Sunday, April 27, 2014

Sunday PM update on severe weather potential today and tonight

Current Mid-South radar - continuously updated
Current watch/warning map - continuously updated

Latest MWN Significant Weather Outlook updated at 5:30pm

UPDATED 4/27/14, 9:00pm

A Tornado Watch has been issued for the western half of west TN and all of northeast AR and southeast MO until 2am (see map below).  Meanwhile, the watch for north MS has been cancelled. Based on latest high-resolution model data, I'm not convinced that we won't see more watches that take us through the night as a couple of rounds of storms are possible.

Everything is quiet in the metro for now, but the storms in AR have produced damage, some of it devastating northwest of Little Rock. The good news is their environment is much more volatile than ours is or is predicted to be. However, we will actually become more unstable overnight than we are now as the warm front we've talked about all day finally moves to our north and puts us in the warm sector of the approaching front.

Based on all the latest data and discussions from the experts, I look for additional storms to move into the metro somewhere in the midnight-3am timeframe, give or take an hour either way. Then a squall line looks to move in around sunrise, perhaps affecting the Monday morning commute. The main risk overnight will be large hail (garage your vehicles if you are able or pull them under a carport) with a risk of tornadoes as well. The graphic below paints those risks well.

Overnight severe weather risk area shows a Slight Risk for the metro with the most serious weather in AR, weakening some as it moves east. The probability of a tornado within 25 miles is 10% tonight (definitely worth taking note of, but not nearly as high as AR) and the hail probability is 30%.
I can't stress enough that you NEED a way to be woken up tonight if a tornado or severe thunderstorm is headed for your home or wherever you'll be. We hope and pray we don't need them, but we prepare just in case. If you don't have a weather radio, get an app that you can program with your location and alerts only for that location. StormWatch+ is used by thousands of people across the country. Yes, we developed it. I'm proud of it and wouldn't recommend it if I didn't use it myself, and I do. To be sure that you get the full utility from it though, read this about how to set your phone audio overnight to be sure you get warnings.

Set shoes by your bed and the kids' beds in case you have to get up tonight (helmets also a great idea for the kids). Flashlights with fresh batteries should be accessible. Check your weather radio for fresh batteries and make sure your safe place is ready. Have pets inside and within your control.

We'll provide monitoring and coverage throughout the overnight on social media - check us on Facebook or Twitter. For now, I need some rest. My team has things covered and I'll see you in a couple of hours. /EP

UPDATED 4/27/14, 6:30pm

The first found of storms has lifted north of the metro with the warm front creeping closer to the area from the south. Scattered storms are forming in north MS, near where a special atmospheric sounding was launched by the Mississippi State University Mobile Sounding Team. The sounding from Helena, AR at 4:30pm showed strong instability and high levels of low-level wind shear, both necessary ingredients for storms that can produce tornadoes. About the same time as that data was coming back, the Storm Prediction Center issued a Tornado Watch for north MS, including all of our metro counties (DeSoto, Marshall, Tunica,  Tate), until 11pm (yellow counties in the map above). Currently, a rotating supercell over NW MS has prompted Tornado Warnings and is headed northeast (see radar above).

High-resolution model data is indicating that big-time storms over SW MO into AR will shift east this evening and move into the area late tonight, likely after midnight. We expect some weakening as they do, but a weakened rotating supercell could still be a strong storm capable of just about anything. We expect the main brunt of the storms to impact the metro after 10pm and could last much of the night. The same models show a potential line/mesoscale convective system brushing by to the north in the wee hours of the morning and maybe another around dawn. In other words, it could be a long night.

HRRR (high-res) modeled "future radar" valid at 3am with a line of storms from Memphis north.
 THIS IS A MODEL, NOT A FORECAST. Additional storms are not shown before and after this time.

We're hopeful these storms aren't severe, but we caution you to be prepared in case. Before you go to bed tonight, have your weather radio programmed and with fresh backup batteries and your smartphone w/ the MWN app with StormWatch+ programmed and NOT on silent, vibrate, or Do Not Disturb.  The severe weather tips below are also good to review.  It would be a great idea to have kids' shoes and helmets next to their beds or close by tonight and have your shoes, cell phone, photo ID, and flashlight by your bed. Your safe place should be prepared with the other items listed below before retiring for the night.

