From snow to storms - spring returns to the Mid-South

Snow and 40+ degrees??

Wasn't yesterday's snow weird?? I drove through a pretty steady graupel shower around 2:45 yesterday afternoon in Bartlett with temps in the low 40s. Some of you reported sleet or snow shaped like "round balls" yesterday - that was graupel, or "dippin dots" as we are known to call it. The reason for this phenomena with temps above 40 was because the air quickly dropped below freezing by about 1000-1500' up. Thus precip fell as snow with partial melting in the lowest layer of the atmosphere, forming the unique precip type that mixed with sprinkles and snowflakes.

Graupel in Oxford, MS at about 3:30 Saturday. Photo courtesy @larryh921on Twitter.

But now that that is over, it's time to welcome spring back to the Mid-South! And we'll welcome it back, less than 36 hours later, with thunderstorms.

The overall pattern

You will probably be familiar with the sound of thunder by week's end, as it looks like there may be multiple rounds of thunderstorms. The overall pattern will put us in the "warm sector" with low pressure in the Plains and southerly flow across the region for much of the week, starting Tuesday. Impulses in the atmosphere could spark a few rounds of storms, mainly Wednesday through Friday, before a decent cold front finally moves through right before we head into Easter weekend, clearing out in time for a pretty decent weekend it appears.

The pattern this week will mean a few things: 1) much warmer air this week - we're talking lots of 70s! 2) southerly wind that brings more humidity (you probably won't need the lotion as much this week), and 3) those scattered t'storms, a few of which could at times be strong. It's too early to time out these impulses that might bring a few strong storms, but this is just an early heads up.

Tonight's thunderstorms

As for the first round that starts off the week, that happens tonight. A cold front will move through the area just after midnight, which will prompt storms to form, likely right over the metro. The Memphis metro is in a MARGINAL risk of severe storms, or category 1 out of 5. The marginal risk is driven by a 5% probability of hail larger than 1" within 25 miles of any point in the metro. Given that the probability of severe hail within 25 miles of the a point in the metro on April 1st is roughly 1%, this means that our risk of severe hail is about 5 times normal for this time of year.

A reminder of the "new" severe weather outlook classification system. We have a Marginal Risk of hail storms tonight.

A Marginal Risk of severe storms exists ahead of a cold front tonight from the Mid-South (star) to north TX.

The aforementioned cold front that arrives overnight won't run into sufficient warm, moist Gulf air until it gets pretty close to the metro, as most of the ingredients for storms (moisture, instability, and some wind shear) will exist along and south of the I-40 corridor. Thus, the front may be dry, or perhaps just bring a few showers, as it approaches our area. Once it gets close, I expect scattered storms to fire with the necessary lift that the front provides. The most likely area to see storms capable of producing hail will be in north MS, though a few can't be ruled out north of the TN/MS state line. The best timing for storms appears to be between 10pm-4am, though by 1am or so, the threat exits west TN and Crittenden Co. and is confined to north MS.

Instability, a necessary ingredient for storms, is measured here by CAPE, which will be present at 10pm basically along and south of I-40. North MS and southern AR has the highest available energy to fuel storms. With no CAPE to our north, there is little chance of thunderstorms as the front approaches, until it gets to the metro. Graphic courtesy WxBell.

Simulated radar from the HRRR high-res model valid at midnight, showing a broken line of storms from central AR along the TN/MS state line. The cold front will be where the lighter rain (greens) are just to our north. Graphic courtesy WxBell.

This week

Once the front clears the area, clouds depart with it and tomorrow looks to be a perfectly beautiful spring day - sunny and 70! Tuesday may also stay mostly dry, and if it does, we'll look for highs to get into the mid 70s. By Tuesday night or Wednesday, Gulf moisture-laden air is back in place and warm temps will combine with passing impulses to bring the possibility of scattered storms each day until another front moves through late week. For those looking ahead to Easter Sunday, conditions at this time appear dry and cool, but not cold like this morning. Being a week out, that could surely change. Stay tuned!

Erik Proseus
MWN Meteorologist

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Wait, I thought winter was over! The four-letter 'S' word returns...

I'm reminded of an elementary school song (that has always annoyed me as a parent) that goes like this: "This is the song that never ends, yes it goes on and on my friends...", etc. The worst part is that the lyrics are such that it repeats ad nauseum. Despite some recent bouts of spring weather, it seems a slight tweak to those words could apply. "This is the winter that never ends, yes it goes on and on my friends..."

