.

The area in red, as defined by the SPC, has the highest threat for receiving severe weather for the rest of the nighttime hours tonight. However, areas in the yellow areas also need to be vigilantly watching weather conditions and EVERYONE should have a way of receiving watch/warning information tonight.

IW

Severe Weather Discussion - 1/29/13

As can be the case across the Southeastern states, we are facing another threat for severe weather during the Tuesday - Wednesday timeframe. I'll iron out the details/specifics of the system below.

In the images above, the Storm Prediction Center (SPC) has upgraded their day 2 outlook to a categorical moderate risk for the Arklamiss area, with a large slight risk surrounding that extends from Lake Charles north to near Indianapolis. Within the moderate risk, there is a 45% chance of severe weather within 25 miles of a given point in the pink circle on the second map, which is a very significant probability. The hatching actually represents a 10% or greater probability of significant severe weather occurring within 25 miles of a point; that is, wind damage greater than 65kts and tornadoes of EF-2+ intensity.

The image above is a regional view at METAR observations from 23z on 1/28, or 6 PM Monday evening. The surface winds are already out of the south-southeast and have been for much of the day. This has actually helped to advect moisture into the region all day today, with most spots already seeing upper 50s for dewpoints. This moisture content will continue to increase as the storm system to our west deepens and moves our way.

This map represents projected surface dewpoint temperatures from the 18z run of the NAM today. Notice all of Mississippi has 60 degree dewpoints with even 65+ degree dewpoints creeping up to the I-20 corridor by tomorrow late afternoon This moisture is just one of the ingredients that will be in place to support the threat for severe thunderstorms across central and north Mississippi tomorrow evening.

Again taken from the 18z NAM, this is a forecast projection of the jet stream level winds tomorrow evening at Midnight. Notice the relative maximum (160kts) over eastern Texas nosing into the Arklamiss; this is a jet streak that will help to enhance lift  at the surface downstream of it across southeast Arkansas/northeast Louisiana into north-central Mississippi. It should be noted as well that that area also represents the area where the polar jet is actually phasing with the subtropical jet.

This map is a representation of the vertical velocities valid at Midnight tomorrow night. This also shows the answer to the Q-G Omega equation. Notice the widespread colors of yellow across the Arklamiss area. This jives up well with the previous map of the jet stream that shows an area of enhanced uplift.

The above image is a MODELED skew-t for a point in north-central Mississippi valid tomorrow night at Midnight. Notice the wind barbs to the right: above 850mb, the winds are pretty much from the same direction--southwest. But notice the significant increase in wind speeds; that is called speed shear and generally suggests a linear mode of severe thunderstorms. Because the winds are particularly strong with this system, winds could gust up to 80 mph along the squall line of thunderstorms as it sweeps through the area. Also of note: the winds do have some component of veering in the lowest 1km of the atmosphere. This will provide enough shear to promote a tornado threat as well, either with any cells within the squall line, or with any cell that may develop ahead of the main squall line. It should also be noted that if any storms manage to develop ahead of the line, they will pose an enhanced risk for strong to significant tornadoes.

Also, the dotted red line plotted on the map represents the temperature of the air parcel, while the solid red line represents the environmental temperature. When the temperature of the parcel is warmer than the environmental temperature, the airmass is considered unstable. This particularly profile is regarded as a "tall CAPE" profile, which generally suggests low topped thunderstorms, albeit they will still be severe.

BOTTOM LINE: 

Best indications from model data right now are that thunderstorms will begin developing across eastern Oklahoma during the early afternoon hours ahead of an advancing cold front. More thunderstorms will eventually develop into northeast/eastern Texas tomorrow afternoon as well. As the airmass destabilizes and better upper support noses in, thunderstorms will quickly grow severe and likely coalesce into a squall line that will march steadily eastward through the evening and overnight hours.

Timing: Midnight - 6 AM for the GTR locally, but severe weather could begin impacting west Mississippi as early as 8 PM.

Threats: Damaging wind gusts of 60-80 mph. This could wind up being a widespread damaging wind event capable of producing significant damage over a a large area. In addition, tornadoes are possible anywhere along the squall line, AND with any cell that may can develop ahead of the line. Any supercellular tornadoes could be strong, although an isolated strong tornado is possible with any cell embedded within the main squall line.

Have a way of receiving weather information tomorrow evening into the overnight hours like a NOAA Weather Radio, as watches and warnings will likely be required.

Stay safe out there, and God Bless!

IW

70's/Severe Wx Next Week?

That's right. Just as we were dealing with snow this time last week, we could be dealing with some springtime 70's by the time next week rolls along.

