Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
321 am PDT sun Jul 5 2015
Synopsis...unseasonably warm weather will continue through the
middle of this week. Cooler weather will arrive late this week as
an upper trough nears the area and onshore flow increases.
Short term...low level offshore flow will prevail today for a
very warm day. Highs will be in the middle 80s to middle 90s. Even the
coast will be very warm. Weak low level onshore flow will return
on Monday and Tuesday for a minor cooling trend. Highs will remain
well above normal however. A heat advisory is in effect from
Seattle southward through Monday. There should also be some
morning low clouds at the coast each day. Schneider
Long term...a weak upper ridge will move over the area on
Wednesday. This will cause highs to warm a few degrees. A cooling
trend should begin on Thursday as an upper trough slowly moves
over the Pacific northwest. Highs should cool to near normal
Friday and Saturday. There should also be more clouds and maybe
even a few showers. Schneider
Aviation...western Washington will remain between an upper level high
offshore and an upper trough over the northern rockies today. This will
result in northerly flow aloft. Thermally induced low pressure along the coast
will result in low level offshore flow.
Meanwhile...areas of LIFR ceilings/visibilities on the coast will dissipate by
Ksea...VFR. Winds will be northerly today.
Marine...an elongated area of thermally induced low pressure along
the Washington coast will result in north or NE flow across the area
today. There is a possibility that wind speeds could reach low end
advisory criterion late this morning over the northern inland
waters /north of the San Juans/ and parts of the Puget Sound sound.
The area of low pressure will weaken tonight and then move east of the
Cascades Monday afternoon. This will allow the flow to become weak
Fire weather...northeast pressure gradients will peak this
morning. The Bellingham-Williams Lake b.C. Gradient has risen to
around -10.0 mb this morning. This will bring drier Continental
air and gusty northeast winds to parts western Whatcom
County...the San Juan Islands...and the northwest olympic
peninsula today. The most critical conditions will occur late
this morning...during a period before winds die down but after
relative humidities have dropped for the day. The northeast winds
will usher in hot temperatures all the way to the coast...with
Forks expecting a high around 90 degrees today.
Most of the Puget Sound sound lowlands will see relative humidity drop
into the 18-30 percent range this afternoon. That range is about
as dry as we get around here. Meanwhile...the Puget Sound sound lowlands
along and west of I-5...especially the Kitsap peninsula...should
have north to northeast winds of 10-15 miles per hour early in the
afternoon. Winds of that modest magnitude with such low humidity
and such dry fuels could really push grass fires along
quickly...especially where wind and slope align. A red flag
warning is in effect today to handle the combination of wind and
low relative humidity today.
Lastly...the middle-level Haines index will be in the 5-6 range on
this afternoon. This indicates a dry and unstable air mass in the
850-700 mb layer. Fires burning at elevations above about 2000
feet under such conditions can become Rowdy and plume-dominated.
With such hot and dry weather...along with fuels that are
unprecedentedly dry for so early in fire season...please be
careful with flammable materials. Conditions are far beyond the
level of fire danger that has been experienced in the lifetime of
many who will be playing with fireworks this weekend. By the same
token...firefighters responding to wildland fires should expect
unusually volatile conditions. Haner/Schneider
Climate...there have been four consecutive days with highs of 90
degrees or above at SeaTac through Saturday...July 4. The record
for consecutive days with a high at or above 90 degrees is five.
This was set in August of 1981. It is almost certain we will tie
this record today with a forecast high of 95. There is a chance we
will break the record on Monday when the high will be near 90
Washington...red flag warning until 5 PM today for central and south Puget Sound
sound-Puget Sound sound lowlands-north coastal lowlands-Strait of
Juan Delaware fuca and northwest interior lowlands-west slopes of the
Heat advisory until 9 PM PDT Monday for Bellevue and vicinity-
east Puget Sound sound lowlands-Seattle and vicinity-Tacoma area.
You can see an illustrated version of this discussion...