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Area forecast discussion 
National Weather Service Eureka California
349 PM PST Sat Jan 31 2015

Synopsis...
dry weather will continue through at least Sunday in most
areas...but a weakening upper ridge and several approaching
systems are expected to bring light to moderate rainfall to areas
north of Cape Mendocino late Sunday through late Tuesday. Dry
weather will return briefly on Wednesday...followed by another wet
pattern late in the week.

&&

Short term...through Monday...
benign weather is expected to continue through the day tomorrow...but
big changes are on the way through the remainder of the week as
several storm systems will move into the Pacific northwest along
the periphery of a slowly weakening upper ridge. In the immediate
future...another round of fog can be expected in the interior
valleys and along the Redwood coast...although its extent may be
somewhat limited by the approach of building high clouds
associated with the first in a series of approaching shortwaves.
While parts of del norte and perhaps extreme northern Humboldt
County may be able to squeeze some rain out of this initial system
early Sunday...the strength of the ambient upper ridge is expected
to preclude any significant accumulations until the arrival of a
slightly stronger shortwave and a weak front sometime early
Monday. At this point...deeper moisture and a slightly weaker
upper ridge will likely allow for a period of moderate rainfall
across primarily del norte...northwestern Trinity...and northern
Humboldt counties...with a bit more uncertainty from central
Humboldt County southward. Model guidance is relatively uniform in
keeping the bulk of significant accumulations well north of Cape
Mendocino...and given the aforementioned stubborn upper ridge this
seems reasonable. In any case...rain rates are not expected to be
high enough to result in any flooding concerns even in the wettest
areas.

Long term...Monday night through Sunday...
two small shortwaves will advance toward northwest California
Monday evening through Tuesday bringing periods of rain for most
areas north of Mendocino County. The European model (ecmwf) is drier and farther
north with each shortwave than the GFS and NAM models...but in
general at least some accumulating rain is expected north of the
cape through early Tuesday afternoon. As such...chances for
precipitation have been increased with this update north of the
cape and particularly in del Norte County.

The last shortwave will be accompanied by a warm front which will
shift north into Oregon leaving northwest California in the warm
sector of a strengthening system off the West Coast. Model ensemble
and deterministic runs all indicate a deep moisture plume will move
into northwest California late Thursday and continue into the
weekend as upper level ridging builds over the inter-mountain west
leaving southwest flow aimed at the northern half of California. The
West Coast atmospheric river landfall tool indicates high
probabilities of integrated vapor transport (ivt) over 250 kg/M/S
Thursday night through Monday as the moisture plume undulates north
and south from southern Oregon to north-central California. With the
prolonged period of high ivt in the GFS model...the model is
producing widespread heavy rain in the 6-12 inch range over the
region. The currently more conservative European model (ecmwf) only indicates 3-7
inches. Thus there is high uncertainty with the rainfall amounts
but not with whether or not it is going to rain. Probability of precipitation have been
further increased Friday through Sunday and if model trends
continue will be increased again with future updates. It should be
noted that the weather pattern depicted in the models is a very
favorable pattern for heavy rainfall in northwest California.

Gusty south winds are also expected just ahead of and with the heavy
rain Thursday into Friday. 925 mb winds are predicted to peak
between 40 and 60 knots along the immediate coast with even stronger
winds farther aloft in the GFS. If the GFS is correct wind
advisories or warnings may be required.

&&

Aviation...
VFR conditions will continue to prevail across the region through
tonight. Some of the interior river valleys will still see fog and
low clouds as a result of the long wave cooling. Kuki may see see
some by daybreak Sunday too. Coastal sites may see interludes of
misty haze this evening as winds go light and squirrelly. However
wind fields will favor a light offshore direction which will
hinder misty air and low cloud development. High and middle clouds
will spread over the region on Sunday. Cross sections indicate
increasing onshore flow and lower level relative humidity through the day on
Sunday as a weakening front approaches from the northwest. This may
result in MVFR cloud decks at kcec and kacv...though exactly when
and how low are uncertain. Going into Sunday night the lower
levels will continue to moisten up which should eventually result
in IFR at kacv and kcec.

&&

Marine...
going forecast looked to be in good shape and on track through
middle week. No changes were necessary. The press gradient between an
offshore ridge and a thermal trough across California will
gradually weaken the next few days. As a result...north winds and
short period seas will diminish. Seas will increase early next
week as a long period westerly swell traverses the waters. The
long period swell may result in greater run up on the beaches;
I.E. Sneaker waves. Not real confident in this as the sneaker wave
index indicates a low threat with the swell building to 8 feet
around 15 seconds by Monday afternoon. A period increase of just a
few seconds would put US in a moderate risk. The nwps and enp
models indicate no shorter period wave groups which may not be
incorrect. The winds will be light southerly early next week and
perhaps short period wind waves will need to be added into the
forecast. Southerly and southeasterly winds will likely increase
significantly toward the latter portion of the week. How much and
how fast are not certain. The GFS model has been consistently
showing a fairly strong system approaching the waters, so for now
will stay close to the averaged GFS surface winds which indicate south-southeast
wind of 30-40kt and the potential for gusts to 45 and 50
knots...mostly in the outer waters. If the wind forecast pans
out...seas will hit 18-20 feet with periods around 10 seconds in the
outer waters. It is still 5 days out and there is a chance all of
the models are completely wrong. Stay tuned.

&&

Eka watches/warnings/advisories...
California...none.
Northwest California coastal waters...none.
&&

$$

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