This evening, temperatures across the Magnolia state have quite a spread to them--low and mid 60s across the southern portion of the state while some spots in north Mississippi are in the 40s. This is thanks to a frontal boundary that has stalled across the central portion of the state. Indications are that it will try to inch back north as a warm front overnight into tomorrow morning and this may bring a few passing showers across central and northeast Mississippi.  This front will get one final "kick" from the upper levels to help push it down towards the Gulf Coast before it will stall again thanks to the upper flow transitioning into a more zonal flow pattern.

Behind this system, high pressure will settle in from the north and keep us dry at least for the remainder of the weekend into the first part of Monday. Models show this high pressure center sliding off to our east during the day Sunday setting up a southerly flow across the Lower Mississippi River Valley. This will help gradually increase moisture across the region, as well as warm the temperatures up into the middle 60s.

The map above is taken from today's run of the 18z GFS model valid for Midnight Sunday evening. The colors represent temperatures at 850mb with the winds overlaid as well. Given a southwesterly wind direction and warmer temperatures located to our west (relative to our north), we can assume some subtle warm air advection is occurring.

This map represents vorticity at 500mb Sunday evening. This map jives up well with the previous one in that we can expect synoptic uplift across portions of the Lower Mississippi River Valley during the evening hours of Sunday. With increasing moisture thanks to the southerly flow aloft, it appears some rain showers may break out as this warm front continues to lift toward the north. It won't rain everywhere, but be aware of a few passing showers Sunday evening into Monday morning.

Monday and Tuesday look to be "primer" days; that is, increasing southerly flow helping to bring additional moisture in advance of an approaching cold front that looks to sweep through the Deep South Tuesday night into Wednesday morning. This will help push temperatures near the 70 degree mark along with mid to high level clouds as well, but no rain showers are expected during this timeframe.

Parented by a strong upper-air trough, this cold front looks to have sufficient instability and wind shear to provide the potential for strong to severe storms. Of particular note is the broadly diffluent flow over the LMRV overnight Tuesday; that is, the spreading out of the wind vectors over a specified area. Additionally, notice the green across Mississippi; this represents slower winds relative to those immediately southwest of that region. This seems to indicate speed divergence as well; both of these entities help to enhance rising air/lift at the surface.

(source: www.spc.noaa.gov)

The above image is the SPC Day 4-8 severe weather outlook. The green area represents a relatively high confidence of severe weather; specifically, it represents at least a 30% or greater probability of severe weather within 25 miles of any given point within that green circle.

Please be weather aware Tuesday into Wednesday of next week as it appears parts if not all of Mississippi could be dealing with a threat for severe weather. It is too early yet to be specific on exact timing and the specific threats, so please check back for further information throughout the weekend!

Have a wonderful Friday, and God Bless!

IW

River Flooding: An Introduction

Changing pace a little from actual weather, this provides a very basic overview of river flooding and some information on the Big Black River in Mississippi:

Over the past week or so, a large part of Mississippi and Alabama has received excessive amounts of rain that not only led to flash flooding, but it also has resulted in many area rivers surpassing flood stage and becoming very swelled. The ground can only hold so much water, and when it reaches its capacity/becomes saturated/, excess water just flows as runoff which creates short-term flash flooding and can create longer-term river flooding because of the excess amount of volume of water trying to flow through the river channel.

More particularly, large amounts of rain over a long period of time, such as what has occurred in this case, promotes rivers to slowly rise. With larger river basins, there is generally a lag between the peak in precipitation and the peak in the river's stage, or height. As outlined below, the Big Black River is actually not cresting until this weekend (Bentonia) and early next week (Bovina). Once rivers crest, it slowly will begin to recede its stage; however, a certain river basin could actually be in flood stage for over a week.

For example, the Big Black River's stage, near Bentonia, MS, was 26.9 feet which is almost 5.0 feet above the flood stage of 22.0 feet. Moderate river flooding has been ongoing around this basin, and additional rising of the water is expected through Saturday. The river's stage is expected to crest near 28.0 feet which will most likely result in catastrophic flooding especially to surrounding agricultural lands. (source, NWS Jackson www.srh.noaa.gov/jan )

Another example, the Big Black River 's stage, near Bovina, is 32.4 feet with a crest of 37.5 feet occurring by early next week. This crest is almost 10 feet above its flood stage and at stages near 38 feet, the water will actually flow across the road near the old Highway 80 bridge as well as cause catastrophic agricultural devastation to the lands surrounding the river basin. (source, NWS Jackson www.srh.noaa.gov/jan )

As seen in the graph above, the Big Black River at Bovina has actually been above flood stage for the last 6 days. Our most recent bout of heavy rain actually began on Sunday 1/13, but as discussed above, there is a lag in the river stage's response because of its larger basin relative to other rivers/streams. (source www.usgs.gov )

Given the recent snowfall amounts of 1-4" that have occurred over central portions of Mississippi, as this begins to melt a little later this afternoon, it could also cause some additional rises to area streams/rivers due to excess water.

