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Posted at 11:45 AM ET, 01/12/2011

Why was there ice before snow Tues. night?

By Wes Junker

Some may be wondering why during the onset of the precipitation yesterday evening we had freezing rain and sleet in places instead of snow or plain rain.

On Sunday I mentioned the possibility for freezing rain during part of the storm:

The warm layer [that usually causes freezing rain problems] is not present [except southeast of D.C.] but one potential problem is the depth of the clouds and the cloud top temperatures. Water vapor does not freeze easily in the atmosphere without dust and other small particles for crystal growth to occur. The temperatures of the cloud tops are pretty close to the temperatures where it gets hard to form snow crystals and instead you get supercooled water droplets [which results in freezing rain].

However, we didn't mention freezing rain much after that as later model runs indicated colder cloud tops and heavier precipitation supportive of all snow. When the models started backing off on the precipitation amounts, I didn't pay quite enough attention to the vertical temperature profiles which suggested freezing rain might be a problem during part of the storm despite below freezing temperatures from the surface to cloud level.

The vertical temperature (solid lines) and dewpoint (dashed) profiles for a typical freezing rain event (left panel) versus last night (right hand panel).

The two atmospheric profiles above, also referred to as soundings, show the typical freezing rain structure on the left compared with, on the right, what was actually observed last night at 5:00 p.m.

Freezing rain and sleet usually develop when there is a warm layer (mostly above freezing) that melts the snow falling from above as shown in the sounding on the left at 850 mb or 5,000 feet. Usually, for sleet the maximum temperature in the warm layer is between +1 C and +3 C. Sleet occurs when the snowflake almost completely melts and then falls back into a below freezing layer. The remains of the ice associated with the old snowflake then refreezes usually into a round ball with the color of a miniature ice cube.

Typical temperatures of the warm layer for freezing rain is usually above 3 C (as shown in the sounding above left). In these cases, the snow flake completely melts and cannot refreeze because it has no small ice crystal to get start forming. In the free atmosphere, without a some speck of ice or dust, water doesn't freeze until the temperature reaches around minus 40 degrees.

Yesterday, the warm layer never got above freezing as shown on the right panel sounding so the freezing rain didn't form due to melting snow. A second mechanism resulted in the reports of freezing rain and sleet.

Around 30 to 40 percent of freezing rain events do not have any temperatures in the atmosphere above freezing. Instead, they have rather shallow clouds with a dry layer above. Note on last night's sounding (on the right) that the temperature (red line) and dew point (dashed line) overlap each other in the lower parts of the atmosphere and then diverge at around -6 C (at the height of 750 mb or 8,000 feet). This coincided with the cloud top level where precipitation formation occurs.

Most particles in the atmosphere that act as ice nuclei, places where snow crystal formation can occur, require temperatures that are colder than - 6 C. In fact, when the cloud top temperatures (that region of the sounding where the temperature and dew point are no longer overlapping) are around -10 C, about 50 percent of the clouds will be composed of supercooled water without any ice crystals present. Deeper clouds with cloud top temperatures that are colder than -18 C almost always have ice crystals and produce snow.

So why did we get freezing rain? The dreaded dry slot came in and produced drying in the mid-levels which limited the depth of the clouds. The shallower clouds kept the tops of the clouds too warm to allow snow crystal formation. Only when the tail of the comma head associated with the upper level system came through did the clouds deepen. That's when we got into the band of heavier precipitation and the freezing rain changed to snow as the cloud tops cooled.

What about the reports of sleet? I suspect that sometimes when we were getting precipitation the clouds were a mixture of ice crystals and supercooled water (water that is below freezing but liquid), with more of the latter than the former because most of the cloud was a little warmer than optimal for crystal growth. When these crystals bumped into supercooled droplets, the super cooled water instantly froze on the crystal. That's not the typical way sleet forms (in most cases, it forms as described above, when a snowflake partially melts in the warm layer and refreezes lower in the atmosphere). I'd be interested in whether people noticed whether their sleet was white (i.e. mixed with snow) or the color of an ice cube. I suspect it was the former. Both sort of bounce, but the way the two types of "sleet" balls form is different.

How's that for a nerdy explanation?

By Wes Junker  | January 12, 2011; 11:45 AM ET
Categories:  Latest, Science, Winter Storms  
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You're probably on to something, the sleet was white here in Columbia as I was leaving work (4:15 or 4:30). Saw it on the hood of my car and thought that it was odd, given how cold it was. Thanks for the explanation.

Posted by: Hokie_Ryan | January 12, 2011 11:50 AM | Report abuse

Wes is awesome.

Posted by: DullesARC | January 12, 2011 12:10 PM | Report abuse

A slightly different tack on weather reporting:

It seems that because of last year's snowpocalypses that most of the reporting this year goes into snowfall number of inches. Yes, on Sunday Wes did point out the possibility of freezing rain... but that was not the headlines everywhere else, for the several days preceding yesterday's precipitation.

But if I think back to February 1999 or was it February 2000, when we had the massive ice storm, in the few years after that a lot of weather reporting concentrated on the icing risks. That overemphasis on icing probably kept up through President's Day 2003 blizzard, when we started thinking about snow again :-)

Posted by: Virginiadude1 | January 12, 2011 12:11 PM | Report abuse

We had bouncy white sleet for a very short time in Centreville before the switch over to big flakes.
Many people complaining about PM rush hour iciness yesterday. They were not at all prepared. But how do you prepare for freezing rain (besides just staying home)?

