Area forecast discussion
National Weather Service San Francisco Bay area
308 am PDT Sat may 25 2013
..cool Holiday weekend with showers possible Memorial Day...
Discussion...as of 02:39 am PDT Saturday...
500 mb pattern over the West Coast continues to be dominated by a
longwave trough which shows up well on the current h20 vapor loop.
The weather over the next several days will be tied into the
evolution of the upper low. This low has brought several days of
cooler than normal weather and this will continue through the
Holiday weekend.
For today and into Sunday there will be little change to the
previous forecast package. A shortwave trough pushes towards the
northern California coast tonight and this will keep higher than
zero probability of precipitation but nothing significant except in the up slope favored
areas of the northwest slopes of coastal mountains where some
light rain or drizzle may be wrung out. The upper trough and the
shortwave energy will keep the low levels mixed and prevent a well
defined marine layer from developing. Behind this shortwave...a
slight cooling of the area will take with weak cold advection
during the day.
The forecast focus continues to be on rain chances on Memorial
Day. The general trend has continued to be slower on the onset of
precipitation. Of the European model (ecmwf)...NAM...and GFS solutions...there remains a
fair amount of disagreement and this contributes to a lower
confidence forecast. The NAM and GFS are in fairly close agreement
on the southward progress of the rain during the day on Monday
with the European model (ecmwf) continuing to bring it farther south with higher
amounts. Timing is in a bit better agreement with even the European model (ecmwf)
slowing the onset for the forecast area until primarily after 18z
Monday. So its likely that Monday morning and early morning will
be primarily dry except for the Bay area north where shower
chances will be higher. Percipitable water amounts 1-1.25 inches
so there will be enough moisture present along with the lift
present from the surface cold front and the upper trough energy.
The models do show what looks like better rain chances later
Monday and overnight but even so there remains uncertainty over
the amounts which will not be great. So the bottom line with the
Monday forecast is that there is better confidence in the timing
being more focused on higher rain chances Monday afternoon but the
southward extent of measurable rain remains in question but with a
blended solution the the North Bay looks in line for the best rain
chances. Will keep low probability of precipitation from the Bay southward.
After the unsettled weather and shower chances diminish on
Tuesday...the European model (ecmwf) and GFS are in decent agreement on the trough
progressing into the inter mountain west and this leaves our area
in the transitional zone between the trough to the east and the
building ridge over the Pacific. This will contribute to a
continued lower confidence forecast given the potential for large
changes in the extended with the highly amplified pattern. For now
will reflect a blended MOS solution which supports a slow middle week
warming trend followed by a return to hotter weather by late week.
&&
Aviation...as of 10:45 PM PDT Friday...skies are mainly clear. VFR
is holding so far but localized low clouds are forecast overnight.
Vicinity of ksfo...west winds have decreased. VFR.
Ksfo bridge approach...VFR.
Monterey Bay area terminals...MVFR ceilings forecast this evening. IFR
overnight.
&&
Mtr watches/warnings/advisories...
... Pt to pt pinos 0-10 nm until 3 am
Small Craft Advisory...pt Arena to Pigeon pt 10-60 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...Pigeon pt to pt Piedras Blancas 10-60 nm
Small Craft Advisory...pt Arena to pt Reyes 0-10 nm
Small Craft Advisory...pt Reyes to Pigeon pt 0-10 nm
Small Craft Advisory...sf Bay from 5 PM
&&
$$
Public forecast: Johnson
aviation/marine: Smith
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