Weather It Is (One Storm And Then The Next)

Our rainy day is about to come to an end, and the skies should clear overnight.  However, the temperatures are not headed upwards, but on a downward slide.  Our next storm, which should arrive Wednesday evening, is forecast to be about 2-3 C colder than today's storm (near -27 C at 500 mb).  It is also suppose to produce quite heavy precipitation (50 mm in some locations that received 15-20 mm with this storm).

The storm should intensify towards Thursday morning and continue as heavy rain and wind throughout the day on Thursday.  At the higher elevations, the temperatures will approach 1-2 C near the surface, but only -9 C at 700 mb.  This means that the atmosphere will be comparatively warm in the lower levels and comparatively cold in the upper levels, which is a recipe for strong storms, including lightning and thunder.   It's also a recipe for graupel and hail, mixed with snow.

I can't forecast an accumulation of ice when temperatures are generally above freezing, but it is not out of the question that in the heavier storms the roads could become briefly ice-covered (and, of course, briefly is enough to be dangerous).

The storm will continue as rain into Friday, and then we'll be in for a period of pleasant and warm weather before the next winter storm.

Here's a blurb  from the BBC.

http://www.metoffice.gov.uk/learning/snow/how-is-snow-formed

How is snow formed?

Snow is formed when temperatures are low and there is moisture - in the form of tiny ice crystals - in the atmosphere.

• Share:
• 
  
• Share this page on Facebook
• Share this page on Twitter
• Share by email
• Print

Related articles

• Severe weather warnings
• Snow forecasting in the UK
• UK snow
• Will it be a white Christmas?
• Snow at Christmas

Related links

• Snow pictures on Flickr

When these tiny ice crystals collide they stick together in clouds to become snowflakes. If enough ice crystals stick together, they'll become heavy enough to fall to the ground.

How cold does it have to be to snow?

Precipitation falls as snow when the air temperature is below 2 °C. It is a myth that it needs to be below zero to snow. In fact, in this country, the heaviest snow falls tend to occur when the air temperature is between zero and 2 °C. The falling snow does begin to melt as soon as the temperature rises above freezing, but as the melting process begins, the air around the snowflake is cooled.
If the temperature is warmer than 2 °C then the snowflake will melt and fall as sleet rather than snow, and if it's warmer still, it will be rain.

'Wet' snow vs. 'dry' snow

The size and make up of a snowflake depends on how many ice crystals group together and this will be determined by air temperatures. Snowflakes that fall through dry, cool air will be small, powdery snowflakes that don't stick together. This 'dry' snow is ideal for snow sports but is more likely to drift in windy weather.
When the temperature is slightly warmer than 0 °C, the snowflakes will melt around the edges and stick together to become big, heavy flakes. This creates 'wet' snow which sticks together easily and is good for making snow men.

Snowflakes

Snowflakes are collections of ice crystals that can occur in an infinite variety of shapes and forms - including prisms, hexagonal plates or stars. Every snowflake is unique, but because they join together in a hexagonal structure they always have six sides.
At very low temperatures snowflakes are small and their structure is simple. At higher temperatures the individual flakes may be composed of a very large number of ice crystals - making a complex star shape - and can have a diameter of several inches.

Here's a quick now about why we "need" it to be a bit colder if we want to get fluffy, large flakes.

 http://whyfiles.org/2010/make-a-snowflake/

The curious growth of a snow crystal
Temperature and humidity affects the shape of snowflake crystals. The temperature of formation determines the original crystal shape. Large (“dendritic”) flakes grow best between -10° and -12° C. Plates grow at warmer or colder conditions.
Humidity — water vapor pressure in the cloud — affects the growth rate due to deposition. A flake that passes through dry air may sublimate (lose mass by converting from solid to gas).
Because temperature and humidity change as a flake bounces around in a cloud, the basic shapes can blend into countless crystal shapes. An ice crystal may also collide with another crystal and aggregate into a snowflake of a different shape. Or crystals may grow as they accrete (collide and adhere) tiny drops of liquid water in the cloud.

Here's some more about snow types

 http://www.its.caltech.edu/~atomic/snowcrystals/class/class.htm