Friday, February 14, 2014

Alaska (and United States) Record Low Wind Chill at Howard Pass RAWS?

On February 14th, the National Weather Service in Alaska noted that the Howard Pass RAWS station achieved a wind chill value of -92°F at 5:39 a.m. That value was close the the presumed statewide record of -96°F set at Prudhoe Bay on 1/28/89 (temp -56°F and wind speed of 21 mph at Prudhoe Bay). Later in the afternoon of the 14th, the Howard Pass station measured a wind chill value of -97°F at 3:39 p.m. (temp -42°F wind speed of 71 mph). If verified, this would represent a state of Alaska and possibly a United States record.

The Howard Pass RAWS weather station is in the Brooks Range of northern Alaska. The station is located at an elevation of 2,062' along the edge of the Noatak River valley and can be found here via Google Maps. Figure 1 shows some pictures of the station that were posted on the NWS Facebook page on 2/15/14. Figure 2 shows a satellite image taken near the time of the record low wind chill value.

Figure 1. Howard Pass station photos posted the the NWS Alaska Facebook page.

Figure 2. Suomi NPP satellite image from 2/14/14 at 2:17 p.m. Alaska Standard Time.

At first glance the wind chill values seems quite improbable. The temperature is not too surprising but the wind speed is remarkably high. This station has data going back several years and unfortunately the temperature data quality is not always very good. However, looking at the wind frequency distribution tables on the RAWS site (not shown), it appears that wind speeds over 60 mph occur pretty regularly during the winter months and a month-long average winter wind speed of 20-25 mph is typical. Based on the station climatology, extreme winds are not uncommon. Additionally, the wind speed show in Figure 3 does not appear to resemble the chart of a malfunctioning station anemometer. Note: I was not able to find metadata about the station's equipment.

Figure 3. Four-day chart of temperature, wind speed, wind gust speed, and wind chill values (updated 9:00 p.m. on 2/15/14).

The station is located north of the Arctic Circle and by mid-February it receives only a small amount of solar radiation. Therefore, the diurnal temperature is primarily a function of advection, clouds, and vertical mixing. However, the little bit of solar energy they did receive on February 14th correlates with the spike in wind speeds which suggests some thermal mixing occurred; although the soundings indicated winds under 20 knots in the lowest 3,200'. Local topographic constriction likely influences the channeling of northerly winds through the passes along the northern slopes of the mountains. Figure 4 shows the surface map near the time of maximum wind chill. Temperatures measured in the lowest 5,000' feet as recorded by weather balloons were quite cold. The Kotzebue and Barrow RAOB soundings showed temperatures between -20°C and -25°C at 850 mb.

Figure 4. NWS surface map near time of maximum wind chill.


Figure 5. Maximum 24-hour wind speed beginning at 2 p.m. on February 14, 2014. Howard Pass had a reading of 103 mph. (retrieved from NOAA interface of Mesowest data server)

Regionally, wind speeds were high, but not necessarily extreme (see Figure 5). The direction of the maximum wind speed was north or northeast at nearly every station. Wind speeds of these magnitudes are uncommon, but not unheard of.


 Figure 6. Low temperatures on February 14, 2014, in western Brooks Range. Howard Pass is -43°F. (retrieved from NOAA interface of Mesowest data server)

The Howard Pass station had a high temperature of -38°F (not shown) and a low of -43°F (Figure 6). Given the high wind speeds and the 850 mb temperatures in the -20s°C, temperatures in the -40s°F seems a little cold in my opinion.

Is it a valid reading?

That is the question. Neither the high wind speed nor the low temperature are especially surprising by themselves but the combination of the two is a little curious. On the surface there doesn't appear to be a 'smoking gun' to disqualify the reading but further investigation is warranted. Richard has conducted an impressive assessment of the pre-event numerical guidance. I highly recommend reading it.

* 2/18/14 Update *

Preliminary communication with the NPS technician  (not by this blog entry's author) who installed and maintains the station indicated that the equipment is designed for this type of environment and that the wind readings are unlikely to be revised downward. The anemometer is replaced annually and the design makes readings that are too high improbable. In addition, data from the previous several seasons appears to confirm the fact that wind is funneled through the north-south oriented mountain gaps and that especially low temperatures are entirely plausible. As the data continues to accumulate, the confidence in the observations in increasing.


15 comments:

  1. http://pafg.arh.noaa.gov/arhdata/sat/hrpt/14046014547/4f1f.jpg

    0145 2/15 POES IR pass shows some wind and temp action NE of Feniak Lake in the vicinity of Howard Pass. Cold in the Colville River Basin, and relatively warmer in the Noatak from my understanding.

