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Area forecast discussion 
National Weather Service Seattle Washington
330 am PDT Wednesday Jul 23 2014 upper level low pressure system just offshore today
will produce widespread showers and a chance of thunderstorms over
western Washington. The low will migrate eastward across central
Washington tonight producing additional rainfall. Precipitation
amounts through tonight will be locally heavy...especially in the
mountains and along the coast. The low will move into southeast
British Columbia Thursday but wrap around moisture will still bring
a threat of showers into the area. An upper ridge of high pressure
will build Friday through the weekend and into early next week. Dry
and much warmer weather is expected.


Short unseasonably strong low pressure system will affect
the region today through much of Thursday. Various models are in
good agreement with the basic evolution of the synoptic scale
pattern the next 24 hours and it will be active. Water vapor imagery
indicates the closed upper low center and associated main vorticity maximum
now centered along 130 west offshore. The trough has become more north/S
oriented and is expected to become somewhat negative tilt as it
slowly moves toward the coast today. This is turning the flow more
southerly and is increasingly diffluent aloft over Washington. Strong vorticity
advection coupled with the 85+ knots jet shifting overhead by later
this morning will increase vertical motion dramatically. In
addition...precipitable water on the 00z kuil sounding was already
near an inch and a Plum of 1.5 inches seen on the satellite precipitable water
product is becoming entrained from a source region off the California coast.
The combination of moisture and strong dynamic lift will produce
widespread showers today with categorical probability of precipitation indicated.

At 230 am radar was already picking up some lightning strikes over
the Cascades of Snohomish County. The upper level forcing increases
ahead of and near the low center which supports a chance of thunder
in the forecast through this evening. Radar also shows a slug of
moderate rainfall expanding over western Oregon which will be the first
round of appreciable rain as it reaches western Washington over the next few
hours. Additional short waves riding northward through the area will
maintain bursts of moderate or heavier rainfall at times. Mesoscale
models including the NAM-12 and wrfgfs tally up .50 to .75 inches
across most The Lowlands and locally over 2 inches near the coast
and mountains. This system has the potential to set record rainfall
over the area. See the climate discussion below.

Showers will begin to taper over parts of western Washington later tonight and
Thursday morning. Some models show considerable wrap around moisture
as the upper low lifts NE into southeast British Columbia. The Post trough
surface pattern also becomes favorable for convergence zone
development anywhere from Seattle-Tacoma northward...mainly in the
afternoon. Some areas south of Tacoma and toward the coast may see
improving conditions with some afternoon sunbreaks. It will remain
quite cool with highs only in the 60s today and Thursday.

Split westerly flow aloft dominates the pattern Thursday night...then
upper heights start to build on Friday making it the transition day
toward dry and warmer weather. Highs will reach the 70s with partly
to mostly sunny skies by afternoon.

Long term...upper level high pressure amplifies over the northern
rockies and interior Pacific northwest. Models show rising heights over western Washington
through the weekend but the ridge axis remains far enough inland to
prevent any strong offshore flow from developing. looks
like a nice gradual warm up to the 80s. Some models such as the
European model (ecmwf) indicate the ridge could expand far enough west to induce a
thermal low over western Washington and even warmer temperatures. This is rather
uncertain this far out but above average temperatures appear likely
with dry conditions. Mercer


Climate...there have been 23 July days with 0.50 inches of rain or
more since records started at sea-tac in 1945. Only 3 of those days
have been since 2000...0.65 inches on July 16th 2001....0.60 inches
on July 20th 2012 and 0.58 inches on July 7th 2002. The record for
the wettest July day at sea-tac is 0.85 inches on July 13th 1981.
The wettest July day on record in Seattle is 0.98 inches on July 5th
1897 when records were kept at the federal building downtown. Felton


Aviation...upper level low about 200 nm west of khqm at 12z moving
east today. The low will move inland tonight. Southerly flow aloft
today becoming light tonight. Air mass somewhat unstable with
increasing moisture today.

Satellite imagery and Doppler radar shows long band of showers from
near Mount Baker to North Bend Oregon at 10z. This band will slowly
move east this morning. As the low approaches later this morning
into this afternoon showers will become more widespread. Isolated
thunderstorms are possible throughout the day. Ceilings at or above 5000
feet will lower later this morning into this afternoon down to
1000-2000 feet with the increasing shower coverage. Some improvement
tonight as the low moves overhead. The low will move off to the
northeast early Thursday morning but wrap around moisture on the
backside of the low will prevent ceilings from rapidly improving
until Thursday afternoon.

Ksea...ceilings 6000-8000 feet lowering down to 1000-2000 feet late
morning/early afternoon as the shower activity increases.
Thunderstorm chance at the terminal too low to put in the taf at
this point. Some improvement in the ceilings tonight. Light winds
this morning becoming southerly 10-15 knots this afternoon into the
evening hours. Felton


Marine...a weak 1015 mb low will remain over the coastal waters
today then dissipate tonight. With the low dissipating the low level
onshore flow pattern will redevelop tonight with Small Craft
Advisory winds possible over the most of the waters. Onshore flow of
varying degrees will continue Thursday into the weekend with high
pressure over the coastal waters and lower pressure inland. Small Craft
Advisory westerlies in the central and eastern Strait are likely
each day late in the day with Small Craft Advisory westerlies over
the northern coastal waters on Thursday. Felton


Sew watches/warnings/advisories...
Pz...Small Craft Advisory Puget Sound sound and Hood Canal.


Sew watches/warnings/advisories...



You can see an illustrated version of the forecast discussion at

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