fire season in the Southern Oregon Area typically begins in mid-June coinciding
with the longer warmer and drier months of June July August and September
generally temperatures are higher in the southern parts of the Area than they are
in the northern part in addition to this difference the area can be understood as
having two distinct climatic zones the coastal strip west of the Coast Range
and the interior valleys of the southern Willamette Umpqua and Rogue
the coastal strip extends from Florence in the north to Gold Beach in the far south and is
bounded by the Pacific Ocean on the west and the Coast Range to the east
temperatures along the coastal strip are more moderate than those of the interior
valleys of the Southern Oregon Area average daily high temperatures in the
summer months are above 65 degrees with the hottest day of the year typically
falling in the third week of August the dry period in which the chance of
measurable precipitation is considerably less begins the first week of July and
extends through most of August on average the driest day of the year for
cities of Florence Coos Bay Bandon and Gold Beach is July 29th
in the summer months winds are predominantly out of the North
however wind speed and direction along the coast are highly variable and
dependent on localized topography and other factors in general the coastal
strip is windy hourly wind speed averages are above six and a half to
seven miles per hour mornings are usually calm with wind
speeds picking up through the afternoon as temperatures rise in the interior
valleys of the Southern Oregon Area the hot dry period normally lasts from late
June through the latter part of September average daily temperatures
range from 78 degrees in the north and increase to the mid-80s in the south the
hottest day of the year usually occurs in the first week of August
with an average high temperature of 79 degrees in the north and 93 degrees in
the South the rainless period in which there is little to no measurable
precipitation begins a mid July for the northern interior valleys and lasts
until mid-august in the southern parts of the area this dry period commonly
begins in late June and lasts into the middle or latter part of September
like the coastal strip prevailing winds of the interior valleys are commonly
from the north during the summer months but are heavily influenced by local
terrain and other weather factors on average the winds are more moderate in
the interior valleys with average hourly wind speeds of between two and six miles
per hour while there are many factors that influence specific local weather
conditions there are general weather patterns to be aware of that can impact
fire suppression efforts in the Southern Oregon Area these generalized patterns
reoccur annually during the warmer months the development of the West Coast
Thermal trough a large-scale weather feature significantly affects
temperature wind cloud cover and precipitation this feature is
characterized by a ridge of higher temperatures in the lower atmosphere it
develops when a strong upper-level high pressure moves into position along and
near the west coast and remains there for an extended period of time
due to heating of the landmass surface pressures lower in California and extend
into Western Oregon this is sometimes referred to as an inverted trough
West Coast Thermal Trough events are commonly tied to offshore winds and downslope
flows resulting in warmer drier air and less cloud cover
this causes abnormally hot and dry conditions west of the Cascade Mountains
when a strong thermal trough is coupled with east winds fire danger is greatly
increased the development and movement of the west coast thermal trough affects
a number of other meteorological measures that affect fire behavior and
has profound impacts on local weather conditions one of the most significant
effects of the West Coast Thermal trough is the role that it plays in the
development of strong east winds that occur annually in the Pacific Northwest
these winds occur during the warm season when a strong surface high pressure
develops out over the Pacific and areas of it move inland and eventually travel
east of the Cascades it's the pressure differences over a relatively short
distance that are its main driver of the East offshore winds the strongest east
winds usually occur first as the thermal trough strengthens coinciding with the
drying of the air mass aloft this is why the ridge tops of the mid slopes dry out
first east wind events usually last for a few
days but can stand for longer periods of time if the
thermal trough is especially strong in the fall when the west coast thermal
trough begins to weaken and move inland the east winds can be particularly
strong this produces hot dry windy conditions along with instability as the
thermal trough moves east and crosses the Cascades a change in wind direction
from the east to west also occurs this event typically signals the end of the
warm season as it brings with it a marine layer push with higher humidity
cloud cover and an end to the east wind
however this fall transition period is a particularly dangerous time for fire
occurring when fuel moistures are at their lowest points
another significant weather event in the Southern Oregon Area are lightning storms
the Area averages approximately 100 lightning- caused fires a year
this is about a quarter of the Area's 10-year average of nearly 500 fires per year
lightning storms can occur year round but are most frequent beginning in June and peak the
last week of July through the first few weeks of August
lightning storm activity commonly begins to decline significantly by the third week of September
these storms begin as offshore lows in the Pacific and develop into maturity by
pulling warm moist air from the deserts in the South and Southeast
as a general rule lightning activity is more frequently in the area's southern
districts centered around Medford Grants Pass and Roseburg and also along the
spine of the Cascades where there is the most instability in these more lightning-
prone areas it is not uncommon for lightning to start dozens of fires over
the course of a 24 hour period the coast range while not immune to lightning
activity sees significantly fewer lightning-caused fires
the prevailing north winds typical of this summer months can be interrupted by dry frontal
passages in which the air of the valley floors heats faster than that of the
mountains causing strong gusty winds as these air masses collide wind speeds
increase and change direction as the name implies dry frontal passages occur
with little to no precipitation and lower relative humidities these type of
frontal passages are more common in the fall and can significantly impact
efforts and increased occurrence of human-caused fires
the depth of the marine layer along the Pacific Coast varies daily commonly extending up river
valleys into the interior during the nights and decreasing as the day warms
into the late mornings and afternoons the moist air of the marine layer
increases humidity and greatly inhibits fire growth at its greatest extent this
effect can be seen up the Siuslaw Coos Umpqua Coquille Rogue and Chetco Rivers
for many miles into the interior valleys as the marine layer begins to dissipate
it is accompanied by increased wind speeds that align with the afternoon
burn period and increased fire behavior
this generalized description of weather
conditions in the Southern Oregon area is not intended to replace the specific
information contained in a local briefing in addition to an up-to-date
fire weather briefing firefighters need to be aware of current energy release
component indices this information can be found on local national fire danger
rating system pocket cards
you