Pesky Halloween Shower Forecast

For the first time in three weeks we have rain to talk about. Increasing moisture and a weak upper air disturbance will cause increasing clouds tomorrow, but the showers won’t arrive until Saturday.

The graphic below — which looks like a radar but isn’t — shows the ECMWF model forecast of rainfall for a 3-hour period ending at 4 PM and one ending at 10 PM on Saturday. Though these are not the “gospel truth”, this model and several others are showing widely scattered, light afternoon showers mainly inland on the Northshore. Then, perhaps a break (not shown) as the heat of the day dies down, then more showers breaking out in in the 8-10 PM time frame moving strongly from west to east produced by the upper air disturbance. This could mean that there’s a decent chance that the little ones can get some trick or treating in during the 5:30-7:30 time frame, but older teens who like the 8 and 9 PM hours may have problems. There’s more of a chance of that happening along the MS Gulf Coast and in Eastern St. Tammany than Western. That’s really splitting meteorological hairs so only time will tell if my forecast offers a trick or treat. Oh…the good news in all of this is that rainfall totals will be light (a few tenths of an inch) and that there probably won’t be any lightning because upward motion will be light.

But for this weather geek, the real danger could happen Tuesday night or Wednesday morning when a cold front approaches our area. Though it is still almost a week a way, it looks to have the right ingredients for severe weather, including the possibility of tornadoes. The ECMWF for Wednesday morning shows the north-south line of storms down the Mississippi Valley along with a strong upper air low behind it over Southern Arkansas. That upper feature will give us rapidly divergent winds aloft leading to strong updrafts, severe thunderstorms, and lowering pressures. Something worth watching.

Did you know that November is one of the few times of the year when we can get tornadoes right along the Gulf Coast? Unlike the early spring, when cold Gulf water temperatures often saves us from the threats that Central Mississippi and Alabama face, the water is still relatively warm. It’s not all that frequent - the last time our area saw powerful tornadoes from this setup was the Nov. 21, 1997 Covington, LA EF-2.

The Foreca graphic looks fine until late Tuesday. It doesn’t show the thunderstorms with the cold front early Wednesday. It also looks like it’s from yesterday - maybe it’s a holiday in Finland? I’ve added my expected high and low temperatures. Tomorrow looks fairly cloudy though we may see a bit of morning sun. Saturday should see some breaks in the clouds and the showers will be widely scattered mostly in inland areas of the Northshore during the afternoon.

Highs won’t be much different from what we’ve experienced, but it will be more humid with lows in the 60s after tonight (add a few degrees for the Southshore). The thunderstorms could arrive a few hours earlier than what the ECMWF advertises - perhaps in the wee hours of Wednesday morning. My early guess at rainfall totals would be close to an inch. Then, just a little bit cooler and less humid for Wednesday afternoon through next Friday.

There will be another chance of showers in the Nov. 11 or 12 time frame followed by cooler weather with highs in the low to mid 70s and lows in the 40s and 50s. Expect to see a considerably wetter November and December than October with temperatures eventually turning below normal. There will be an active storm track across the South with some severe weather threats. Could this be the first year in two decades where rain will ruin outdoor Thanksgiving grilling and frying?