Area Forecast Discussion
Issued by NWS Portland, OR

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FXUS66 KPQR 122259 RRA
AFDPQR

Area Forecast Discussion...
National Weather Service Portland OR
258 PM PST Wed Dec 12 2018

.SYNOPSIS...A few showers continue over the region today, mainly over
the higher terrain. A  warm front will produce rain for southwest
Washington and far northwest Oregon tonight and Thursday. More
widespread rain is expected on Friday when a cold front moves across
the region. This front may also produce strong gusty winds for the
coast and coastal mountains. Most of Saturday will be dry before
another frontal system arrives on Sunday. Active weather continues
through early next week.

&&

.SHORT TERM...Tonight through Saturday...There has been a relative
lull in the weather today. There have been some light post-frontal
showers around, generally focused in the higher terrain. However, the
next moist frontal system is already bearing down on the Pac NW. The
main low pressure center is in the Gulf of Alaska, with an extensive
frontal boundary draping down through the Pac NW offshore waters.
Satellite derived precipitable water values show another 1.2-1.3 inch
plume of subtropical moisture feeding into this system. Light rain is
beginning to spread into western Washington, and it will increase in
intensity later this evening and overnight. The focus of the heavy
precipitation axis will again remain to the north of our CWA up
towards the Olympic Peninsula. However, it looks like the south
Washington coast and Willapa Hills will remain on the southern
periphery of the main rain band, and these areas should pick up some
significant rainfall totals tonight through Thursday. Some locations
in the Willapa Hills may end up with 1.5-2 inches. We anticipate some
steep rises on the Grays and Willapa Rivers, but no flooding issues
are anticipated at this time. There will also be some gusty winds
developing tonight into Thu morning over the south Washington coast,
with gusts 40 to 45 mph expected.

There may be some light rain at times tonight and Thu over far NW
Oregon, but think the southern half of our CWA will remain dry. In
addition, mild southerly winds on the south side of the frontal
boundary should result in rather mild conditions Thu, with highs into
the mid 50s for many locations, especially south of the PDX metro
area. The frontal boundary will lift even further north later Thu and
Thu night, which should result in dry conditions across the CWA
through early Fri morning.

A rather sharp shortwave trough currently out near 160W will approach
the Pac NW from the southwest Thu night. The fcst models are in good
agreement that this feature will interact with the stalled frontal
boundary, producing a rapidly strengthening surface low that will
move along the front through the offshore waters. There is some
uncertainty on the exact strength and track of this low. Based on the
12Z models and ensemble guidance, we decided to follow the 12Z GFS
track at this time, which strengthens the low to around 980-985 mb
inside of 130W and moves it onshore on northern Vancouver Island.
This solution would result in some gusty coastal winds for us Fri
morning, especially for the south Washington and far north Oregon
coastal areas. Currently thinking winds will peak around 50-55 mph
along the beaches and coastal headlands. However, there are some
tracks that keep the low closer to the coast, including the 12Z NAM,
which if it verified, with probably result in high wind criteria
being reached.

As the surface low moves inland early Fri, it will pivot the frontal
boundary to be oriented north to south. The front is modeled to push
inland during the day Fri into Fri evening. The upper level trough
looks likely to be splitting as it moves inland, so the current model
QPF values are not really all that impressive. That being said,
confidence is high that it will be a cloudy and wet day across the
region. Snow levels should generally remain below the Cascades passes
and expect several inches of snow through Fri night. However, current
QPF totals would keep accumulations well below advisory criteria.

Post-frontal showers look to continue through Fri night and into
early Sat. But it currently looks like most of the region will stay
mainly dry during the day Sat. There will probably be plenty of
clouds around though, as the next frontal system will be rapidly
approaching the coast. Pyle


.LONG TERM...Friday night through Tuesday...
Both the GFS 12Z and ECMWF 12Z show a strong upper level low in the
Gulf of Alaska narrowing and reaching further south as it moves
east. The resulting cold front will reach down the entirety of the
Oregon Coast bringing precipitation into the area by Saturday
evening. Southerly winds through Monday along the coast are pushed
to a more westerly regime as the axis of the trough moves over the
forecast area.

This upper level zonal flow will form atmospheric river conditions
bringing a steady flow of precipitation into the area through the
forecast period. By Wednesday the two models start to diverge on the
location of where the atmospheric river is pushing the moisture. The
GFS 12Z shows precipitation moving northward as a result of a high
pressure to the southeast which will dry out the area by Wednesday
morning, whereas the ECMWF stays on the wetter side with persistent
rain through Thursday. Although there is slight disagreement near
the end of the forecast, rain will be the primary feature of the
extended. -BPhillips

&&

.AVIATION...VFR conditions continuing across much of the area
this afternoon with increasing mid-level moisture pushing into
the coast and northern parts of the area ahead of an approaching
weak front that will clip the northern portions of the forecast
area. This front will bring widespread rain and lowering cigs
with MVFR cigs becoming common, along the coast and into portions
of the interior -  mostly north of a KTMK to K4S2 line. As the
warm front lifts north, expect increasing gusty south winds along
the coastal areas and, although to a lesser extent, inland.

PDX AND APPROACHES...VFR cigs continue with some showers under
onshore flow aloft through the period. South winds become
southeast increase a bit overnight/early Thursday to around 15
kt.

&&

.MARINE...Another fast moving warm front pushes across the
region beginning early tonight. Expect to see another round of
gusty southerly winds, with gusts 35 kt. A large westerly swell
combined with a fresh southerly swell, will keep seas near 17-20
ft.

Models continue to show a 980 MB surface low moving northeast,
but well northwest of the waters on Thursday. The low will bring
a cold front with stronger gales and possible storm force gusts
across the waters on Friday. Currently, there is no long term
break in sight as additional systems line up across the Pacific
well into next week. The strong low on Thursday night/Friday
seems to be a more potent swell generator with combined seas
around 20 feet. Regardless, winds likely to reach gales again
with seas building toward 20-25 feet by late Friday. The key
message remains that a very active pattern will continue through
the period.

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory until 10 AM PST Thursday for Central Oregon
     Coast-North Oregon Coast.

WA...High Surf Advisory until 10 AM PST Thursday for South
     Washington Coast.

PZ...Gale Warning until 6 PM PST Thursday for Coastal Waters from
     Cape Shoalwater WA to Cascade Head OR out 60 NM.

     Small Craft Advisory until 10 PM PST Thursday for Coastal
     Waters from Cascade Head OR to Florence OR out 60 NM.

     Small Craft Advisory for Rough Columbia River Bar until 6 PM
     PST Thursday.

&&

$$

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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 forecast area.


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