Tornado in Washington, May 1, 2024

Area Forecast Discussion
National Weather Service Pendleton OR
1102 AM PDT Wed May 1 2024

Updated Aviation Discussion

.AVIATION...18Z TAFS...VFR conditions for all sites during this
period before another system arrives tonight. Confidence is moderate
for KRDM/KBDN decreasing to MVFR due to rain bringing low VSBYs and
CIGs Thursday morning(50%), but lowers for snow (30-40%). Winds will
generally be light, but KDLS/KPDT/KALW could have winds gusting
around 17-22kts into this evening before decreasing. Feaster/97

&&

.SHORT TERM...Today through Friday...Satellite and radar imagery
tonight show an upper trough continuing to pull a band of light to
moderate showers across northeast OR, while a drier northwest flow
is setting up across western and central PacNW. While in the lower
elevations this band has produced mostly rain, the northern Blues
and Wallowa county can expect to see light snow accumulations of 1
to 2 inches through the remainder of the nighttime hours.

Today, conditions will dry out across the forecast area as a dry
northwest flow aloft sets up on the backside of the departing
upper trough. That said, a weak plume of moisture is expected to
wrap-around an upper low centered over southern Saskatchewan,
resulting light snow showers over Wallow county and the northern
Blues this afternoon. Otherwise, breezy west to northwest winds
will develop through the Cascade gaps into the Columbia Basin,
with strongest winds of 20-25mph and gusts up to 40mph possible
through the Kittitas valley.

The next system in the form of an open wave in northwest flow
aloft will dive across OR around midnight, and exit the area by
Thursday evening. There is good agreement amongst global and hires
short term guidance that the open wave will keep much of the
rain and mountain snow showers across the OR side of the forecast
area, though the south central WA Cascades will see very light
snow showers. The heaviest precipitation is expected to impact the
OR Cascades as the wave tracks over the region, with HREF
probabilities indicating a ~80% chance of 4 inches of snow above
4.5kft-5kt. Further east, the HREF shows the Ochoco-John Day
Highlands having a 45-55% chance of 2 inches of snow; the
remainder of the northeast OR mountains will see only up to an
inch of snow (confidence 70-80%).

Behind the open wave exit late Thursday, a transient ridge will
slide into the PacNW bringing another brief period of dry
conditions. By Friday afternoon, guidance shows an occluded
frontal system pushing into the PacNW ahead of a closed
developing in the northeast Pacific. Ahead of the front, the
combination of the transient ridge then southwest flow aloft will
result in warm air advection across the PacNW, pushing snow levels
to between 6kft-7kft Friday afternoon and night. This will result
in mostly rain showers with snow showers over the higher crest
areas along the Cascades. Rain showers will spread east in the
overnight hours Friday into Saturday. Only other notable
phenomena will be breezy winds switching to the south and
southeast across the forecast area ahead of the upper closed low
and occluded frontal system Friday. Lawhorn/82

.LONG TERM...Saturday through Wednesday...Ensemble and deterministic
models are showing better agreement for the weather pattern over the
weekend before uncertainty increases Monday through Wednesday.
Saturday will see a closed low off the PacNW coast moving SE and
into SW Oregon by the afternoon and evening then into the Great
Basin on Sunday. This will slowly push a cold front across the
forecast area Saturday and into Sunday. The slow moving nature of
the front and the path of the low will allow moisture wrapping
around the low to advect back into the forecast area Saturday night
and Sunday. This could provide some meaningful precipitation amounts
as the 48 hour NBM probabilities indicate a 40-70% chance of half an
inch of rain over the eastern mountains under this pattern and
possibly around an inch for some mountain locations. River level
forecasts show some rises on the main stem rivers as a result
of this but as of now none are expected to reach bankfull.

The low moves off into the Rockies Monday leaving the PacNW under a
northwest flow. This is where the uncertainty begins to increase as
timing of features in the flow become more apparent. Most likely
scenario will be for small shortwaves to continue providing
occasional showers Monday through Wednesday especially over the
mountains. Snow levels will be down to 3000 to 4000 feet so could
see some accumulation snow impacting travel over the higher
passes.

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  35  60  36 /   0  10  20   0
ALW  61  37  64  40 /   0   0  20  10
PSC  65  41  67  42 /   0   0  10   0
YKM  63  37  66  37 /   0  10  20   0
HRI  64  40  66  39 /   0  10  20   0
ELN  58  34  63  36 /   0   0   0   0
RDM  55  34  54  33 /   0  60  40   0
LGD  53  31  56  32 /  20   0  30  10
GCD  54  34  56  33 /   0  30  50  20
DLS  61  42  66  42 /   0  40  20   0

&&

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

SHORT TERM...82
LONG TERM....91
AVIATION...97

Preliminary Local Storm Report
National Weather Service Pendleton OR
1139 AM PDT Wed May 1 2024

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0600 AM     Snow             2 ESE Lostine           45.47N 117.38W
05/01/2024  M4.2 Inch        Wallowa            OR   Cocorahs

            OR-WL-6 reported 4.2 inches of new snow
            overnight.

0700 AM     Snow             3 SSW Wallowa           45.54N 117.56W
05/01/2024  M3.9 Inch        Wallowa            OR   Cocorahs

            OR-WL-10 reported 3.9 inches of new snow
            overnight.

0800 AM     Snow             3 SE Lostine            45.46N 117.39W
05/01/2024  M3.5 Inch        Wallowa            OR   Cocorahs

            OR-WL-9 reported 3.5 inches of new snow
            overnight.

&&