Feb 14 2024 Forecast

forecast.txt

.DISCUSSION...Today through next Tuesday...Well, we mentioned a
nonzero chance of lowland snow in the last few discussions, and that
appears to be coming to fruition in what will be an extremely
challenging forecast for the lowlands north of about Salem. The
addition of high resolution guidance has significantly increased the
probabilities of snow accumulation for these areas, including for the
greater Portland and Vancouver metro area. Several inches of snow are
likely in the Columbia River Gorge east of Multnomah Falls, with over
a foot likely for the Cascades and upper portions of the Hood River
Valley by the time snow diminishes late Thursday or early Friday.
Meanwhile, latest probabilistic guidance continues to suggest very
low probabilities of impactful snow south of about Wilsonville. This
leaves SW Washington, the Portland/Vancouver metro area, and lower
portions of the Columbia River, where the forecast is so challenging
that 1-2 deg C of error for any portion of the lower atmosphere could
make the difference between a cold rain and several inches of heavy,
wet snow. Models have trended colder and snowier in the past 12-24
hours.

To set the stage, strong inversions have locked low clouds and fog
into most of our inland valleys. Based on latest (2 AM PST) RAWS
observations, the top of the inversion layer appears to be around
1500-2000 ft. Keep this inversion depth in mind, as it will be very
important to the potential for lowland snow today through Thursday.
The inversions are unlikely to break today as high clouds spread into
the forecast area from the southwest, leading to poor mixing. Many
locations in the Willamette Valley remained in the lower to mid 40s
Tuesday due to the strong inversions and persistent low cloudiness.
Fog and low clouds lingered for much of valley, expanding overnight.
Therefore it appears temperatures will struggle to climb much more
than a couple degrees from this morning`s temperatures in the mid to
upper 30s. Areas of dense fog have increased over the past hour, so
we have issued a Dense Fog Advisory early this morning for the
Portland/Vancouver metro.

Synoptically, we have low pressure near 40N/130W, advecting mild,
moist air northeast toward SW Oregon. The deterministic 06z GFS, NAM,
and 00z ECMWF all keep the low near 1000 mb or slightly weaken it
while it approaches the Pac NW coast. This low appears to be right in
that "Goldilocks" spot where it will remain weak enough to prevent
strong warm advection, while remaining strong enough to enhance
frontogenesis, allowing significant QPF to impact much of SW
Washington and NW Oregon. Looking at Q-vector convergence in low-res
GFS, EC, and NAM data, there will be excellent forcing for synoptic
lift over nearly the entire CWA. Therefore the NBM mean QPF of around
1 inch for the inland valleys, 1-2" for the coast, and 1.5-3" for the
Coast Range and Cascades seems reasonable for totals through Thursday
night. NBM 90th percentile QPF runs from around 1.5" for the inland
valleys, 1.5-3" for the coast, and 2-5" of rain for the higher
terrain. These high QPF totals show the likelihood of sustained
moderate to locally heavy precip rates, which tend to gradually drag
down snow levels due to latent heat processes associated with melting
and evaporation. Areas where precipitation remains heaviest the
longest stand the best chance for snow levels to reach the suburban
hills or even valley floors - and at this point, that area appears to
be the greater Portland/Vancouver metro area where low-level
frontogenesis will remain the strongest for the longest.

Therefore it should not be a surprise that an increasing number of
models and ensembles, especially the high-resolution ones, are
beginning to show accumulating snow infringing on the Portland metro
and the Clark County Lowlands. 00z HREF means are showing anything
from a trace of snow for the inner metro to 2 inches I-205 eastward.
Ratcheting these totals up to HREF 90th percentile values now shows
1-2 inches of snow in Portland/Vancouver and 3-6" of snow east of
I-205. These snow accumulations assume a 10:1 snow-to-liquid ratio,
but the marginal temps and thus the wet nature of the snow should
yield ratios more in the 3:1 to 5:1 range. Therefore it is probably
safe to assume these models are actually depicting half the snow they
are showing. Regardless, this is a significant development, agreed
upon now by nearly all 00z ECMWF and GEFS members. Therefore, we will
be issuing a Winter Weather Advisory for the potential of up to 3
inches of snow in the Portland metro area tonight into Thursday.
Temperatures will be extremely marginal, if not too warm, for snow to
accumulate on roads. However, heavy precipitation rates can change
road temps quickly for similar reasons as for the atmosphere. The
most likely areas for accumulating lowland snow will be north and
east of I-205, elevations above 500 feet (including Portland`s West
Hills), and perhaps far western Washington County along the east
slopes of the Coast Range and the Gales Creek/Banks area. That said,
any elevation could see accumulating snow where heavier precipitation
remains persistent in the Portland metro, along the Columbia River,
or in SW Washington. High-end (95th percentile) snowfall totals
approach 8 inches for portions of the Portland metro. All in all, the
setup with this system is very similar to the Jan 2017 surprise
Portland snowstorm, where decaying low pressure failed to advect
enough mild air to counter the lowering snow levels due to heavy
precip rates. That event led to 8 inches of snow at PDX, with over a
foot of snow in some portions of the metro area. With all this in
mind,

All this talk about snow should not mask the fact that there is still
a 40-50% chance elevations below 500 feet will not see any
accumulating snow. So don`t be surprised if wet snow mixes in from
time to time but the p-type never fully changes over. While
confidence in all the details of this system remain remarkably low
for something that begins to affect us in Periods 1-2, confidence is
high in THIS IMPORTANT DETAIL: If precipitation fully changes from
rain to snow in your location, assume precipitation will remain snow
for the remainder of this event. This could happen as early as this
afternoon, but is more likely tonight into Thursday. If rain fully
changes to snow, it means the necessary work has been done by latent
heat processes to cool the column enough to bring snow down to your
elevation. Therefore, with the lack of significant warm advection,
the column should remain cold enough for precipitation to continue as
snow until it tapers off.