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000
FXUS66 KPDT 131716
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1016 AM PDT Sat Apr 13 2024

Updated Aviation Discussion

.AVIATION...18z TAFs...Main concern for the period will be the
potential for thunderstorms over BDN and RDM. Weather models are a
bit divided on timing and location of thunderstorm activity, but
sensed a good enough signal to at least mention the possibility
between 23 and 05z this afternoon and evening. Some isolated
storms could be strong to severe and produce frequent lightning,
strong outflow winds, and even small hail. Rain and thunderstorm
threat diminishes during the overnight hours. Elsewhere, expect
VFR conditions with sct-bkn high clouds and light N to NW winds.
Evans/74

&&

.PREV DISCUSSION... /issued 935 AM PDT Sat Apr 13 2024/

MORNING UPDATE...Day cloud phase distinction RGB satellite
imagery reveals low clouds draped across mountain and river
valleys of the Blue Mountains region of eastern Oregon this
morning. While surface observations and web cameras indicated
areas of fog, locally dense, earlier this morning, visibilities
are generally improving and lingering fog is anticipated to lift
through mid-morning.

Focus will then shift to convection this afternoon, primarily
across central and north-central OR where a Marginal risk of
severe thunderstorms is being highlighted by the Storm Prediction
Center. The main hazards will be a low-end chance (5%) of severe
hail (1" or greater in diameter) and severe wind (58 mph or
greater) with any organized convection that develops in an
environment characterized by steep mid-level lapse rates (7-8
C/km) and ample effective shear (35-45 kts). 12Z HREF members
continue to advertise neighborhood 30% neighborhood probabilities
of exceeding 3-hr precipitation of 1" for the Bend-Redmond-
Prineville metro region so an isolated flash flood threat with any
training thunderstorms remains a possibility. Additionally, low-
level hodographs are supportive of a very low (2%) chance of a
tornado.

Initiation is expected this afternoon in Lake, Klamath, and
Deschutes counties on the northwest side of a developing surface
low placed beneath diffluent flow aloft in the exit region of an
upper-level jet associated with the deep offshore low centered
just west of the SF Bay area in CA. This surface low is already
becoming evident in latest surface observations in south-central
and southeast OR into northeast CA and northwest NV. The threat of
severe storms tapers off later this evening, though some
lingering showers and isolated thunderstorms may continue (20%
chance) into the overnight hours.

Looking east across the Blue Mountains, while 12Z HREF
mean and 50th percentile surface-based CAPE increases to around
500-1000 J/kg, locally higher than 1000 J/kg, and becomes
uncapped, there is a lower chance of thunderstorms due to a lack
of synoptic forcing. Isolated air-mass type cells may develop,
but confidence is low (<30% chance), and given weak effective
shear (<25 kts), chances of severe storms are very low (<5%
chance). Plunkett/86

PREV DISCUSSION... /issued 247 AM PDT Sat Apr 13 2024/

SYNOPSIS...Multiple weather concerns are anticipated for the
short term period:  (1) thunderstorms with a marginal threat of
isolated severe today; (2) strong winds associated with a
vigorous cold front beginning late Sunday then more widespread
windy conditions on Monday; (3) patchy blowing dust associated
with the winds; and (4) cold air lowering snow levels Monday and
Monday night. Other than light mountain snow showers on Tuesday,
the long term period that extends to next Saturday is looking dry.
There will be a gradual warm during this time.

SHORT TERM...Today through Monday night...A large closed low has
developed off the CA coast with the center a little over 200
miles west of the SF Bay area. Meanwhile, a strong westerly flow
is currently over southern Canada. This has left our forecast area
between both synoptic features. A southerly flow aloft on the
front side of the trough will set the stage for numerous showers
and scattered thunderstorms this afternoon and evening over most
of Oregon. SPC continues to show central and north central OR
under a marginal risk for severe weather today. HREF mean SBCAPES
are between 300-600 J/kg with 40-50kt of shear between 0-6km which
means thunderstorms are capable of sustained updrafts. The
diffluent flow is particularly disconcerting, as storms could
train with heavy rainfall and localized flooding. MFR 00Z RAOB
shows 0.73" PWATs which is a good indicator for heavy rain
potential. Overall confidence for today`s forecast is very high
for showers (100%) and thunderstorms (70%), moderate for heavy
downpours (40%), and low for large hail and damaging winds
(5-10%). Otherwise, it will be unseasonably warm today due to an
inverted surface thermal trough over the region, and today`s highs
will be about 10-15 degrees above seasonal average.

