Heavy Snow in Washington, April 1 to 2, 2023

Area Forecast Discussion
National Weather Service Pendleton OR
1035 PM PDT Fri Mar 31 2023

updated the aviation discussion

.UPDATE...A vigorous low pressure system will impact the pacnw
with periods of snow...heavy at times across the area mountains.
Winter storm warnings remain in place across our WA/OR cascades
and blue mountains. Across the southern blue mountains and Ochocos
a winter weather advisory is in place.

These winter storm warnings and winter weather advisories
continue into Sunday. There remains around a 60 percent chance of
greater than 6 inches of snow at MEH and around a 50 percent
chance of greater than 8 inches of snow across the cascades with
high confidence. Locally higher amounts will occur as an upper
level jet LFQ remains over the region this weekend. Elsewhere
valley rain can be expected but locations over central Oregon
such as Bend and Redmond could also experience light snow tonight
with minor accumulations. Another challenge is winds and
pressure gradients tighten creating breezy to windy conditions
tomorrow.

Otherwise minor changes were done to the forecast tonight as low
temperatures are forecast in the upper 30s to low 40s...with upper
20s in the mountains.

&&

.PREV DISCUSSION... /issued 556 PM PDT Fri Mar 31 2023/

SHORT TERM...Tonight through Sunday night...The short range
deterministic models and their ensembles are in very good
agreement with the next weather system to move into the region.
This next system contains abundant moisture, and a strong upper
jet stream. This jet stream will provide good upper level support
for dynamic lift, which when combined with a lower level jet max
will provide efficient lift into the Cascades and the northeast
mountains. This will result in heavy snow over the next couple of
days over the Cascades and the northeast mountains. Snow levels
will start out rather high, around 3500-4000 feet MSL this
evening, but then they will lower to near 500-1000 feet MSL
northwest areas and to 1500-2000 feet MSL in southeast portions of
the forecast area by late Saturday night. Heavy snow is expected
in the Cascades and the Northern Blue Mountains, with moderate to
locally heavy amounts in the Ochoco-John Day Highlands and the
Southern Blue Mountains. Winter Storm Warnings are in effect for
the Upper East Slopes of the southern WA Cascades, and for the
east slopes of the northern OR Cascades beginning at 5 PM PDT this
afternoon. A Winter Storm Warning is also in effect, beginning at
11 PM PDT this evening, for the Northern Blue Mountains of OR and
WA above 4000 feet MSL. Finally, Winter Weather Advisories are in
effect above 4000 feet MSL, beginning at 11 PM PDT this evening,
for the Ochoco-John Day Highlands and the Southern Blue Mountains.
These highlights will be in effect until Sunday afternoon or
evening. Conditions will become drier Sunday night before another
weather system moves into the region early next week.

The deterministic model's ensembles support a high probability of
heavy snow amounts for the warning areas and moderate to locally
heavy snow amounts for the advisory areas. The NBM 4.1 supports
up to a 100 percent chance of greater than 8 inches of snow for
both the OR and WA Cascades' warning areas, and 80-90 percent for
the higher elevations (above 4000 feet MSL) of the northern Blue
Mountains. They support high probabilities mostly from 70-90
percent of greater than 4 inches of snow over the Southern Blue
Mountains and the Ochoco-John Day Highlands, above 4000 feet MSL.
As such, the current warnings and advisories look good as they
are and will continue them.

The other concern for the short term period will be gusty windy
conditions on Saturday, especially with the passage of this
weather system's cold front and behind it. The mixing from the
cold front will cause stronger winds at between 5000-1000 feet MSL
to be mixed downward to the surface. However, these winds are
mostly in the 30-40 mph range, which is just under advisory
criteria wind speeds. Though there could be some local gusts
around 45 mph over the higher exposed ridge tops, most winds will
be from 20-30 mph with gusts to 40 mph. The NBM 4.1 wind maximum
gust probabilities are in agreement and do support these speeds,
with just isolated locations with gusts to 45 mph. The winds will
increase overnight tonight and Saturday morning, and then peak in
the afternoon and early evening, before diminishing overnight and
on Sunday. The next few shifts will need to monitor forecast wind
speed trends in case winds become expected to reach or exceed
advisory criteria.

