Monday, February 28, 2011

Mapping Out The Next Several Hours

Tornado Watch Cancelled

The Tornado Watch has now been cancelled. Showers and occasional downpours will persist for the next few hours across the entire state. The threat for severe storms is slowly shifting down into Southeast Virginia and Northeast North Carolina, where the air is more unstable. You can spot the unstable air on the map below - indicated by the red circles.

Regardless, isolated thunderstorms are still possible for Central and South Central Virginia for the next couple hours. We will be tracking radar very closely into the evening and will bring you live updates in our Storm Room.

Sunday, February 27, 2011

Severe Thunderstorms Possible Monday

A line of thunderstorms will race across the area from 3 PM to 10 PM on Monday. The strongest storms will be capable of damaging straight line winds and small hail.

Temperatures will soar into the upper 70s to near 80 on Monday. This heat will provide *just* enough fuel for thunderstorms later in the afternoon and evening.

Storms will initially start west of the Blue Ridge around midday. They should form into a squall line and then race our way. The leading edge of the storms will be moving into western parts of our area around 3 PM.

The atmosphere will only be marginally unstable once the storms arrive, so we could see some weakening of the squall. Nonetheless, any storms that survive the trek across the Blue Ridge will be capable of damaging wind gusts over 70 mph. There could also be some small hail with the strongest storms. Most of the rain will quickly taper after midnight.

Limiting Factors (technical):
Just like last time, we will have a highly sheared environment with only marginal instability. However, upper-level "lift" could overcome this obstacle.

CAPE, or energy in the atmosphere, will be somewhat limited. Here is a plot for tomorrow evening, which shows almost 1000 CAPE west of I-95 (not too impressive). In addition, CAPE is much less in central VA. This could lead to some weakening of storms, especially after sunset. The key will be sunshine...if we see more, then the air could become more unstable.

Shear is going to be very high. The upper level winds will be howling! This will add to the potential of some powerful downbursts. It could also help to maintain the strength of the squall line. On the flip side, it could also rip apart storms as they try to develop. This is something to watch closely tomorrow - shear vs instability.

Friday, February 25, 2011

Explanation Of Today's Winds

Here is a list of some of the highest gusts from Friday. You can find a full list by clicking on the following links:

Winds are driven by differences in pressure. Today we had a powerful low pressure parked off the coast of Boston. At the same time, a strong area of high pressure settled into the Upper Midwest. The entire Mid-Atlantic region was stuck in the middle of this clash.

Here are the gusts from 4 PM, when we experienced our strongest winds. Around this time our Sky Tracker camera in downtown Richmond recorded a gust of 62 mph! These winds extended all the way north into Pennsylvania and down south into North Carolina.

Our winds will quickly subside tonight as the core of the high pressure nudges closer to our area and the low tracks into the North Atlantic.

Strong Winds, Rain, & T-Storms Friday

Strong winds are occurring today in the Commonwealth as a potent low pressure storm system sweeps through our region. High Wind Warnings (yellow shade on map) and Wind Advisories (tan shade on map) are posted through 8PM in the area. Sustained winds most of the day will remain in the 25-35 mph range!

Not only will it be windy today, but after our first round of morning rain we could get thunderstorms, some of which could be severe from mid-day through 4PM (damaging straight-line winds are the primary threat). There is a slight risk area today in yellow for part of central Virginia (highlighted in yellow below), where isolated, brief tornadoes may also occur in addition to the straight-line damaging wind threat that exists for most of central Virginia.

Here's a timeline of both the rain and the winds for the rest of today:

And here are the expected sustained winds at that same time (higher gusts will be occurring):

As the cold front nears the mountains in western Virginia, winds will be at their strongest from the southwest, bringing in a surge of moisture and warmth from the south to serve as fuel for thunderstorm development. In addition, the surface cold front will serve as a forcing mechanism for thunderstorms to develop, especially as we get breaks in the clouds mid-day into early this afternoon allowing for some heating and instability to form.

Winds mid-day will be sustained at 25-35 mph, with gusts as high as 45 mph.

Severe threat will continue through mid-afternoon as the strong cold front rushes through central Virginia. The primary threat out of any storms that can form ahead of the front will be damaging wind gusts and downbursts (70mph+).

As the front passes through the heart of central Virginia mid-afternoon, winds will still be running 25-35 mph with gusts to 45-50 mph:

Our severe threat will end by 4PM as the front reaches southeast Virginia, forcing the moisture and instability out to sea. Much cooler, drier air will surge into the region behind the front, and you will notice the change in humidity and temperatures this evening.

Even after the cold front passes, it will take until closer to midnight for winds to subside below 10 mph. If you have evening plans, these are your expected sustained winds at 7PM:

It looks like we could get another quarter of an inch in central Virginia today from the rounds of rain, but those of you who get the possible t-storms, you could get locally higher amounts of rainfall! We will be closely monitoring the severe threat today and provide updates both on air and on-line. Stay with CBS 6, we'll keep you ahead of the storm. --Carrie Rose--

Thursday, February 24, 2011

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.