Cold temperatures

Many of us woke up to frost this morning as temps fell into the mid to upper 30s. Now a Freeze Warning is in effect for areas northeast of Shelby County (including Tipton and Fayette Counties) and a Frost Advisory for the rest of the area Saturday morning. A repeat of these cold conditions can be expected Sunday morning until finally a bit of a pattern shift will mean some warmer temps return.

Freeze Warning tonight for the counties in light blue. Frost Advisory for the darker blue counties including Memphis and Shelby County.

But that's not all...

However, cold is not all we're dealing with unfortunately. Yes, we're putting a chance of snow in the forecast for tomorrow - March 28th.  I've been looking at the possibility all week actually, and doing my best to ignore it like a middle school pimple, in the hopes that it goes away. Alas, it hasn't. Kudos to the GFS model for not budging all week on the upper-level disturbance that brings the precip chance tomorrow.

Our favorite response in Facebook comments to this story... :-)

Atmospheric setup

The setup looks like this: the pattern that brought brutally cold air and rounds of ice and snow to the eastern U.S. over the past couple of months has re-appeared. It features a large trough of low pressure in the upper and mid-levels east of the Mississippi River and a ridge of high pressure over the western states. The Mid-South happens to be positioned in the "sweet spot" - northwest flow between the ridge and trough where the jet stream is setup and upper-level disturbances track like trains on an atmospheric train track (one such disturbance is responsible for our showers today in nearly the same setup).

At 500mb (18,000') a large ridge (and warmer air) is to our west, while a trough (and cooler air) covers the Midwest and Northeast. The Mid-South is in "northwest flow" with wind blowing from the northwest. Graphic courtesy WxBell.

At 30,000' (300mb), the huge dip in the jet stream over the eastern U.S./Canada is evident with the Mid-South on the western periphery of that trough. The let stream overhead is the "train track" for upper level disturbances to move along.

As we head into tomorrow, we'll start off a few degrees colder than today, and with increased cloud cover and a bit better chance of precip, surface temperatures won't climb quite as quickly tomorrow. In addition, temperatures at 5000' (a measuring stick of sorts for winter weather) are well below freezing, meaning that precip that falls through that level is snow.

Precipitation forecast by the GFS model between 7am-1pm Saturday, along with 5000' temperatures in the red/blue lines. The 5000' freezing line is sold blue and southwest of the metro. Temps at this level over Memphis are about -4°C (28°F). Precip at this level is snow, and falling into slightly warmer air. 

The flies in the ointment

There are two main model discrepancies to consider: 1) the height of the freezing level above ground and 2) the exact track of the main axis of precip. In the colder solutions that promote a better chance of snow, the freezing level is closer to 500-1000' (low enough to get snow at the surface), while in the warmer models, it's closer to 2000' (more of a chance of the snow melting before reaching the ground). The exact track of the narrow band of precip that is expected is also hard to determine, but appears to be from northeast AR across the metro into north-central MS.

The track of the precip on the GFS model is in the previous graphic. This one shows the NAM model track with "simulated radar" valid at 1pm. It is a little warmer and indicates more rain than snow possibilities.  It also has the precip slightly southwest of the GFS track.

Bottom line

So how do we think this plays out? The most likely scenario sees a light rain/snow mix beginning after 8am with temperatures in the mid 30s. As temps slowly rise to near 40, precip should change over to just rain by noon or so. Because rain is mixed with snow, precip is light, and temperatures are above freezing, no accumulation is expected, especially on roads. A low probability exists for a dusting on grass/mulch beds, etc. but this would mainly be to the northwest of the metro in northeast AR. Tomorrow afternoon sees a chance of rain showers continuing until about mid-afternoon with temperatures remaining in the 40s. It'll be a cold day.

Another word on near or below freezing temps

Besides the snow, for those who have tender vegetation already in the ground, frost (or a light freeze east of the Bluff City) is possible tonight and again Sunday morning. Make plans to cover those plants. By Sunday afternoon, we should be back in the 60s and hopefully FINALLY putting winter in the rearview mirror! By the way, the average last freeze at Memphis International Airport is March 19, but it's March 28 (tomorrow) at the Agricenter, which is also representative of most of the suburbs. The latest snow ever recorded in Memphis is April 25, so this is not unprecedented by any means. I personally am looking forward to 70s again next week!

Erik Proseus
MWN Meteorologist

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Visit MemphisWeather.net on the web or m.memphisweather.net on your mobile phone.
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