    That Howard Pass area can get breezy as I recall from my fish survey days on the Noatak River.

    But I wouldn't bet $ on the accuracy of anything electronic subject to dark, wind, and cold temps without a recent calibration.

    Gary

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    1. Here's a map locator and station data set for the Howard Pass RAWS.

      http://amazon.nws.noaa.gov/cgi-bin/hads/interactiveDisplays/displayMetaData.pl?table=dcp&nesdis_id=39617266

      Gary

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    2. http://www.wrh.noaa.gov/mesowest/getobext.php?sid=HOWA2

      Now it's gusting to 93 ~9:30 AM 2/15...anybody want to head up there for some ground truth for a once in a lifetime event? With wind like that I'd expect the ground scouring would be intense.

      Gary

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    3. The chart has been updated Gary. I don't even know how anyone could get there? You would have to go by snow machine since the winds are too strong for flying.

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    4. The latest POES IR shows wind in that location. The other nearby stations report less wind but somewhat similar temps. I'm surprised the Noatak RAWS isn't as windy, same for the others.

      It is a funnel location on elevated terrain. The cold advection may be driving some intense winds. I'd think it'd effect the other RAWS and not disperse as quickly as it appears to be doing in that valley.

      With that wind orographic turbulence would be extreme.

      Gary

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  2. Previously it was stated that most temp stations are rated to about -40. What would be the rating for a typical anemeter? And how often are these stations calibrated? Case in point: the Fairbanks airport temp sign has been running 5 degrees warmer than actual conditions since they changed the display. I have a photo of it saying 104ºF from last summer when it should have been around 95 - 96 (sun was on it). If a very visible sign is off then what about those remote ones? Who and how often are they calibrated?

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    1. Eric, you have a good memory. The -40° temperature threshold was specific to the lithium battery that the Vantage weather station uses. Below that temperature the voltage drops and the reading are unreliable. The RAWS stations do not have that issue. I believe there are a couple of 12V batteries at those stations but don't quote me on that. According to Rick the anemometer is designed for the conditions at Howard Pass and is replaced annually. He spoke with the NPS technician and he feels that the equipment will not indicated winds stronger than they actually are. Also, looking at a fair amount of RAWS data, they appear to have temperature problems in the summer with poor ventilation but not in the winter.

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    2. RAWS are generally powered by 12V batteries held in insulated coolers, and will be covered with snow this time of year. It is a real question on how the anemometer (RAWS standard is 20' AGL) can survive these repeated extreme wind events. An independent sensor/station is needed, though that is not likely to happen.

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    3. The RAWS pic above looks like the mast is less than 20', based upon the battery box, solar, and anemometer's size.

      The wind measuring unit appears to be a Young product (http://www.youngusa.com/products/7/). Wonder which one they use...might be good for their future business advertising..."This unit survived the recent record wind chill in Alaska."

      Gary

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    4. I now see the wind data is missing for this morning...maybe I spoke too soon regarding the anemometer's longevity in the face of adverse weather. QC indicates quality issues as well. Maybe it's time to inspect the device.

      Gary

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    5. At 6:39 a.m. this morning the wind chill dropped to -96°F. Since the temperature is now -41°F, the record set on 2/14 might be in jeopardy.

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    6. Jeopardy? Not without an estimate of wind speed. Maybe it's simply a matter of Internet data logging not keeping up with the unit's output.

      The 07:39 AKST reading of 23G96 seems a bit suspect. With that short a mast maybe the reported snow depth from drift (93") is impairing the anemometer. But that's unlikely as the lower photovoltaic unit is apparently still functioning, and snow tends to accrete downstream of a solid object, not upstream. Woods Rule #1: face the outhouse door into the prevailing wind or prepare to shovel.

      Well, we'll just have to wait and see if the wind info returns.

      Gary

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  3. It must be a sturdy little devil to withstand that wind, especially given the flat plate area of the solar panel. Sitting on a rise in terrain in a valley subject to contained flow of wind. Bernoulli would love the airfoil effect on wind flow that hill provides.

    Still, today's gust of 103 is a brisk wind if true. The recent 93" of snow reported is suspect, but might just be drift. I'm amazed it's still intact.

    Gary

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  4. How History is written:

    http://www.adn.com/2014/02/15/3328190/howard-pass-wind-chill-reaches.html

    Gary

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    1. Someone at the NWS asked if they could use the chart I made and I said yes. As you have probably noticed, I do put my names on maps but not charts. They added my name to the chart. I feel uncomfortable about the attention because Rick is the one who noticed and investigated the event.

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