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Winter threat: 1/9-12/ 25

Question for those more knowledgeable than me - does the ECMWF just not do freezing rain? Assumes it's all going to be snow or rain?
 
UncleJuJu98 said:
I'm wondering if those upper 20s and low 30s will actually stay socked in; in the central Alabama area throughout Friday during the day. If the precip is too heavy to raise temps. (Wet bulb temp..? I think)
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"Evaporative cooling" or "Wet bulb-ing". Tends to lose out to WAA in our neck of the woods. The warm nose coming up out of the gulf that rolls through the south as the low passes west to east tends to overwhelm it.

I'm going to show my age with this link - it's so old most of the images no longer work - but I always thought it was a good piece on thermodynamic processes. Nerd out. :)

"THERMODYNAMIC/TEMPERATURE PROCESSES

Temperature advection, adiabatic warming/cooling, and diabatic effects can have major influences on atmospheric temperatures and precipitation type in the cool season, especially during borderline rain/snow situations.

Effects of these processes on temperature: Warm (cold) air advection causes a temperature increase (decrease) at a particular level or in a layer. Adiabatic cooling (warming) occurs when air rises (descends), resulting in a temperature decrease (increase). When the air is stable, then adiabatic cooling due to lift will have a more substantial effect on temperature than for an unstable atmosphere. Diabatic effects include diurnal heating/nocturnal cooling, condensation, evaporative cooling, and melting.

Diurnal heating/nocturnal cooling affects temperature (especially in low levels).

Condensation produces latent heat release, which produces warming that can counteract somewhat the effects of adiabatic cooling from lift. Latent heat release is most noteworthy in convection.

Evaporative cooling occurs as precipitation falls into relatively dry low levels. The precipitation evaporates in the drier air which causes cooling in low levels and at the surface. In borderline rain/snow cases, this can cause frozen precipitation to remain as such until low-level saturation occurs, or it could cause liquid precipitation to temporarily change to frozen or freezing precipitation. Once the low-level air mass saturates, then evaporative cooling no longer is a factor.

Melting (of snow to rain aloft or snow on the ground) causes a small amount of cooling in the atmosphere since heat from the environment is needed to melt the ice crystals. If significant melting occurs aloft, then an isothermal layer at or below 0 deg C could result. A saturated isothermal layer is important for heavy precipitation production since the layer will be associated with a larger absolute moisture content than one in which temperature and mixing ratio decrease with height.

Advection and vertical motion often oppose each other. Warm advection usually causes ascent, which in turn produces adiabatic cooling to at least partially counteract the warming. However, vertical motion due only to warm advection likely will not be strong enough to completely counteract the warming and any latent heat release. Thus, low-level (e.g., 850 mb) temperatures and thicknesses usually will rise during warm advection situations. However, occasionally temperatures and thicknesses may not rise (perhaps even fall) in the face of warm advection (models can show this). For this to occur, other forcing mechanisms must be present to produce much stronger vertical motion (often on a smaller scale), including significant jet streaks, frontogenesis, and/or CSI/convective instability. Therefore, models can hint that significant mesoscale processes may be present to produce strong enough lift and adiabatic cooling to overwhelm warm advection. This often is a scenario, given adequate moisture, for heavy precipitation production, such as was the case during the January 16-17, 1994 snowstorm in Kentucky in which 1 to 2 feet of snow fell across parts of north-central Kentucky in less than a 12-hour period.

Strong adiabatic cooling could cause precipitation to fall as or change to snow during the period of maximum lift during borderline rain/snow situations."
Click to expand...

https://www.weather.gov/lmk/winterpt1

So many different thermodynamic variables it's hard to keep track, for sure. But that's the reason I plot the wet bulb 32, dewpoint 32, and surface 32 degree lines on my weatherscope when watching winter weather. The drier (lower) the dewpoint, the more evap cooling there will be, and the greater distance there will be between those lines. Where the 32 wetbulb line is, heavy precip will lower the surface temps to that wetbulb temp. That's an overgeneralization, but close enough for this. Clear as mud? LOL
 
Blountwolf said:
"Evaporative cooling" or "Wet bulb-ing". Tends to lose out to WAA in our neck of the woods. The warm nose coming up out of the gulf that rolls through the south as the low passes west to east tends to overwhelm it.

I'm going to show my age with this link - it's so old most of the images no longer work - but I always thought it was a good piece on thermodynamic processes. Nerd out. :)

"THERMODYNAMIC/TEMPERATURE PROCESSES

Temperature advection, adiabatic warming/cooling, and diabatic effects can have major influences on atmospheric temperatures and precipitation type in the cool season, especially during borderline rain/snow situations.

Effects of these processes on temperature: Warm (cold) air advection causes a temperature increase (decrease) at a particular level or in a layer. Adiabatic cooling (warming) occurs when air rises (descends), resulting in a temperature decrease (increase). When the air is stable, then adiabatic cooling due to lift will have a more substantial effect on temperature than for an unstable atmosphere. Diabatic effects include diurnal heating/nocturnal cooling, condensation, evaporative cooling, and melting.

