Why did the temperature go UP last night?

Something interesting happened to the temperatures along the Blue Ridge Mountains this morning. CHO was reporting a temperature of 61 at 3 AM after being at 50 at 9 PM the previous night. The temperature did drop back in to the mid 50s by 7 AM, but that’s not anywhere close to the mid to upper 30s we were forecasting. I mean, some of us did (as shown below), but a good majority of us were in the 50s, close to 60, at 5 AM. So, what went wrong!?

Well, we had a wind from the southwest much of the overnight. This is a traditionally warmer wind for us and is part of the reason we stayed on the warmer side of our forecast. The other part is we had a downslope wind.

Before we talk about a downslope wind, first let’s talk about the geography of Central Virginia. We have the Blue Ridge Mountains in our backyard and are home to the Blue Ridge Parkway, Skyline Drive and Shenandoah National Park. A lesser known mountain chain, though, is the Southwest Mountains, which extends from Nelson County, through eastern Albemarle County and western Orange County. You can drive along this chain on Route 231 (Gordonsville Rd.) from Route 250 to US 33.

Charlottesville is in a unique position being between these two mountain chains; it’s part of the reason why cold air damming (CAD) is so tough to scour out (I’ll elaborate on that in a different blog post) and why temperature forecasts can be blown.

I mentioned the blown temperature forecast from last night (Thursday, November 10^th) being due to the downslope winds. So, what are downslope winds? Here ya go:

Meteorologists break down the atmosphere in to parcels of air. As one of these parcels of air, think of a square box, rise over the mountains, the pressure above it lessens. According to the ideal gas law (PV=nRT), that means the temperature (T) must cool when the pressure (P) decreases thanks to the volume (V), number of moles (n), and universal gas constant (R) all remaining constant. The inverse is true when a parcel of air goes down a mountain; the pressure above the parcel becomes greater which means the temperature must increase. Here’s a rough schematic of what I’m talking about: http://www.weather.gov/images/fsd/news/ridge2.png

 

Another thing to consider is the temperature inside that parcel of air cooling as it goes up the mountains; the temperature gets closer to the dew point which then produces clouds. However, the temperature warms as it goes down the mountain (inverse relationship), meaning there is a wider gap between the temperature and dew point (dew point depression in meteorological terms) as the air dries out after having a good bit of moisture wrung out of it from the parcel’s trip up the mountain. The larger the dew point depression is the less humidity is inside the parcel of air, which means clouds evaporate and the air dries out. Need some evidence of this? Check out the dew point drop from 9 PM last night through the morning hours and how it goes back up by 9 AM:

We see this all the time along the Appalachian Mountains. A strong northwesterly flow across the Great Lakes runs in to the higher peaks of the mountain chain. As soon as these parcels of air are lifted up along the mountains, more clouds form; some of these clouds could also produce showers or snow depending on the temperature. These clouds then evaporate as they go down the mountain and are the reason why the Appalachian Mountains could be cloudy, but we are sunny here across Central Virginia.

Leave any questions or comments below!
David Reese

November 4th Evening Forecast

July 11th Evening Forecast Video

A few more clouds roll in from the south in the overnight as a warm front slowly pushes north. This warm front will bring back the humidity, which also ups our rain chances the next couple of days. Temperatures Tuesday morning will be in the mid to upper 60s with afternoon highs in the mid to upper 80s. A few showers are possible in the afternoon, especially along the mountain tops and points just east and west of there.

A few more showers and thunderstorms are expected area wide Wednesday as a piece of energy rolls our way. This piece of energy will also bring more clouds along with it, thus keeping temperatures on the cooler side as we expect highs in the mid to upper 80s once again. High pressure builds in Thursday afternoon, bringing the heat back with it. What do we mean by heat? Well, we’re looking at forecast highs back in the low to mid 90s. There will also be a small chance for rain Thursday afternoon. In other words, this will be very similar to what we experienced most of last week. Friday will be a touch drier as some mid-level dry air arrives from the west. We’ll then bump up rain chances as another disturbance rolls our way this weekend. Yes, the weekend will have better shower and thunderstorm chances. The silver lining, though, is that we’ll see temperatures in the upper 80s to near 90. Which, isn’t all that bad considering where we could be this time of year?

Tonight: Clear to partly cloudy. Lows near 70.
Tuesday: Partly sunny with isolated afternoon showers and storms. Highs in the upper 80s.
Wednesday: A mix of sun and clouds with afternoon showers and storms. Highs in the mid to upper 80s.
Thursday: Hot and humid with isolated afternoon showers and storms. Highs in the low to mid 90s.
Friday: Partly cloudy. Highs near 90.
Saturday: Partly cloudy with afternoon showers and storms. Highs near 90.
Sunday: Partly cloudy with a chance for rain. Highs in the upper 80s.
Monday: Partly cloudy. Highs in the upper 80s.

