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Weather forecasts and information for all of Loudoun County, Virginia. Including Leesburg, Aldie, Waterford, Ashburn, Sterling, Dulles, Hamilton, Purcellville, South Riding. Loudoun Weather is your source for Loudoun County weather forecasts, weather radar and more.

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Category: Forecast

Loudoun Weather Outlook for May 27, 2024

Posted on May 27, 2024May 27, 2024 By admin No Comments on Loudoun Weather Outlook for May 27, 2024

We have a risk of isolated severe weather today, but the clouds should help keep the severe not as widespread. Despite this, there could still be an isolated storm with high winds, hail, and even an isolated tornado. I think that the greater risk is south and east of Loudoun. Showers and some thunderstorms do look likely, but most of these may hold off until this evening, which will make most of the day dry, though a few small showers could pass through in the next few hours.

PLEASE STAY WEATHER-AWARE TODAY

ALSO NOTE – The NOAA NWS Storm Prediction Center has already outlined the area for a likely Severe Watch in the region.

If we clear out and the sun helps to destabilize the atmosphere, the chances for severe weather will increase.

The week should get better as we go forward.

Forecast:

Memorial Day: 🌦️ Hi 84 Lo 61 – Scattered showers and thunderstorms will become likely later in the afternoon and evening. Mostly cloudy, becoming clear overnight.

Tuesday: 🌦️ Hi 81 Lo 55 – Slight chance of showers and thunderstorms in the afternoon. Sunny during the day, mostly clear at night.

Wednesday: ⛅ Hi 74 Lo 52 – Chance of showers, with possible thunderstorms in the afternoon. Partly sunny during the day, mostly cloudy at night.

Thursday: ☀️ Hi 71 Lo 49 – Sunny during the day, mostly clear at night.

Friday: ☀️ Hi 71 Lo 49 – Sunny during the day, mostly clear at night.

Saturday: 🌤️ Hi 76 Lo 53 – Sunny during the day, partly cloudy at night.

Sunday: 🌤️ Hi 79 Lo 55- Mostly sunny

The more extended range looks near average but will trend warmer again and wetter.

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Forecast

Winter 2023 – 2024 – PRELIMINARY

Posted on October 1, 2023October 1, 2023 By admin

Quick Summary

I said I would summarize and so here we go:

I think we will struggle again to keep it cold here in the Mid-Atlantic. The Oceans remain warm, and I seriously think they were a huge factor last year in messing up a few borderline events for the region. I think we have the same issue this year.

The biggest factor for this year, beyond the warm Atlantic Ocean, is El Nino (Warm Equatorial Pacific Ocean). This year looks to have a strong El Nino. NOW, there are types of El Nino’s and there are hints that this once could become better situated later in the season. This basically is a question of an east based verses a west based El Nino. There were signs that it would be east based, but I am seeing it forecast to become more west over time, and that could be great thing (for snow lovers)!

The Outlook

In general, my first outlook is super preliminary! So, I will do a better job after some additional data comes in.I am expecting average temperatures and average snow. That was a lot of words to get here. What does that mean exactly?

Our area sees snow totals close to 24 inches on average. Temp will average out with a few cold spells and some warmer spells.

Things I need more time to review!

  • Will El Nino set up more easterly?
  • Will we see a favorable Pacific Ocean? That could help us a lot for snow.
  • The Snow Advanced Index (measured in October) can help predict the way that the Atlantic blocking pattern works.

I am just too early trying to pinpoint this! What I have discovered is this year has many conflicting signals. I am sorry for the hype on the outlook to be boring in general! We will get a lot of data over the next several weeks and by the End of October, I will revise and have some better ideas!

Forecast

The 2022 – 2023 Winter Outlook Factors

Posted on November 5, 2022November 5, 2022 By admin No Comments on The 2022 – 2023 Winter Outlook Factors

I will post the conclusion here:

  • Normal to slightly above normal Temperatures
  • Normal Snow
  • The Wild card is the influence of the Tunga volcano

The BIGGEST concern I have for below-normal snow this year is the presence of La Niña

What is La Niña?

La Niña is a cold pool of water in the equatorial Pacific region. When the equatorial Pacific is Warm (El Niño) this region can supply a strong sub-tropical jet stream and usually a better chance of big snowstorms due to the amount of moisture. Although El Niño winters are not particularly cold, if the timing is right, we can get some whoppers! YES, 2016 was an El Niño year, and we got the last HUGE storm then.

La Niña, on the other hand, tends to make the Jet to the north push milder air into our region. We are often in the Battle Zone between colder to the north and warmer to the south when storms come along. The storms tend to turn to a mix or rain if they have any wintry weather in them at all. In general, we end up being slightly above normal with temperatures. The Pacific Northwest tends to get blasted with rain and Mountain snows. That does not do us many favors for keeping it cold in the east, as we get a ridge of High Pressure (Southeastern US warmer high pressures) pumping warmer air here.

