Scientific Forecaster Discussion

NWS Discussion
			
				

Area forecast discussion 
National Weather Service Portland or 
234 PM PDT Sat may 18 2013 


Synopsis...plenty of clouds and occasional rain today as a weak low 
pressure system slowly moves into Oregon and Washington. Showers will 
slowly decrease tonight and Sunday as the system moves into the 
Great Basin and falls apart. More sunbreaks Sunday should lead to 
more seasonable temperatures. Monday is shaping up to be a pleasant 
sunny and warm day under high pressure with temperatures in the lower to middle 
70s. High clouds will begin to increase from the northwest later in 
the day in advance of our next system...which will bring a dramatic 
change back to chilly and unsettled weather Tuesday through Friday. 


&& 


Short term...today through Tuesday night...visible satellite imagery 
shows solid cloud cover across the district this afternoon. Krtx 
radar shows an area of light rain spreading across Washington County 
and entering the greater Portland and eventually Vancouver metropolitan 
area. Additionally areas of light rain persist along the coast and 
much of the rest of our forecast area. Snow levels remain rather 
high...odot pass cams show occasional rain and it appears to be 
raining at timberline as well. 


Weak and poorly defined low pressure circulation just offshore from 
Astoria will drag a weak cold front onshore over the next few hours 
with decreasing showers thereafter. What is left of the front will 
finally lumber east across the Cascades tonight with decreasing 
showers behind the front. By Sunday morning showers will be mainly 
confined to the higher terrain...though a stray shower is still 
possible elsewhere due to weak shallow instability. 


Made very few changes for Monday and Tuesday. The move the midnight 
shift made to bump up Monday temperatures into the 70s appears to be a good 
one. The air mass looks rather dry throughout Monday on NAM bufr 
soundings...from Eugene all the way up to Portland. With 850 mb temperatures 
pushing +10 degree c in the Willamette Valley...highs could get well 
into the 70s some areas with plenty of sunshine. Onshore gradients 
will keep the coast from warming up too much. 


Models showing good agreement on rain from our next system holding 
off until Tuesday morning. As Tuesday progresses it appears an 
unseasonably deep and cold upper low will sink southeast into Washington/or 
spreading rain across the region. Rain appears likely a good portion 
of the day Tuesday...with snow levels lowering to the Cascade passes 
Tuesday night. There is an outside chance we may need to eventually 
consider snow advisories for the Cascades as early as Tuesday night. 
Weagle 


Long term...Tuesday evening through Saturday...few changes. Closed 
low pressure system will linger over the Pacific northwest through the week. 
With the cold nature of the low...record low maximum temperatures 
are possible on Wednesday...with snow down to pass level likely in 
the Cascades. Thursday and beyond models begin to disagree on where 
the low will propagate...however the overall trend will 
remain...with showery and cool conditions persisting through at 
least Saturday. Burgess 


&& 


Aviation...mix of VFR and MVFR ceilings across the region. A 
weakening front is pushing across the region...but back edge of 
the front is still offshore and not likely to clear the Cascades 
until this evening. Not much change...with scattered showers and 
mostly low VFR and pockets of MVFR through tonight. Cascades and 
passes will frequently be obscured in clouds. Showers should 
diminish overnight behind the front...with MVFR ceilings at the coast 
and mostly VFR inland. Some patchy fog possible tomorrow morning 
in the southern Willamette Valley if any clearing 
occurs...possibly affecting keug. 


Kpdx and approaches...expect a mix of VFR and MVFR in scattered 
showers through this afternoon as a cold works its way through. 
Shower activity should decrease later this evening behind the 
frontal passage. Mostly VFR conditions overnight. Pyle 




Expect light rain or showers off and on through the day. 
Predominantly VFR conditions may drop to MVFR at times during heavier 
showers. Shower activity should taper off somewhat this evening... 
with mainly VFR conditions during the evening and overnight hours. 
Pyle 


&& 


Marine...a cold front is pushing through the coastal waters this 
afternoon. The front has been slower than expected...so decided 
to extend the Small Craft Advisory for winds into the early 
evening hours. Expect winds to decrease and turn northwest later this 
evening behind the front. High pressure will build over the NE Pacific 
sun into early next week. It appears that there will be a few 
chances for small craft advisories for winds over the next few 
days. Sunday and Monday may see wind gusts into the 20 to 25 knots 
range during the late afternoon and early evening hours. 


Seas remain at 5 to 6 feet through the weekend. Seas may bump up a 
bit during the periods of gusty winds on sun and Monday. A large 
low pressure system may bring another small craft for winds on 
Wednesday...along with seas above 10 feet. Pyle 


&& 


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


&& 


$$ 


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.