Tuesday, December 20, 2011

Freezing drizzle with an all snow sounding?

Freezing drizzle once again proves to be a difficult-to-forecast, and extremely hazardous form of precipitation.  This point was made all to clearly as 30 cars went off the road on I-70 near Limon, CO resulting in two fatalities during the midday as reported by News9 in Denver.   A prison van containing 9 inmates and two guards was traveling with a trailer when it careened off the road and rolled.

Why was the freezing drizzle so hazardous?  I can easily imagine drivers taking note of drizzle freezing on impact on their windshields and external mirrors.  But they probably didn't take the drizzle seriously.  After all the highway appeared relatively clear according to video from the Huffington Post taken just after the accident. And the freezing drizzle didn't have the dramatic appearance of huge amounts of ice forming that most drivers look for as cues to take action to slow down as if they were going to be driving on a skating rink.  In addition, I suspect road crews didn't respond to freezing drizzle by treating I-70.  In this case, I see no evidence in the video that the highway was pretreated.  If it was then the treatment appeared insufficient to melt the ice given the number of accidents reported.  If they were waiting to time their treatment for the arrival of the snow, they may have completely missed the opportunity to prepare for the freezing drizzle.  The road crews time their treatment according to forecasted timing of winter precipitation.

 The NWS had a winter weather advisory out for the area at the time but there was no indication that freezing drizzle was expected.  From the wording of the advisory issued at 5 am MST, it is apparent to me that the forecasters were expecting snow and not freezing drizzle with an expected arrival time in the early afternoon. When reports came in of freezing drizzle around Yuma, CO around 8:30 to 9 am MST the NWS responded by including freezing drizzle in northeast Colorado.  The NWS eventually included freezing drizzle in their winter weather advisory updates at 1 pm MST for the Limon area.  Had the road crews depended on the forecasts, they may have been caught unprepared for the freezing drizzle.
How was the freezing drizzle detected?  Really the first observations were from  the Automatic Surface Observing System (ASOS) and from spotters.  The ASOS at Limon started to report freezing drizzle around 10 am and Akron a half hour earlier. In this event, the spotters in Yuma, CO  and other places proved invaluable as they beat the ASOS stations in reporting freezing drizzle by almost an hour.  Numerous other places in eastern CO not served by an ASOS would depend entirely on spotters to report freezing drizzle.

The WSR-88D network failed to adequately detect the freezing drizzle.  The areas impacted were too far away to allow the network to effectively detect the freezing drizzle.  In Figure 1 below, the KPUX radar showed no freezing drizzle.  What KPUX did show was an area of snow sublimating as it fell from the higher to the lowest beam.  There was no ability to scan below about perhaps 3000' above the radar level (5298' MSL).  Most of the freezing drizzle likely developed below that level.

Figure 1.  On the top:  a loop of half hourly reflectivity from Pueblo, CO centered over Limon, CO (red dot) from 1517 to 2123 UTC 19 December 2011.  The four panels represent different elevation scans starting with 0.5 deg (upper left) to 1.5 deg (upper right), 2.4 deg (lower right) and 3.4 deg (lower left).  On the bottom:  a meteogram from Limon, CO with temperature, dewpoint in the top pane followed by precipitation type, wind direction/speed (kts), and then ceiling/visibility in the lower panel.

This freezing drizzle was stealthy, falling from shallow clouds easily evading the radar network.  It also fell under conditions that would appear at first to be too cold to allow liquid precipitation to fall.  But once it did fall under such cold weather (see the meteogram from Figure 1), it froze quickly on impact to any surface provided the ground is cold enough as it was in this event.  And as we saw, it was also difficult to forecast leaving little time for the various departments of transportation to pretreat the roads.

But could this event have been forecast?  Difficult is not impossible in my book so let's see what could've helped.  The location of the freezing drizzle was not typical of what would be expected relative to major synoptic features.  Typically freezing rain occurs along a belt poleward of a frontal boundary where some warm air resides overhead to melt the falling snow.  But freezing drizzle can also occur with the entire sounding profile below freezing.  In this case, the freezing drizzle occurred entirely north of the snow shield expanding northward from southeast Colorado (Figure 2). From a standard synoptic climatology, this event was atypical.

Figure 2.  A surface observation plot on 19 December 2011 - 18 UTC with mosaic radar, sealevel pressure isobars, and a frontal analysis.  The freezing drizzle region is outlined in red and the red arrow points toward Limon, CO.

