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Cool post about snowflakes. Snow growth zone is where temps are -10 to -20C up in the atmosphere.. But because dewpoints were dry in that zone this morning,(green lines jog to the left away from the red temperature line) (gap between green and red line) it led to the formation of the snowflake type that was falling.
At the center of this one is something called ‘latent heat.’ And that energy falls under the larger umbrella of ‘phase changes.’ When we’re talking about water, we have something that can exist in three different states – a liquid, gas, or solid. The liquid is of course the water we drink every day or that falls from a storm. The gaseous state is water vapor, which is always in the air around us in varying quantities. And the solid state is ice.
When molecules of H20 switch between these different phases, energy is being transferred. Some of these processes require energy (such as evaporation or melting). It takes an input of energy to melt ice and snow, or to dry up that puddle left behind by a summer storm. When breaking up those icy bonds, energy is taken out of the surrounding environment and in turn cools the ambient air. The same happens when water evaporates. A good example is when you get out of the ocean after a swim – you’re freezing! That evaporating water is taking energy out of the air around your body, and is leaving behind a chill
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But when a molecule of water vapor condenses and takes on its liquid form, energy is actually released. This is the key to the rapid snow melt you witness when dew points soar into the 40s and 50s. The dew point is the temperature that air needs to achieve in order to condense. If you have a temp of 40F and a dew point of 40F, the air is said to be saturated and you’re going to be seeing some fog, mist, rain, or snow. In situations like we often have in March, there’s snow pack on the ground and milder, higher dew point air easily can cool to this point as it wafts over the snow. A condensation frenzy ensues! This is also why we tend to see a lot of fog (advection fog) when milder air invades over the snow.
Here’s the real trick – condensation involves 7.5 times more energy than melting does. What does that mean? For every molecule of water vapor that condenses, 7.5 bonds that are holding ice molecules together breaks. It works even better if there is a breeze keeping that mild air flowing up over the snow. That way there is a fresh supply of moist air condensing over the snow pack, releasing its latent heat and helping to power the melting process.
Jet stream finds a shortcut and Canada goes cold. We explain
Thursday, April 23, 2015, 1:26 PM - As you follow the forecast this week, you may hear this unfamiliar weather term coming up over and over again: upper level low. Here’s what it means.
Meteorologists talk a lot about the upper levels of the atmosphere, and you may find yourself wondering why you should care about what’s going on so far above the earth's surface. After all, we spend the vast majority of our time down here on the ground. So, unless you’re a pilot (or a bird), why would you be interested in what’s going on so far above us?
The answer is that what goes on high up in the atmosphere directly drives the weather that we experience. So understanding what’s going on up there provides insight into why things are happening down here. This week is an excellent example of how this is the case.
This Week’s Pattern
I’ve drawn a line to highlight the flow of the jet stream at this level. You’ll notice that path is very contorted with lots of sharp curves, including a trough over the B.C. coast, a steep ridge centered on Alberta, and a deep trough over the Great Lakes. As a general rule, the jet stream would prefer to flow directly west to east. But as you can see this heavily amplified pattern causes it to divert wildly to the north and south.
In scenarios such as this, you’ll often find that the jet stream wants to look for a shortcut, to carve out a more direct path from west to east. In this image above, I’ve highlighted two areas where a shorter path is available for the jet stream, (1) and (2). In the case of (2), you can see how the shortcut is especially appealing. If you want to cross Hudson Bay, there are much easier ways of doing it than to go through Kentucky.
So let’s take a look at how the ground-weather pattern develops in response to this situation, in the following image.
This shows the pattern (as forecast by a model) for the morning of April 23, about 36 hours after the first image. The jet stream has taken advantage of the two shortcuts, to form a more direct path from west to east. But in doing so, it has left something behind. What was formerly a pair of troughs, or southward dips in the flow, have become cut off from the main jet stream and are now separate lows. These are the features we are interested in – the upper level lows – that will be driving much of the weather across Canada for the remainder of the week.
Because these lows have been cut off from the main jet stream flow, we also refer to them as “cutoff lows.” Without the connection to the jet stream, they tend to stall in place, and a cutoff low like this can hang around for several days until it is eventually reabsorbed into the jet stream flow
As you can see, the smaller West Coast low has been reabsorbed into the Gulf of Alaska trough, but the East Coast low has made very little eastward progress by the weekend.
Weather impacts for Canada
In addition to being slow moving, cutoff lows have another characteristic that is causing some unusual weather across Canada this week. Because they originate in the arctic air mass north of the jet stream, these cutoff lows are associated with a pool of very cold air. And adding a slug of cold air to the upper atmosphere is a sure way to create weather chaos.
When we have a situation in the atmosphere with cold air above warm air at the surface, the result is instability. The warm air is less dense, so it wants to rise like a hot air balloon. In this situation, with the cold air aloft being provided by the cutoff low, and the relatively warm air at the surface being provided by the strong late April sun (yes, temperatures are well below seasonal, but they are still warm compared to the very cold air aloft), we have a situation that tends to produce a lot of widespread rising air.
The result is a radar map that looks like this below.
Check out the areas in green which means the atmosphere is loaded with moisture and can lead to some very heavy downpours when above 1.70.
