Tuesday, November 18, 2014

Another Freeze Warning Tonight

By Jay Grymes & Steve Caparotta

WAFB First Alert Quickcast:

- another freeze for Wednesday morning
- warming trend for the rest of the week
- stormy weather possible for the weekend

Fool me once: shame on you … fool me twice: shame on me. But fool me three times, and we say, “Hey, what’s going on here?”

Yep -- we know -- our early morning temperature forecast was a bust! In fact, our forecast lows for each of the three recent morning freezes (last Friday and Saturday mornings plus earlier this morning) have all been too cold. Our guidance tools haven’t done well at all: one of the normally more-reliable models, the American GFS, has been calling for lows that have been as much as 3° to 6° too low. What’s more, we think it may be doing it again for tomorrow morning.

We remind you that last night we mentioned the potential that some high clouds and light winds might be enough to keep Baton Rouge in the upper 20°s … in the end, those clouds were considerably heavier than we had expected. As a result, the low didn’t drop below 30° for almost all WAFB communities and the freeze line barely extended south of the Capital City. 

While that’s good news for keeping the home a little warmer overnight and it means that freeze-protection efforts were more than adequate, being “too cold” with our forecast by 3° to 4° is a bit disappointing for us. And some of you may be grumbling about the preparations that you undertook that weren’t really necessary.

As a reminder: how do the clouds impact the falling temperatures? Essentially, they “capture and return” some of the heat lost at night from the ground and air near the surface. In reality, the ground gives off heat to the atmosphere at about the same rate whether we are under clear or cloudy skies. However, when clouds are present they re-direct some of that energy back towards the surface. In the end, under cloudy skies, the energy being “sent back” by the clouds means that the NET energy loss from the ground and low-level air is lower. A lower NET energy loss means a slower drop in air temperatures near the ground. 

In addition, just a little wind at night helps mix some of the slightly warmer air well above the ground down towards the surface. (Remember, during a routine afternoon, the air near the ground is warmer than the air aloft. But at night, the air near the ground usually becomes a little cooler than the air just a few tens of feet above the surface). Mixing the warmer air from above downward towards the ground further slows the “effective” cooling rate and can keep overnight and early morning temperatures a bit warmer than had there been no wind.

And late last night into early this morning, we had both: a decent cloud deck and a little wind.

So we’ll prepare for another freeze tonight into tomorrow morning, and we’ll go with a Wednesday morning low that proves colder than this morning’s minimum … but we won’t go as cold as the latest GFS and NAM models are suggesting. As of this afternoon, both models were calling for a Baton Rouge low of 25°. 

We are expecting mainly clear skies tonight. In addition, winds for most of the night should be near calm thanks to surface high pressure sitting right on top of us for much of the overnight hours (before shifting to the east early Wednesday). So, we’ll concede an overnight low in the upper 20°s but not the GFS’s mid 20°s.

After that, it’s “Goodbye freezes!” … probably for at least for the next 7 to 10 days or more.

A warming trend begins in earnest on Wednesday with Red Stick area highs expected around 60° or so under sunny skies. For Thursday, after a morning start around 40° we’ll plan for a sun/cloud mix with highs in the upper 60°s for metro BR. Then it’s back to the 70°s for Friday, Saturday and Sunday ... and possibly Monday too.

We’ll call for spotty showers on Friday afternoon with the weather becoming much more active as we head into mid-day Saturday. Our current guidance is suggesting that a fairly potent storm system will be sliding across the mid-Mississippi Valley through the weekend, promoting an active thunderstorm event across the Bayou State.

Our current thinking is that the weather could become very active from the latter half of Saturday into Sunday morning, with the potential for another round of severe weather that will be much like what we saw on Sunday evening. The NWS Weather Prediction Center is currently calling for widespread rains in excess of 2” between Saturday and early Monday for our viewing area and the NWS Storm Prediction Center is already calling for a 30% chance of severe storms this weekend over southeast Texas and southwest Louisiana. Given the 5-day window, that’s close enough to get our attention.

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