Tuesday, March 29, 2011

A Rainy Forecast Is A Good Thing

Although rain can be disruptive to our day-to-day lives, we desperately need some in our area. You can see on the graph below that we are 1.7" below normal for the year.

We aren't the only ones dealing with dry conditions. You can see on the map below that most of the Southeast is 2 to 8" below normal for rainfall this year.

Not surprisingly, numerous rivers and streams are now running below normal. The ones to the west are near normal from the heavy rains we saw earlier this month.

Right now, moderate drought extends across central and southern Virginia.

Luckily, we will see an active weather pattern over the next several days. In fact, two back-to-back storm systems could bring 1 to 2" of rain from Wednesday into Thursday. This is great news!

Sunday, March 27, 2011

Snow Possible Early Monday

Weather Update - Last Call

It is 6:30 Sunday morning and it is currently raining and sleeting in Richmond with temperatures at 35 degrees. Here is the timeline for the rest of today:

8 to 11 AM
Wintry mix of rain, sleet and wet snow. This will be the best chance to see big wet snow flakes. Temperatures will hover in the mid 30s and lower 30s across the entire area. The magic number is 35 degrees, which is usually the rain/snow line in these types of situations.

11 AM to 2 PM
Precip will quickly taper across the entire area. Temperatures will warm into the low 40s, which will melt most if not all of the snow. I’m expecting 1” or less across most of our area (click here for my latest accumulation forecast), with all snow sticking only on grassy surfaces and other things detached from the ground. Roads will remain wet but drivable.

Saturday, March 26, 2011

Weather Update - Saturday Afternoon

This is my latest forecast for the area. Again, there are no major changes. One thing to note - a lot of the snow will melt in the afternoon, so these numbers can be deceiving. Also, most of the accumulation will remain on grassy surfaces and other things detached from the ground. Temperatures will drop below freezing Monday night, though ground temperatures will remain well above freezing. Most of the major roads will remain wet, while untreated bridges and overpasses could become icy or dangerous.

Midnight to 4 AM
Rain with a few ice pellets will overspread central VA. It will be snowing in northwest VA.

4 AM to 10 AM
This will be the peak of the storm. Central VA will see cold rain and wet snow with a few ice pellets mixed in. It will be all snow for the northern half of the state.

10 AM to 2 PM
Precip will slowly taper - mix in central VA, rain farther south.

Here are several things to consider before this storm hits tonight and early tomorrow:

1) Duration of event

This storm system will race past the area, which will shrink the window of opportunity to see an accumulating snow. In fact, the best chances for Richmond to see an accumulating snow will be during the peak of the storm from 5 AM to 9 AM.

2) Intensity of precip

Most of the precip will be light to moderate, though we could see an occasional burst of rain or snow. This will also impact our accumulations.

3) Surface temperatures

Temperatures will hover around the freezing point in central VA during the peak of the storm (upper 20s in northern VA and mid 30s southern VA). Remember, the magic number is typically 35 for the rain/snow line. Most of the precip will be gone by early afternoon, so we could see temps rise to near 40, which will melt a lot of the snow.

4) Ground temperatures

Ground temps will remain well above freezing for the entire event. This means that most accumulations will stay on things detached from the ground: grassy surfaces, cars, rooftops. Ground temps will stay well above freezing Sunday night, so most roads should remain wet. However, bridges and overpasses could quickly become icy/dangerous.

Winter Weather Update Saturday Morning

A Winter Weather Advisory is in effect for western parts of Virginia for this evening through Sunday afternoon for a developing storm system in the Southeast US that will expand a winter mix of precipitation into the Commonwealth tonight into Sunday. Snowfall of 1"-3" is expected in the purple highlighted counties, but elevations higher than 2000 feet can expect snowfall accumulations as high as 4"-5" by Sunday evening.

Much of the rest of central Virginia will experience a winter mix of wet snow and rain Sunday, too. Precipitation will expand into central Virginia from the southwest late Saturday night into early Sunday morning. Most temperatures will still be in the upper 30s to around 40 degrees with the onset of wet snow and cold rain. These air temperatures combined with mild ground temperatures will inhibit a big snowfall accumulation.

