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Area forecast discussion 
National Weather Service Seattle Washington
330 am PDT Tuesday Jul 22 2014

Synopsis...an upper trough off the central British Columbia coast
will shift southward off the Washington coast later today and
tonight. The trough will move inland over the Pacific northwest
Wednesday and Wednesday night resulting in a moist and unstable air
mass. Clouds and showers will taper off on Thursday as the trough
moves east of the area. High pressure will build into the area
Friday through Monday. Expect dry weather and above average
temperatures beginning this weekend.

&&

Short term...current satellite and radar show a band of showers
extending from near Portland northeast across the South Washington Cascades
and some areas east of the crest. Water vapor imagery shows the
short wave over Oregon is becoming entrained in southwest flow aloft
over Washington. This flow pattern will probably keep most the precipitation
south and east of the Puget Sound sound region most of the day. A couple
models hint at a few showers reaching the foothills and possibly
south of Puget Sound sound in the afternoon so low probability of precipitation remain in the
forecast. Otherwise...mostly dry with increasing middle and high
clouds.

The forecast becomes tricky beginning tonight as the upper low off
British Columbia moves south off the Washington coast. This position will
turn the flow more southerly over western Washington allowing moisture off the
California coast to become entrained. Most models including the wrfgfs show
this pattern developing tonight. The amount of instability aloft is
in question as any convection will not be surface based. The NAM-12
does show a more unstable lapse rate mainly between 700-500 mb but
other models would imply more stratiform type precipitation. Decided
to increase shower coverage and put in some isolated thunder after
midnight given the favorable southerly flow pattern.

Upper level low pressure off the coast shifts eastward right across
Washington on Wednesday. The flow aloft is strongly diffluent ahead of the
low center which in itself implies some risk of a thunderstorm.
Models pretty well agree on strong vorticity advection and vertical
lift increasing during the day. Decided to increase probability of precipitation and add a
chance of thunderstorms. Model quantitative precipitation forecast amounts are impressive for this
time of year and could challenge some rainfall records. The 4 km
wrfgfs seems to have a good handle on the system and is showing 24
hour totals from Wednesday morning through Thursday morning of
around a half inch over The Lowlands and one to two inches in the
mountains.

Moist onshore flow lingers into Thursday with wrap around moisture
possible as the upper low becomes negative tilt and exits the area.
The forecast is trended toward increased probability of precipitation and cloud cover.
However...some improvement is indicated for the afternoon. This will
depend on how quickly the system exits and strength of Post-low
onshore flow. High temperatures Wednesday and Thursday will remain
cool for this time of year...struggling to reach out of the 60s.

Long term...the long range forecast looks dry with a warming trend
still in the cards. The GFS/European model (ecmwf) both show gradually rising upper
heights and weak onshore or northwest flow. This should steer any
systems well north of the area. The Four Corners high strengthens
over the weekends and expands north and westward. 500 mb heights
could reach above 5990 mb by Monday if the European model (ecmwf) is correct. High
temperatures should easily reach into the 80s this weekend and into
Monday...and possibly warmer by Monday. Mercer

&&

Climate...unusual rain event for late July possible on Wednesday.
The record rainfall for July 23rd at Seattle is 0.54 inches set in
1949. The second highest rainfall total for July 23rd in Seattle
including the federal building records which go back to 1891 is 0.06
inches in 1959...1952 and 1912. There is the potential on Wednesday
for the daily precipitation record in Seattle...as well as other
locations in western Washington...to be broken. Felton

$$

Aviation...upper level trough offshore with the southwesterly flow
aloft. Air mass stable becoming somewhat unstable by late afternoon
with increasing middle level moisture especially south part.

Middle and high level clouds embedded in the southwesterly flow aloft
moving over mainly the southern portion of the area this morning
with the cloud cover spreading north in the afternoon. Shower
activity increasing tonight with ceilings lowering down into the
3500-5000 foot range.

Ksea...middle and high level clouds today with light winds becoming
northwesterly 4 to 8 knots in the afternoon. Ceilings lowering
tonight but still remaining above 3500 feet. Felton

&&

Marine...a weak 1015 mb surface low will move into the coastal
waters tonight and remain through Wednesday morning. This pattern
will create light pressure gradients over the waters today through
early Wednesday. Pressure gradients increasing later Wednesday into
Wednesday night with Small Craft Advisory winds developing over most
of the waters Wednesday night or Thursday. Winds easing Thursday
night. The onshore flow pattern will return Friday into the weekend
with Small Craft Advisory westerlies in the central and eastern
Strait late in the day. Felton

&&

Sew watches/warnings/advisories...
Washington...none.
Pz...none.

&&

$$

You can see an illustrated version of the forecast discussion at
www.Weather.Gov/Seattle/gafd/latest_webafd.Html

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