Scientific Forecaster Discussion
fxus66 kpqr 290428
Area forecast discussion
National Weather Service Portland or
928 PM PDT Tue Mar 28 2017
Synopsis...rain increasing this evening as front slowly approaches
from the west. This front will ensure quite a bit of rain tonight and
most of Wednesday. Then decreasing showers on Thursday. Still looks
like a nice and mild few days for Fri and Sat, though will see
increasing clouds on Sat. Rain threat, while not all that great,
returns for sun into early next week.
Short term...(tonight through friday)...
new model quantitative precipitation forecast are a little higher than reality so far this evening,
but the radar is showing a rapid increase in rain moving inland over
the last hour.This is the leading edge of increasing warm advection
that will move into the area overnight and Wednesday. Rap model show
decent isentropic lift targeted at the central Oregon coast later
tonight and Wed. Also this feature will be fairly slow moving to the
south and east, so expect abundant rainfall amount in that area.
Also the Cascades around Mt. Jefferson will get a fair share of rain
as well. Snow level will be above 7000 ft so no snow impacts at pass
levels. Have updated rain amount for the evening updated to reflect
new forecast. Through Wed afternoon should see about 1" to 2.5" rain
for the Coast Range and Cascades while the valleys see 0.6" to 0.8".
So far this fall/winter season, we have had two months with record or
near record rainfall (october and february). Now, with rain expected
over the next 36 hours, could add a third month.
Models agree on dragging the trailing cold front into the region on
Wed am, so will transition to showers in the north zones. But,
another wave on the boundary will approach and move into southwest
Oregon later Wed and Wed evening. Net effect will be to slow the
fronts eastward progression, hanging it up over Lane and Linn
counties at that time. So, will keep rain into early evening for that
area, then transition to showers late Wed evening once the front
Behind the front, a cooler air mass moves in, with the cold upper
trough moving through late Wed night into Thu morning. There is
marginal instability over the coastal water for thunderstorms Wed
night. Expect showers to become more orographically favored once
the upper trough passes Thu morning. In fact, Thu likely to be lot
like it was yesterday, with mix of sun and clouds, with decreasing
threat of showers.
Now, for last day of March, or Friday. Appears March will go out like
a Lamb, under partly to mostly sunny skies and mild temperatures.
High pressure will bring lighter winds, and good deal of sunshine.
Temperatures in the 50s on the coast, with upper 50s to lower 60s
inland. If mother nature cooperates, could even get into the middle
60s from Salem southward. Rockey.
Long term...(fri night through tuesday)...no changes. Previous
discussion follows. Model agreement is better through Sat,
with a more progressive pattern over the region. Models continue to
show ridging east of the region on Saturday. Still looks dry and mild
on Saturday, with just increasing mid and high clouds. Now, while
there are still differences in the models, will have to keep some
minor threat of showers in forecast for Sat night into early next
Aviation...generally VFR across the area as of 03z, except for
areas of MVFR along the S Washington and far north Oregon coast.
Coastal sites will lower to MVFR by 05z and then become a mix of
IFR and low-end MVFR overnight through Wed morning, or possibly
into the afternoon. Konp most likely to see IFR conditions late
tonight through much of Wed. Inland taf sites drop to MVFR late
tonight and will likely remain at MVFR into Wed afternoon. Areas
of IFR possible 12z through 20z Wed, especially in the Tualatin
valley. Higher terrain becomes obscured late tonight. Secondary
trough or cold front reaches the coast around 03z Thu, for
improved flight conditions, albeit more unstable.
Kpdx and approaches...VFR conditions as of 03z and will prevail
until at least 08z. Flight conditions deteriorate to MVFR
overnight as precipitation increases. IFR will be possible late
tonight through Wed morning in the Tualatin valley. Improvement
to VFR at the terminal likely Wed afternoon. Weishaar
Marine...minimal changes to the current forecast. A developing
low pres out near buoy 46404 at 03z is expected to migrate east
along a frontal boundary and make landfall along the North Washington
coast Wed morning. South wind will increase to gale force around
06z as the low approaches the waters. GFS shows 25 to 35 kt
boundary layer wind over the waters at 09z, then becoming sub-
gale around 15z Wed. There will be an additional burst of wind with
the frontal passage early Wednesday morning. The NAM suggests 45
to 50 kt 975 mb wind speeds late tonight and early Wed morning,
but will cap gusts at 40 kt for now.
Wind speeds continue to ease Wed afternoon, but veer to the northwest
late Wed afternoon and early Thu evening. Models show a secondary
cold front rotating through the waters during that time. The back
side of the upper trough moves across the waters late Wed night
and Thu morning, which will likely produce solid Small Craft
Advisory wind speeds. High pres settles over the waters Thu
afternoon for much lighter wind speeds.
Seas are a mix of a long- period (16-18 second)and a medium (12
second) period west swells, although the medium period swell set
appears to be waning. South wind waves will build overnight,
resulting in a chaotic sea state. Combined seas are expected to
build to around 15 ft tonight. Seas fall below 10 ft late Wed
night or Thu morning. Weishaar
Pz...Small Craft Advisory for winds until 10 PM PDT this evening for
coastal waters from Cape Shoalwater Washington to Florence or out
Gale Warning until 7 am PDT Wednesday for coastal waters from
Cape Shoalwater Washington to Florence or out 60 nm.
Small Craft Advisory for hazardous seas until 10 PM PDT this
evening for coastal waters from Cape Shoalwater Washington to
Florence or out 60 nm.
Small Craft Advisory for rough Columbia River bar until 1 PM
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