Sunday, March 29, 2015

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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