PWAT is the entire mass of water vapor in the air. higher dewpoints lead to higher PWAT values especially if those dewpoints are higher in a vertical sense (not just the surface)
0.50 inches or less = very low moisture content 0.50 to 1.25 inches = low moisture content 1.25 to 1.75 inches = moderate moisture content 1.75 to 2.00 inches = high moisture content 2.00 inches or above = very high moisture content
Wasn't sure where to put this but can be used by anyone.. Pretty damn good explanation by Steve regarding what happened to the rain storm last night! We were supposed to get over an inch of rain but I...and many others... got less than 1/2". (I got 0.25"!)
" In the forecast for September 12, 2015; the northern Mid Atlantic was forecasted to have around 0.50″ to 1.50″ throughout the region. Clearly, in southwestern to central New Jersey that was no where near the case, in face some locations got no rainfall at all! The rest of the region under performed as well. Meanwhile, much of eastern Pennsylvania, northwestern New Jersey, and the Hudson River Valley verified nicely. So what’s the deal here?
Well, first let me say that if I did this forecast again, I would have forecasted the same outcome with the same data before the event. There was nothing clear in the model guidance nor observations 12 hours or even 6 hours leading up to the event to support what would eventually happen. Believe me, I checked over all the date. So what happened?
We knew from the start that the best lifting and dynamics was over the eastern Great Lakes and northern New York. The idea here was that a phase was going to start to take shape between the Sub Tropical jet stream and the Polar jet stream in the afternoon. This did not happen as fast as what was forecasted. Instead you had two separate areas of strong lifting.
The first area, focused over eastern Pennsylvania through the central Hudson River Valley, lead to rainfall amounts of 0.50″ to 2.00″ with locally higher amounts. Locations to the east of this area, basically most of the forecast area, ended up with little if any rainfall. So where did the rainfall expected for New Jersey, southeastern New York, and Connecticut go?
Well, because of the location of the best lifting, the secondary low pressure system which was associated with the Sub Tropical jet stream ended up about 50 miles further to the east. What was supposed to happen was the Polar jet stream and upper level low was going to pull this low back to the north and northwest, but that did not happen.
As you can see over Delaware and southern New Jersey, the forecast worked out as expected with 0.50″ to 1.50″ of rain. However, with the lack of phasing and a track further to the east the heavy rain expected for New Jersey, southeastern New York, and Connecticut remains over the coastal waters. As a result, the region was in a hole and everyone wondered where the rain was.
This post is not meant as a an excuse for what is clearly a busted forecast but a learning tool to see how even when you have all the data point to a solution, atmospheric dynamics can throw a twist into your theory and lead to a total bust even when you do as much as you can for the right forecast. "
" When forecasting for the Long Range whether that’s a week, a month, or a whole season; sometimes it is a good idea to step back and really look at the weather pattern in place. Let’s not jump ahead but LOOK at what is happening now.
One area that has my eye is what is happening on the East coast this morning. No, a huge storm is not brewing and the heavy rain threat will stay out over the Atlantic, this time, but the ingredients are what have my attention.
As we enter Fall we have a western Atlantic that is very warm. There is no denying that we are entering a -AMO phase, which can be clearly seen in the rest of the Atlantic, especially the eastern Atlantic, but we still have very warm water off of North America and the Gulf of Mexico. As a result, the surface pressure anomalies have been below normal for several weeks in this area. Not enough to produce significant storms, but enough to signal an area of rising area is becoming established off the East coast. Why is this important?
Well, while we have all sorts of teleconnections and theories from Siberia to Tahiti that impacts our weather, there is one fundamental law that one can not ignore. The Conservation of Mass. The fact that air is rising off the East coast means that the air rising MUST be filled by a denser air somewhere else. Enter your Polar air mass with dense, cold air. This leads to higher pressure over the Continent, like the Great Lakes, to remain in place and send that cooler, denser air towards the coastal waters.
In this case, the process involves a North Atlantic Oscillation that is currently negative, which will then waver around neutral for the rest of September before falling negative again in late October.
The point I’m trying to make is that sometimes you have to stop complicating the equation and go back to one basic law. The Conservation of Mass is king, always has been and always will be. The question is what type of cold air will be present over North America over the next several months and how will the Polar and Sub Tropical jet streams interact. "
Figure I put this in here since some (and I) were wondering how it was snowing (Flurrying) yesterday with surface temps in the low-mid 40sF (7C)
First .... Strong NW flow off the Lakes produced Lake effect moisture/clouds/snow.
Second... As soon as the sun came up BOOM everywhere in PA, NY, New England... instability clouds showed up. Surface warmth and VERY cold aloft.
Third... The moisture was VERY scattered and light in nature but some folks had snow squalls, some just flurries...
Now... Bridgeport, CT at the coast reported flurries (Trace).. so lets take a look at the sounding from 2pm...
You can see that there was a small layer of clouds at about 5000 feet but more importantly.. the temps dropped like a brick as you went up into the atmosphere from the surface..
It was actually 46°F on land and 19°F just 4800' up! Part reason why just flurries is because dewpoints were in the teens near the surface so it was bone dry. Any moisture that was coming down was drying up but a few flakes found their way down!
There wasn't enough time to melt since it was below freezing even at 2500'. To the right is the text data you can see the levels and the temps & dews.
Some folks reported graupel, drizzle, and snow flurries in this area.
Lastly... the clouds I saw at the beach and entire area were basically at 5000' and COLD
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