".SHORT TERM...Tonight through Tuesday...Quite impressive frontextends from near the equator up the east Coast of the CONUS and
Canada and continues past Alaska to the arctic circle. Multiple
surface low pressure centers have developed and continue to develop
along this frontal boundary. Even though it is supposed to be coming
onshore less than 24 hours from now, the surface low has mostly
eluded detection using satellite methods and the Buoy detection
network is quite sparse in the areas of concern. Current thinking is
the surface low will emerge from somewhere within the swath of
brighter WV returns centered near 40N and 135W. Hopefully the
forecast will become more clear overnight as the surface low starts
to show the whites of it`s eyes. In addition, this area of brighter
WV returns would suggest better moisture being advected towards the
area than modeled QPF would suggest. Therefore, did not make any
changes to day shift QPF as any guidance loaded tried to lower QPFwhich would likely be a step in the wrong direction.
Otherwise, surface temperatures east of the Cascades have plummeted
this evening with strong high pressure aloft and fresh snowpack. Mid
level clouds (~5000 feet) building up against the east side of the
Cascades has limited the cooling at Hood River and the Dalles.
However, colder, drier air continues to advect towards that area from
the north and wet bulb temperatures will continue to fall even if air
temperatures do not drop much more. TTD-DLS gradient is finally up to
-7.2, but is still shy of the 9mb progged in 12Z NAM and a far cry
from the 10 to 12mb gradient progged 24 to 36 hours ago. Any slow
lovers should hope to see this gradient increase to help funnel the
cold air into areas west of the Cascades.
Adjusted tonight`s low temperatures down a degree or two to account
for the colder, drier airmass. The boundary which brought
accumulating snow to areas in the Gorge and Hood River Valley
dissipated and settled somewhere between Salem and Eugene. This frontwill start to lift north again near morning and will overspread
either freezing rain or snow across the area. Still considerable
uncertainty with the amount of precip to come out of this band. 00Z
NAM is only showing a few hundredths while the GFS is a bit wetter
and the 00Z GEM and 12Z ECMWF are significantly wetter. Initial
thinking is that the dry air in place will be significantly dry
enough for most precip at the onset to be snow for areas in the
Portland metro area and I-5 corridor. However, this dry airmass will
likely keep any accumulation unlikely until the afternoon period when
the deeper moisture moves into the area. However, do not want to
discount the potential for an inch or two of accumulation with this
initial band, especially given a few of the wetter models.
00Z GEM threw a wrench in the forecast and is now on board with the
ECMWF bringing a double barrel surface low onshore with the southern
low near Lincoln City. This would never quite allow the warmer air
aloft to overcome the cold air and keep precipitation all snow for
areas in the north Willamette Valley northward. This is now shaping
up to be an epic battle between the domestic an international models
with the ECMWF and GEM wanting to dump snow on the Portland/Vancouver
metro area and the NAM/GFS bringing freezing rain.
Freezing rain may be possible for a bit at the onset for areas of the
central Willamette valley, especially west of Salem where the colder
air banks up against the Coast Range. Accumulations will likelyremain less than 0.1" before temperatures warm above freezing by mid
afternoon. The Willapa Hills may stay all snow for this event, so
current forecast of 1 to 3 with local amounts up to 6 inches may be
underdone. Another area where more snow may fall than in the advisory
is for the lower Columbia/I-5 corridor where precip may remain snow a
bit longer than elsewhere.
No changes to the forecast for the Columbia River Gorge and the Hood
River Valley. Regardless of the low pressure track, winter storm
warning criteria appears likely to be met for those areas from snow,
and a combination of snow and freezing rain for the western Gorge.
Confidence is high for the forecast in the Columbia River Gorge and
Hood River Valley, but confidence elsewhere is still quite low. Went
ahead and issued the winter weather advisory for those areas
elsewhere because confidence is high that whatever will fall will be
icy/snowy, but exactly what and for how long it will last and
accumulate is still up in the air.
If the ECMWF and GEM are right, Portland and surrounding areas just
might see a white Christmas yet, but not if the grinch (GFS/NAM) has
anything to say about it. /Bentley
The below previous discussion points remain valid even this evening
and therefore have been kept for this discussion.
It is interesting to note that 40 percent of the ECMWF ensemblemembers are showing at least 2 inches of snow at KPDX and about 85
percent give an inch. Conversely, only 10 percent of the GFS ensemblemembers give an inch for KPDX. Lots of intrigue, but no clear-cut
solutions even this close to the event.
Current MOS guidance temperature and dewpoint forecasts seem to
suggest the greatest impacts for freezing rain will be concentrated
to the east metro. With slightly varying model solutions regarding
the low track, a 50-100 mile shift in the track will result in major
differences regarding precip type. The precipitation wanes from south
to north Mon and Monday night, with minimal precip left by 06Z Tue.
The ECMWF maintains some degree of offshore flow into Tue, while the
GFS shows nearly neutral gradients 06Z Tue, but becoming weakly
offshore again by Tue morning.Weishaar"
Now lets see what the models have to say! The 48 hour snowfall map from UW-MM5 WRF-GFS.
That's just about 2 inches for most of the Portland Metro Area... and again this is the GFS...which doesn't think we get a lot of snow...However...NWS Portland just tweeted this out...
Currently expect little if any snow accum in the valley tomorrow because freezing rain is the most likely scenario. However, there is still a chance (20-30%) that temps stay cold enough for several inches of snow in the I-5 corridor and the PDX/Vancouver metro area. Stay tuned. https://t.co/U2M6bHUFCe— NWS Portland (@NWSPortland) December 24, 2017
It's 11:15pm...We're 45 minutes from tomorrow...about 12 hours out from the first possible wave and we really have no idea what is going to happen. Stay Tuned...It's about to go down.
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