Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Eureka California
847 am PST Thursday Nov 27 2014
Synopsis...a frontal boundary and associated rainfall will stall
over southern Oregon today and tonight with some rain spreading
into far northern California. Friday this front will spread south
across the area bringing widespread rain. Periods of rain will
persist through the weekend.
Update...radar has been showing reflectivity returns moving over
southwestern Oregon this morning. These returns have been
clipping the far northwest corner of del Norte County this morning.
Automated rain gages have registered a few one hundredths north
of the Orca border in SW Oregon...with nothing south in del Norte
County. Satellite imagery showed the front offshore the Oregon
coast extending southwestward just outside norcal coastal waters.
An anticyclonically curved bulge has been forming west of the
north coast indicating that the front is starting to slow down and
stall. Both the GFS and nam12 have latched onto this stalling
trend...showing very little or no rain developing in del Norte
County through the day today. Thus I reduced the precipitation chances
and quantitative precipitation forecast across del Norte County today. The models continue to show
the front edging closer to the norcal coast late tonight into
Friday as a positively tilted middle-level trough centered near 140w
slowly moves eastward. We are still expecting rain to develop
across the region...as early as tonight for del Norte County and
perhaps as late as Friday night for southern mendo County. //End
Previous discussion......issued at 413 am PST...
Short term (through Saturday night)...radar and Sat imagery show
the frontal boundary stalling out just to the north of orcal
border this morning. Some light rain has started to fall over
southern or. The 06z NAM and 00z GFS were in good agreement with
the location of the precipitation staying farther north. Now the 06z GFS
has trended a bit farther north as well so the front may stay in
Oregon a bit longer than expected. Trimmed probability of precipitation back a bit...but
kept them mostly the same. The upshot of this is there will be a
very tight gradient between a lot of rain and nearly nothing. At
this point it looks like Crescent City may be on the dry
side...but confidence is low on this. Elsewhere across the area
will see dry conditions with some middle and high level clouds.
Inland valleys of Trinity County are seeing some fog this
morning...but as southerly winds increase ahead of the front today
this will clear out fairly quickly. Temperatures will be fairly
warm...especially along the coast.
Tonight the front will remain in far southern Oregon in the
evening with the GFS and European model (ecmwf) showing the front spreading rain
south after midnight to Cape Mendocino and into Mendocino County
by sunrise. The NAM is a bit slower...but will discount that for
now. Rainfall amounts still vary from model to model...but
generally favor heavier amounts due to the warm airmass and and
fairly strong onshore flow. The GFS is showing amounts ranging a
third of an inch to nearly an inch by Friday afternoon. Went a bit
higher than this due to reasons stated above. Friday night and
Saturday there is general agreement on the warm and moist onshore
flow continuing. The GFS has rather modest rainfall amounts in
most areas...while the European model (ecmwf) has heavier rain. The differences are
mainly due to the depth of the upper level trough to the north of
the area. This will keep snow levels above 6000 feet in most areas.
Long term (sunday through thursday)...Sunday the model solutions
start to diverge even more. The 06z GFS brings a warm front into
the area and some light rain for much of the day. The European model (ecmwf) has
this front as well...but it is farther south and east. The
Canadian has more of a compromise. Trended probability of precipitation down a bit...but
this will need to watched. Sunday night through Thursday the main
forecast issue will be the an upper level trough that cuts off and
slowly moves east. The GFS is very progressive and plows this low
over the West Coast while the European model (ecmwf) is much slower with it. The
GFS brings heavy rain to the coast Monday night while the European model (ecmwf)
keeps the coast dry until late Tuesday night and then brings a
much weaker front onshore as the low is drifting northwards.
Generally trended the forecast towards the slower solution...but
did not completely discount the faster solution. Confidence is
very low during this time period. Mkk
Aviation...mostly VFR conditions will prevail again today as
valley fog dissipates and pockets of clouds along parts of the
coast erode by this afternoon. High cloud cover will increase over
the region as the approaching front gets closer to the coast.
Ceilings will lower to MVFR and possibly IFR first across del
Norte County this afternoon as the front stalls offshore. Kcec
will possibly see some light rain this afternoon, however, if the
front slides farther east then kcec can expect longer lasting
rainfall. Areas to the east and south of del Norte County can
expect deteriorating conditions as the rain bearing front pushes
farther east late tonight into early tomorrow morning. \Kml
Marine...an approaching storm system will maintain elevated
southerly winds and building short period seas today and Friday.
In addition, a swell will impact the coastal waters today causing
seas to build near 10 to 13 feet by late this afternoon.
Consequently, a Small Craft Advisory was added to marine zone 455
beginning late this afternoon. Conditions will ease briefly on
Saturday and winds should become northerly behind the passing
shortwave. Then winds should become southerly again on Sunday.
There is still considerable uncertainty with the wind shifts over
the weekend as models have been doing a poor job sticking to a
solution. The little changes made in the forecast were to remain
in coordination with the neighbors. Waves should remain elevated
early next week. But how high, direction, steepness, et cetera. Is too
hard to determine at this point given the variability in the
Northwest California coastal waters...
Small Craft Advisory from 3 PM this afternoon to 9 PM PST Friday
Small Craft Advisory until 9 PM PST Friday for pzz470.
Small Craft Advisory from 3 PM this afternoon to 9 PM PST Friday
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