A Transition Ahead

7 pm temperatures already approaching double digit negatives!

7 pm temperatures already approaching double digit negatives!

You wouldn't know it, but a transition to warmer temperatures has already begun, hundreds of metres above your head...

Our current temperatures are VERY impressive for so early in the evening. Negligible difference compared to last night.

Folks, these are 7 pm readings, to the right.

Already double digits in the valley as well.

If we can spend as many hours as possible without cloud cover tonight it will go a long way in us keeping the colder air entrenched in the area for longer, significantly increasing our chances for snow over the next 48 hours.

If only we had snow cover, these temperatures would even be more impressive, but snow and cold snaps don't always go hand in hand.

One of the most impressive temperatures so close to the coast was at CYBL (Campbell River Airport), which recorded a chilly -14C reading early this morning. They have the precious snow cover advantage, though.

Models often overestimate the amount of mixing in the atmosphere with regards to cold air, and this poses a real challenge for weather forecasters, especially when it comes to assigning probable precipitation types to Vancouver and the surrounding area. Cold air loves to linger in valleys and lower elevations.

Our inversion is beginning to show up on soundings, enhanced by strong radiative cooling tonight.

This skew-t plot shows many, many confusing lines, but I've highlighted the temperature inversion on the temperature profile. Normally, temperatures decrease with increasing height, but our lower levels are significantly colder right now and upper levels of the atmosphere have warmed the past 12 hours. This type of inversion is occurring as our air mass is changing, setting up a strong inversion aloft, with a large dew point depression above the inversion.

This skew-t plot shows many, many confusing lines, but I've highlighted the temperature inversion on the temperature profile. Normally, temperatures decrease with increasing height, but our lower levels are significantly colder right now and upper levels of the atmosphere have warmed the past 12 hours. This type of inversion is occurring as our air mass is changing, setting up a strong inversion aloft, with a large dew point depression above the inversion.

The skew-T/log P diagram above is one of the most useful diagrams for a meteorologist. This standard thermodynamic diagram can be used to calculate values including important moisture and humidity parameters like saturation mixing ratio, mixing ratio, relative humidity, dew point depression, saturation vapor pressure, and vapor pressure.

Temperature and level parameters include virtual temperature, potential temperature, equivalent temperature, equivalent potential temperature, wet bulb temperature, wet bulb potential temperature, lifting condensation level, convective condensation level, convective temperature, thickness, freezing level, wet-bulb zero level, level of free convection, mixing condensation level, equilibrium level, and maximum parcel level.

It's enough to make your head spin!

And, I didn't even mention the many, many stability and shear assessments.

It goes on, and on, and on.

The shallow layer of cool air may be too thin to support snow, so supercooled water droplets may fall through the cool air as freezing drizzle or light freezing rain. Temperatures in the upper levels are not forecasted to rise much above freezing. 

Always be aware of the temperatures at the surface, because the supercooled droplets may be freezing on contract during the next 48 hours which may pose a significant travel hazard. Just be aware and monitor the latest forecasts, as chances are they will change rapidly over the next few days.

It's always an interesting to watch a transition event unfold, when we switch back to a weak onshore flow. The interaction between the warmer marine air and cool polar continental air mass always creates interesting weather.

Right now I'm forecasting between 1-4 cm of snow as a rough guide, as precipitation is expected to remain very light and somewhat spotty, and sometimes varying by type (freezing drizzle, freezing rain, snow pellets, snow grains etc.). The depth of the sub-freezing layer at the surface along with the depth of the moisture layer will have to be watched carefully.

First thinking suggests higher amounts to the south and east of the City of Vancouver

What's the general NAM model saying to get a rough, first estimate of snowfall:

A little overdone, I think.

A little overdone, I think.

And what about the MM5-NAM 24 hour precipitation amounts to 4 am Tuesday:

mm5nam total precip.png

And, we can't leave the WRF-GFS mesoscale model out of the equation, just for completeness:

WRF-GFS.png

Last one, I promise...

How about a high resolution LAM-WEST high resolution Canadian model through Tuesday evening:

LAMWEST.png

Take-home message. This isn't going to be a heavy snowfall event.

A job of a forecaster in this type of setup to to determine all the plausible outcomes, using a variety of forecasting techniques such as looking at short-range ensemble forecasts to determine the probability of a high impact event, analyzing current observations, and using deterministic models.

For example, a forecaster might choose to use a short range ensemble forecast (SREF) to confirm something that a deterministic forecast is showing. Even though a high resolution model may look super realistic and plausible, it's impossible to tell that it will verify with absolute certainty. That's where short range ensemble forecasting comes into play, which is advantageous to use in the short range (1 to 3 days) time frame:

SREF plume diagram. Great for analyzing uncertainty (spread), onset of precipitation, and most importantly precipitation type

SREF plume diagram. Great for analyzing uncertainty (spread), onset of precipitation, and most importantly precipitation type

Dry-Cold and Wet-cold Debate

I promised I would talk a little bit about the difference between a wet-cold and a dry cold during this blog. I appreciated your feedback and would like to share a few thoughtful answers. Thank you for taking the time to answer.

1) Assuming calm winds, a damp cold is much worse than a dry cold, in my opinion. As for why, I'm not completely sure, but I would guess that the moisture makes the cold "stick" to my skin.

I find dry cold is actually quite comfortable if winds are light, but usually we have outflow winds when we have a dry (arctic) air mass, so I can understand the perception that dry cold might be worse.

I also find that the wind chill index vastly underestimates the feeling of the cold. For example, -10C with calm winds doesn't feel bad at all, but 0C with a wind chill of -10C feels much colder than -10C, to me.

I guess perception of temperature is a very subjective science. It probably comes down to body chemistry, skin type, metabolism, body fat ratio, and probably many other factors.

2) In my opinion, the reason "we" complain about the wet cold in Vancouver is because we simply don't dress for it. Even if there were a difference, the windchill on the prairies more than makes up for it.

3) After having lived back east for a couple winters previously, I can say that the moist cold is much worse. I would rather it be continental cold and -10 than it be that damp cold here in Vancouver and +1. It goes right through your bones: you shiver. Continental cold is only bad when there is wind. If there is no wind you can't even tell; it feels as it's around 0 degrees even if it's -10. Now, when you get to -20 it really does feel cold regardless and there is no comparison, but wet-cold is THE WORST!

It's not what the experts tell us though:

1) News 660

2) CBC Article

If you'd like to chime in on the debate, feel free to comment below.

Next blog, will be inspired by a shocking recent event. In fact several of them. Have any of you Vancouverites noticed the influx of static shocks with the cool, dry air. I have, and have been shocked more often the past few days than during the past few months combined.

Comment below if you've noticed much more static electricity lately!

#50shadesofVan