Area forecast discussion 
National Weather Service Portland or 
308 am PDT Tuesday may 21 2013 


Synopsis...a front is spreading rain across the region this morning. 
A cold upper low will move over northwest Oregon this afternoon...resulting 
in lowering snow levels...convective showers containing small 
hail...and possibly a few thunderstorms through this evening. Cool 
and unsettled weather will continue through the work week as the low 
remains over the Pacific northwest. The low will weaken and shift further 
inland Friday...and the showers will decrease over the weekend. 
However...another approaching low will maintain at least a slight 
chance for showers Saturday through Monday. 


&& 


Short term...cool and wet weather is expected the next several days 
as an upper level low wobbles over the Pacific northwest. The upper low is 
near 52n 132w early this morning with a surface low further 
south...around 200 miles offshore of New Port Oregon. A front is 
quickly pushing through the region this morning spreading rain across 
Washington and or. 


The upper low will move south this morning and move over northwest Oregon 
this afternoon...bringing with it a pocket of very cold air...-22f at 
500 mb. Instability will increase due to the difference between the 
air aloft and at the surface...and the rain will turn to convective 
showers this afternoon. Many of these showers will contain small 
hail.. there is a possibility that a few thunderstorms may sprout up 
in the afternoon and evening...especially since there is no cap to 
restrict vertical lift. Added a slight chance of thunderstorms to the 
forecast. Do not have a good sense of where the likeliest location 
for a thunderstorm will be but think the Willamette Valley may be a 
more likely target than the coast...but added the slight chance to 
the entire forecast area. 


Snow levels will lower today and tonight and are expected to fall 
below the Cascade passes around sunset tonight and down to around 
3000 feet by Wednesday morning. Snow will be slow to accumulate on 
roads due to preceding warm Road temperatures but melting snow may 
still result in slick driving conditions and those traveling over the 
Cascades should prepare for winter conditions through Thursday 
morning. The late season snow will also be a threat for 
hikers...climbers...and campers. Th 


Long term...Thursday evening through Monday...several shortwave 
systems will rotate around a prominent closed low that will 
stubbornly linger across the region late this week. This means there 
will be periods of showers mixed with sporadic sun breaks through 
this weekend...with the bulk of the moisture pushing through the 
area Friday and Saturday. Behind the main frontal system...snow 
levels rise above Cascade Pass level and temperatures warm up closer 
to normal. Wet and unsettled weather appears to continue through 
early next week. /27 


&& 


Aviation...an elevated front is spreading light rain and clouds 
into the region this morning. The front is currently centered over 
the Willamette Valley. Expect a mix of VFR and MVFR along with 
continued periods of light rain to continue through the early 
morning hours. The front should push into the Cascades by 
daybreak and dissipate. A deep upper level trough will then push 
into the region...bringing widespread shower activity later today. 
Expect conditions to trend toward VFR as the day progresses and 
flow turns northwesterly. There is a chance of small hail in any heavier 
showers along with a slight chance of a few claps of 
thunder...primarily during the late afternoon and evening hours. 


Kpdx and approaches...expect a mix of VFR and MVFR conditions this 
morning with light rain as a front moves through early this 
morning. Conditions should improve to mainly VFR this afternoon. 
Widespread showers may bring a brief period of MVFR ceilings along 
with a chance for small hail during the late afternoon or evening. 
Pyle 


&& 


Marine...an approaching low pressure system will tighten the 
pressure gradient over the waters this morning...bringing 
increasing northwest winds in the 15 to 25 knots range. A Small Craft 
Advisory for winds will be in effect from just after daybreak 
through the early evening hours. There will be a chance for small 
hail and a few lightning strikes over the waters late this 
afternoon and evening. 


Borderline Small Craft Advisory winds will continue at times 
through Wednesday as the upper level low pressure remains parked 
over the Pacific northwest. The trend will be toward more benign weather late 
this week into next weekend as the upper low high pressure weakens 
and higher surface pressure builds into the waters. 


Seas should hold in the 4 to 6 feet through midday Tuesday 
increasing quickly Tuesday afternoon and evening in response to 
the gusty northwest winds. A small craft for hazardous seas will likely 
be necessary later Tuesday through Wednesday as fresh choppy swell 
pushes seas into the 10 to 12 feet range. 


&& 


Pqr watches/warnings/advisories... 
or...none. 
Washington...none. 
Pz...Small Craft Advisory for winds from 7 am this morning to 8 PM 
PDT this evening for coastal waters from Cape Shoalwater Washington to 
Florence or out 60 nm. 


Small Craft Advisory for rough Columbia River bar from midnight 
tonight to 4 am PDT Wednesday. 


&& 




$$ 


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.