Deep South Winterwx Discussion 2015-2016

[BigB0882]

Thanks for those graphics. The NAM looks very wet. If I add them up I see over .5 QPF. Of course the NAM has the horrible warm nose. .5 QPF of ice would be awful. Am I adding that up correctly? I think you add them, I learned about this earlier today.

[cycloneye]

NAM went up in total precip.

I had to take out the wxbell graphic because is not permitted in this board.

[Kennethb]

The key to the wintry precip is the dewpoints. Here in Baton Rouge the dewpoint has dropped to 24, 5 degrees in the past hour. Dewpoints are near zero in north LA. The dewpoint will continue to drop with the temperature. Wetbulb when precip begins will drop the temps to support wintry precip.

[HurriGuy]

NAM went up in total precip.

I had to take out the wxbell graphic because is not permitted in this board.

My apologies. I was not aware.

Please remove snow accumulation too.

[HurriGuy]

The key to the wintry precip is the dewpoints. Here in Baton Rouge the dewpoint has dropped to 24, 5 degrees in the past hour. Dewpoints are near zero in north LA. The dewpoint will continue to drop with the temperature. Wetbulb when precip begins will drop the temps to support wintry precip.

do you mind explaining to me why dew points are key to supporting wintry precip in a little more detail? Thanks

[GTStorm]

Does anyone know how to translate those images of the NAM to QPF? Just rough estimates if possible.

I think this graphic is better.

12 hours.

http://www.tropicaltidbits.com/analysis ... n_us_3.png

18 hours.

http://www.tropicaltidbits.com/analysis ... n_us_5.png

24 hours.

http://www.tropicaltidbits.com/analysis ... n_us_7.png

36 hours.

http://www.tropicaltidbits.com/analysis ... _us_11.png

36 hrs cannot be right. Not gonna happen in Savannah. It never does.

[BigB0882]

I love that run of the NAM and it is the last one to really watch before the event begins. Good one to end on. Let's hope it is right. It shows 4+ for Baton Rouge. I realize that is not all snow, maybe no snow even, but it bodes well for a chance of seeing SOME snow.

[hurricanehunter69]

Yes!!! The cold air advection has finally arrived to the coast! Game on!

[ktulu909]

what do you guys think of the Gretna, LA area chances of getting in on anything other than slush? Seems it looks like I'm in for the absolute worst part of this whole thing.

[Janie2006]

I'll give it a go, but sometimes it's as dry reading as you'll find. Basic air parcel theory, really.

The wet bulb temperature is a combination of the air temperature and the amount of water vapour in the atmosphere at a given point. So, it's a combination of the dry-bulb temperature (air temperature) and the dew point. If you cooled the air temperature to the dew point you'd have 100% humidity, which yields condensation of the moisture in the atmosphere.

The wet bulb temperature is between the dry bulb temperature and the dew point and it is effectively the lowest temperature that can be reached by evaporation alone. As rain (or snow) falls from clouds, evaporation takes place as the precipitation falls into drier air. The "parcel of air" (the theory assumes that "parcels of air" rise into the atmosphere based on heating processes) releases any moisture it carries. Evaporation. As the evaporation occurs, the amount of moisture in the atmosphere increases. The dew point necessarily increases as a result. The air temperature decreases. The dry bulb temperature and the wet bulb temperature draw closer together. Dynamic evaporative cooling takes place. Or I just like saying the word "dynamic". You decide.

Now then, imagine a scenario in which precipitation falls through drier air that happens to be in the upper 30s, and the dew point of that same air is in the upper 20s. The precipitation falls through the air, increasing the dew point and lowering the temperature, and let's say you have enough precipitation to reach 100% relative humidity....condensation takes place, well, precipitation reaches the ground *but* at the same time we find that the air temperature has lowered to 32*F and the dew point has *risen* to 32*F. Winter precipitation. Actually, that's how a lot of precipitation occurs. Heh.

So, the wet bulb temperature gives us another good measure of the factors we need to have in place before precipitation (or any variety) can take place.

(And I think I have all of that correct. I don't think I missed much, but someone feel free to bust my chops if I have.)

[BigB0882]

Great explanation, thanks so much! I have never understood that and now I have a much better understanding.

[beagleagle23]

00Z GFS starting to come in....

[BigB0882]

00Z GFS starting to come in....

Last one I plan on watching carefully. Please give us one last good run!

[HurriGuy]

GFS went up. BTR 3+ INCHES rather than 18z had us at about 2.4 inches.

[HurriGuy]

GFS definitely wants to give us at least some flurries unlike NAM's huge ice storm.

[windnrain]

Really? GFS just went up? What's the QPF?

Awesome!

[cycloneye]

00z GFS.

I am done posting the model runs for the deep south event. It has been a pleasure to colaborate from outside the U.S with the region folks with the information. Now is time to watch the weather evolve from now. Stay safe.

18 hours.



24 hours.

[Janie2006]

Well, don't thank me quite yet.....General Meteorology was a looooong time ago!

Let's see if I can't think aloud for a moment....

At last check I noticed that the air temperature at KMOB was 54*F and the dew point was 37*F, with a steady North wind, indicating that the front has passed and the air is drying considerably. The dew point dropped 12*F in the last 2 hours. This is entirely consistent with the passage of a cold front, especially along the Northern Gulf Coast. However, I notice back to the west that moisture levels are increasing in Texas and I know disturbances in the flow are moving this way. So, what do I expect? I expect continuing cold air advection from the arctic front, and I expect the dew point to continue to lower for the same reason.

However, at the same time moisture begins to over-run the arctic air, so I'd expect the radar to begin lighting up towards morning. Initially it's indicating virga, because the atmosphere is relatively dry, *but* for the same reasons I mentioned earlier I expect that moisture to increase, thereby increasing the dew point and lowering the air temperature. So, when the precipitation finally condenses out of the clouds and reaches the surface, according the winter weather criteria, the air temperature will be at or below 32*F and the place starts to rock and roll.

How'm I doin' so far?

That's *not* the same thing as saying what *type* of precipitation it might be, cos that's dependent on the temperature profile of the air column. BUFKIT and Skew-T are your friends. Right now, what is significant is the temperature at the surface, because whatever hits the ground will start accumulating. Rain that freezes on contact, sleet, snow, mice, etcetera.

And I'm done with really long posts tonight.

[windnrain]

Uh, you sure?

http://weather.rap.ucar.edu/model/displ ... hr036hr048

The 00z looks like we in BR only are forecast for .11 QPF, lower than even the last one.

[BigB0882]

Uh, you sure?

http://weather.rap.ucar.edu/model/displ ... hr036hr048

The 00z looks like we in BR only are forecast for .11 QPF, lower than even the last one.

That looks like a 12 hour forecast. Isn't that missing the part of the period where most of our QPF comes in?