Scientific Forecaster Discussion
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Area forecast discussion
National Weather Service Portland or
844 am PDT sun Sep 15 2019
Aviation and marine discussions updated.
Synopsis...wet and cooler weather is expected today through
Wednesday as a couple of moist fronts move across the region. There
is a slight chance for thunderstorms with the Post-frontal showers on
Monday and Wednesday. A couple of weak disturbances will continue
rain chances late in the week, but the airmass should be slightly
warmer with daytime highs peaking in the low to mid 70s.
Short term...today through Wednesday...wet and cooler weather is
expected the next few days as a couple of moist cold fronts move
across the Pacific northwest. Satellite imagery shows the first front
approaching the coast early this morning. Rain along the coast early
this morning will move inland throughout the morning. Rain intensity
will increase in the afternoon when an upper jet streak will enhance
lift over the area and better tap into the 1+ inch precipitable
water. Satellite derived precipitable water showed values of 1.3
inches with the front around 10 PM Saturday night and the GFS
forecasts these values will increase to around 1.5 inches early this
morning. The front will move east of the Cascades tonight. Rainfall
totals of 0.75 to 1.25 inches can be expected from this morning
through this evening with the highest amounts along the coast,Coast
Range, and the Cascades.
A cool upper trough moves over the area Sunday night and Monday for
showers, lowering snow levels, and a slight chance for thunderstorms.
An additional 0.10 to 0.25 inch of rain can be expected with the
showers Sunday night through Monday afternoon with snow levels
lowering to around 6500 feet.
Another moist front will bring another round of moderate to heavy
rain on Tuesday followed by another cool upper trough Tuesday night
and Wednesday. The current model runs have sped up the movement of
this front from the previous runs, but the available moisture and
upper jet dynamics still support widespread heavy rain, and expect an
additional 0.75 to 1.5 inches of rain Tuesday morning through Tuesday
evening. Winds will be breezy with the front passage on Tuesday with
gusts of 40 mph possible along the coast and 30 mph along the valley.
The wind combined with the saturated soils may result in the Downing
of weakened trees and tree limbs.
Another cool upper trough follows the front for continued showers and
possibly isolated thunderstorms Tuesday night and Wednesday. Expected
rainfall totals today through Wednesday afternoon from the two cold
fronts and Post-frontal showers are 2 to 4 inches for the coast,
Coast Range, and Cascades, and 1 to 2 inches for the Willamette
Valley. Heavy rain rates with the fronts today and Tuesday may
challenge low lying areas with poor drainage for minor urban
flooding. Ponding on the roads is also possible with the heavy rain,
as well as hydroplaning of vehicles. Daytime highs will be in the 60s
through Wednesday. ~Tj
Long term...no changes. Previous discussion follows...Wednesday
night through Saturday...unsettled weather looks to linger into the
upcoming weekend as additional disturbances move across the region.
These disturbances may bring periods of showers to the forecast area
and should generally keep temperatures near normal. There is a chance
that we may see some dry weather late next week as an upper level
ridge tries to build into the pac NW, but confidence is low due as
guidance continues to show weak systems moving through the flow. Due
to the uncertainty will continue to go with a blended forecast. /64
Aviation...starting to see improving conditions starting in the
northwest around kast as a cold front moves through. Other areas
still have a ways to go before returning to VFR, though. Expect rain
with IFR or low MVFR vis through much of the afternoon. For inland
areas, rain turns to -shra and VFR after 00z Monday. Unstable air mass
behind the front may bring ts to coastal sites Sunday night.
Kpdx and approaches...IFR or low MVFR vis expected through at least
21z with rain. Rain turns to showers after 00z with at least temporary
improvement to VFR. May see MVFR ceiling overnight. Bowen
Marine...a front over the waters continue to move slowly east
early this morning. Winds have dropped, likely due to the wind shift
with the front cutting off the coastal jet. Have dropped small craft
advisories. Seas will come up some today but remain below 10 ft.
Previous discussion follows. Bowen
Following the front, a broad and weak surface low pressure will
drop southward across the waters through Monday. This should
bring relatively benign winds and allow seas to subside slightly,
though cold air aloft will probably result in some squally
showers and thunderstorms tonight and Monday.
Forecast models continue to trend stronger with the next front,
which will impact the coastal waters later Monday night and
Tuesday. The 00z NAM and GFS models suggest gale force gusts are
a good possibility with this system. 06z NAM continues with this
trend so confidence is increasing for possibility of gale force
winds for early Tuesday. Enp wave guidance appears a bit
conservative considering the strength of this system, so we
leaned more toward nwps output based on our forecast winds, which
shows seas peaking around 11 feet Tuesday morning. /Mh weagle
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