Wednesday, February 23, 2011

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

Sunday, February 20, 2011

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.

Saturday, February 19, 2011

Highest Gusts From Saturday

Wintergreen saw a gust of 71 mph...that's almost hurricane force winds! Here is a list of highest gusts from other areas across the state. You'll notice that winds were not as strong in southern Virginia.

Wind Forecast For Tonight

Here is the forecast for sustained winds the rest of tonight. Gusts could easily exceed sustained winds by about 10 to 15 mph, especially from 8 PM to midnight. Conditions will quickly improve in central Virginia from midnight to 4 AM, though it will remain breezy along the Chesapeake Bay.

Friday, February 18, 2011

Fire Danger On Saturday

A front will sweep through the area early tonight and drag in extremely dry air for Saturday. In addition, we will see strong winds from the west/northwest sustained at 25 to 35 mph with gusts up to 45 mph. For these reasons, a Red Flag Warning has been issued from 9 AM to 9 PM on Saturday for high fire danger. Winds will eventually diminish by Saturday night.

Extreme Fire Danger Saturday

A dry cold front will pass through Virginia overnight tonight, ushering in extremely dry air and whipping winds up from the northwest on Saturday at 20-30 mph with gusts to 40 mph. Even though the front will allow cooler air to begin moving into the Mid-Atlantic, relative humidity values will still drop Saturday afternoon into the 10%-20% range in the Commonwealth. These factors will create an extreme fire danger threat Saturday, and there is a Fire Weather Watch in effect for tomorrow. This watch will likely be upgraded to a Red Flag Warning later.

More than 65% of the Commonwealth is now in a moderate drought, with the rest of the state abnormally dry (except most of the Eastern Shore).

We will be monitoring the fire danger today (which is high) and especially Saturday (critical) in central Virginia, and provide updates on any fires that start and spread out of control. You are advised to postpone any outdoor burning until after the weekend. We will have a shot at some rain on Monday and Tuesday next week, and it will be cooler by the middle of next week. Even so, outdoor burning is still banned before 4 p.m. each day in Virginia.

Thursday, February 17, 2011

Drought Conditions Worsen in Virginia

As we continue to fall behind for our year-to-date precipitation this month, drought conditions are expanding eastward in central Virginia. Now everywhere along I-95 is included in Moderate Drought, the first level of drought. 65% of the Commonwealth is now officially considered to be in a drought, up from 57% percent at the report a week ago.

Here is how Richmond compares for our year-to-date precipitation with the normal and last year:

Also, the eastern Peninsulas are now considered to be "Abnormally Dry" after another week without any significant precipitation. This does not bode well for the continued high fire danger expected through Saturday in the state. A dry cold front will pass through the Commonwealth overnight Friday into early Saturday, generating gusty winds around 35 mph (both Friday and Saturday afternoons) and reinforcing the dry air mass. The worst day for fire danger should be Saturday, when very low relative humidity will combine with strong west-northwesterly winds. These are the three critical fire danger factors that are a result of the weather, and that we pay close attention to during a day that meets these three criteria:

Tuesday, February 15, 2011

Three-Day Warm-Up Starts Wednesday

We began the work week with highs in the low 70s, but a dip in the jetstream and an associated low pressure system, allowed a strong cold front to cool us down on Tuesday. The graphic below indicates the upper-level features Tuesday evening, with troughs in the eastern and western U.S., and a flat ridge across the Plains states.

A broad ridge will build into the southern and southeastern U.S. by the end of the week, allowing much warmer air to move back into the Mid-Atlantic. The graphic below indicates how the upper features will look on Friday. Highs will reach the mid-70s in central Virginia on Friday under this ridge. Enjoy!

Monday, February 14, 2011

High Fire Danger Monday

A low pressure system tracking through the northern Great Lakes along the Canadian border will bring a dry cold front through the Commonwealth later this afternoon and early evening. As this front approaches, winds will pick up from the west and be quite gusty. We will also be unseasonably warm today with highs in the 60s, and with dry air in the region, this will create low relative humidity values this afternoon in the 20%-30% range. All of these factors combined will result in high fire danger today, especially between Noon and 5 p.m. Therefore, there is a Red Flag Warning in effect today for nearly all of central Virginia.

The highest gusts should occur in the Piedmont and higher terrain of western Virginia, especially along ridge-tops. If you're driving in a high-profile vehicle today, particularly along I-81, be prepared to battle some tough cross-winds!
Part of the reason why we have this high fire danger is because we're running below-normal for year-to-date precipitation:

The last times we had good soaking rains were on January 26th and on December 12th. It's no wonder we're a little dry! Here's the latest drought monitor for Virginia, which shows 57% of the state is in Moderate Drought:

Notice how the eastern Peninsulas of Virginia are doing okay for precipitation? That's partly why they are excluded from the Red Flag Warning today. No rain chances are on the horizon until next week, when our pattern should shift to a more active one conducive for storm systems and moisture to make it into the Commonwealth. Let's hope we pick up some much-needed rainfall next week!