Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 240946 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
237 AM PDT Tue Apr 24 2018

.SYNOPSIS...Strong high pressure aloft will remain over the Pacific
Northwest through Thursday. Offshore low-level flow will persist west
of the Cascades today. The surface thermal trough drifts inland
Wednesday, in conjunction with a southerly flow reversal along much
of the coast. A southwest marine surge develops Wednesday night and
Thursday. An upper level low approaching the north California coast
late in the week will spread a chance for showers and maybe even a
few thunderstorms to the region Thursday evening. The upper low
eventually slides southeast into the Great Basin over the weekend.

&&

.SHORT TERM...Today through Thursday...An early taste of summer will
continue for the next couple of days. Water vapor imagery early this
morning showed 500 mb high pressure centered over SW Oregon to NE
Washington. A 500 mb low was noted near 40N 139W. The 03 hr NAM MSLP
forecast valid 09Z depicted a surface thermal trough extending from
near KOTH north to the Washington coast. This has resulted in
offshore low-level flow across the forecast area. At 09Z the
KTTD-KDLS gradient was about -6 mb. Gusts to 50 mph were noted at
Crown Point. Three Corner Rock RAWS, in the South Washington Cascades
registered gusts to 55 mph. Many areas had humidity values of 25
percent or less and offshore wind gusts 20 mph or more at 09Z. The
06Z NAM indicates the KTTD-KDLS offshore gradient peaks near -7 mb
between 15Z and 18Z today. Model 850 mb temps valid 00Z Wed are
expected to be around 12C in the north to 16C in the south. Thus, max
temps today will be around 4-8 degrees warmer than Mon, with several
high temp records sure to fall. Would not be surprised if KAST and
KTMK hit 80 degrees today, which would be a record for KAST.

The NAM shifts the thermal trough into the Willamette Valley and SW
Washington interior lowlands by 06Z Wed. In addition, south flow
reversal along the south Oregon coast is forecast to reach at least
KONP or even a bit more north by 12Z Wed. Typically, models tend to
be too agressive moving the thermal trough inland. In any event,
subsidence inversions should be quite prevalent Tue night in the S
Washington and North Oregon Cascades and foothills. Model forecast
850 mb temps valid 00Z Thu rise to around 15C at KPDX to 19C in the
Lane County Cascades. By 18Z Wed the low-level offshore flow is
nearly gone, mainly confined to the west end of the Gorge and SW
Washington Cascade foothills. The thermal trough axis is forecast to
be centered from the S Washington Cascade foothills to the Lane
County Cascade foothills. This will maintain low to mid 80s Wed for
much of the interior lowlands. The coast will be much cooler Wed,
except for the S Washington and extreme N Oregon coast where light
offshore flow is expected to continue.

The NAM shows some interesting developments Wed night. First, a
southwest marine surge strengthens, mainly impacting the coastline
and Central Oregon Coast Range and coastal valleys. Should this
occur, some of this marine air would likely spill through the Central
Coast Range gaps and leak into the west side of the South Willamette
Valley. Second, surface low pressure settles over the north
Willamette Valley and induces stronger offshore flow through the
Gorge. Onshore flow gradually deepens through the day Thu, but the SW
Washington interior lowlands and North Willamette Valley will remain
under the thermal trough. Thus, expect minimal cooling in these areas
Thu. The Central Coast Range and South Willamette Valleys will be
several degrees cooler due to the deeper marine layer. The GFS, ECMWF
and NAM show SE-S 500 and 700 mb flow Thu afternoon. Will need to
watch for potential convection over Northern California and the South
Oregon Cascades drifting north into the forecast area. The GFS has a
hint of this in the Central Oregon Coast Range 00Z Fri. The more
favorable instability looks to occur in the 00Z-06Z Fri time frame.
Weishaar

.LONG TERM...Thursday night through Monday...Models are in
general agreement showing the upper low drifting closer to the North
California and South Oregon coast Thu night. However, the QPF
details differ. The GFS shows an axis of 500-800 J/kg CAPE over the
Lane County Cascades 00Z Fri spreading north to northwest into the
Willamette Valley. However, by 06Z there is little to no CAPE. The
NAM shows an area of QPF developing along the the boundary of the
thermal trough and incoming marine layer at 03Z Fri and expanding it
north overnight. There may be the potential for elevated convection,
especially in the Cascades. By Fri morning a much deeper marine
layer is expected as the upper low continues to drift inland. The
ECWMF is much slower moving the low inland compared to the GFS.
However, Fri looks to be much cooler with scattered showers. The
unsettled pattern continues through the weekend, but Saturday looks
to have the highest POPS. Weishaar

&&

.AVIATION...12z TAFs: VFR conditions expected for the next 24
hours. Offshore flow will ease Tuesday afternoon.

KPDX AND APPROACHES...VFR conditions through early Wednesday.
Fairly strong easterly wind gusts of 20-30 kt will continue
through 18z-20z Tuesday, then ease off.

&&

.MARINE...A surface thermal trough over the waters with east to
northeast winds. Some isolated gusts 25 kts expected downwind of
the coastal river gaps including the mouth of the Columbia River
through this morning. Winds will ease during the afternoon and
evening as the thermal trough weakens. A southerly wind reversal
may push into the central waters tonight. A more substantial
southerly surge will likely occur Wednesday afternoon into
Thursday morning more widespread fog and low clouds. Small
craft advisory level winds could return towards the
weekend, as a front moves into the north Oregon and south
Washington waters.

A longer period westerly swell moved into the waters overnight.
All buoys in the coastal waters now showing a period of 17
seconds with significant wave height of 6 to 8 ft. Wave heights
are expected to peak below 10 ft today. The swell period and
height will then gradually lower over the next few days. /mh /tw

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
PZ...None.

&&

$$

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This discussion is for Northwest Oregon and Southwest Washington
from the Cascade crest to 60 nautical miles offshore. This area is
commonly referred to as the CWA or forecast area.



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