Tuesday, February 24, 2015

Thunder Snow in Nome

This morning the National Weather Service noted the occurrence two lightning strikes concurrent with snow in Nome – thunder snow. This is quite the event. Figure 1 shows the text of the Special Weather Statement that was issued by the NWS office in Nome. Since the event was observed by NWS personnel, we assume that the report is valid. Unfortunately the two closest lightning detectors to Nome are both non-operational at the moment so we cannot tell how widespread the event was.


Figure 1. Special Weather Statement issued this morning by the Nome, Alaska, NWS office.

The METARs (see Figure 2) indicate the beginning and ending of the event. Several "special" METARs were issued that note the presence moderate snow and blowing snow concurrent with the lightning. The temperature was noted to be -2°C (28°F) during the thunderstorm.


Figure 2. Screenshot of METARs for Nome, Alaska, between 17:15 Z and 17:31 Z.

Thunder snow requires an unusual set of atmospheric occurrence to come together at just the right time. I am not a forecaster and am not qualified to provide context on the specific atmospheric dynamics at the time. However, for those out there more qualified than I am, here is the 12 Z (3 a.m.) upper air sounding from Nome this morning.


Figure 3. Upper air Skew T/Log P plot from Nome, Alaska, for February 24, 2015 (12 Z).

Just how rare of an occurrence is this? Surprisingly, it is not unprecedented. In fact there are 11 instances of thunderstorms in Nome with a temperature below freezing prior to today's occurrence. Figure 4 shows all of the hourly observations (1960-present) with snow and/or sub-freezing temperatures and a thunderstorm. Only 69 days in Nome have recorded a thunderstorm since 1960; the fact that 11 of those days were winter occurrences is remarkable. Even more amazing is that three instances occurred with a temperature of 2°F or colder!


Figure 4. Hourly observations from Nome, Alaska, that contain a thunderstorm and sub-freezing temperatures.

8 comments:

  1. Very interesting Brian. I read this a few days ago and wondered why it happens:

    http://www.adn.com/article/20150221/thundersnow-may-be-cause-flashes-arctic-sky

    Probably led early inhabitants to wonder what was coming next?

    Gary

    ReplyDelete
  2. The snowthunder events seem to be clustered around the few years around 1980. I would invoke something to do with PDO, but the dates don't support that. It seems that it might be a unique weather pattern within the larger patterns. What does the reanslysis say about the general conditions then as compared to the decade before and after? Are there weather patterns similar to what is found today? Is there records of thundersnow outside the Bering sea?

    ReplyDelete
  3. Very interesting. I noticed that this afternoon's Fairbanks AFD mentioned the thunder as follows:

    "SURFACE... A 1003 MB LOW 100 NM NORTHWEST OF POINT HOPE WILL MOVE TO 200 NM NORTH OF BARROW BY 3AM WED. A COLD FRONT ASSOCIATED WITH THIS LOW STRETCHING FROM POINT HOPE TO ANVIK WILL MOVE TO DEADHORSE TO FAIRBANKS BY 3AM WED THEN DISSIPATE ALONG THE ALCAN BORDER WED MORNING. CURRENTLY SNOW SHOWERS AND SCATTERED THUNDERSTORMS ARE CAUSING HEAVY SNOW ALONG THE FRONT ALONG WITH SOUTH WINDS GUSTING 20-40 KT AND 1-3 INCHES ALONG THE WEST COAST. THE STRONGEST WIDNS AND HEAVIEST SNOW ARE NORTH OF KOTZEBUE. THESE CONDITIONS WILL DIMINISH SIGNIFICANTLY AS THE FRONT MOVES INLAND TONIGHT WITH JUST SCATTERED SNOW SHOWERS BY THE TIMES IT REACHES THE EASTERN INTERIOR LATE TONIGHT. CONDITIONS WILL CLEAR BRIEFLY WEST OF THE FRONT."

    I looked very briefly at the data from Kotzebue, Fairbanks, and McGrath, and these all have some reports of winter thunder, but again mostly in the 1970s and 80s, and none in the ASOS period. Some of them must be bogus (I would think) because of the low temperatures, e.g. thunder at -7F in Fairbanks a couple of times, and once at -20F in Kotzebue! I wonder if there is any way to be confident as to which reports are real.

    The Nome sounding shows a pretty steep lapse rate above the isothermal layer (close to moist-neutral through a deep layer). Presumably there was enough instability somewhere nearby in the frontal zone to generate the rapid overturning necessary for cloud electrification. Too bad the lightning detection network is down!

    ReplyDelete
    Replies
    1. Thank you for looking into the other station obs. With stations that are unmanned, it is easy to dismiss a -20°F with thunder observation. But Nome and Fairbanks (not sure about Kotz) have NWS personnel on site. Also, I'm not sure when automated lightning detection systems were installed at Alaska stations.

      Delete
  4. Very cool. Great writeup. The past history of this at Nome is surprising to me, though I'm wondering about the cases where it was near zero F. Sure it is a manned station, but things happen. It looks like you have translated the old codes to the new ones (ie in the 70s the report would have read TS which you show for 1976 vs -TSSN, (METAR code) as shown on dates before and after in your table.) Maybe there are issues with pulling these reports?

    ReplyDelete
  5. I agree Jim. It's difficult to imagine thunder with temperatures near zero. The data came from the Iowa State IEM data archive. Here are the first four raw observations from Figure 4 in the post. I do not know if the observations were interpreted from another format or not – presumably they were.

