Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
322 PM PDT sun Oct 19 2014
Synopsis...a cold front will bring widespread rain to northwest
California overnight and into Monday. This front will also cause
gusty mountain winds as it crosses the region. A Post frontal
trough will follow the cold front...bringing a few strong showers
and a possible thunderstorm...which may contain small hail.
Temperatures will cool Monday night as the clouds clear and cold
air fills in behind this system. More rain is expected Tuesday
night into Wednesday...then again late in the week.
Short term (tonight through wednesday)...afternoon satellite
imagery shows the next cold front about 7o miles offshore.
However, dry air being entrained from the south continues to erode
the clouds from the front over the coastal waters. This will not
be long lasting this evening though as the upper support to the
surface front helps to force the front to the east tonight. Models
have sped up the front slightly, which seems to be in good
agreement with visual data and buoy data. Buoy 2 which 275nm west
of Coos Bay showed the front passing around 5 to 6 am with a steep
pressure rise following the front. Went ahead and sped the front
up slightly in the forecast as well, bringing rain to the
immediate coast as early as this evening. The bulk of the rain
will come overnight though. The quantitative precipitation forecast continues to be a tough nut to
crack. Frontal rains seems to be consistent between models, but
Post frontal showers associated with a trailing trough show a
significant difference. The ec continues to show ample quantitative precipitation forecast over
del norte and northern Humboldt, while the GFS and NAM seem to be
somewhat less. There is a hint of some cyclogenisis on the trough
off of Cape Mendocino which could cause some enhanced precipitation over
coastal Mendocino. Confidence is low here, so did not hit this in
the forecast, but will need to be watched. This Post frontal
trough will also provide a focus for instability in the Post
frontal cold pool. This means there will be a shot of a
thunderstorm along the Redwood coast through the day on Monday.
Any thunderstorm that does develop will have the chance of
producing small hail. At elevation in this cold pool of air, there
is a shot of seeing some snow showers with snow levels dropping
briefly to around 6kft. Any snow that does fall will not stick
around long due to warm ground and a quickly modifying air mass.
Winds will remain an issue with the front as well. Pre frontal
winds are already gusting this afternoon along coastal mountain
ranges of Humboldt and del norte with gusts as high as the low 30
miles per hour range. This will likely enhance tonight as the front
approaches. Peak gusts can be expected just ahead of front passage
tonight with ridge top gusts as high as 45 miles per hour. These will be very
isolated, but breezy conditions in the mountains can be expected.
With saturating soils and strong gusty winds, there is the chance
of falling tree limbs or falling trees. If in the mountains
tonight or on Monday, be careful. The Post frontal trough will
bring another hit of winds on Monday as well, but the gusts will
not be as great.
Overall, look for wet and breezy conditions through Monday due a
cold front and a Post frontal trough. Precipitation amounts will range
from a quarter inch to a few inches over upslope prone mountains.
Area beaches will also see building surf due to large Post frontal
Monday night into Tuesday northwest cal should see a brief drying trend.
Overnight lows may drop into the middle to upper 30s in Trinity
County valleys and into the 40s in the coastal County valleys
under clearing skies and cool air loft mixing down. Frosty
conditions are possible Monday night in the eastern Trinity
valleys. There is a possibility that the eastern Trinity valleys
stay cool on Tuesday as cool air may become trapped. This all
depends on how stable the Tuesday air mass becomes. At this point
there looks like there will be enough mixing to scour out the cool
air and cause some warming. High temperatures on Tuesday could be rather
warm as southerly flow develops, bringing a strong air mass
modification. Another frontal boundary will sink into northwest cal late
Tuesday into Wed, bringing more rain to del norte and Humboldt
counties. At this time it looks like the vast majority of precipitation
will remain out of Mendocino on Tuesday into Wednesday. Bfg
Long term (thur through sun)...both the GFS and European model (ecmwf) were wet on
Thursday. However there were significant differences between the
two...both temporally and spatially. The GFS has the front and
bulk of rain over southern mendo County by Thursday morning...with the
north coast drying out. Meanwhile the European model (ecmwf) has the frontal band
dangling offshore the north coast with the bulls-eye of rain aimed
at del Norte County. Eventually the emcwf does spread the rain
farther southward and eastward by Thursday afternoon and
evening...though it keeps most of mendo County dry. We beefed up
the precipitation chances across the board...leaning slightly in favor of
the slower European model (ecmwf). Bigger differences between the two models arise
on Friday. The GFS brings an upper trough toward the coast much
faster with rain or showers increasing substantially County Warning Area- wide.
The ecwmf is much slower with the progression of the 500mb trough
and has the main band of rain still off the coast. Averaged the
wet GFS with the dry European model (ecmwf) for essentially middle of the Road
precipitation chances on Friday. The models do not come into real good
agreement over the weekend...but both indicate an upper trough
impacting the area. The European model (ecmwf) finally brings the front onto coast
on Sat...while the GFS brings a cold upper trough down from the northwest
for periodic showers. Confidence is increasing that next weekend
will be wet with bouts of precipitation both Sat and sun. Precipitation chances
have been beefed up.
Aviation...better conditions along much of the north coast this
afternoon after morning wide widespread marine clouds. However
north of Trinidad...cu/SC clouds persisted into the afternoon.
Widespread clouds also persisted south of Cape
Mendocino...especially in the Ukiah area where ceilings were IFR to
low MVFR conditions through late morning...then mostly VFR after
midday. Terminals will gradually deteriorate as an approaching
frontal system from the Pacific northwest moves into the region.
Rain and gusty winds will impact the cec and north coast late
tonight- overnight. Along the Humboldt and del norte
coast...airfields may see some limited convection during forecast
Marine...seas have been trending downward today with buoys
reporting 5 to 7 feet at 11 seconds this afternoon. Southerly winds
have been increasing and should reach 20-25 knots in the northern
outer waters later this afternoon and evening. The nwps continues
to indicate short period southerly waves hitting 9 feet in the outer
waters this evening. See no reason to second guess the model at
this point. Thus will maintain the advisory for both winds and
seas through this evening in pzz470. Next issue is the large
westerly swell expected to arrive later tonight into Monday
morning. Jason-2 altimetry pass from 18z yesterday showed 20 feet
around the periphery of the wave maximum. This is in pretty good
agreement with the ww3 model and will continue to follow the
guidance. We will have a much better idea on how good the model is
doing once 15-17 feet swell GOES by buoy 2 tonight. Confidence is
high the model is not too far off the mark and an advisory for
seas has been posted for all waters starting late tonight and
continuing through Tuesday. If the swell forecast pans out...high surf
conditions with breakers near 20 feet can be expected on Monday.
Both the GFS and nam12 have come in stronger with southerly winds
Tuesday night into Wednesday as a new frontal system develops across the Pacific
northwest. Beefed up the wind forecast in the northern waters. It will
need to be watched for low end gales.
Northwest California coastal waters...
Small Craft Advisory from 3 am Monday to 11 PM PDT Tuesday for
Small Craft Advisory until 11 PM PDT Tuesday for pzz470.
Small Craft Advisory from 3 am Monday to 11 PM PDT Tuesday for
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