Applying products like "Rain-X" to car windows helps some, but the driving is always a nightmare.
At minimum have spare hat, gloves & a long handled scraper in your car at all times Nov - March.

Posted by: FIREDRAGON47 | January 12, 2011 12:12 PM | Report abuse

Wes Junker

Could you possibly put out a long range forecast of what you think the possibilities are for the rest of the winter? Are there any upcoming snow events coming? What the temperatures will be like? And so on. That would b nice!

Posted by: bobdello01 | January 12, 2011 12:13 PM | Report abuse

very interesting, wes. just the right amount of nerdiness, imo. one thing though: you give such great verbal explanations of things and so forth, but i love pictures/graphs/charts etc... and i can't make out any of the units etc... on those rain profile graphs. any way you could make those into links?

Posted by: walter-in-fallschurch | January 12, 2011 12:15 PM | Report abuse

I was pretty sure that I saw some graupel, especially in the afternoon before the heaviest of the preciptiation started. Is that possibe?

Posted by: jerryfrombaltimore | January 12, 2011 12:17 PM | Report abuse


You raise a good point. I gotta shrink all of the graphics Wes sends me for these posts to 452 pixels wide for them to fit in the blog. I'll try to make them enlargeable moving forward when having them bigger would help readability.

Posted by: Jason-CapitalWeatherGang | January 12, 2011 12:20 PM | Report abuse

Walter, Here's a simplified graphic of what is important.

and a link that shows what the lines mean. The one explanation the link leaves off is the temperature lines. If you look at the bottom of graphic you'll see temperature, above the values is a line that slant off the the right. That is the line that tells you what the temperature and dewpoint is if those to lines cross it.

Posted by: wjunker | January 12, 2011 12:29 PM | Report abuse

Jerryfrombaltimore, that is what snowflakes that are encrusted in ice/rime and lose their snowflake look is called. It's not really sleet though many people call it that but it bounces and falls faster than a regular snowflake.

bobdello01, I don't see any snow on the immediate horizon. If nothing pops up then maybe I'll do something on the longer range Friday or sometime early next week if I feel confident enough to make a post.

Posted by: wjunker | January 12, 2011 12:38 PM | Report abuse


What are the next snow event that is coming towards dc?

Will next tuesday be ice or snow? How much?


Posted by: SNOWLUVER | January 12, 2011 12:41 PM | Report abuse

thanks wes. i'll try to digest that. are those meant to be the same link? i see the generic "explanation" one, but not the "simplified graphic of what's important" from yesterday.

Posted by: walter-in-fallschurch | January 12, 2011 12:49 PM | Report abuse

This winter has been disappointing so far esp. with all the misses but just as an example are there stats on our total seasonal snowfall as of today's date looking back at say the past 10-15yrs? I feel like about half the time we had to wait until the 2nd half of January till we got any meaningful snowfall. This would help us snow loves know that winter's not yet over and a few inches through mid january is typical in DC.

Posted by: lobp | January 12, 2011 12:52 PM | Report abuse

Next Tues looks like a rain event. No snow for the next 7-9 days.

Posted by: VaTechBob | January 12, 2011 12:59 PM | Report abuse

here's something that says we're .8" (3.5" vs 4.3") behind "normal" for snowfall this winter.

Posted by: walter-in-fallschurch | January 12, 2011 1:02 PM | Report abuse

I'm not sure what we got in Silver Spring between 6:30 and 7:00, before the real snow started. It was a mist of ice particles that were like little strands. Not flakes. They weren't fluffy like snow nor did they make a clicking sound when they hit the ground. What do you call that?

Posted by: cassander | January 12, 2011 1:06 PM | Report abuse

Walter, here are a couple of graphics. One for the typical freezing rain sounding., one for liek yesterday's freezing drizzle.


Posted by: wjunker | January 12, 2011 1:10 PM | Report abuse

cassander, I'm not sure. One possibility would be needles that were rimed. They tend to form in the portion of the cloud where its kind of warm providing they get seeding (little ice crystals) from above.

Posted by: wjunker | January 12, 2011 1:13 PM | Report abuse

thanks wes.

Posted by: walter-in-fallschurch | January 12, 2011 1:17 PM | Report abuse

Some great photos of the blizzard up in MA. (Jealous snow lovers, proceed with caution.)

Posted by: CuseFan07 | January 12, 2011 1:39 PM | Report abuse

thanks only .8 behind at DCA. Feels a lot worse than that. NOVA looks like the big loser so far (IAD is -2.2)

Posted by: lobp | January 12, 2011 1:48 PM | Report abuse

RE: Graupel

Thanks, Wes, that's exactly what the hood of my car looked like yesterday afternoon. Never heard of that before!

Posted by: Hokie_Ryan | January 12, 2011 1:54 PM | Report abuse

Given a sounding - vertical profile of temperature/moisture - forecast for yourself the type of precipitation:

Posted by: SteveT-CapitalWeatherGang | January 12, 2011 3:36 PM | Report abuse

Interesting post, thanks... I've seen graupel on my deck before and didn't know the science behind it.

I know there are some detractors, but Henry M's final snow map made Tuesday morning did show freezing drizzle in VA approaching DC.

Posted by: spgass1 | January 12, 2011 5:26 PM | Report abuse

The sleet was definitely white. Just looking at it at first, I thought the public works guys in Greenbelt had gotten carried away with salting the streets.

Posted by: kevinwparker | January 12, 2011 5:38 PM | Report abuse

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