    METAR PAOM 231200Z 03005KT 7SM -TSSN OVC/// M08/M11 A//// RMK SLP009 60083 T10831106

    METAR PAOM 240000Z 09012KT 7SM -TSSN OVC080 M18/M21 A2943 RMK 10AS RADAT ZERO SLP965 60000 T11781211 52024

    METAR PAOM 201800Z 36012KT 1SM TS CLR M17/M22 A3030 RMK XPEPX IPT OPWPE I RIUI SLP261 T11661216

    METAR PAOM 280000Z 07016KT 35SM TSSNPL OVC020 M17/M26 A2990 RMK THN SPTS IOVC RADAT ZER 9 SLP124 8/007 T11721256 411721350 57025

    ReplyDelete
    Replies
    1. Brian, All of the obs on table 4 were originally encoded in the old SA code and at least the earlier ones were hand written. (I know this since I worked as an observer in the early 80's...In Nome coincidentally). How they got into a computer and translated into the METAR code is unknown to me, but you can bet there were plenty of mistakes made in the process. I pulled up the context to the 1973 report from table 4. What I see is several things that cause me to highly doubt there was a thunderstorm that day, The weather progression is typical stratiform snow...only gradual changes in weather. And there are several cases of garbled remarks, and also important parameters in the body of the obs missing for no conceivable reason including the ob in question which is missing the ceiling hgt and the altimeter setting. Also, there are no remarks pertaining to a thunderstorm, something that would have be required ie., TB14. I think this one is a result of sloppy handwriting on a midnight shift and an automated conversion of the code. (wasn't me--this time!) If I find the time I'll look at the others, or try to dig up the soundings.

      METAR PAOM 222000Z 23005KT 35SM OVC035 M09/M11 A2947 RMK 10SCOME3/1 SLP980 T10881111
      METAR PAOM 222100Z 13005KT 15SM OVC035 M08/M11 A//// RMK -EPTXORIXQPSC/ Q-K SLP984 T10831106 53005
      METAR PAOM 222200Z 12003KT 15SM OVC035 M07/M11 A2948 RMK 10SCOME3/1 SLP984 T10721105
      METAR PAOM 222300Z 18004KT 15SM OVC035 M07/M11 A2948 RMK 10SCOME3/1 SLP984 T10721105
      METAR PAOM 230000Z 22005KT 15SM OVC/// M07/M09 A//// RMK SLP984 T10721089
      METAR PAOM 230100Z 33004KT 15SM OVC030 M06/M10 A2948 RMK 10SC SLP982 T10611100
      METAR PAOM 230200Z 34005KT 15SM -SN OVC028 M06/M11 A2948 RMK 10SC/SB39 SLP982 T10611105
      METAR PAOM 230300Z 32003KT 15SM -SN OVC030 M06/M09 A2948 RMK 10SC/ SLP982 T10611094 56002
      METAR PAOM 230400Z /////KT 15SM SN OVC027 M/M A//// RMK OIWXWQXQRXWUPEXORIXQPSC SLPNO T////////
      METAR PAOM 230500Z 05003KT 10SM -SN OVC025 M07/M11 A2949 RMK 10SC SLP985 T10721105
      METAR PAOM 230600Z 32003KT 15SM OVC/// M07/M09 A//// RMK SLP985 T10721094
      METAR PAOM 230700Z 12003KT 8SM OVC023 M08/M09 A2950 RMK 10SC SLP989 T10771088
      METAR PAOM 230800Z 05003KT 5SM -SN OVC021 M08/M09 A2950 RMK 10SC SB50 SLP989 T10831094
      METAR PAOM 230900Z 05003KT 3SM -SN OVC012 M08/M10 A2952 RMK / SLP996 T10831100 51010
      METAR PAOM 231000Z 34003KT 7SM -SN OVC023 M08/M10 A2953 RMK SLP001 T10831100
      METAR PAOM 231100Z 36003KT 5SM -SN OVC024 M08/M11 A2955 RMK 10SC SLP006 T10831105
      METAR PAOM 231200Z 03005KT 7SM -TSSN OVC/// M08/M11 A//// RMK SLP009 60083 T10831106
      METAR PAOM 231300Z 28003KT 7SM -SN OVC025 M08/M11 A2957 RMK 10SC OTZ BRW SLP013 T10831105
      METAR PAOM 231400Z 36005KT 7SM -SN OVC023 M09/M11 A2958 RMK 10SC OTZ SLP016 T10941105
      METAR PAOM 231500Z 00000KT 7SM -SN OVC023 M11/M12 A2958 RMK 10SC/ OTZ SLP018 T11061117 51008
      METAR PAOM 231700Z 00000KT 3SM -SN OVC010 M11/M12 A2961 RMK 10NS OTZ SLP026 T11051116
      METAR PAOM 231800Z 00000KT 4SM -SN BKN/// OVC/// M09/M12 A//// RMK SLP029 T10941117
      METAR PAOM 231900Z 00000KT 3SM -SN SCT010 SCT150 OVC200 M12/M13 A2964 RMK 2SF2AC6CS SLP036 T11161127
      METAR PAOM 232000Z 17003KT 6SM SN SCT012 SCT150 BKN200 M12/M12 A2966 RMK 1SF2AC6CS SLP043 T11161122
      METAR PAOM 232100Z 32003KT 7SM SCT014 SCT150 OVC200 M10/M12 A2968 RMK 1SC2AC8CS/INTMT S-/ SLP050 T11001117 52020

      Delete