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Next 12 Hours

Forecast data from the National Digital Forecast Database.

5 Day Forecast

• Sunday
• 
• High: 63 °
• Low: 50 °
• Rain

• Monday
• 
• High: 59 °
• Low: 50 °
• Rain

• Tuesday
• 
• High: 61 °
• Low: 48 °
• Rain

• Wednesday
• 
• High: 59 °
• Low: 50 °
• Rain

• Thursday
• 
• High: 59 °
• Low: 39 °
• Chance of Rain

Area forecast discussion 
National Weather Service Portland or 
215 PM PDT sun may 26 2013 


Synopsis... showers will taper off this evening as one low pressure 
system exits the area. A stronger...more organized system is expected 
to bring more rain to the region Memorial Day...with the potential 
for windy conditions along the coast. Zonal flow across the North 
Pacific will continue to guide systems onshore...keeping US wet at 
times through midweek. There is a hint we may dry out and warm up by 
next weekend...but there is still some 
uncertainty in this. 


&& 


Short term... tonight through Wednesday...scattered showers are 
about this afternoon. Aircraft soundings tell the story with moderate 
low level lapse rates with a cap around 7000-8000 feet. This confirms 
the popcorn low topped showers that do not last long in one 
area...but briefly accumulating with light to moderate rainfall due 
to the high precipitable water content in the lower level clouds. 
Activity has been least active along the central Oregon coast and 
Coast Range where surface high pressure is building...and most active 
along the Cascades...foothills...and northwest into the I-5 corridor of SW 
Washington. It will also remain breezy for a little while until the 
surface high nosing in along the coast pushes into the valley. This 
will shift the breezy conditions and confine them offshore. Showers 
will also decrease in coverage as the sun sets...but this dry period 
will be short lived. 


Confidence is high that Memorial Day will be a wet one. Rain will 
spread in from southwest to northeast during the day. The reason can 
be found by looking at the expansive cloud shield moving inside of 
130w this afternoon...out ahead of pronounced drying on the water 
vapor loop. The upper low is in the Gulf of Alaska...but energy ahead 
of the low is driving an occluding frontal system that also has a 
decent moisture tap with it as well. Best rains in the morning will 
be along the coast and Coast Range...spreading into the valley 
through the morning...then to the Cascades in the afternoon. Expect a 
transition to showery rainfall later in the day and more orographic 
driven precipitation in the evening. When all is said and done... 
with 1-1.5 inches possible in the Coast Range/Cascades and 0.25-0.50 
inch or a few spots with a little more in the valleys. Steady rain 
Monday morning will probably taper to showers for the valleys by late 
afternoon as synoptic lift weakens and SW flow aloft results in 
rainshadowing. Showers gradually decrease in coverage Monday 
night and Tuesday. 


This system is a bit more dynamic then the past few...so expect 
breezy conditions on the beaches and headlands...and even for a time 
inland in the later morning and afternoon as the pressure 
gradient...which is pretty easterly initially opens up. The gradient 
does weaken at this time as the low is lifting north. Could get some 
gusts in the 30-40 miles per hour on the coast...especially farther south...with 
gusts in the 20s in the valley in the afternoon. 


Moist SW flow continues Monday night into early Tuesday...and with high 
precipitable water values warrants the continuation of at least likely probability of precipitation...with 
the highest probability of precipitation in the elevated terrain. Probability of precipitation ease from SW to NE 
through the day again as high pressure noses back in. 


The strong zonal jet will likely push another similar system onshore 
Tuesday night or Wednesday...but the timing/best locations are a bit uncertain 
due to the fast flow across the Pacific. Small initialization errors 
can become large timing errors much beyond 24-36 hours when a strong 
zonal jet on the order of 150 knots is dominating the North Pacific. 
Models are trending to focus the energy with this system south of the 
area...but not south enough to suggest will be spared any rain with 
this one...thus a continued chance for wet and cooler than normal 
conditions through at least middle week. The best chance for any "drier" 
periods would be Tuesday afternoon and evening...though confidence on 
this is low to moderate at best. Kmd 


Long term...Wednesday evening through Sunday...lingering showers 
gradually subside late Wednesday and Thursday as high pressure noses 
in over the east Pacific. The main story for the extended period 
lies in the possibility for this weak ridge to develop over the 
region Friday and Saturday. Latest model runs have flattened this 
ridge a bit...with the European model (ecmwf) suggesting a more amplified ridge. Even 
though the GFS and Gem are now showing more zonal flow developing... 
we will still likely see a pleasant Friday with temperatures 
flirting back near normal and dry conditions overall. MOS guidance 
is going a little strong with high temperatures for Friday and even 
Saturday...have bumped highs up a few degrees but not going as high 
as the guidance due to model discrepancies. The next shortwave 
brings the next batch of showers and cooler temperatures Saturday. 
Now it looks like cool and showery conditions may linger through 
next weekend. /27 


&& 


Aviation...VFR clouds across the region with pockets of MVFR 
under scattered showers. Cascades frequently obscured in clouds 
into this evening. Not much change tonight...though will see 
showers decreasing so MVFR also decreases. Next system approaches 
later tonight and early Monday with rain. At that time...expect 
increasing MVFR on coast after midnight...while inland ceilings drop 
to 4000 to 6000 feet. Will have to wait until 12z to 16z for MVFR 
ceilings to increase over the interior. 


Kpdx and approaches...showery Mode today...with predominately VFR 
conditions. Will have brief MVFR under showers. Lower clouds 
clearing up this evening...then increasing middle and high clouds 
after midnight...with low VFR or MVFR ceilings after 12z Monday. Rockey. 


&& 


Marine...weak low now 40 miles northwest of Astoria still strong enough 
to pop S winds of 15 to 20 knots over north waters...with gusts near 25 
knots. Strongest winds within 30 miles of shore...and north of Tillamook. 
Have Small Craft Advisory up for that until 8 PM this evening. 
Otherwise...S to SW winds 15 knots or less. Seas holding near 5 feet. 


Next front well offshore will push across the coastal waters early 
Monday am. Winds S to southeast tonight...but as front nears...winds will 
increase...running 20 to 25 knots after midnight with gusts around 30 
knots. These winds continue through middle to late afternoon on Monday. Seas 
will build a bit...running 6 to 9 feet...mainly due to choppy wind 
waves. Highest seas will be over the outer waters. 


Winds relax Monday night behind the front...with seas falling back to 
5 to 6 feet. Another front Tuesday night and Wednesday may pop up winds 
again... but will wait until that front draws closer before making 
too much detail on winds. Rockey. 
&& 


Pqr watches/warnings/advisories... 
or...none. 
Washington...none. 
Pz...Small Craft Advisory for winds until 8 PM this evening for 
north coastal waters...or from Cape Shoalwater to Cascade 
Head from 0 to 60 nm offshore. 
Small Craft Advisory for winds from 1 am to 5 PM Monday on 
all coastal waters. 


&& 


$$ 


More weather information online at... 
http://weather.Gov/Portland 


This discussion is for northwest Oregon and southwest Washington from 
the Cascade crest to 60 nautical miles offshore. The area is commonly 
referred to as the forecast area.