184
FXUS66 KPDT 010335
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
835 PM PDT Tue Apr 30 2024

.EVENING UPDATE...A few changes made to the POPs in the forecast
to better represent the current radar. Much of the precipitation
is lingering across the eastern mountains with a few cells running
along the lower Columbia Basin. GLM lightning data has shown a
few in cloud pulses of lightning have been associated with a few
of the cells however, upper level conditions have tamped down the
cells abilities to grow vertically. Many of these cells put down
some rain and graupel with ground observations showing up to 0.05
inches of precipitation has fallen along the eastern mountains
and through portions of the foothills of the Blues. Guidance does
show some light chances (10-15%) chances of a few isolated
nocturnal thunderstorms over the Blue Mountains, Union County and
the Wallowas. Bennese/90

&&

.PREV DISCUSSION... /issued 416 PM PDT Tue Apr 30 2024/

UPDATED AVIATION...00Z TAFs...Main interest through the forecast
period will be the breezy conditions across RDM/BDN through
04-08Z. Elsewhere winds will continue to be elevated between
06-12kts. Most TAF sites will see light shower activity on and off
through the period as well. Even with the showers, VIS will remain
P6SM, CIGs will be above OVC/BKN050 so conditions will remain VFR.
Bennese/90

PREV DISCUSSION... /issued 257 PM PDT Tue Apr 30 2024/

SHORT TERM...Today through Thursday night...Satellite and radar
reveal scattered showers over the mountains with more isolated
activity across the lower elevations. From a synoptic perspective,
mid-level water vapor imagery shows an upper-level trough over
the PacNW, accompanied by a circulation center over northern OR
and southern WA. This trough will track east-southeast across the
PacNW through the evening and is providing synoptic-scale lift
across the region. Moreover, cool temperatures aloft coupled with
daytime heating have produced steep low- and mid-level lapse
rates. At the surface, observations indicate mostly light snow or
rain/snow mix over the mountains due to cool temperatures and a
dry boundary layer; elsewhere, rain or graupel showers are
forecast.

While most convective activity has been shallow, one cell in the
Columbia Basin has managed to produce a brief signal on GLM
products. The expectation is that with modest surface-based CAPE
(100-350 J/kg HREF-advertised), and steep low- and mid-level
lapse rates (9-10 C/km, and 7-8.5 C/km, respectively) the
convection will be sustained through the afternoon before waning
this evening as instability diminishes and the trough exits to our
east. Most shower activity will be shallow, but there is a low
(10-20%) chance of cloud-to-ground lightning, highest chance over
the Blue Mountains.

Overnight, NBM probabilities paint a low to medium chance (25-60%)
of sub-freezing minimum temperatures for portions of the Yakima
and Kittitas valleys, the lower Columbia Basin, and the foothills
of the northern Blue Mountains. Confidence in widespread sub-
freezing temperatures is too low to issue a Freeze Warning,
especially with westerly winds expected to keep the boundary layer
mixed through much of the night.

Wednesday night and early Thursday morning, the next system from
the Pacific is progged to arrive from the Gulf of Alaska. Guidance
is in good agreement that the open wave or weak closed low will
track southeast across OR mainly producing precipitation for the
Oregon half of our forecast area. Snow levels will be low enough
that the mountains will see another round of light snow while
lower elevations will see light rain with perhaps a rain/snow mix
for portions of central OR.

The system is anticipated to exit by Thursday night with dry
conditions forecast. Plunkett/86

LONG TERM...Friday through Tuesday...Run-to-run discontinuity of
the medium range models continues, leading to uncertainties in the
forecast for this coming weekend into early next week. Overall,
there will be spring-like conditions, but forecasting details have
been challenging the past several days.

Confidence is moderate to high that most of Friday will be dry
with near to slightly above seasonal temperatures. A weak
transitory ridge will provide a brief break in precipitation and
winds. Meanwhile, an upper low will deepen offshore and will
increase the southwest flow aloft west of the Cascades.
Precipitation will spill over the Cascade Range Friday evening,
and the rest of the forecast area will have an increasing chance
of precipitation Friday night and Saturday. Current forecast will
show 80-90% PoPs along the Cascades, 60-80% from the Cascade east
slopes to the Blue Mtns, and 40-60% east of the Blues on Saturday.
The ECMWF and a majority of its ensembles advertise more QPF
compared to the GFS/GEFS based on the position of the low that is
farther north along the OR coast compared to northern CA. The
ECMWF EFI from the 00Z runs paints an area of 0.6-0.8 of the QPF
over portions of eastern OR, indicating there is a chance of
moderate to heavy showers with this system. There are also hints
of a late snow event for the Blue Mountains this weekend on the
EFI (shift of tails around 2.0), but the snow-to-liquid ratios
should be very low this time of year (5:1) that any chance of
appreciable accumulations over the passes is low.

The models are out of sync Saturday night through Tuesday,
therefore it`s difficult to describe the forecast with
certainty. PoPs decrease Saturday night and Sunday, but there
still is around a 40-60% chance of mountain showers. There will
be an upper low and large trough over the western U.S., but the
question will be the position. A majority of the ensembles favor
the center of the low over the Great Basin, leaving a cyclonic
flow or split flow aloft over WA/OR. The area most likely to
receive precipitation during this time will be from the Blue Mtns
eastward. The low is progged to weaken and track eastward early
next week, but now the question is another trough off the coast.
The Canadian and GEPS are the strongest with the offshore trough
while the others show a more westerly flow across WA/OR or a weak
shortwave trough. Since it will be a WSW flow with variable
amounts of moisture, will forecast a chance of mountain rain/snow
showers and a slight chance of showers elsewhere. NBM has breezy
conditions early next week which looks reasonable based on the
westerly flow aloft. Wister/85

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  34  59  34  61 /  70   0  10  30
ALW  37  62  38  65 /  30  10  10  30
PSC  38  67  40  68 /  20   0  10  20
YKM  32  65  35  67 /  40   0  10  10
HRI  36  65  39  66 /  60   0  10  20
ELN  34  60  33  66 /  60   0  10  10
RDM  27  56  35  55 /  10   0  40  30
LGD  33  55  30  57 /  90  10  10  40
GCD  31  54  34  54 /  80  10  30  50
DLS  39  61  43  65 /  30   0  30  20

&&

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

$$

SHORT TERM...86
LONG TERM....85
AVIATION...90