Typical La Niña Pattern:

Typical La Niña Weather Patterns
Typical La Niña Weather Patterns

So this is Factor One for the 2022 – 2023 Winter weather outlook!

You can see in the National Weather Service’s Winter Outlook that they are weighing heavily on the La Niña pattern. If you have been watching the weather the last week or so, you can see that this is what the pattern has looked like!

NWS Winter Temperature Outlook

And, I have to say, we have had a horrible set of winters when there is a La Niña! I saw a meteorologist show that 60% produced below normal snow for the area.

My Thinking On La Nina:

There are a few reasons why we may see some early sneak peeks of winter in November and December, but they are unlikely to be long-lived.

Polar Vortex

The Polar Vortex is not something new and makes visits into the Lower 48 states every winter. The news media LOVES to over hype when it comes into play for the United States. Additional names to this Arctic region airmass could be the Polar Express (the streams of air that bring in the Vortex, or at least portions of it), or the circumpolar vortex.

What is it exactly?

The polar vortex is a large area of low pressure and cold air surrounding both of the Earth’s poles. (that comes straight from the NWS) In the winter it strengthens.

When it is very circular, it is considered stable and the jet stream is usually very strong. Storms likely will rip across the United States and the coldest air stays “locked” in Canada.

Polar vortex Information from Climate.gov
Polar vortex Information from Climate.gov

Disruptions

If you want there to be colder air masses in the United States, you will want the Polar Vortex to be weaker than normal so that storm systems can disrupt and dislodge the colder air into the United States. Another thing that can help to split and disrupt the Polar Vortex is something known as Sudden Stratospheric Warming. This is warming above the vortex that can “split” the vortex in some situations. When it splits, this also will bring colder air into the lower latitudes. This does not always mean the United States! The Warming and Splitting can take some time and air masses and weather patterns can take some time to respond.

SO WHAT!! What about our winter?

We have already seen some impacts of the disruption in October. And now, all models are showing that an elongation/disruption mid-month. The disruption could cause a link to the cold in Siberia and provide some real cold for the US. It appears that cold will settle in for a lot of the Nation by mid and late month. Will it link with some moisture? There are some hints that it *MAY* – but I would not bet on snow.. but I am going to say that the risk is not zero!

Arctic Oscillation, North Atlantic Oscillation, and the Pacific North American Index, Eastern Pacific Oscillation

These features would take a very long time to explain, but they can work together.

Arctic Oscillation

“The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55°N latitude. When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions. This belt of winds becomes weaker and more distorted in the negative phase of the AO, which allows an easier southward penetration of colder, arctic airmasses and increased storminess into the mid-latitudes.” – noaa.gov

My Thoughts:

It seems like the Arctic Oscillation appears that it will have some negative periods this winter due to the weaker Polar Vortex. That could mean some colder intrusions.

North Atlantic Oscillation

“The North Atlantic Oscillation (NAO) index is based on the surface sea-level pressure difference between the Subtropical (Azores) High and the Subpolar Low. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track” – NCDC

We want a negative NAO (if you want snow) mainly because it will block the retreat of cold air in the area and cause storms to ride up the east coast. That tends to be our snow making helper! When the NAO starts to turn positive, we end up usually doing the best.

My thoughts:

The NAO looks like it will have some negative phases as well. We had a good negative run in October, but that changed at the end and as we have gone into the first 2 weeks of November.

Pacific North American

“The Pacific-North America (PNA) pattern is one of the most prominent modes of low-frequency variability in the Northern Hemisphere extra tropics, appearing in all months except June and July. The spatial scale of the PNA pattern is most expansive in winter.” – NCDC

We basically want a positive phase which means a Ridge on the West Coast and a trough bringing colder conditions and the winter storm track would be good for the area. (if you like snow!) It could also connect to the cold over the Arctic and from Siberia.

My Thoughts

I think this will depend on the way that the Ocean temperatures impact the region out west.

Ocean Temps

La Nina is the big one. The cold area of water in the Equatorial Pacific is very prevalent. The other interesting areas are the Pacific Northwest warmer air temperatures. The other area is the warmer Ocean temperatures off the East Coast. The different areas seem to offset each other.

Huge Volcano Tonga moisture Input

This is a huge unknown this year. When Tonga exploded it put a ton of moisture into the atmosphere. This is something I think already impacted the Hurricane season and the southern Hemisphere winter. I do not know how it will impact, but it could help it be colder.

CONCULSION

I think we will see an early start to colder temps in late November and I would not be surprised to see some snow late November and early December. I think we still end above normal in temperatures. Additionally, the La Nina impacts will be the biggest one. So we end up with 16 to 25 inches of snow which is just normal amounts.

