Western Washington Weather for Wilderness Wanderers

 

  1. Know signs and patterns:
    1. Falling barometer (altimeter reading high), south or southeast wind, layer clouds thickening and descending beginning with cirrus è low or trough or (warm) front approaching.  Onset of bad weather may be from eight to forty-eight hours; fastest in fall and winter.
    2. Rising barometer, winds from west or northwest, convective clouds (towering cumulus, thunderheads) è period of showers, gradually decreasing.  May last a few to twenty-four hours (even more).  Convergence zone possible west slopes of Cascades somewhere (depending on low-altitude wind direction) between roughly Three Fingers and north side of Mount Rainier, but most frequently between S. Fork Stillaguamish and Snoqualmie Rivers.
    3. Lenticular or other mountain wave clouds, clear or high clouds above è humidity increasing, strong pressure gradient with higher pressure upwind.  Often harbinger of storm.  High winds on ridge tops and isolated summits.
    4. (a), (b), and (c) require rising air è cools at rate 9.8°C/1000m = 5.5°F/thousand feet while unsaturated, then more gradually (say about 6°C/1000 m = 3.3°F/thousand feet) in cloud.
    5. Stratus or stratocumulus, calm air, steady barometer, clear skies above è fair weather, clouds probably disappearing by afternoon.

 

  1. Peculiarities of western Washington weather:
    1. Compared to almost anywhere else, equable—rarely hot, rarely cold, rarely windy, rarely subject to heavy precipitation.  But humid, even clammy. Temperature changes small.  Showers and thunderstorms typically post-frontal, not subject to pronounced regular diurnal (afternoon buildup) cycle as in other mountains.
    2. Susceptible to remarkable geographic variability, large differences occurring over short distances.
    3. East and northeast slopes generally much drier than west and southwest.
    4. Precipitation subject to remarkable local variation, with little consistency from one storm to the next.

 

  1. Value of altimeter for forecasting:
    1. Altimeter reading too high è air pressure is falling; too low è rising.
    2. Unless weather really hot or cold, error translates to (sea level) pressure change, in ratio: 8 meters (or 25 feet) of error = 1 millibar change in S.L.P.
    3. A fall of one millibar or more per hour sustained for several hours is a warning to be heeded.

 

  1.   Resources to check before an outing:
    1. Start from University of Washington Atmospheric Sciences Department site: http://www.atmos.washington.edu/data/ .
    2. (Minimal) essential preparation: read “National Weather Service Zone Forecasts” for your intended geographic zone; read “Washington State Forecast Discussions”; get the detailed avalanche forecast from “Northwest Weather and Avalanche Center”. 
    3. More really great information: “Northwest MM5 Regional Prediction System”; driving conditions from “WSDOT Road Weather Information System”.  Also twenty-four hour snow depth telemetry data, available from the Northwest Weather and Avalanche Center Web site.
    4. As a general rule, check commercial weather “information” sources, weather satellite photographs, and radar images last; less informative for our specific purposes.

 

  1. Further information:
    1. Northwest Mountain Weather, by Jeff Renner, published by The Mountaineers Books. A fine, well-written introduction.
    2. Cloud photographs at numerous Web sites. Especially recommended: http://www.atmos.washington.edu/gcg/Atlas/  and http://ozthunder.com/photo/clouds.htm .
    3. For weather and altimetry, a basic introduction is http://www.wrh.noaa.gov/Portland/firedata/ALT.HTM .
    4. For equations involving Standard Atmosphere and general altitude-versus-pressure relationship, e-mail me. (If you know of a good non-technical source of information about this, please also e-mail me.)
    5. For help in interpreting weather maps and charts, start with http://weather.unisys.com/  and rummage around, clicking on “More Information” often. 
    6. Better information is usually quite a bit more technical, requiring some understanding of basic mathematics, physics, and chemistry. Numerous texts are available for those willing to confront education at this level.  I myself rely on Atmospheric Science: An Introductory Survey by John M. Wallace and Peter V. Hobbs, published by Academic Press.  Look for new edition soon. Also have a look at http://www.auf.asn.au/meteorology/index.html .
    7. A degree in meteorology is not necessary to benefit from the National Weather Service “Forecast Discussions”, but a translating dictionary is. The basics can be found at http://www.hpc.ncep.noaa.gov/html/contract.html  and http://www.crh.noaa.gov/dtx/afdterms.htm .  A few little things you may find confusing at first: (a) Times are almost always given as UTC or Z, i.e. Greenwich.  Subtract eight hours for Seattle, seven when daylight time is in effect.  (b) Weather maps, other than surface, are plotted at constant pressure, rather than constant altitude, so, for example, a useful map for many of our purposes is the 850-millibar constant-pressure map.  Highs and lows are shown by height contours.  (If the altitude is low at 850 millibars, then at a higher, more typical altitude for 850 millibars, pressure will be lower than 850.)  (c) It’s useful to know that 850 millibars is roughly 1500 meters or 5,000 feet and 700 millibars is roughly 3000 meters or 10,000 feet.  (d) “Thickness” is the difference in height between two pressure levels, for example between 1,000 millibars and 500 millibars.  (That’s essentially the lower half of the atmosphere.) When the thickness of a layer of air is low, that layer is cold, so thickness maps are often used as simple guides to determine where the upper air may be unusually cold (or warm). (e) Meteograms derived from the local executions of the MM5 are plotted backwards (from right to left). 
    8. With questions or suggestions, e-mail BillFortney@aol.com .