Perhaps the models could've provided some guidance for a forecaster faced with an atypical event.  According to the precipitation type forecast from the Short Range Ensemble Prediction System (SREF), the answer would be no.  None of the ensemble members from the 09 UTC forecast that morning showed any freezing drizzle in eastern Colorado for 18 UTC (1 pm MST).  The only areas with mixed phased precipitation that the SREF forecasted was for the synoptically traditional areas north of the polar front between the rain/snow boundary in western KS (Figure 3).  
Figure 3.  An SREF precipitation type forecast from 19 December 2011 - 09 UTC valid for 18 UTC.  The precipitation type shows as green shading for rain, blue shading for snow, and orange shading for freezing precipitation.  Each small panel represents a different ensemble member while the large panel shows the ensemble mean freezing temperature location.  Graphic provided by Penn-state e-wall website (http://www.meteo.psu.edu/~gadomski/ewall.html).
A forecaster depending on the precipitation type algorithms within the models would not likely anticipate the freezing drizzle event.  Even storm-scale high-resolution models had difficulty in predicting the freezing drizzle.  The High Resolution Rapid Refresh Model (HRRR) didn't provide any help (Figure 4) even when considering multiple consecutive runs.

Figure 4.  Three precipitation type forecasts from the HRRR starting valid for 18 UTC starting with the 14 UTC run (left), 15 UTC run (middle) and 16 UTC run (right).  Blue hatched areas represent snow, green hatching represents rain and red hatching represents freezing precipitation.

Perhaps this was a time to jettison the model precipitation type output and go it alone.  Consider the same SREF forecast output above but now we'll view the sounding profiles through BUFKIT.  And instead of looking at direct model output we use the top down method for determining precipitation type (should precipitation fall).  As it turns out the SREF output contained some clues about the likelihood of freezing drizzle as long as a forecaster understood the precipitation type top-down forecasting method as presented in this course in AWOC Winter (See slide 38).  In a vertical visualization mode for a model site at Limon, a shallow layer of high RH air was forecast by almost every model member in the lowest 3000' above ground for the 09 UTC forecast cycle valid at 17 UTC (10 am MST).  Then almost every run also depicted a dry layer above that indicated a lack of precipitation falling from the elevated moisture in the dendrite production zone (snow production layer in Figure 5). The lower moist layer was a little too warm to efficiently generate snow on its own, and without introduction of ice nuclei by either snow from above or other sources like power plants, liquid precipitation was the most likely result if precipitation were to form.

This scenario seemed to be supported by the echoes depicted by the KPUX radar where occasional spits of precipitation seemed to dry and then reemerge before finally closing in as the freezing drizzle was replaced by snow.  The lowest beam, after all, was sampling the dry layer forecasted by the SREF returns where the precipitation was initially sublimating. After some hours, the precipitation aloft finally managed to saturate the dry layer and the freezing drizzle turned to snow at Limon, but only after the damage was done.
BUFKIT has further guidance to help determine precipitation type.  The panel on the left side of Figure 5 shows an energy diagram indicating the amount of integrated energy above freezing vs the height of the freezing level.  Clearly since the sounding was entirely below freezing, the output of the precipitation schemes depicted snow.  Well, that initially doesn't seem like much help.  But recall, this was one sounding amongst an ensemble for one time period.

Figure 5.  The SREF 8 hour forecast from the 19 December 2011 09 UTC run shown in vertical profiles for Limon, CO.  The skewt on the right shows one of the ensemble member runs indicating the low-level moist layer separated from the high level moisture arriving from the south by a dry layer.  The energy diagram on the left shows a traditional precipitation type of snow if precipitation from the snow production layer was able to survive the descent through the dry layer. 

This run didn't trigger that alert because the coldest temperature of the lowest moist layer was just cold enough to trick BUFKIT to thinking that snow production may have been efficient enough to produce snow.  However, several other ensemble members did trigger an alert in BUFKIT to notify forecasters to consider freezing drizzle (See Figure 6).  Quickly scanning through the SREF members would've that at least a significant minority of members triggered BUFKIT in showing this message.  Even in the absence of this message, just about all members showed the background setup for freezing drizzle.

Figure 6.  Similar to figure 5 except one of the spectral model ensemble members is shown that triggers BUFKIT to warn the user that freezing drizzle is possible (yellow oval).

Even several deterministic runs of the RUC model also showed similar alerts as this model run in Figure 7 showed from the 12 UTC model run valid at 17 UTC.

Figure 7.  Similar to figure 5 except this is a single deterministic RUC model 5 hour forecast from the 12 UTC run.  This model run also triggered BUFKIT to warn the user of possible freezing drizzle.  The moisture profile is similar to the SREF runs.

As we see, the thermal and moisture profiles set the stage to allow freezing drizzle to form.  The problem was that precipitation was not guaranteed.  How could a forecaster be confident that the shallow moist layer would've produced precipitation?  That's the hard part.  The satellite image at 1745 UTC, showed the low-level upslope clouds banked up against the front range and also with implications of that layer extending east along the north side of the Palmer divide (Figure 8).  Northeast winds along the divide clearly provided a good upslope component into the divide.  Provide sufficient moisture for saturation at some starting point away from the divide and the forced upslope would've forced drizzle to form as that air ascended.  It's not a sure bet but given saturation in a shallow cold airstream that's about to ascend for one reason or another, shallow precipitation production seemed to be a possibility.  

Figure 7.  A GOES visible image and surface plot for 19 December 2011 - 18 UTC (1745 UTC for the satellite image) provided by NCAR/RAP (http://weather.rap.ucar.edu/). 

