wrobstory · February 14, 2024 15:17
diff --git a/forecast.txt b/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.
	.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.