NWS Portland, OR wrote its last area forecast discussion before the first event at Dec 25, 2:50 AM PST, 3 hours 10 minutes earlier.
Area forecast discussion, Dec 25, 2:50 AM PST, 3 hours 10 minutes before
Area Forecast Discussion
National Weather Service Portland OR
250 AM PST Wed Dec 25 2024
.SYNOPSIS...Next frontal system arrives today, with rain increasing
along the coast later this morning, and inland this afternoon. Rain
and breezy tonight into early Thursday, then showers. Another strong
low will push into the region late Thursday night into early Friday,
with more rain and breezy winds. Then showers for rest of Friday.
Unsettled weather continues through the weekend into early next
week, though heaviest rainfall likely to shift southward, being over
southwest Oregon and far northwest California. Snow continues at
times over Cascades.
&&
.SHORT TERM...(Today through Friday)...
Active storm track over the north Pacific continues, with next front
approaching. Will see lingering showers this am, mainly over the
higher terrain. Otherwise, increasing clouds this am, with rain
increasing along the coast by noon, then across the interior this
afternoon. This as the strong baroclinic zone gradually shifts to the
coast and into the interior tonight. As such, rainy night on tap for
the region. Models have backed off on the amount of rainfall
expected, and with the rather fast movement of the front, rainfall
across region look mostly in the 0.50 to 1.00 range for today through
Thu am, with 1-3 inches across the Willapa Hills/Coast Range. As
such, rivers will come up, but no flooding expected.
As the surface low will move off the north Pacific to south side of
Vancouver Island by 7 am Thu. Cold front will push to the coast
between 1 and 2 am, then race inland overnight. As it does, pressure
gradients will tighten, with gusty south to southeast winds on the
coast this evening, increasing towards midnight. From late evening
into the overnight hours, southerly winds will gusty 45 to 55 mph,
with gusts 55 to 65 mph on the headlands/open beaches. The winds will
east after about 3 or 4 am Thu. But, farther north over western
Washington and far north Oregon coast, will see the gusty winds
persist longer, as the rather tight gradient associated with the
bent-back occlusion shifts inland over western Washington. As such,
west to southwest winds will gust 50 to 60 mph over the Willapa Hills
and south Washington coast until about 7 am, then gradually east
through rest of the morning. With all this, will upgrade the High
Wind Watch on the Headlands/Open Beaches for all the central Oregon
coast for 8 pm this evening to 4 am Thu, north Oregon and south
Washington coast from 10 pm to 8 am Thu, and from 12 am to 10 am for
the Willapa Hills. Again, winds will be easing after 8 am across
south Washington and Willapa Hills, but will keep warning to 10 am in
case wind hold up a bit longer than expected.
Rest of Thursday will remain showery, along with modest potential of
isolated thunderstorms. Cooler across the Cascades with snow above
4000 feet. Will maintain Winter Weather Advisory for the Cascades,
where will see 2 to 8 inches of snow on Thu. But, higher up (say,
above 5500 ft), could see 10 to 20 inches of snow for this afternoon
through Thursday afternoon. Either way, those with travel plans over
the Cascades should be ready for snow covered roads and somewhat
hazardous driving conditions at times.
Not much of any break in the afternoon and early evening, as next
system will be fast approaching. Models in fairly good agreement in
bring a strong low off the north Pacific and into the Pacific NW late
Thu night. But, do have differences in models, where GFS and ECMWF
being more aggressive (stronger) with the low as it moves onshore on
south Washington coast, where as the NAM has been shifting the low
farther to the north. If buy into the GFS/ECMWF runs, would have
modest potential for high winds along the coast late Thu night into
early Fri with gusts 55 to 70 mph, and gusts 35 to 45 mph across
Willamette Valley. But, if NAM is the winner, much less wind would be
expected, with gusts 45 to 60 mph on the coast, and 25 to 35 mph
across the interior Fri am. So, will go with a blend of the models,
and keep it breezy, but not ready to get all that dragged into
supporting one or the other models at moment. As for the rainfall,
likely to be similar to tonight/early Thu front, with 0.50 to 1.00
inch across region with 1 to 3 inches over the Coast Range and
Willapa Hills, as well the Cascades to some degree.
