At first glance, the enhanced water vapor loop below would probably have snow lovers in the northern Mid-Atlantic and the Northeast jumping for joy. An active subtropical jet stream streaming across central and southern Mexico is acting like a fire hose and injecting a tremendous amount of moisture and energy into a developing storm moving northeast over south Texas. Normally, in the winter, that would mean a serious winter storm threat for those in our area. But unfortunately, there are several factors stacked against those north of the mason dixon line for significant, if any snow.
Let’s look at this morning’s upper level map from the 6z NAM model run:
There are 5 features that are important with regards to the behavior and development of this storm (or non-storm):
- The initial southern jet stream shortwave energy responsible for the storm itself, now over southern Texas. This area is easily seen on the water vapor (WV) loop above.
- A pretty potent shortwave over the Great Lakes, also well seen on the WV loop.
- A small focus of energy from the northern jet stream over Nebraska. This is a little harder to see, but is the small swirling area over southwest Nebraska with a darker green center.
- Energy associated with a large low pressure system over the Pacific northwest, which is only partially visualized on the WV loop above, but not hard to pick out.
- An area of confluence, represented by the two red lines on the upper level map, and the streak of blue on the WV loop above. Confluence just refers to winds that come together or meet up. You can see on the upper level map that the lines (representing upper level winds) that bend up over central Canada come down and meet up with the lines bending down over Texas in the Midwest region. This is important because confluence is not a good thing for promoting precipitation. The sinking air that results chokes off precip, and in some cases it can be pretty abrupt.
The upper level map above corresponds to the surface map below:
And this morning’s radar, which is pretty impressive by the way:
You can see above that the area from the central plains to Ohio is devoid of precipitation – that’s the result of the confluence I talked about above. The precip shield is hitting a brick wall from central Kansas to central Missouri. This is very important with regards to our weather tomorrow.
Now back to those five feature describe above:
The map above is the forecasted upper level pattern for tomorrow afternoon around 2 pm. I know it’s a little busy, but I’ve tried to simplify it as best I could. By this time, the energy from Texas on the previous map (#1) is trying to combine or phase with the energy that was over Nebraska (#3). This causes a deeper trough, or U shaped structure, over the southeast US, and rising heights, or upper level winds to flow southwest to northeast, along the east coast. Normally, this would carry our developing low pressure area straight towards us. But there are two problems. First, the energy over the Great Lakes on the previous map (#2) has now become incorporated into a 50/50 low (so named because of its position at 50 north, 50 west). This (A) prevents our storm from climbing up the entire eastern seaboard, and (B) creates confluence right in our backyard. Think of it like two opposing magnets – they bounce off one another.
The low is only able to gain a certain degree of latitude, and then is forced off the coast. The second problem is the energy in the Pacific Northwest we talked about earlier – it’s racing in behind our storm. The low over the southeast climbs to a position to northern Georgia, hits the resistance from the 50/50 low, and is pushed out by the energy to our west, or kicker, due east, off the southeast North Carolina coast.
So, in the end, we’re left with some clouds and not much else, while eastern Kentucky, West Virginia, and a large part of Virginia see significant snows.
Can this change? Perhaps. If the energy over the Great Lakes moves out faster, the 50/50 low is weaker or further east, or the kicker system is a little slower, this could bring the precip shield up into our area enough for a few inches of snow. The 12z NAM just came in a little further north, and brings some light snows into the southern 1/3 of PA. I’ll be watching the next few model runs through this evening and will let you know if there are any major changes.