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Winter threat: 1/9-12/ 25

Here is what I put out to my social media followers:
UPDATE ON COMING WINTRY WEATHER
A mixture of ice and snow is looking likely for much of north and possibly central Alabama.
Very cold air has invaded the southeast and will remain in place through the week.
An area of low pressure will track near the gulf coast later this week. The exact track will ultimately determine how far south we see ice and snow. The further north the low tracks, the less snow and ice; the further south, the more snow and ice for much of north and central Alabama
Forecast models nudged the track a little further north overnight, but another nudge south is possible, a track shift as little as 50 miles will make a huge difference, especially in the I-20 corridor.
Later tonight, I should have more conifdence on the track and talk more specifically about what is expected.
Everyone should be aware that travel issues are likely as precip rapidly overspreads the area after midnight Friday morning and wll last through the day. If the low pressure takes a further south track, then widespread travel issues are expected. There will be a bit of a thaw Sunday, but more cold air will come in on Monday, this could be a long duration event, again depending on the eventual track of the low pressure to our south.Mainly Rain (1).jpg
 
Let's face it - the models are going to windshield-wiper back and forth between boom and Forecasted Convective Amplification Deficiency, as both are plausible outcomes for snow in the south. Absent any major shifts colder or warmer, we'll see lots of possible outcomes. Things to watch for this week in reality, not the models:

1. Note the temps this week, and how they compare to predictions. If they Forecasted Convective Amplification Deficiency high, then likely so will the warm nose. Because of the water temp in the Gulf, that's likely the biggest threat to snow in our area, a lot of WAA coming up off the gulf. Always is the potential snow killer.
2. Note where the low pressure forms in the SW gulf. The strength and track of the low is a huge factor - further south puts the apex of the warm nose further south, and moves the snow line south.
3. Note the dewpoints out ahead of the system. If the air is dry at the surface with low dewpoints, the evaporative cooling effect can be quite dramatic - enough to make the entire column freeze to height and just DUMP snow where you would have thought only rain. But that only lasts until the column is saturated, so there will need to be a big difference between temp and dewpoints for that to matter.
 
Blountwolf said:
Let's face it - the models are going to windshield-wiper back and forth between boom and Forecasted Convective Amplification Deficiency, as both are plausible outcomes for snow in the south. Absent any major shifts colder or warmer, we'll see lots of possible outcomes. Things to watch for this week in reality, not the models:

1. Note the temps this week, and how they compare to predictions. If they Forecasted Convective Amplification Deficiency high, then likely so will the warm nose. Because of the water temp in the Gulf, that's likely the biggest threat to snow in our area, a lot of WAA coming up off the gulf. Always is the potential snow killer.
2. Note where the low pressure forms in the SW gulf. The strength and track of the low is a huge factor - further south puts the apex of the warm nose further south, and moves the snow line south.
3. Note the dewpoints out ahead of the system. If the air is dry at the surface with low dewpoints, the evaporative cooling effect can be quite dramatic - enough to make the entire column freeze to height and just DUMP snow where you would have thought only rain. But that only lasts until the column is saturated, so there will need to be a big difference between temp and dewpoints for that to matter.
Click to expand...
Yes also onset timing of precip can offset some of that waa and just how far north the warm nose gets....the earlier the precip into the drier air, the more likely the cold air stay, causing the warmer air to go up and over....
 
Let's talk about bombogenesis. Any time there is a gulf low, there's also the possibility that the cyclone will "bomb out". Much like RI in a hurricane, "bombing out" means a rapid drop in pressure.

Bombogenesis Overview:​


Bombogenesis refers to the rapid intensification of a low-pressure system. Specifically, it occurs when the central pressure of a system drops by at least 24 millibars in 24 hours, a process often associated with the development of a "bomb cyclone." This process can result in a stronger, more dynamic storm system with enhanced wind speeds, heavy precipitation, and abrupt weather changes.


When a low-pressure system in the Gulf of Mexico (GOM) undergoes bombogenesis, it can significantly affect the strength and impact of a winter storm system in Alabama.



How Bombogenesis Applies to Winter Storms in Alabama:​


  1. Moisture Source from the GOM:
    • The GOM provides abundant warm, moist air. As a developing low-pressure system moves across or near the Gulf, it taps into this moisture.
    • Bombogenesis increases the system’s capacity to draw in moisture, fueling heavy precipitation, including rain, snow, or freezing rain, depending on the thermal structure of the atmosphere.
  2. Intensified Lift and Precipitation:
    • Rapid intensification causes stronger atmospheric dynamics, including enhanced lift. This can result in heavy precipitation across Alabama.
    • In winter, a cold air mass interacting with this moist air creates the conditions for snow, sleet, or freezing rain, depending on the temperature profiles.
  3. Strengthened Cold Air Advection:
    • Bombogenesis intensifies the pressure gradient around the system, driving stronger northerly winds behind the low.
    • In Alabama, this can transport Arctic air more effectively, lowering temperatures and increasing the likelihood of winter precipitation.
  4. Enhanced Frontal Boundaries:
    • A rapidly strengthening Gulf low often interacts with existing cold fronts or Arctic air masses.
    • This enhances the contrast between warm, moist Gulf air and cold, dry air from the north, leading to sharp frontal boundaries and intense weather conditions (e.g., snow squalls or freezing rain).
  5. Track of the System:
    • The track of the bombogenesis low is critical:
      • To the South and East of Alabama: A colder setup, with snow or sleet more likely.
      • Closer to the State or Over the Gulf Coast: Warm air intrusions aloft can create freezing rain or sleet.
      • Too Far North: A rain event is more likely as warm air dominates.