There may be some sleet (ice pellets) that mix in with the snow and rain in the pink areas.

Temperatures will be in the 30s above freezing Sunday while the slushy snow is falling and melting at the ground, except for northern locations from around Tappahannock to Charlottesville and north through Fredericksburg.

That is where more snowfall could stick to the ground and elevated objects when temperatures there fall below freezing Sunday while the snow is still coming down. Snow and rain should taper off from northwest to southeast during the afternoon. Notice the temperatures in the afternoon are in the mid to upper 30s where the snow is still falling, so some of that will melt on its way to the ground.

Here is the potential accumulated snowfall for Sunday, with most of this sticking to the grass and elevated objects:

Notice that the Metro Richmond area is right on the 1" borderline for that wet snow. Our surface temperature will be very close to freezing for a few hours Sunday. If we can get to the freezing mark, that will help more of the snow to stick on the grass and objects off the ground.

Any wetness on pavement Sunday night that doesn't evaporate will freeze for Monday morning as low temperatures plummet into the mid to upper 20s. This will likely create black ice and slick spots on the roads for your Monday morning commute. But any snow that does stay on the ground into Monday morning will melt off quickly Monday afternoon with highs in the upper 40s and low 50s in central Virginia. Stay with CBS 6, we'll keep you ahead of the storm.

Friday, March 25, 2011

The Latest On This Weekend's Wintry Mix

I want to first start off by saying that it is not uncommon to see wintry weather this late in the year. In fact, check out some notable snowfalls I found while digging through climate data this afternoon.

The forecast hasn't changed much from earlier this morning. I have posted temperatures along with the precipitation forecast below (Rain=green, Mix=pink, Snow=blue). Most of the precip should taper by early Sunday evening.

You can see that temperatures will hover above freezing. In addition, ground temperatures are relatively warm. This will be a tough forecast to verify because a lot of the sleet (ice pellets) and snow will melt. Nonetheless, here is my early forecast for accumulations. I want to emphasize that this is not etched in stone and you should check back on Saturday for the very latest information.

Most of the accumulations will stay on things detached from the ground, however, there could be slick spots on the roads...especially to the north and west. We will continue to track this and have another update tomorrow morning!

Friday Morning Update on Weekend Winter Weather

Here is the latest data as of Friday morning on the rain and winter mix expected this weekend in central Virginia. This data will likely change some over the next model runs today and tomorrow.

Overnight Friday into pre-sunrise Saturday, a weak upper wave currently tracking through the Midwest and Great Lakes will pass overhead and could produce some rain showers that may also have a few sleet or wet snowflakes mixed in.

However, temperatures at and near the ground will be above freezing in the mid to upper 30s and low 40s when this precipitation falls, so nothing will stick and there will not be icy patches Saturday morning. The light rain and mix will exit south before sunrise Saturday.

The rest of Saturday looks mostly cloudy but dry, so if you have outdoor plans during the day, you should be good to go!

Rain will begin moving back into the Commonwealth late Saturday night, and by Sunday morning, there should be a wintry mix of precipitation occurring in the area. On the maps to follow, green is rain, pink is a mix (snow, sleet, rain), and blues/whites are snow:

Sunday does look like it will be a cold washout day with a winter mix likely.

However, surface temperatures look like they will remain above freezing and hinder much, if any, accumulation or ice development during the day Sunday.

As colder air aloft is pulled southward into the Commonwealth Sunday night, a changeover to all snow may occur from north to south. Yet with surface temperatures still above freezing, accumulation may only occur on the grass and elevated objects.

Precipitation should quickly taper off after Midnight:

The snow map below should be taken lightly as it will likely change. Also, I think this represents how much snow may fall while it's mixing in with the rain, but not this much snow will literally "accumulate" on the ground because the surface temperature will remain above freezing Sunday and Sunday evening. We shouldn't fall below freezing until after the precipitation is done shortly after Midnight early Monday morning.

There should be wetness on the pavement Monday morning, and with temperatures by then falling to and below freezing, we could certainly develop icy patches on pavement Monday morning.
Stay with CBS 6, we'll keep you ahead of the storm!

