Area forecast discussion 
National Weather Service Medford or 
313 am PDT Thursday may 23 2013 


Discussion...current water vapor image shows the upper low spinning 
near the Oregon/Washington border and this is expected to continue 
through Friday morning before weakening and lifting northeast into 
Washington state Friday afternoon. A weak upper trough remains over 
the forecast area Friday night through Saturday. 


Precipitation has decreased and ended during the overnight hours. 
However expect showers to increase again late with a series of 
shortwaves rotating around the upper low. Most of the showers should 
be concentrated along and west of the Cascades and northern Klamath 
and lake counties and more isolated in northern cal. There may be 
enough instability for isolated thunderstorms east of the Cascades 
in the afternoon...but this will be the exception. Most hours and 
locations will have just instability showers. 


Another shortwave rotating around the upper low is expected to move 
into the northwest part of the forecast area tonight resulting in a 
continuation of showers mainly north of the Umpqua Divide and 
Cascades near and north of Crater Lake. Elsewhere showers are 
expected to be more isolated. Plenty of cloud cover remains over the 
area which will limit the amount of cooling at night. Not as 
confident we'll get a sustained period of freezing temperatures in 
the Klamath basin and Shasta Valley. 


The upper low will move northeast into Washington state Friday and 
as it does showers will decrease from south to north with most 
exiting the forecast area by Friday evening. We'll catch a break in 
shower activity Friday night. 


Another...but weaker upper trough swings into the area Saturday 
morning with showers expected to increase west of the Cascades. The 
NAM shows quantitative precipitation forecast breaking out east of the Cascades in the afternoon 
which is most likely a result of increasing instability and weak 
shortwaves passing by. 


Weak trofing remains over the area Sunday. However the GFS suggest 
the trough axis will move east in the afternoon with weak ridging 
building from the west. Confidence is not high on the GFS solution 
therefore we'll keep the chance of showers in throughout the day. 
-Petrucelli 




&& 


Aviation... 


Based on the 23/06z taf cycle. 


A persistent stream of moisture will continue to move into the north 
coast, Umpqua, and north Cascades this morning. This will keep 
light rain going along the coast with scattered to numerous 
showers inland to the Cascades with MVFR ceilings and occasional MVFR 
visible over this area. Mountain obscuration will also be fairly 
widespread on the west side. Showers with MVFR ceilings are also 
ongoing in the Klamath basin, but these showers will decrease 
around daybreak. Showers are much more isolated with general VFR 
conditions elsewhere on the east side and California. -Wright 


&& 


Mfr watches/warnings/advisories... 
or...freeze warning until 9 am PDT this morning for orz029. 


California...freeze warning until 9 am PDT this morning for caz084. 


Pacific coastal waters...Small Craft Advisory for hazardous seas 
until 11 PM PDT this evening for pzz350. 
Small Craft Advisory for hazardous seas until 11 am PDT this 
morning for pzz356. 
Small Craft Advisory for hazardous seas until 11 PM PDT this 
evening for pzz370-376. 


$$