Tricky forecast with snow nearby on Monday

Published 7:00 am Saturday, January 9, 2021

By Skip Rigney

When it’s cold enough, there’s not enough moisture. When there’s enough moisture, it’s not cold enough. This is the meteorological rule of thumb for forecasting snow on the central Gulf Coast. It’s probably more accurate to say that it’s the rule of thumb for why it’s usually a safe bet to forecast that it won’t snow in our neck of the woods, even in the middle of winter.

But, rules are made to be broken, and this rule will be put to the test late Sunday night and Monday morning. That’s when the weather map features that are usually associated with snow in our area will be coming together.

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Cold air has been filtering into the Deep South since a cold front passed through on Thursday. A large, sprawling ridge of high pressure to our north seems to be anchored around Missouri and Kentucky. Since that high isn’t going anywhere fast, neither is the cold air in our area.

Meanwhile on Sunday, low pressure at several miles altitude will be moving over Texas, providing the lift needed to spread snow from Lubbock to Dallas.

In the Texas coastal zone, there is a large change in temperature between the cold land and the relatively warm waters of the western Gulf. Where the southeastern fringe of an upper-altitude low pressure circulation moves over a surface temperature boundary is prime territory for the formation of low pressure at the surface. When that surface low forms over the western Gulf on Sunday, its counterclockwise winds will lift moist air and push it toward the cold central Gulf Coast.

But, here’s where our almost-always-true rule of thumb comes into play. The moist air that will be streaming over us on Sunday night and Monday morning will also be warmer than the air that has been over us the last couple of days. Computer weather models predict the temperature will be above freezing in a layer nearly two miles thick from the ground upward. While precipitation looks likely for Pearl River County Sunday night into Monday morning, it appears that part two of the snow forecasting rule-of-thumb will apply, namely, when there’s enough moisture, it’s not cold enough.

However, it doesn’t take a meteorologist to correctly guess that the further north you go the colder it will be. That’s why if you plan to drive Monday morning northwest of a line extending from about Baton Rouge to Hattiesburg to Meridian, keep up on the latest weather forecast. Roads in the central and northern parts of Louisiana and Mississippi could get icy.

Exactly where that line sets up is very difficult to forecast. Just like during hurricane season, predictions vary from model to model, and between runs of the same model. It won’t take much change in the position or strength of any of the weather systems discussed above to bring a little snow, or more likely sleet, into Pearl River County on Monday morning, even if our forecasting rule-of-thumb says probably not.

By Monday evening, the low pressure system responsible for our brief flirtation with frozen precipitation will be pulling out of the Gulf into the Florida panhandle. On its backside, much drier air will be flooding across our area setting up a dry mid-week with lows in the 30s and highs in the 50s.