Wintry Mix This Weekend?

Over the past couple days, computer data has been hinting at the possibility of wintry weather this weekend. I just got done looking at the latest data this evening, and it continues to show this scenario.

A few rain showers could clip the area during the day Saturday, especially in southern VA. Precip will intensify Saturday night into early Sunday as an upper-level storm system approaches. During this time, central VA could see a mix of sleet (ice pellets) and rain. Northern VA could see a mix of wet snow and rain.

A lot can change from now until Sunday, so we will continue to track this system and bring you more updates over the next couple days.

Legend for the maps below: Rain (green/yellow), Mix (pink), Snow (blue).

Technical Discussion

The key to the forecast in central VA will be the mid-level temperatures. I've posted a Skew-T diagram below. This shows temperatures and humidity from the ground (bottom of the diagram) all the way up 50,000 feet (top of the diagram). I've marked the freezing line in red. You can see the temperature line (pink line) is very close to freezing at 6000 feet, then much below freezing at around 4000 feet. This is your typical sleet profile (snow melts at 6000 feet then refreezes at 4000 feet). This will be something to watch when the new data comes out tomorrow.

Wednesday, March 23, 2011

Strong Storms Likely Wednesday Evening

Strong to severe thunderstorms are likely in central Virginia beginning late this afternoon in northern and western Virginia and lasting into this evening as a low pressure storm system sweeps into the region and forces a strong cold front through the Commonwealth late tonight.

The first piece of the puzzle you'll notice today for our severe weather setup is the wind shift back to the southwest as yesterday's cold front lifts back northward through the Commonwealth as a warm front.

With a warmer, moist air-mass in place by late this afternoon in central Virginia, along with sufficient upper lift ahead of the approaching low pressure storm system, conditions will be ripe for storms to initiate during peak heating. You can see by 4PM, our first storms should be developing in the mountains and begin tracking southeast toward central Virginia:

The first round of storms could reach the Metro by 5PM at the earliest and affect us through 11PM. Some of those storms could be severe.

The strongest storms could become severe, capable of damaging winds in excess of 60 mph. Any of these thunderstorms could also have hail, frequent lightning, and heavy downpours.

As soon as the cold front passes your location, your severe threat will end. This front should move quickly through the Commonwealth tonight, clearing the Metro by Midnight.

Locations that get the strongest storms will also likely receive the most rainfall from the heaviest downpours, anywhere from a half an inch to near an inch. Stay with CBS6, we'll keep you ahead of the storm.
--Carrie Rose

Friday, March 18, 2011

From the Moon to the Sun: Cool Astronomy Events This Weekend!

We have a couple of interesting astronomical events this weekend, from the Moon to the Sun. The Full Moon is Saturday night, rising in the east in Richmond at 7:40 PM.
This will be a rare perigee full moon, meaning that the Moon is at its closest proximity to the earth in its elliptical orbit around us. As a result of literally being closer to the Earth and also being a Full Moon, it will be 14% bigger and 30% brighter than when the Moon is at apogee (farthest away from Earth in its elliptical orbit).

In addition, there is an optical illusion that occurs when the moon is at the horizon: it looks to our eyes to be huge, even though it really isn't (camera photos can prove this illusion by showing the moon is the same size at the horizon as when it's higher aloft)!

(Image above: Lloyd Kaufman & James H. Kaufman)
So this combination of literally being bigger and brighter plus merely appearing to be bigger should lead to a great Full Moon viewing Saturday at sunset. It looks like cloud-cover on Saturday should be moving out of the area by sunset, so hopefully we'll get to see this Full Moon on the eastern horizon as it rises.

Our second astronomical note for this weekend is the Vernal Equinox.
This is the official astronomical start of Spring on Sunday at 7:21 PM EDT.
This is the exact moment when the Sun's most direct rays are over the Earth's Equator. After this, the most direct rays continue to creep northward into the Northern Hemisphere, leading us into our Spring and Summer months until the Autumnal Equinox, when the most direct rays are back over the Equator on their trek southward into the Southern Hemisphere.