Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Seattle Washington
405 am PDT Friday Oct 31 2014
Synopsis...a Pacific frontal system will bring rainfall to western
Washington this morning. The cold front will cross the Cascades by
noon with the rain turning to showers. Saturday should be partly
sunny with light spotty showers. Another front will approach the
area on Sunday with rain developing along the coast late in the day.
This will be the first in a series of weather systems that will
bring rainy and locally windy weather to the area most of next week.
Short term...satellite imagery shows stalled front over the
interior of western Washington this morning. A wave developed along
the backside of the front overnight which brought the eastward
movement of the system to a halt. That wave is currently along the
coast moving north. Temperatures at 3 am were in the lower to middle
Once the wave along the backside of the front moves through later
this morning the front will continue its slow eastward movement.
Current timing puts the front east of the Cascades by midday. There
is a bit of a break in the satellite imagery between the front and
the cold air cumulus associated with the trough offshore so there could
be a break in the precipitation midday. As the cooler air mass
filters into western Washington this afternoon the air mass will
become unstable with showers developing. Temperatures will not rise
much from the current readings with the cooler air moving inland.
Highs in the middle 50s will be common.
Shower activity decreasing tonight as the main energy associated
with the cold trough offshore dives south into California. With the
trough inside 130w can not take showers completely out of the
forecast. Will go with just chance probability of precipitation overnight. With the cooler
air mass over the area any breaks in the cloud cover will allow
temperatures to drop significantly. By Saturday morning expect the
colder locations to get into the upper 30s with 40s common over most
of the area.
Models in better agreement on the 00z run with what is left of the
trough moving through western Washington Saturday afternoon. Once
again the main energy associated with the trough is well south of
the area leaving western Washington with a very weak trough passage.
European model (ecmwf) still leaves some shower activity over the area while the
remainder of the model suite is mostly dry. Will start chipping away
at the chance of showers forecast and dry out the northern portion
of the area on Saturday. If the model trends continue look for the
chance probability of precipitation to be taken out of the entire area for Saturday. Highs
will continue to be only in the middle 50s.
Not much going on Saturday night with the weak trough to the east
and zonal flow aloft offshore. There is a chance of some residual
showers in the wake of the trough Saturday evening so have kept
chance probability of precipitation in the forecast. The chances are not very high and will
not be surprised if Saturday night turns out ot be dry. Lows will
again be cool with 40s common.
Next system getting organized off the coast on Sunday with the
leading edge of the precipitation associated with the system
spreading on to the coast in the afternoon. The European model (ecmwf) is a little
faster with this system versus the other models with showing
precipitation getting into the interior Sunday afternoon. Will stay
with the chance of rain forecast for Sunday afternoon for the
interior. Highs will be in the 50s.
Long term...extended models have a general theme of a wet period
during the extended forecast and confidence is high in this general
trend. The devil is in the details. Both the European model (ecmwf) and GFS show the
system Sunday night into Monday with another system right on its
heels for Monday night into Tuesday. The solutions start to diverge
on Tuesday with the GFS moving the front through the area while the
European model (ecmwf) stalls the front over western Washington with the pattern
showing an atmospheric river setting up across the eastern Pacific.
The GFS lifts another system into the area Tuesday night into
Wednesday while the European model (ecmwf) keeps the long fetch of moisture aimed
right at the area. The GFS lifts the front north of the area
Wednesday night and Thursday with a upper level low developing off
the coast moving toward haida gwaii. The European model (ecmwf) is much further south
with this feature resulting in a slower lifting of the front to the
north. The extended forecast will have rain or rain likely in every
period Sunday night through Thursday. Felton
Hydrology...a Flood Warning is in effect for the Skokomish River
early this morning. The river has recede slightly in the last couple
of hours but with another shot of rain moving in and the river only
a half a foot or so below flood stage will leave the warning up for
the morning hours.
The other river we have been concerned about is the Nooksack river.
Rainfall amounts overnight have been running below the predicted
values. With the front moving through later this morning flooding on
the Nooksack looks unlikely.
Flooding is not expected on other area rivers during the next 7 days
although if the European model (ecmwf) solution verifies some of the more flood prone
rivers will need watch in the middle of the week. Felton
Aviation...SW flow aloft. The air mass is stable and moist but will
becoming more unstable today as cooler air moves in aloft. The areas
of rain over western Washington early this morning will turn to showers
today. There is a trough just offshore that should weaken and near
the coast later today.
Ksea...the rain will should turn to showers this morning...although
to be sure the area of unstable air with the upper trough is still
offshore and may take awhile to work inland over western Washington today.
Marine...pressure gradients have become quite light early this
morning and there are no advisories in effect now. The trough just
offshore might be quite weak by the time it finally moves ashore.
Weak high pressure will build Sat...and then a series of stronger
systems is likely for next week.
Washington...Flood Warning Skokomish River.
You can see an illustrated version of this forecast discussion at