Maximum temperatures will cool to about 5 to 10 degrees below
normal by Sunday, with highs mostly in the upper 40s to mid 50s
lower elevations, and mostly in the mid 20s to mid 30s mountains.
Low temperatures will mostly be in the 30s lower elevations, and
mid teens higher mountains to mid 20s mountains lower ridges. 88

LONG TERM...Monday through Thursday...The extended period remains
characterized by unsettled conditions with ample shower activity to
begin the week, especially over the elevated terrain. Due to the
broad troughing across the western CONUS and the cooler airmass
aloft locally, confidence remains high that both daytime and
nighttime temperatures stay below normal into the middle of next
week. This is the primary feature highlighted by the ECMWF
Extreme Forecast Index(EFI) with values of -0.7 to -0.9 blanketing
the CWA; lowest over Central Oregon and the Ochoco-John Day
Highlands. In addition to cold temperatures, the EFI depicts a
weak signal for anomalous snowfall in the Oregon Cascades Monday
and Tuesday. Otherwise, variables like wind and QPF are expected
to stay within climatological norms. There is moderate
confidence(40-50%) the pattern trends at least a little calmer and
drier around the middle of next week, although some discrepancies
still exist past Tuesday.

On Monday guidance is in excellent agreement regarding the overall
upper-level pattern, placing a closed low smack-dab in the middle of
the CWA. Cold temperatures aloft will support instability and ample
shower activity, especially in the Cascades and eastern Mountains,
but at this point it’s tough to tell whether showers will be robust
enough in the afternoon hours to produce any lightning with any sort
of confidence. Models depict this upper-low quickly exiting the
region to the southeast Monday night but then having another
developing area of lower pressure drop into the Pacific Northwest
from British Columbia. The latter feature is associated with a
much greater degree of uncertainty as both deterministic and
ensemble guidance vary in the overall depth of the wave/closed low
but show similarities in timing. Generally, expect another burst
of increased shower activity focused in the Oregon Cascades. This
area in particular is one to watch as the NBM paints it with a
40-60% chance to exceed 12 inches of snowfall Monday through
Tuesday; significantly lower probabilities for the rest of the
Cascades and eastern Mountains.

Once this secondary upper level shortwave/low pressure feature moves
out of the region Tuesday night into Wednesday, a ridge of high
pressure over the eastern Pacific begins progressing towards the
Pacific Northwest. It's important to note model uncertainty
continues to increase Tuesday onward. Exploring the ensemble cluster
analysis reveals a few potential scenarios with nearly ~65% of the
grand ensemble members favoring a generally drier solution
Wednesday and Thursday outside of the chance for diurnally driven
showers over the elevated terrain. The other solutions either
present a much weaker ridge or no ridge at all which would lead to
better chances for precipitation by comparison. Confidence should
increase in the coming days as to which scenario comes to
fruition and whether we’ll finally see more of a distinct reprieve
from the active and cooler conditions. Schuldt/99

AVIATION...mainly vfr conditions are expected early before a
vigorous weather system continues to bring rain to the taf sites
into Saturday. The rain will create areas of mvfr conditions with
lowering ceilings and vsby. Ceilings may lower between BKN030-050 in
heavier rain showers. For taf sites KRDM and KBDN ceilings may lower
to BKN020-040 along with vsby between 3SM-5SM in snow between 06Z-
18Z. In addition surface winds will increase tomorrow after 15z and
become sustained between 15 mph to 25 mph with gusts to 30 mph at
most taf sites especially after 20z.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  38  51  33  50 /  90  80  40  60
ALW  40  53  35  52 / 100  90  60  80
PSC  43  57  36  55 /  60  40  10  30
YKM  32  54  27  51 /  40  30  20  40
HRI  40  56  34  55 /  70  50  20  40
ELN  32  49  26  47 /  50  40  20  40
RDM  32  46  27  43 /  80  80  40  70
LGD  37  44  27  42 / 100 100  60  90
GCD  35  43  26  42 /  90 100  60  90
DLS  40  52  34  50 /  80  80  70  80

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Warning until 5 PM PDT Sunday for ORZ502.

     Winter Weather Advisory until 5 PM PDT Sunday for ORZ503-506.

     Winter Storm Warning until 11 PM PDT Sunday for ORZ509.

WA...Winter Storm Warning until 5 PM PDT Sunday for WAZ030.

     Winter Storm Warning until 5 PM PDT Sunday for WAZ522.

&&

SHORT TERM...97
LONG TERM....99
AVIATION...97