The data used for this post were collected on the afternoon of 1/17.

IW

Snow Begins...

As anticipated, many many reports of snow have been received by NWS Jackson across central and west-central Mississippi. The much-awaited upper level low is now located over north-central Louisiana and is moving toward the east. Rain developed late last night across west Mississippi and transitioned into snow fairly quickly; therefore, some spots have already received upwards of 1" of snow such as Yazoo City. Rain has persisted across the GTR area for the last couple of hours, but finally transitioned into a rain/sleet/snow mix and now has transitioned to all snow with even a clap of thunder heard in the last 15 minutes.

Here's the RAP forecast sounding valid for 14z (8 AM) across the GTR area. Notice the red and green lines are side by side from the surface all the way to around 550mb. More importantly, they are side by side from -10 C to -15 C which favors snow growth/development as dendritically shaped flakes. These flakes will fall through the column that's sub-freezing and make it to the surface as snow. This suggests snow will continue for several more hours across central and eastern Mississippi as forecasted.

A side note: Some heavier bursts of snow are possible, and some traffic accidents have already occurred in and around the Jackson metro as well as in portions of Yazoo County. Some roads across the GTR may become treacherous later this morning as any slushing on the roads could complicate the morning commute.

IW

A Snowy Thursday!

Wow..what a wintry weather week it has been across the northern half of Mississippi and even Alabama as well. Earlier this week, several waves of freezing rain impacted the area. Now, we have yet another chance of wintry weather, except this time it will be in the form of snow. Let's talk about some of the why's and how's concerning our snow potential for overnight tonight into the morning hours of Thursday.

Here's the observed sounding from KJAN (Jackson, MS) from 12z (6 AM) this morning. The red line is the environmental temperature with the green line (it's basically on top of the red line) is the environmental dewpoint temperature. When the lines are so close, such as shown here, that means the atmosphere is nearly saturated up until the point where they spread apart. In this case, the atmosphere was saturated up to about 500mb (18,000 ft). Notice the temperature lines (the blue straight lines) that run diagonally from bottom left to top right. From the surface up to about 650mb, the temperature is actually above freezing (above 0 C) which is why we have been dealing with rain showers all day today. But notice above the 650mb level, the temperature is below freezing so today's rain is actually starting as snowflakes but as they descend through the atmosphere into the warmer layer, they melt and turn into raindrops. 

However, things will begin to change dramatically over the next 12 hours. A strong upper level low pressure center, currently over eastern Texas, will begin moving our direction tonight and actually close off/strengthen. A good moisture channel is established ahead of this feature as seen in the water vapor imagery below, which measures water vapor content in the atmosphere at roughly 600mb. The white/lighter colors represent moisture. 

This moisture transport will be enhanced with the approach of the aforementioned upper-level low. As the low approaches from the west, very strong uplift will result in rain showers developing once more after Midnight, possibly starting as early as 10 PM tonight over west Mississippi. This precipitation will gradually expand in coverage to the east and northeast as more uplift overspreads the rest of the state. Initially, the precipitation will be rain thanks to the warm layer aloft hanging tough. However, evaporative cooling will begin to take effect in the mid-levels of the atmosphere and this will begin to lower temperatures sufficiently for the rain to begin to change over to snow.

Here is a forecasted skew-t model sounding for 15z (9 AM) tomorrow morning, for the KGTR airport. Remember, this is a modeled skew-t and these data could be flawed. However, notice the atmosphere is saturated from the surface all the way to 600mb. If you follow the -10 C to - 15 C isotherm up to where it intersects the temperature/dewpoint lines, you can see it is saturated at that location which is commonly referred to as the dendritic growth zone, or the snow growth zone. In this region, dendrite-shaped snow flakes can develop and begin falling. Since the atmosphere is saturated all the way to the surface AND is below freezing, it is conceivable to assume that the precipitation will be snow by this point and probably a few hours before this sounding time as well. 

I think the primary timeframe for snow, at least locally in the GTR, is sometime between 4 AM and 10 AM. After this timeframe, the best combination of cold air/moisture/dynamics will be shifting eastward into Alabama. Accumulations of 1-4" are possible across east-central Mississippi, but trying to pinpoint the exact location of the heaviest snowfall is very difficult, especially with these types of setups. 

