Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 251606 AAA
AFDPQR

Area Forecast Discussion
National Weather Service Portland OR
905 AM PDT Wed Apr 25 2018

.SYNOPSIS...Strong high pressure aloft will remain over the region
today and then drift east Thursday. An upper level low pressure
system is expected to impact the forecast area late Thursday through
the weekend. Longer range models suggest a return to warmer and drier
conditions around the middle of next week.

&&

.SHORT TERM...Today through Friday...Early morning water vapor loop
showed a 500 mb ridge axis extending from Southern California through
Central Oregon and Washington. Meanwhile, a closed upper low was
centered near 36N 135W. There was also a weak short-wave feature
dropping southeast across Southern British Columbia as noted by the
associated dry slot. The surface pattern is much different this
morning compared to the previous night. At 12Z Tue the KOTH-KGEG
gradient was around -15 mb. At 08Z it was -1.8 mb. The KTTD-KDLS
gradient is -2.7 mb, compared to about -7 mb last night. Needless to
say there is much less wind early this morning compared to yesterday.
This is reflective in the overall improved humidity recoveries along
the coast and the interior lowlands.

The 03 hr NAM MSLP forecast valid for 09Z indicates surface low
pressure over the south Oregon waters and a secondary surface low
near KTMK. 08Z metars indicate fog and stratus along the coast to
just north of KONP. This matches well with the 09 hr Hi-Res ARW
boundary layer moisture forecast. The thermal trough seems to be
centered from the South Washington coast through the North Oregon
Coast Range to the Rogue Valley at 08Z. The NAM holds the thermal
trough over the Willamette Valley and SW Washington lowlands this
morning and then shifts it to near the Cascade foothills by 00Z Thu.
Model 850 mb temps are forecast to warm around 2C today compared to
yesterday. The 00Z/24 KSLE sounding came in with an 850 mb temp of
14C. Thus, expect a little more warming today for the interior. The
Hi-Res ARW shows a thin marine layer extending to near KAST by 18Z.
However, it also indicates slight offshore flow developing over the S
Washington and far North Oregon coast. This will make for a tricky
max temp forecast at KAST today. High confidence that coastal areas
south of KTMK will see significant cooling today.

NAM and other high-resolution models show an deepening southwest
marine surge Wed evening, coupled with increasing offshore flow
through the Columbia Gorge. By 09Z Thu the Hi-Res ARW suggests marine
air spilling into the Central coastal valleys and potentially
reaching the South Willamette Valley by 12Z. Meanwhile, the surface
thermal trough is forecast to retreat back into the North Willamette
Valley. The marine layer continues to deepen Thu, which will push the
thermal trough to the South Washington and North Oregon Cascades by
early Thu evening. Max temps Thu will be warmest in Clark County and
the South Washington Cascades and foothills and areas north and east
of KSLE. The South Willamette Valley and Central Oregon Range will be
several degrees cooler due to the southwest marine surge.

Models are not all that excited about convective potential Thu
evening. the GFS shows the best surface-based CAPE (500-1000 J/kg)
over the Cascades and foothills 00Z Fri. likely tied to low-level
convergence in the vicinity of the thermal trough. However, by 03Z
max CAPE values diminish and are confined to near the Cascade Crest.
Will need to evaluate for elevated convection above the intruding
marine layer. By 06Z Fri the marine layer appears to be deep enough
to eliminate any convective threat.

Fri will be signficantly cooler inland with 850 mb plunging from
around 15C Thu to 2C. The upper low meanders toward SW Oregon and NW
California Fri, but models suggest a possible deformation band
drifting north over the forecast area Fri. There is a slim threat of
convection developing over the Lane County Cascade crest Fri
afternoon, but the GFS 700 mb streamlines shift to a slightly more SW
component. Have not included thunder in the forecast, but next few
shifts will need to evaluate the need for its inclusion. Weishaar

.LONG TERM...Friday night through Tuesday...Models consistent in
showing the upper low progressing slowly across the Pacific NW
through the early part of the weekend, augmented by another
shortwave attaching to the low Sat night and Sun. This will keep a
decent chance of showers over the region into Sun, which combined
with the low level onshore flow keeps temps a little below seasonal
normals. Sun night into Mon models in pretty good agreement that the
upper trough shifts east as a ridge of high pres presses in from the
west. This will allow temps to moderate some during the early part
of next week, while the chances for showers diminish.

&&

.AVIATION...18Z TAFs: A southerly marine surge brought low
stratus and fog to the coast this morning. KAST and KONP are
currently LIFR. Based on the latest high-res models, think that
the clouds may push away from the north Oregon coast during the
late morning or early afternoon hours, returning KAST to VFR.
However, think that the clouds will remain through most of the
day at KONP, although they may lift a bit to IFR. A stronger
marine surge tonight should push the clouds up the coast again,
returning KAST to IFR by 03Z-06Z. Cigs may lift a bit through
the night at the coast as the marine layer deepens, but expect
IFR or low MVFR into Thu morning. IFR or low MVFR stratus may
seep inland to the south Willamette Valley late tonight or early
Thursday morning. Otherwise, the interior lowlands to remain VFR
through the next 24 hrs.

KPDX AND APPROACHES...VFR conditions through 26/18z. Pyle

&&

.MARINE...Weak low pressure resulted in a southerly wind surge
last night, bringing a surge of fog and low clouds. Expect
reduced visibilities especially within 30 NM of the coast today.
Southerly winds will continue to ease this morning then veer to
the west or northwest as high pressure strengthens offshore.
Another southerly surge returns tonight as a weak low lifts
northward offshore. Winds will generally be less then 15 kt.

Showery low pressure returns for Friday into the weekend. A weak
front is expected to arrive over the weekend, which may bring a
period of small craft advisory winds. Seas should remain in the 5
to 7 ft range through the remainder of the week. Pyle

&&

.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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