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Area forecast discussion 
National Weather Service Sacramento California 
330 am PST Thursday Nov 20 2008 


Discussion... 
upper level trough approaching the northern California coast moving east-northeastward at about 
40 kts...forecast to move into eastern Oregon by 00z Friday. High level 
moisture entrained into the southwesterly flow ahead of the trough is mainly 
spreading across the southern portion of the forecast area from I-80 
southward...with drier middle level flow to the north. Satellite trends indicate 
the southern moisture will gradually be thinning from west to east during the 
early morning...although Lee enhanced ac/cirrus will continue east of the 
Sierra Nevada crest. Given the narrow temperature/dewpoint spreads won't give up 
on possible fog forming during the early morning...although southeasterly winds 
from 5-7 kts make it less favorable than yesterday morning's calm 
winds. Last look at the fog product showed a couple of patches of 
stratus/possible fog developing around Folsom and in the central SAC 
Valley West of kbab where the areal coverage was becoming more extensive. 


The trough is associated with a cold front that will be moving inland 
during the morning hours. There is a lack of enhancement with the 
offshore front S of 40n...and SSMI rain rates were rapidly 
disappearing. However...eka radar detecting some light echoes along 
the north coast. Since the cold front is running into dry/low precipitable water air off 
the coast as it moves eastward...we are not looking at significant precipitation 
with this system. The 20/00z GFS cross sectional analysis indicates 
moisture/clouds below 850-700 mbs...mainly ahead of and with the 
front...will be spreading inland this morning...then decreasing this 
afternoon/evening. Dynamics/upward vertical motion forecast to 
weaken rapidly during the afternoon as the upper trough moves into eastern Oregon. 
Thus we have limited probability of precipitation mainly to the morning over the northern portion 
of the forecast area...and decreased areal coverage of probability of precipitation in the 
afternoon. Synoptic cooling from this system expected to lower maximum 
temperatures today some 3 to 6 degree over most of the area. 


In the wake of the upper system and cold front...nly/downslope flow 
will develop over the area...likely precluding fog development... 
except where the gradient is weaker in the northern San Joaquin Valley this 
coming night. Post frontal air mass will also result in a chilly 
night. The next upstream trough is forecast to track further north late 
Friday night and early Sat...and we have confined our mention of low 
probability of precipitation to Sat morning for now. 


Medium range models taking a more similar approach than previous 
runs with development of a closed-low vicinity 30n/130w by 00z 
Tuesday...although the Canadian model is about 10 degree further west than the 
Euro/GFS. While the Canadian keeps norcal dry and warm under a 
strong upper level ridge through the 144 hour period...the Euro/GFS 
wobble the closed eastward toward the California coast Tuesday/Wednesday increasing the 
chances for precipitation in norcal. These solutions have some support from 
GFS ensemble runs...but with closed/and or cut-off lows off the 
coast this time of year...confidence is not very high on this 
timing. If memory recall is accurate...we only had 2 days of rain in 
Nov at dts in 2007 with just under one inch of precipitation for the 
month...as an Omega-Rex block formed off the West Coast resulting in a 
dry Nov...and a very late start to our wet season. The Canadian may 
be hinting at that scenario. We are cautiously optimistic that the 
wetter-looking models will verify based on some ensemble support. 
Jhm 


&& 


Aviation... 
patchy fog...mist and low stratus clouds are developing this morning 
in portions of the Central Valley. Areas with IFR ceilings and visible are 
expected across portions of Sacramento and northern San Joaquin 
valleys and Delta through the morning. Currently areas of stratus 
and fog are near and east of kmhr...west of kbab...and east and 
north of krdd. Expect these areas to grow as skies clear some as 
high clouds shift eastward. Clouds will increase later this morning 
over the northern mountains and the northern Sacramento Valley with 
a slight chance of light rain into the afternoon. Fog will diminish 
with the increasing clouds by late morning. Southwest winds gusts up 
to 35 miles per hour over mountain ridges this afternoon as a cold front passes 
through. Ek 


&& 


Sto watches/warnings/advisories...none. 


$$