Given the nasty impacts of freezing drizzle, and its capability to cause havoc on unsuspecting travelers, we should pay special attention to the possibilities of its formation.  Forecasting drizzle is still a tough job and I'll be the last person to say that I would've forecasted this event had I been in the hot seat.  So my blog post is really to show that there are clues in the forecast guidance that might help for the next event. And these can give a forecaster the chance to score a major victory by calling attention to the possibility of freezing drizzle. Then its up to the road crews to respond in kind at least to the sensitive areas where they can make a difference even if the lead-time is relatively short.

Addendum - 21 December

I've received a few comments which I should relay.  Al Pietrycha commented that this event is not so rare after-all with respect to the synoptic conceptual models of freezing drizzle events in the Central High Plains.  Often there is freezing drizzle in the upslope ahead of the deep saturated lifting associated with a synoptic system.  I agree with his statement because the low-level upslope often saturates the boundary layer prior to the arrival of the large scale precipitation shield.  In the absence of the upslope, the airmass would likely remain unsaturated, as it often does further east.

A climatology of freezing drizzle was created by Cortinas et al. (2004) from which I include one of their figures below showing the number of days per annum of freezing drizzle.  The Central High Plains reported about one day.  The frequency increased further east to northeast.  However the climatology doesn't address the relative location of the freezing drizzle relative to synoptic systems.  Thus we don't know the relative impacts to transportation of other freezing drizzle events compared to this one.  It's likely many freezing drizzle events may have occurred after road treatments were applied.

With respect to other observational platforms that could detect freezing drizzle, Al mentioned two platforms: mesonet stations and webcams.  The former doesn't have a precipitation type detector directly but the anemometers often stop spinning upon the accumulation of ice and thus the onset of suddenly calm winds across more than one station would be a good indicator of freezing drizzle and rain.  Al mentioned that webcams are useful because the lenses often ice up during freezing precipitation events.

Cortinas Jr., J. V., B. C. Bernstein, C. C. Robbins, J. Walter Strapp, 2004: An Analysis of Freezing Rain, Freezing Drizzle, and Ice Pellets across the United States and Canada: 1976–90. Wea. Forecasting, 19, 377–390.        


  1. off topic, but I wrote it after reading your blog: http://not-that-sane.blogspot.com/2011/12/travails-of-english-as-second-language.html

  2. Lak, Thanks for pointing out in an interesting way that I really meant 'cues', not 'queues'.

  3. Jim - I thought you might be interested in a first-hand driving account for this event. I left Denver at ~10:20am. Light snow began just east of Denver and continued until I got to Strasburg. By the time I got to Deer Trail, the precip type was completely freezing drizzle with car saying it was 27 degrees. The roads looked fine, and had not been treated. Around Deer Trail was when I saw numerous cars that had skidded off. Between Deer Trail and Limon their were 20-30 vehicles involved in either a multi-car accident or had rolled. Almost all of the roll-overs were SUVs that were packed to the hilt. The valley through Limon was actually dry and ice free (no freezing drizzle), but as you climbed the hill east of Limon freezing drizzle made roads icy once again. However, it was the first time I had seen any road treatment in the form of gravel/salt mix (approx. 12:15pm). The rolled van was west of Genoa. By the time I reached Flagler it was becoming all snow again with temp still reading 27 degrees. While in a convenience store in Stratton (2:15pm), the tornado sirens started wailing. Burlington had a Sheriff's vehicle traveling as a pace car for east bounders for nearly 20 miles before turning around and heading back west (2:35pm). Just thought I would relay a time-line and some of the events while in transit. :-)

  4. Pam, Thanks for you most excellent account. I'm also glad you got through okay. It sounds as if the road was just treated west of Genoa and that the police car escort was mainly responding to the snow forecast since they should've been further west. But why were the tornado sirens blaring?

  5. After discussing the tornado siren issue with Al Pietrycha, it was relayed back to me that GLD had this happen during a winter storm in 2007(?). You may contact Al or Scott Blair for the details, but I guess power outages were responsible during the GLD event. I didn't notice the power flicker while I was in Stratton, but it might have flickered somewhere else in the town.

    Another thought I had but forgot to mention above was the number of SUVs that were involved in accidents or had slid off the road compared to the number of cars. It was the cars that tended to slow down while the SUVs and trucks continued at an inappropriate speed for the conditions. This brings up the whole "I have 4-wheel drive" false sense of security/safety concern. While 4-wheel drive does help in certain situations, driving on ice would not be among those (4 wheels in motion versus 2). Also interesting was that I saw no tractor trailers involved in any of the accidents I passed.

    On a side note, I agree with your addendum. Having lived in the Denver area for 25 years, it wasn't uncommon to have upslope along the front range produce a few hours of freezing drizzle before becoming all snow. It always made the work commute interesting. And you always knew it was going to start within the hour if you could smell a certain part of eastern CO. :)