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Area forecast discussion 
National Weather Service Medford or 
849 PM PST Friday Nov 21 2008 


Discussion...a weak cold front is just about to reach North Bend. 
It will move across southwest Oregon through late tonight then 
dissipate across south central Oregon and northern California 
Saturday afternoon. 


The 00z GFS and 12z European model (ecmwf) are in good agreement through Tuesday night. 
High pressure will build into the region Saturday night through 
the weekend while the next trough offshore drops southward. This 
will result in areas of night and morning valley fog Saturday 
night through Monday morning. 


The upper low is expected to reach the central California coast 
early Tuesday. Slight chance to chance probability of precipitation seem in order for 
Tuesday into Wednesday with wrap around moisture in southerly flow 
giving The Mount Shasta area the highest probability of measuring 
precipitation from showers. 


The model differences become significant Wednesday but at this 
point the extent of the similarities lead ME to expect some 
semblance of weak ridging between systems Wednesday night with 
valley fog developing...then the next trough approaching from the 
west-northwest on Thanksgiving day. The current forecast of chance 
probability of precipitation on the west side of the Cascades and slight chance on the 
east side looks appropriate given the uncertainty of the track and 
strength of this trough but growing probability that there will be 
a system moving into the region. Beyond Thursday night there are 
few model similarities. 




&& 


Aviation...the trend is for ceilings to lower overnight tonight as 
the cold front moves into the area. Ceilings will lower to MVFR at the 
coast this evening with IFR overnight tonight. Ceilings tonight could 
lower to near 2000 feet with a scattered deck near 1000 feet. 
Inland ceilings should remain above 3500 feet tonight into Saturday 
morning. However there will be occasions where ceilings from broken cloud 
cover will lower to 2000 feet. There is the potential for patchy 
fog to develop in the Rogue Valley early this evening...before the 
cloud deck lowers from the incoming front tonight. 




&& 


Previous discussion... /issued 222 PM PST Friday Nov 21 2008/ 


Discussion...the leading edge of the cold front is reaching our 
outer marine waters. The NAM and GFS appear to be too slow with 
the timing of the front. They both suggest the front will reach 
the coast until around 7 PM PST. So have opted to go with 
increasing probability of precipitation at the coast this evening...then spreading inland 
overnight tonight as the front continues to make progress inland. 


The front is expected to slow down Saturday morning. So expect 
showers to continue into the morning hours...then slowly decrease 
in the afternoon as 500mb heights rise in response to a shortwave 
ridge moving towards the County Warning Area. Conditions will be dry Saturday 
night into Sunday. However...conditions will be favorable for fog 
to develop in the Westside valleys. 


The ridge axis will move over the region Sunday morning...then 
shift east Sunday afternoon. Conditions will be dry and cool with 
fog lingering for most of the morning hours in the valley followed 
by partly cloudy skies in the afternoon. 


There is some uncertainty for the first part of next week. The 
NAM/GFS and European model (ecmwf) suggest a split flow will develop with an upper 
level low sinking south towards the central California coast. If 
this pans out...then precipitation will be delayed until Monday 
night at the earliest and at the same time could weaken to the 
point where there will not be much more than sprinkles developing 
at the coast. 


Petrucelli 


Tuesday through Friday... 
there is a high level of uncertainty in the timing...and moreso in 
the details...of the forecast in this critical time period. Model 
guidance has had a lot of run to run inconsistencies. 


At this point...have sided with the 12z European model (ecmwf) as it appears to be 
the consensus solution of the 12z GFS and the 12z Canadian. The 
GFS splits the Tuesday system and then brings in a system on 
Thursday with flow from the south. The Canadian...on the other 
hand...indicates a progressive northwest flow storm track over the 
area with snow levels that would be problematic for elevations 
above 4000 feet. The European model (ecmwf) indicates the Tuesday system 
splitting...but not until its over our area. It then brings in a 
system under northwest flow on Thursday that would bring snow 
levels around 3500 to 4500 feet. The concern...at this time...is 
the possibility that a blended solution could occur...in which the 
remnant cut-off low over California from the Tuesday system gets 
pulled northward ahead of the northwest flow storm system 
indicated. This would lead to travel troubles on Thursday. 


Thus...as mentioned...confidence is not high in the details of the 
forecast for Thanksgiving just yet. Instead...its low...but there 
is the potential of travel troubles for Thursday...so stay tuned. 
Lutz 






&& 


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


California...none. 


Pacific coastal waters...Small Craft Advisory for winds until 10 PM 
PST this evening for pzz350-pzz356-pzz370-pzz376. 
Small Craft Advisory for hazardous seas until 4 PM PST Saturday 
for pzz350-pzz356-pzz370-pzz376. 


$$ 


Dw