Forecast

What will Impact Winter 2021 -2022 – #5 Ocean and Great Lakes Temps

Posted on October 28, 2021November 10, 2021 By admin No Comments on What will Impact Winter 2021 -2022 – #5 Ocean and Great Lakes Temps

The Mid-Atlantic has a lot of factors because of the proximity to the Atlantic Ocean, Mountains, and even the Great Lakes. I want to outline the impacts of these AND the Pacific which will all impact this winter, especially early in the year!

Atlantic Temps are WAY WARM!

The Atlantic Ocean is currently much warmer than normal. I think this will impact us in several ways:

  1. Storm systems will have a lot more moisture to work with! The heat should evaporate more moisture and juice up any storm system. This is part of why I think each time the forecast models say it will be a dry period, they back off pretty quickly
  2. The Western Atlantic Ridge (which at times is known as the Bermuda High) will be strengthened by the warmer air. This could slow the infiltration of colder air and stall fronts away from the coast and make them just inland. This could have a significant effect on the rain/snow line!
  3. Warmer temps from the ocean will also make marginal (close to frozen events) more likely to be a mix or even rain. That heat will make it inland! This becomes less of a factor for the western zones, but does impact the upper levels which could make sleet and freezing rain a problem. This is not new, but could be a more common set up

The Greenland Temperature Gap

There is often a cold pool off the Northeastern United States that is linked, many times, to a better negative NAO (See previous posts for information on the NAO)

Great Lakes

You would not think the Great Lakes would have too much impact on our weather, but it really does! The Great Lakes can provide additional moisture to some storm systems, and, if it is warm, it can help moderate temps that are being impacted by winds that go across them. I am not saying this is a huge factor, but it does have an impact.

The Pacific Looks Poor

Probably one of the worst looks I have seen in a long while. The colder weather will encourage cold storms and the Jet Stream to be in a poor setup for us. I will discuss the Pacific Decadal Oscillation, the Pacific North American weather patterns, and the Eastern Pacific Oscillation in a post soon. All I can say is the pattern will favor milder terms for us and drier than normal conditions. I just do not like it.

Of course, we also have La Nina which will not bode well for big storms.

Summary

Most of the ocean looks terrible for us for winter. It is also the biggest signal that the winter may be warmer than normal and have less snow.

Forecast

What will impact Winter 2021 – 2022 #3 Snow Advance Index (SAI) & #4 North Atlantic Oscillation(NAO) and Atlantic Oscillation (AO)

Posted on October 27, 2021November 10, 2021 By admin No Comments on What will impact Winter 2021 – 2022 #3 Snow Advance Index (SAI) & #4 North Atlantic Oscillation(NAO) and Atlantic Oscillation (AO)

Snow Advance Index

The Snow Advance Index is a feature that is not always the best indicator for how our winter will be. The basic principle is that the advancement of snow cover in October is closely linked to the phase of the North Atlantic Oscillation.

North Atlantic Oscillation

The North Atlantic Oscillation (NAO) measurement is a KEY FEATURE needed to help get snowstorms on the East Coast of the United States. For example, the NAO is currently negative, which helps to slow down and aid in the development of big Nor’easters like the one we are having this week (October 26 – 27, 2021)

Really, though, the negative NAO (NAO-) is not the key to these storms, it is as the NAO flips back and forth between the negative and positive (NAO+) that usually signals a storm

Anyways, when the Snow Advance Index shows a great increase, the NAO *COULD* end up being more negative for the winter.

So what is the Snow Advanced Index Looking Like?

Snow Progression


The index is actually below where I would like to see it, but it is higher than last year!

imageywstm.png
Source – https://www.aer.com/science-research/climate-weather/arctic-oscillation/

The NAO Has been running Negative for a long while now. We will see if this can continue into November and Beyond! That would be a good sign for offsetting the La Nina impacts. (The line is below the 0)

NAO Index
NAO Index from Climate Prediction Center

Arctic Oscillation

Another factor for colder conditions is the Arctic Oscillation! As noted yesterday, when the Polar Vortex gets disrupted, it can turn negative! WE NEED THE NEGATIVE (if you like snow)! That is a better indication for more snow in the east! It has been running back and forth between positive and negative phases. We will see how this works out, but it looks like it is heading to negative again so the colder long-range seems to be linked to this as well! (Observed in black and red the various model predictions)

I am not super convinced we get non-stop help from the Arctic Oscillation this year, but I do expect a lot of variabilities which could help us at times be colder and have a chance of winter weather.

In Summary

I do not think we have much to count on from the Snow Advanced Index, North Atlantic Oscillation, or Arctic Oscillation. The negative NAO has been encouraging, but that alone will not set us right for winter weather.

Forecast

Posts pagination

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Latest Radar from the National Weather Service
National Weather Forecast from https://www.wpc.ncep.noaa.gov/#page=ovw
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Precipitation Forecast from the National Weather Service
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