Forecasting snow in the south is always challenging, and the common meteorology maxim is "upper-level low, weatherman's woe." So, some surprises are certainly to be had with events like these, so it will be fun to watch and wait to see how things unfold tonight! 

Enjoy any snow you see, and God Bless!

IW

Wintry Monday, and Thursday?

A peek at the radar this afternoon shows lots of rain across the southeastern states and Lower Mississippi Valley. The cold front that helped usher in the much colder air we are currently experiencing stalled over south-central Mississippi into central Alabama. However, our upper air flow continues to be out of the southwest which is helping to create an "over-running" situation where warm, moist air overrides shallow cold air at the surface.

This particular setup actually is conducive for frozen precipitation. Temperatures across west-central Mississippi, mainly along and west of I-55 and north of I-20, are at or below freezing at this Noon hour.  As a result, the National Weather Service in Jackson has issued an Ice Storm Warning with freezing rain advisories surrounding that as seen on the map below with the shades of dark purple.

Here, the greatest intensity of freezing rain is expected and upwards of between a quarter and half inch of ice is expected to accumulate in these areas. This will create very hazardous driving conditions as roads will become slippery. Additionally, ice will accumulate on trees and power lines which may cause power outages and some trees to topple. This is a very serious winter weather situation and if anyone is traveling towards this region, or anywhere shaded in purple above, should exercise extreme caution and consider delaying any commutes until tomorrow afternoon.

Looking ahead to tomorrow, a southwesterly flow will continue aloft. This will continue to create widespread showers across not only the GTR but most of Mississippi and Alabama as well. Here's a look at projected rain for tomorrow afternoon from the 12z NAM this morning:

This broad southwesterly flow will continue through mid-week before a strong upper level approaches the area from the west. This will bring another chance of widespread rain Wednesday night into Thursday morning. Despite surface temperatures projected to be in the upper 30s during this timeframe, some evaporative cooling could occur and a brief rain/snow mix before the system departs later Thursday. The suns will finally return to our area by the end of the week along with some moderating temperatures, into the mid 50s by Saturday.

Stay safe out there, and God Bless!

IW

Gravity Wave Wind Damage

Across west and southwest Alabama, rain showers earlier this afternoon have, for the most part, dissipated. However, winds continue to gust to almost severe limits particularly across west-central Alabama even with no rain or thunderstorms present. These winds are due to a gravity wave; a simple explanation goes as follows: a strong pressure indifference develops following rain showers and thus winds can gust upwards of 40 or even 50 mph.

The National Weather Service has at least a dozen reports of non-thunderstorm wind damage across parts of Sumter, Pickens, Tuscaloosa, Greene, and Hale counties including some homes and businesses without power.

This area of enhanced wind gusts will continue eastward toward the Birmingham metro over the next 30 to 45 minutes with isolated instances of wind damage possible. No severe weather or tornadoes will result.

A Soggy End to the Week

This radar picture tells the story for our weather over the next several days--periodic episodes of moderate to heavy amounts of rain. In fact, the HPC believes upwards of 4-5 inches of rain could fall over the next five days in a corridor from southeast Texas into southern Arkansas. For this reason, flash flood watches have been posted for much of eastern Texas into western and central portions of Louisiana, including the cities of Lake Charles, Houston, and Shreveport. A few thunderstorms could be thrown into the mix as well, but a widespread severe weather event is especially likely with this system--the primary concern is for heavy rain.

All of this rain is being driven by a strong upper level low over northern Mexico that has actually been cut off from the primary jet stream which is located across the northern tier states this afternoon. Because of this setup, this feature will be very slow to move over the next several days thus posing a threat for widespread rainfall in areas that actually could use some. Broad diffluence aloft juxtaposed with increasingly moist and slightly unstable air off the Gulf of Mexico is actually enhancing lift over portions of southeast Texas and is leading to some stronger thunderstorms on the immediate Texas coast near Galveston.

What does all of this mean locally across the GTR? Models show the trough currently over northern Mexico gradually ejecting northeast and weaken as it gets absorbed into the larger upper air flow pattern. However, moisture already in place across the area combined with the extra lift from the approaching shortwave will probably lead to an uptick in the rain intensity and coverage especially Thursday into Friday. There should not be any widespread flooding issues from this system around here but a few pockets of localized heavy rain are certainly possible as are a few thunderstorms but again no organized severe weather episode is expected.

Looks like we dry out temporarily Friday afternoon into Saturday before another storm system approaches from the west. This, unfortunately, will lead to another prolonged chance of rain across the Southeastern states with additional rain amounts of over 2" possible for most locations from Sunday - Wednesday.

Stay dry and God Bless!
IW