Have a great Monday night!
Meteorologist David Reese

June 27th Evening Forecast Video

Slow moving showers and storms will continue to push through the region tonight. The slow motion of these storms could produce locally heavy rain, meaning localized flooding and flash flooding are possible later tonight through the early morning hours of your Tuesday. A few lingering showers and storms are possible Tuesday afternoon as a cold front eases through the region. Temperatures ahead of the cold front will top out in the mid to upper 80s. A few locations could push that 90° mark.

Wednesday will be mostly sunny with forecast highs in the mid 80s, which is average for this time of year. We’ll then end the month of June with a chance for a few stray showers and storms Thursday afternoon as highs top out in the low to mid 80s. Another cold front heads our way for Friday and Saturday, increasing the rain chances and coverage around Central Virginia. This cold front then settles to the south of us for Sunday and the 4th of July. The ultimate location where the cold front stalls will greatly dictate who sees rain for the 4th and who sees drier, more average conditions for the first part of July. This will definitely be something we keep an eye on as we get closer to Patriotism in the Park next Monday evening.

Tonight: Mostly cloudy with showers slowly moving out. Lows in the upper 60s.
Tuesday: Partly cloudy with isolated afternoon thunderstorms. Highs in the upper 80s.
Wednesday: Mostly sunny. Highs in the mid 80s.
Thursday: Partly cloudy with stray afternoon showers/storms. Highs in the low to mid 80s.
Friday: Partly cloudy with the chance for a few showers and storms in the afternoon. Highs in the mid 80s.
Saturday: Partly to mostly cloudy with scattered showers and storms. Highs in the middle 80s.
Sunday: Partly cloudy with showers and storms. Highs in the mid 80s.
Independence Day: Partly cloudy. Small chance of showers/storms south of Charlottesville. Highs in the mid 80s.

Have a fantastic Monday evening!
Meteorologist David Reese

June 25th 10 PM Weathercast

Temperatures will tumble in to the mid 60s early Saturday morning thanks to a cold front easing through the region. This cold front will help scour out the cloud cover as we go throughout the day Saturday. Some late-day sunshine will allow temperatures to top out in the low to mid 80s around the area. There will also be low humidity greeting us this weekend as well!

More sunshine is in the works Sunday afternoon with highs back in the mid to upper 80s but with low humidity. A southerly wind develops later Sunday night through Monday afternoon in advance of a cold front. This cold front will help spark off a few more showers and thunderstorms Monday afternoon with forecast highs topping out in the upper 80s to near 90 degrees. It remains to be seen, but a few of the thunderstorms Monday afternoon could be on the stronger side, so we’ll fine-tune that forecast over the weekend. A couple more showers are possible Tuesday afternoon as the cold front is slow to clear the region, but I think most will stay dry Tuesday. A dry day is in the work Wednesday and most of Thursday before another storm system provides us with the chance for another round of showers and storms to close out June and kick-off July.

Tonight: Gradual clearing. Lows in the mid 60s.
Saturday: Partly cloudy. Highs in the low to mid 80s.
Sunday: Plenty of sunshine. Highs in the mid 80s.
Monday: Partly cloudy with the chance for afternoon showers and thunderstorms. Highs in the upper 80s.
Tuesday: Partly cloudy with a stray afternoon shower or thunderstorm. Highs in the upper 80s.
Wednesday: Partly cloudy. Highs in the mid 80s.
Thursday: Partly to mostly cloudy with showers and storms possible. Highs in the low to mid 80s.
Friday: Partly cloudy with the chance for a few showers and storms in the afternoon. Highs in the mid to upper 80s.

Have a great weekend!
Meteorologist David Reese

June 22nd 6 PM Weather

A cluster of showers and storms will develop out west tonight and surge east in the overnight. We expect the batch of storms to be over us between 4-8 AM and to be severe limits. That said, some gusty wind, heavy rain, and lightning are expected during that time-frame. We’ll then try to break out in to a bit of sunshine around lunchtime. This is *VERY* important, because the more sunshine we see, the better the chance will be for strong to severe thunderstorms across Central Virginia. If that sunshine materializes, and temperatures get in to the mid to upper 80s, we could be talking about storm dropping large hail, damaging wind gusts and an isolated tornado. Yes, all modes of severe weather are possible tomorrow afternoon *IF* we see lots of sunshine. That second round of storms will be between 3-8 PM before diving south and east.

A few more showers are possible Friday morning before we finally start to see a lot more sunshine and lower humidity in the afternoon. The lower humidity and pleasant weather conditions will hold throughout the weekend with forecast highs in the mid 80s. Another storm system arrives early next work week and provides us with another chance for showers and storms next Monday and Tuesday.

June 13th 6 PM Weathercast

We kicked off the work week with a beautiful day. Highs topped out in the low to mid 80s under a ton of sunshine and low humidity. We'll slowly increase the heat and humidity the next few days, allowing for pop-up showers and thunderstorms starting Wednesday.

June 11th Forecast Video

The big story this weekend is the heat. Yes, we are looking at highs back in the low to mid 90s for the first time this year. Oh, and if we get warmer than 93° at CHO today or tomorrow, it would be the warmest we've been since 2014!

June 9, 2016 Forecast Video