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Area forecast discussion 
National Weather Service Medford or 
848 PM PDT sun Mar 21 2010 


Discussion...an update is note necessary. The latest radar and 
satellite images are in line with the 18z GFS and support the 
forecast of isolated to scattered showers through Monday morning... 
mainly over the coastal range and Oregon Cascades. The snow level 
forecast is also on track with them expected to drop to 3000 to 
3500 feet overnight. This shower activity is due to a few weak 
shortwaves arriving in northwest flow on the back side of an upper 
level trough. The trough is presently centered near Vancouver 
Island and will continue moving quickly eastward. 


A stronger shortwave will drop southeastward across Washington and 
northern Oregon on Monday. The majority of showers generated by 
the shortwave will occur north of our region but its passage will 
favor the development of a batch of isolated showers and snow 
showers Monday near Crater Lake and across northern portions of 
Klamath and lake counties. A high pressure ridge will build into 
the area Monday night and persist through Tuesday night with a 
thermal trough developing along the coast. 


&& 


Aviation...there will be areas of terrain obscured...especially 
the Oregon Cascades through Monday afternoon along with isolated 
showers and snow showers that will result in decks of lower clouds 
with MVFR ceilings. 


&& 


Previous discussion... /issued 250 PM PDT sun Mar 21 2010/ 


Short term...numerous showers observed over the region this 
afternoon as cooler air aloft filters in behind the front and a 
strong upper short wave approaches. Nearest lightning strike 
recently occurred near Portland. There will continue to be some 
showers into the night mainly west of the Cascades with heights 
bottoming out around 06z. A few lingering showers are possible 
across the west side into Monday under northwest flow with continued 
cooler temperatures...but high pressure returns through Tuesday with 
temperatures quickly rebounding to 5-10 degrees above normal. 
Continue mild and dry Wednesday with increasing south flow. 


The next front is timed similarly by the models to arrive 
Wednesday evening with most areas west of the Cascades getting 
wet by midnight. This front should allow for some healthy Small 
Craft Advisory southerlies on the coastal waters in addition to 
gusty south winds near Weed and across some of the higher terrain 
inland. Slightly sub-zero temperatures at 850 are expected behind this 
front which indicates the precipitation will turn to snow over the 
mountains and east above about 4000-4500 feet for the Thursday 
early morning commute. A quick few inches of snow at the passes 
look possible as both the GFS and ec are showing about .3" 
falling across the Cascades west of Highway 97 between 11pm Wednesday 
and 5am Thursday...with more possible into late morning Thursday 
mainly west side. Stavish 


Long term...Thursday morning Onward...models and ensemble plots 
remain in fairly good agreement with the notable exception of the 
Thursday evening...Friday time frame. Both models ensemble plots 
are indicating the first upper level low to deepen off the Oregon 
coast Thursday morning. From there the European model (ecmwf) deepens a closed low 
off the northern Oregon coast while the GFS opens the low and 
progresses it through the area. However the European model (ecmwf) solution is more 
of an outlier solution in the plots with the dgex being much more 
northerly with the low. All models are showing another closed low 
to move into the region again by Friday morning...with the European model (ecmwf) 
again being deeper and thus slower to move through. Although the 
operational run is more of the outlier with this system there are 
a number of plots that are similar. With this in mind have cut 
back the probability of precipitation to very ow chance along the coast and Cascades and 
slight chance elsewhere and will continue to walk it back tomorrow 
if models continue to trend in this direction. Models plots are in 
good agreement except for slight timing on the ridge building on 
Saturday ahead of the next system on Sunday which continues the 
trend of a shallower...quicker GFS and deeper...slower European model (ecmwf) 
solution. Sven 


&& 


Mfr watches/warnings/advisories... 
or...none. 


California...none. 


Pacific coastal waters...none. 


$$ 


Dw