Friday, March 27, 2015

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

Follow MWN on Facebook, Twitter, and Google+
Visit on the web or on your mobile phone.
Download our iPhone or Android apps, featuring StormWatch+ severe weather alerts!

No comments: