Update on Mother Nature’s Halloween tricks

A brief update on our Halloween forecast, as I know many of you have kids that are REEALLLY wanting to get some cavities trick-or-treat on Thursday.  (Do you know how difficult it is to be a meteorologist with a crummy forecast on Halloween?? 🙂 )

Precipitation outlook

Overall, the bulk of the day looks wet and nasty. Rain will likely begin overnight Wednesday night (though it should hold off until after the University of Memphis football game at the Liberty Bowl), leading to a wet start to Halloween. Rain will continue, perhaps most of the day, and will be occasionally heavy. In fact, the GFS computer model (which is not out of line with other models and is shown below) predicts 2-4″ of rain with this system.  [The NWS is currently forecasting about 2″.]  Rainfall totals will have to be monitored as downpours could produce flash flooding in localized low-lying areas or areal flooding of urban areas or near streams and rivers.

Tuesday morning GFS computer model showing 24-hour rainfall amounts from 1am Thursday to 1am Friday. Brown colors are 2-3″ with whiter areas indicating 3-4″.

Severe weather probability

As far as severe weather potential, as discussed yesterday, this system will be moving through a “high shear, low instability” environment, which means there will be plenty of wind at all levels to support severe storms, but marginal instability to get them going.  Instability is the potential for air to rise when forced upward by a lifting mechanism like a front.  The higher the instability, the more likely storms are to form.  In other words, if storms can get started, the atmosphere will allow them to become strong to severe based on the shear.

High-shear environments, particularly when abundant shear is present in the lowest layers of the atmosphere, are also supportive of tornadoes. This will be the case on Thursday; therefore storms that form could produce a few tornadoes.  The more likely form of severe weather though will be damaging wind due to the very high wind speeds in the storm environment, which will also contribute to fast-moving storms.

There is a 30% chance of severe weather within 25 miles of the Memphis metro on Thursday, according to the Storm Prediction Center. Damaging wind and a few tornadoes are the main threats.

The good news (if there is some)

Models continue to indicate that the bulk of the rainfall will be during the day and may move out by 6-7pm.  However, a couple models still point to the chance of light rain Thursday night during prime trick-or-treating hours. Timing of the front will ultimately determine when rain comes to an end and we can’t get it down to the hour this far in advance. Cross your fingers that it all moves out by the time the kids are ready to hit the streets!

One additional note of caution, even if things do clear up by evening, is some areas could see excessive standing water and the ground will be saturated. Plan accordingly (footwear for the kids).  Temperatures should still be mild Thursday evening (60s) so fortunately it won’t be a heavy coat Halloween!

Lastly, not only should you plan ahead for your “fun” activities, but this is the first storm system of our secondary severe weather season with the real potential for producing severe weather. Know your plan ahead of time in case warnings are issued, including the possibility of Tornado Warnings and outdoor sirens sounding, in the afternoon hours in particular. Re-read the addendum to yesterday’s blog for more details and make sure your StormWatch+ alerts are setup on your MemphisWeather.net mobile app so that you will receive location-specific warnings if they are issued (link below).

We’ll follow up again tomorrow with more on timing and specific threats.

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