ORIGINALLY POSTED 4/27/14, 2:15pm

This may become somewhat of a live blog with updates appended as we go through the afternoon and evening hours, as necessary. But for now, we'll recap the current scenario and what we expect the rest of the day and night.

Rain moved in this morning as a warm front moved to a position just south of the metro and has now stalled on our southern counties in north MS producing thunderstorms with torrential rain. A scenario of "back-building" has resulted in which new storms form on the front and move away to the northeast, only to have additional storms re-fire on the boundary and repeat the process. Flash Flood Warnings are in effect for most of NW MS, southern Shelby County, and Crittenden County south of Marion this afternoon and a Flash Flood Watch is in effect for the entire metro until Tuesday evening (see watch/warning map above).  Flooding will be a concern anywhere that heavy rain falls for a period of time. Do NOT drive across water-covered roadways. It doesn't take much swiftly-moving water for you to lose control and end up in a ditch that you may not be able to escape from. In fact, the Weather Prediction Center has just issued a discussion that warns of the possibility of severe, perhaps life-threatening, flash flooding across our area.

Area where severe flash flooding may occur due to continuous heavy rain in the same areas

Until the warm front moves north of us, which we expect to occur slowly over the course of the afternoon, additional showers and t'storms will be likely. The good news is that the main threats with these storms will be lightning, very heavy rain and flooding, and possibly some small hail. No tornadoes or damaging wind are expected with this current batch. The other good news is that the rain is helping to stabilize the atmosphere somewhat (for now).  Once the warm front moves north, however, more unstable air arrives as south wind becomes gusty and dewpoints rise to the upper 60s - more than enough juice to fuel severe storms.

A break in the rain SHOULD occur behind the daytime band of storms and before another round overnight. Most likely this will be in the evening hours. As the dryline/cold front moves into Arkansas today, it will fuel supercells capable of all modes of severe weather, including large and violent tornadoes. We have some concern for the state of AR ito southern MO... These storms will move east into the metro region overnight. We're hopeful that today's rain means a less volatile atmosphere as they move east, causing them to weaken.

SPC seems to agree that that may be the case, trimming it's Moderate Risk such that the metro is now in "just"' a Slight Risk (see graphic above). I quoted "just" because it does NOT mean the severe weather threat is gone. There is nothing magic about the edge of a moderate risk zone being moved 50 miles. The risk simply tapers some from west to east (AR into TN/MS). We're close enough that overnight storms, which may be after 10pm into the wee hours Monday, can still produce large hail, damaging wind, and a few tornadoes.We'll root for weakening storms, but realistically have to plan for severe ones and you should too.

So what is your severe weather safety plan for the overnight hours and have your communicated it to/with family? How will you get severe weather notifications overnight if you need to take shelter? We of course recommend StormWatch+ in the MWN app. It can be set to receive just the warnings you want for the location(s) you want and will wake you up for Tornado and Severe Thunderstorm Warnings. (Read this caveat regarding silencing your device and whether you'll still get our warnings.) Click here for more info and download links for MWN with StormWatch+.

I've said it before but there are two things NOT to count on when it comes to life-or-death warning information: outdoor sirens and Facebook. 1) Sirens likely will not be heard in a closed-up home with rain pouring and wind whipping in a storm while you're sleeping. 2) Facebook does too much filtering of posts and won't show you the one you really needed to see. We're even seeing reports of Facebook not delivering notifications from our page in some cases. Boo Facebook! We'll be posting there, but you may not see it.

For tomorrow: the primary threat (Moderate Risk) has been shifted just east of the metro. But re-read what I said above about the delineation between Moderate and Slight Risk zones. We are still looking at a threat of all modes of severe weather again tomorrow, mainly during peak heating into the evening. We'll tackle that more later, but don't let your guard down.

Probability of severe weather within 25 miles of you on Monday
Be prepared, not scared.

Erik Proseus
MWN Meteorologist

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