Sunday will be another warm day, and a few record highs may be
reached. Showers and thunderstorms will once again develop. On
Sunday, the highest probability of thunderstorms (60%) will be
over south central and southeast OR.The parent low off the CA
coast will move south and way out of any concern. However, a
secondary upper low will pinwheel around the main low and set up
over northern NV. This will not have as much impact in terms of
moisture and instability in our forecast area, but our far eastern
zones which includes Wallowa, Union, Grant, Wheeler and Crook
Counties will observe showers and a chance (40-60%) of
thunderstorms. I can`t rule out a strong or even severe storm, but
the threat is minor. The westerly flow over southern Canada will
lower to the south across northern WA. A tight cross Cascade
gradient will set up, resulting in a strong marine push late
Sunday. Deterministic models are showing 5-7mb differences between
SMP-ELN by late Sunday. Confidence is high that the Cascades gaps
which includes the Kittitas Valley and the eastern Columbia River
Gorge will observe very windy conditions, and this may warrant a
Wind Advisory. Due to strong winds that are also forecast for
Monday, will highlight the concerns in wx graphics which still
gives time to analyze both days for any wind highlights.

Monday`s winds continue to look strong and more widespread due to
the approaching cold front moving south across the PacNW. The
marine push will tighten surface gradients for widespread windy
conditions in the lower elevations. There are also signs of
isentropic descent pushing winds aloft to the surface. However,
the factors that may keep winds from reaching high wind warning
criteria are the winds at 850mb and 700mb that models show between
30-35kt. For now, confidence is about 80% for winds reaching
advisory (45-55 mph gusts) but confidence decreases to around 50%
for some areas to have stronger than 55 mph gusts. This is also
supported by the NBM probability for winds.

There will be around a 10-15 degree drop in temperatures from
Sunday to Monday, and snow levels will lower to around 2500-3000
feet Monday night. Despite the lowering snow levels, drier air
behind the front will limit snow accumulations with little to no
accumulations for most of the forecast area. Latest model runs
have trended lower on snow amounts in the Cascades and northeast
mountains. It will be a chilly Monday night with lows in the 20s
and 30s along with a bitter wind chill due to sustained winds of
10-20 mph overnight. Wister/85

LONG TERM...Tuesday through Saturday...The long term is finally
bringing a stretch of quieter weather, with some lingering
mountain precipitation during the middle of next week giving way
to dry and clear conditions with temperatures a few degrees above
normal.

Tuesday will be the most active day of the long term forecast
period as an upper low swings across Canada. With the front
associated with this feature already east of our region, expect
that most appreciable precipitation will occur due to orographic
lift across the mountains of our forecast area. With limited QPF
to begin with and snow levels between 3000-5000 feet, the bulk of
any activity will be light rain or snow. By Wednesday, clouds
begin to clear and precipitation exits with the eastward
progressing low, bringing benign conditions as we lie between
systems with an offshore ridge that may begin moving over by the
end of the period. This should help to slowly raise temperatures,
with highs for our population centers on Tuesday in the upper 50`s
to mid 60`s, raising a couple of degrees each day with upper 60`s
to upper 70`s by a week from today.

Overall there is moderate confidence (60%) in the long term
forecast. There does lie some uncertainty in regards to the
overall pattern after the Canadian low departs, with discrepancies
lying in the placement of the offshore ridge plus some possible
shortwave activity that may move across late next week. Ensemble
guidance overall seems to favor a lack of significant shortwave
activity and ridging encroaching but not quite moving directly
over the PacNW, which is reflected in the above forecast, but the
outliers do pull forecast confidence down for our temperatures.
Goatley/87

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  72  46  73  43 /   0   0  10   0
ALW  77  49  76  47 /   0   0  20  10
PSC  77  50  81  48 /   0   0   0   0
YKM  75  45  79  42 /   0   0   0   0
HRI  76  48  79  45 /   0   0  10   0
ELN  74  46  73  42 /   0   0   0   0
RDM  65  43  66  35 /  50  60  30  10
LGD  73  46  72  41 /  10  10  40  30
GCD  74  45  70  38 /  20  40  60  30
DLS  75  53  72  47 /   0  10   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...None.
WA...None.
&&

$$

SHORT TERM...86
LONG TERM....87
AVIATION...74