Diurnal heating/nocturnal cooling affects temperature (especially in low levels).

Condensation produces latent heat release, which produces warming that can counteract somewhat the effects of adiabatic cooling from lift. Latent heat release is most noteworthy in convection.

Evaporative cooling occurs as precipitation falls into relatively dry low levels. The precipitation evaporates in the drier air which causes cooling in low levels and at the surface. In borderline rain/snow cases, this can cause frozen precipitation to remain as such until low-level saturation occurs, or it could cause liquid precipitation to temporarily change to frozen or freezing precipitation. Once the low-level air mass saturates, then evaporative cooling no longer is a factor.

Melting (of snow to rain aloft or snow on the ground) causes a small amount of cooling in the atmosphere since heat from the environment is needed to melt the ice crystals. If significant melting occurs aloft, then an isothermal layer at or below 0 deg C could result. A saturated isothermal layer is important for heavy precipitation production since the layer will be associated with a larger absolute moisture content than one in which temperature and mixing ratio decrease with height.

Advection and vertical motion often oppose each other. Warm advection usually causes ascent, which in turn produces adiabatic cooling to at least partially counteract the warming. However, vertical motion due only to warm advection likely will not be strong enough to completely counteract the warming and any latent heat release. Thus, low-level (e.g., 850 mb) temperatures and thicknesses usually will rise during warm advection situations. However, occasionally temperatures and thicknesses may not rise (perhaps even fall) in the face of warm advection (models can show this). For this to occur, other forcing mechanisms must be present to produce much stronger vertical motion (often on a smaller scale), including significant jet streaks, frontogenesis, and/or CSI/convective instability. Therefore, models can hint that significant mesoscale processes may be present to produce strong enough lift and adiabatic cooling to overwhelm warm advection. This often is a scenario, given adequate moisture, for heavy precipitation production, such as was the case during the January 16-17, 1994 snowstorm in Kentucky in which 1 to 2 feet of snow fell across parts of north-central Kentucky in less than a 12-hour period.

Strong adiabatic cooling could cause precipitation to fall as or change to snow during the period of maximum lift during borderline rain/snow situations."

https://www.weather.gov/lmk/winterpt1

So many different thermodynamic variables it's hard to keep track, for sure. But that's the reason I plot the wet bulb 32, dewpoint 32, and surface 32 degree lines on my weatherscope when watching winter weather. The drier the dewpoint, the more evap cooling there will be, and the greater distance there will be between those lines. Where the 32 wetbulb line is, heavy precip will lower the surface temps to that wetbulb temp. That's an overgeneralization, but close enough for this. Clear as mud? LOL
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Good explanation/post!
 
Spann sniffing at the same things we are. :)

 
I've only forecasted 2 Southern snowstorms accurately.

January 2011: Just by reading NWS discussions and thinking ahead about what they would do next. I was just a high school junior weather nerd at that time.

February 2015: By relying on what I told my grandpa before he passed away. He always ask if we would see snow each winter. He had asked me that before Thanksgiving in 2014. I said yes, we would. He passed away a few days after Christmas that year. I never forgot what I told him. Then came that big February snowstorm. I saw it as a gift from him to me.

He's helped me a lot with my weather forecasting since then as well.
 
MichelleH said:
It was absolutely horrible. I hope I NEVER see that again in my lifetime.
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I would like to think we are more prepared, but we're not - likely less so, to be honest. There are more folks with generators, but not likely as a percentage of total people here. Imagine being out of power with a foot of snow for a week, because that's reality - our power grid can't handle that much in the south, because 99.9% of the time it wouldn't need to. As a weather nerd I'd love to see it happen, but as a someone who has worked in public preparedness, not so much.
 
Richardjacks said:
93 happened in mid March, much higher sun angle than mid January....a thaw, melt, warmup... will take longer than events later in the season.
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Yep. Really hoping that statement doesn't look prophetic some time next week.
 
Just to be contrarian, 0Z GFS is coming in warmer - rain in N LA and S AR this run.
1736222482662.png
 
Warmer warm nose, quick cutoff to snow without nearly as much ice on this GFS run.
1736222673247.png
 
Oof - I spoke too soon about ice. This run hurts NW GA, W SC and W NC. This run would be a total heartbreaker for B'ham and Tuscaloosa. Nothing but rain.

1736222894638.png
 
Richardjacks said:
Check out the bham sounding literally at 32 with td of 32. literally less than 1 degree from much of it snow...plus starts off warmer but there is plenty of evap cooling aloft.
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Yep, it's razor close, but how many times have we seen it be 33 and rain until somewhere between bham and Cullman? The models seem to be throwing out every possible scenario, but they are all plausible. Maybe they'll agree by Thursday, but I doubt it. :)
 
I note that whole GFS run had the low further north - crossing S LA, MS, AL instead of in the gulf. That as always is going to be critical - placement and strength of the low.
 
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