Update On Friday's Severe Weather Threat

A front will sweep through the area midday Friday and bring us a brief window of opportunity for severe thunderstorms. This will occur between 11 AM to 6 PM from west to east. Any storms that develop will be capable of damaging wind gusts over 70 mph.

Technical Discussion

There are three key ingredients for severe thunderstorms: instability (heat and humidity), atmospheric lift (or a trigger), and wind shear. I'll discuss all three below.

Here is a map for tomorrow at 1 PM, when a front will be moving into the Blue Ridge. This front will provide the "lift" in the atmosphere. Notice the wind arrows on the map, which are strong from the west/southwest at 25 to 35 mph with gusts up to 45 mph. This should bring in moisture at the surface, indicated by the light shades of green.


There should be *just* enough low level moisture to produce minor instability in the atmosphere. Below is an image for tomorrow at 1 PM, which shows low instability. Sunshine could add some heat to the atmosphere, which would increase our instability.


An upper-level disturbance will be passing the area during this time, which will also produce "lift" in the atmosphere. In addition, jet stream winds will be very strong in the upper atmosphere. Notice the reds and purples on the map below, which indicate winds from the southwest at around 100 to 115 mph. This is a prime example of speed shear, or winds that increase with height. Although this is a key ingredient for severe thunderstorms, the shear might be too high for storms to develop.


To Summarize:

1) We will have strong atmospheric lift (surface front, upper-level disturbance)
2) Instability is low, though increased sunshine could help
3) Wind shear is very high...almost too high

We will be watching this very closely tomorrow. Any storms that develop will be capable of powerful downbursts in excess of 70 mph.

Threat for Strong to Severe Storms Friday

A powerful storm system currently located across the far southwestern U.S. will bring central VA a chance for strong to severe thunderstorms on Friday. It appears as though the best combination of wind shear, moisture, and lift will be across the Mississippi Valley on Thursday, where an outbreak of severe weather is likely. Our atmospheric parameters won't be quite as conducive for severe weather in central VA, but modest heating and very strong wind shear will interact with fairly meager amounts of low-level moisture to give us a slight chance for a few severe storms. The lack of rich low-level moisture will be the limiting factor in what otherwise might be a better event for us. Here's how I think things will play out with this system: We'll see increasing clouds on Thursday, with only a slight chance for a few showers before sunset. Showers will increase in coverage Thursday night into Friday morning, but I don't think we'll have the lift and moisture in place quite yet for thunderstorms during the overnight period. Showery weather will taper off in the morning, with a period of sunshine developing. Temperatures will warm into the upper 60s and low 70s as strong southwesterly winds bring more moisture into the area Friday afternoon. The cold front will move through Richmond at 4 PM, likely accompanied by a broken line of showers and storms. A few storms could be strong to severe, especially east of I-95. All of the activity that develops will be east of the area by 7 PM. Cloud bases will be high, so despite strong wind shear, tornadoes don't appear very likely. Strong mid-level forcing and cold air aloft will result in a hail threat, with strong downburst winds possible as well. Whatever happens, Stay With CBS 6, We'll Keep You Ahead of the Storm. ZD

Long Range Outlook

Brief Summary:

La Nina will continue to weaken heading into spring. NAO (North Atlantic Oscillation) will generally remain neutral or positive. Although we will see a brief cool down for the start of March, temperatures will remain roughly near or above average. Precipitation will stay near or below average. The chances for a significant snow (over 6") look slim to none.

Digging into the data (technical):

Meteorological winter is about to end (December through February), so it is now time to look at the overall pattern heading into spring. Earlier this winter, we had a strong La Nina, which typically brings mild and dry weather to our region. However, we experienced a strong -NAO (North Atlantic Oscillation), which is when a strong ridge in the jet stream persists over western Greenland and eastern Canada. This brought us cold and snowy weather for December and early January. This ridge eroded in late January, and we have been mild and dry ever since.

La Nina is starting to quickly weaken. In fact, we could be in neutral conditions by May or June. This is a plot of sea surface temperatures, which shows that Nina still exists, but it isn't as strong as earlier this winter. In addition, what you can't see is the water below, which is starting to warm.

Past La Ninas have brought us slightly cooler and wetter weather for March through May. However, this might not be the case this year because La Nina is weakening.

Here is a plot of NAO, which shows negative conditions early in the winter, and then neutral/positive conditions ever since mid January.


There are indications that NAO could turn slightly negative in early March, which would typically get any snow-lover excited. However, the ridge is too far east. I've posted the long range outlook for the upper-level atmosphere (first map), and temperatures for the lower atmosphere (second map). Notice on the first map how the ridge, indicated by the yellow blob, is located east of Greenland. Although this could bring us slightly below average temperatures, it is not in an ideal location for significant snow. Average highs for early March are in the mid 50s and average lows are in the mid 30s. We should generally stay near these numbers for early March. Of course, there will be some fluctuations.

Obviously, there could be some changes to the forecast, however, this isn't looking good for snow-lovers.