Looks like Friday will be a repeat of Thursday, with showers and
breezy south to southwest winds, along with a chance of
thunderstorms.
.LONG TERM...(Friday Night through Tuesday)...Start to see
differences in the models, with the majority of the ensemble data
suggesting the atmospheric rivers will most likely end up south of
the region. This would in turn result in fewer impacts except that
cooler temperatures aloft would support lower snow levels, higher
snow totals and more travel impacts for those crossing the Cascades.
/Muessle
&&
.AVIATION...As of 10Z Wed, VFR conditions prevail across the area
ahead of the next system. There are spots of MVFR to IFR over the
interior valley. As the next front spread rain into the area
expect to see MVFR conditions develop at the coast after 18-21Z
Wed, then into the interior terminals after 06-09Z Thu. But areas
north of about KUAO should experience MVFR conditions earlier per
the HREF guidance. In additions to rain, this front will bring
gusty south winds and strong low- level wind shear (mainly speed
shear with minimal directional shear) this afternoon through much
of tonight.
PDX AND APPROACHES...VFR conditions giving way to MVFR conditions
after 00Z Thu as rain intensifies with the next system. But there
is about a 30% chance for MVFR to develop after 18Z Wed per HREF.
Also expecting strong low-level wind shear around 2000 ft towards
06z Thu and continuing through 12Z Thu. This will mainly be speed
shear rather than directional shear. /mh -TK
&&
.MARINE...The Hazardous Seas Warning remains on track, with seas
currently around 12-14 ft at 12-14 seconds. Winds and seas will
begin to quickly ramp up later this morning as a strong front
moves eastward over the coastal waters. Storm Warning is in
effect for this system for later this morning through Thursday
morning. By late this afternoon, expect seas between 20-25 ft at
15 seconds due to a combination of increasing wind waves and a
strong westerly swell.
Seas remain elevated through Thursday morning before falling to
near 20 ft by Thursday evening. However, yet another system
arrives Friday afternoon/evening. Winds remain somewhat gusty
during this time, with current model guidance now showing
sustained winds around 30 to 35 kts with gusts between 40-50 kt.
With that said, there is still some uncertainty regarding exact
wind speeds with Friday's system, due to varying track and
strength of the low by the models. Feel moderate to high
confidence in Gales, but can't rule out Storm force winds at least
for part of the coastal waters. The active weather continues into
the weekend, but systems appear slightly less wild. /mh
&&
.BEACH HAZARDS...Large, mid period swell is expected to impact
the region the tonight through Thursday evening. Westerly swells
of 18-22 feet combined with wind waves will produce combined seas
near 25 feet at 14-16 seconds. The westerly swell wane a little
later Thursday to near 20 ft, the ramp up again later Thursday
night into Friday with seas around 25 ft. But depending to the
track and strength of the system generating the westerly swell,
could see seas approaching 30 ft impacting the coast.
For now will extend the High Surf Advisory through Friday
afternoon. But may need to elevate to a High Surf Warning with
more impacts.
Additionally, with Toke Point forecast to reach 12 ft at high
tide around 9 am Thursday, a Coastal Flood Advisory for tidal
overflow has been issued for just the south Washington Coast.
Minor flooding up to one foot above ground level will be possible
at that time.
&&
.PQR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory 4 PM today to 4 PM FRI for ORZ101>103.
High Wind Warning from 10 PM today to 8 AM THU for ORZ101.
High Wind Warning from 8 PM today 4 AM THU for ORZ102-103.
Winter Weather Advisory from 1 PM today to 10 pm THU for
ORZ126>128.
WA...High Surf Advisory 4 PM today to 4 PM FRI for WAZ201.
High Wind Warning from 10 PM today to 8 AM THU for WAZ201.
Winter Weather Advisory 1 PM today to 10 PM THU for WAZ211.
Coastal Flood Advisory today through THU AM for WAZ201.
High Wind Warning midnight tonight to 10 AM THU for WAZ203.
PZ...Hazardous Seas Warning until 10 AM PST today on all waters.
Storm Warning from 10 AM today to 10 am THU on all waters
&&
www.weather.gov/portland
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