Key Atmospheric Indicators:​


  1. Pressure Drop Over the Gulf: Watch for rapid pressure falls in the GOM. A sharp drop signals potential bombogenesis.
  2. Jet Stream Interaction: A strong upper-level trough or jet streak over the Gulf enhances divergence aloft, aiding the rapid deepening of the surface low.
  3. Temperature Gradients: A strong thermal gradient (baroclinic zone) between the Arctic air to the north and Gulf warmth provides the energy for bombogenesis.
  4. Surface Observations: Increased wind speeds, convergence, and cloud cover in the Gulf region often precede rapid low development.


Potential Impacts in Alabama:​


  1. Heavy Precipitation: Bombogenesis increases moisture transport, leading to potential snowfall totals or ice accumulations if the cold air holds.
  2. Strong Winds: Enhanced pressure gradients can bring damaging winds, even if precipitation is limited.
  3. Temperature Plunge: Rapid advection of Arctic air behind the system can result in a quick freeze, causing hazardous conditions.
  4. Sharp Transition Zones: The interaction between warm Gulf air and Arctic cold often creates a narrow zone of mixed precipitation (rain, sleet, freezing rain, snow).


Example Scenario:​


  1. Setup: A low-pressure system forms in the western GOM and begins intensifying as it moves northeast, undergoing bombogenesis.
  2. Impact: Alabama sees:
    • Arctic air drawn in from the north.
    • A sharp temperature gradient, producing a wintry mix in central Alabama and snow in northern areas.
    • Enhanced wind speeds due to the steep pressure gradient, increasing the risk of power outages.
Click to expand...

Now - do we have any factors in play that would aid bombogenesis with this low? Yes - very warm SSTs in the GOM for winter.

How Warm Gulf Waters Influence Bombogenesis​

  1. Increased Heat and Moisture Supply:
    • Warmer Gulf waters mean higher sea surface temperatures (SSTs). These temperatures increase the evaporation rate, putting more water vapor into the atmosphere.
    • This additional moisture is a key ingredient for bombogenesis, as it fuels the storm's development by enhancing latent heat release during condensation.
  2. Stronger Baroclinic Zone (Temperature Gradient):
    • When warm, moist Gulf air interacts with colder Arctic air to the north, the resulting temperature gradient strengthens. This intensifies the baroclinic zone, a region where bombogenesis is most likely to occur.
    • A stronger gradient amplifies the storm’s dynamics, making the low-pressure system deepen more rapidly.
  3. Enhanced Atmospheric Instability:
    • Warmer Gulf waters contribute to greater instability by increasing the difference between the warm, moist lower atmosphere and cooler upper-level air.
    • This instability promotes stronger rising motion (convection) and aids in the rapid deepening of the storm.
  4. Stronger Warm Conveyor Belt:
    • A warm GOM feeds the storm’s warm conveyor belt, a stream of warm, moist air rising ahead of the system. This conveyor belt enhances precipitation and fuels the storm's intensification.
  5. Jet Stream Interactions:
    • Warm Gulf waters can help "anchor" the development of a low-pressure system under a favorable jet stream configuration (e.g., a trough over the southern U.S.).
    • The additional warmth creates a feedback loop: enhanced surface heating encourages rising air, which further intensifies the low when the jet stream evacuates air aloft.
Click to expand...

Now don't take this as a prediction that we're going to get a bomb cyclone in the gulf out of this, but understand it's something to watch for that if it occurs would have a huge impact and that there is some atmospheric support for with this system. The 1993 storm was a bomb cyclone in the gulf. I typically think of that storm as Voldemort and do not speak much of it, but this particular setup is similar in potential low placement and the potential impact if it did is so high I believe it warrants the mention.
 
Blountwolf said:
Let's talk about bombogenesis. Any time there is a gulf low, there's also the possibility that the cyclone will "bomb out". Much like RI in a hurricane, "bombing out" means a rapid drop in pressure.


Now - do we have any factors in play that would aid bombogenesis with this low? Yes - very warm SSTs in the GOM for winter.



Now don't take this as a prediction that we're going to get a bomb cyclone in the gulf out of this, but understand it's something to watch for that if it occurs would have a huge impact and that there is some atmospheric support for with this system. The 1993 storm was a bomb cyclone in the gulf. I typically think of that storm as Voldemort and do not speak much of it, but this particular setup is similar in potential low placement and the potential impact if it did is so high I believe it warrants the mention.
Click to expand...
The upper air pattern isnt quite the same or as explosive as it was in 93...ther were actually 3 shortwaves that phased. Here is only two with some additional energy feeding but the tilt is coming much earlier...so pressure falls aren't as quick.
 
Richardjacks said:
The upper air pattern isnt quite the same or as explosive as it was in 93...ther were actually 3 shortwaves that phased. Here is only two with some additional energy feeding but the tilt is coming much earlier...so pressure falls aren't as quick.
Click to expand...
True, but GOM SSTs... sure don't look like your typical January either.
 
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