038
FXUS66 KPDT 070509
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
1009 PM PDT Tue May 6 2025

.UPDATED AVIATION DISCUSSION...


.AVIATION...06Z TAFs...VFR conditions continue with diurnally and
terrain-driven winds through Wednesday morning as cloud coverage
increases. However, sustained winds of 12-20kts with gusts at 20-
30kts may develop at all sites Wednesday afternoon through the
evening from the increased southwest flow. Feaster/97


&&

.PREV DISCUSSION... /issued 227 PM PDT Tue May 6 2025/

.SHORT TERM...Tonight through Thursday...Current satellite
imagery this afternoon shows mostly clear skies across the PacNW,
though some cirrus layers are starting to increase across western
WA.

Upper level ridging continues to move across the area this
afternoon, which has resulted in drying and warming conditions at
the surface. By tomorrow morning, the upper level ridge will be
pushed east of the area by a strengthening upper level low in the
Gulf of Alaska, with southwest flow developing as a result.
Tomorrow afternoon, an upper shortwave trough will swing around
the bottom of the low and lift across the PacNW and forecast area
through Wednesday night. The main moisture plume with this system
will mainly stay north of the forecast area, however, there is a
25-50% chance that light rain showers will develop over the
Cascade crest and Northern Blues Wednesday afternoon and
overnight. That said, confidence is mod-high (70-90%) that total
accumulations will be less than 0.1 inches in these areas.
Elevated instability is also expected to develop ahead of the
shortwave trough late Wednesday afternoon into the evening.
Showers developing over the northern Blues will be able to tap
into this instability aloft, resulting in around a 15% chance of
an isolated thunderstorm or two developing through the evening
hours. Otherwise, a cold front with this system will tighten
surface pressure gradients as it approaches the PacNW, resulting
in breezy to to locally gusty winds (gusts 30-45mph) through the
Cascade gaps with breezy conditions (guts 20-30mph) elsewhere in
the lower elevations.

By Thursday morning, the upper shortwave trough will have exit the
region with shower activity having ended across all mountain
areas. Upper level ridging will build from the south throughout
Thursday into Friday, resulting in another drying and warming
trend with light winds that will persist into the weekend.
Lawhorn/82

.LONG TERM...Friday through Monday...Little in the way of
sensible weather concerns Friday through Saturday as ensemble
guidance is in excellent agreement that a 500-mb ridge will be
over the Pacific Northwest. Will note Minor (level 1 of 4)
HeatRisk encompassing all population centers Friday and Saturday;
forecast afternoon high temperatures are 10-15 degrees above
normal, in the upper 70s to mid-80s.

Uncertainty in the 500-mb pattern is notable late Saturday as
ensemble NWP struggles to resolve the evolution of a trough or
closed low in the Pacific. Have opted to include a mention (slight
chance, or 15%) of thunder for limited portions of the Blue
Mountains region late Saturday given the approaching trough/low and
advertised instability and lift.

Sunday through Monday, ensemble spread burgeons. While all clusters
of ensemble scenarios depict some form of a closed low or open wave
trough approaching the Pacific Northwest from the west, they differ
with regard to timing/magnitude of the feature. Sunday, roughly 25%
of members indicate a more progressive solution with a closed low
moving onshore; this solution would see widespread showers, cooler
temperatures, and a slight chance of thunderstorms (primarily for
the Blue Mountains region). Less than 10% retain the upper ridge,
which would keep hot, dry conditions across our CWA. The remaining
members show the low offshore with warm temperatures and mostly dry
weather. Looking ahead to Monday, 95% of ensemble members show the
low moving onshore with 15% of those members indicating a weaker low
overhead. The remaining 5% of dissenting members retain a ridge
overhead with a closed low offshore of SW Oregon or NW California.
The members that place the low/trough over the PacNW would result in
cool, unsettled conditions with area-wide precipitation chances.

When/if the trough/low moves through, breezy to windy westerly winds
will develop through the Cascade gaps. The NBM shows these
conditions Sunday and Monday. Ensemble guidance shows quite a lot of
spread, however, and the ECMWF EFI is not highlighting the Cascade
gaps for any climatologically atypical winds. Plunkett/86

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  44  81  43  71 /   0   0  20   0
ALW  49  82  46  70 /   0   0  30   0
PSC  44  85  43  73 /   0   0  10   0
YKM  46  82  39  70 /   0   0  10   0
HRI  44  84  44  74 /   0   0  10   0
ELN  43  77  36  68 /   0  10  10   0
RDM  39  80  37  75 /   0  10   0   0
LGD  43  80  44  72 /   0   0  20   0
GCD  41  82  41  77 /   0   0  10   0
DLS  47  79  42  74 /   0   0   0   0

&&

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

$$

SHORT TERM...86
LONG TERM....82
AVIATION...97