033
FXUS66 KPDT 021019
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
319 AM PDT Sun Jun 2 2024

.SHORT TERM...Today through Tuesday...

Key Messages:

1. Windy conditions Monday. *Wind Advisories issued*

2. Significant widespread rainfall Sunday and Monday.

3. Rises on area rivers associated with enhanced rainfall.

Current radar and infrared satellite imagery showing some light
returns along the east slopes of the Cascades under partly cloudy
skies. These light returns, which rainfall is not actually
reaching the ground surface due to a present dry air at lower
elevations, are a result of an approaching warm front that will
bring widespread showers across our area later today. However, the
main concern with this robust incoming system is associated windy
conditions expected Monday as the cold front passes through the
Pacific Northwest. This frontal system, coupled with the upper
level shortwave trough that will be approaching the coast later
this evening, will tighten isobars and develop a pressure gradient
along the Cascades to produce windy conditions across the Gorge,
Basin, Simcoe Highlands, northern Blue Mountain foothills, and the
Kittitas/Yakima Valleys. Wind gusts of 45-55 mph will be possible,
which has warranted a Wind Advisory on Monday between 8 AM and 11
PM over the aforementioned areas. Confidence in these wind values
and advisory-level winds is high (80-90%) as the GFS and NAM both
highlight a pressure gradient of 11.5-13 mb between Portland and
Spokane, which is well above the normal advisory threshold of 11
mb. There is also good consistency with 850mb winds as both the
ECMWF and GFS highlight a large swath of 35-45 mph winds over the
Basin, foothills, Simcoe Highlands, and the Yakima/Kittitas
Valleys. Further confidence in gleamed from the NBM, which
suggests a 70-100% chance of earlier mentioned areas experiencing
wind gusts of 48 mph or greater. When gusts are bumped up to 55
mph, a 70-90% chance still resides over the Simcoe Highlands with
a 45-65% chance along the Lower Columbia Basin of Oregon and the
northern Blue Mountain foothills. This will be further analyzed
over the next 24 hours as these areas may need to be upgraded to
to warnings.

As mentioned earlier, widespread rainfall will occur by late
morning today and extend through Monday morning as significant
rain amounts are anticipated. This is a result of a moderate
atmospheric river (AR) suppling the moisture to the passing upper
level trough and associated frontal system. Both the ECMWF and GFS
showcase a Integrated Vapor Transport (IVT) of between 600-800
kg/m*s as both ensemble products produce an 85-100% of IVT
greater than 500 kg/m*s. These values translate to widespread
precipitable water amounts of 1-1.5 inches, which is 175-225%
above normal. Rain is forecast to peak between 11 AM Sunday
through 11 AM Monday. Rain amounts of 1-2 inches over the Cascades
and northern Blue Mountains, 0.50-1 inch along the northern Blue
Mountain foothills of Washington and the Simcoe Highlands,
0.20-0.60 of an inch through the northern Blue Mountain foothills
of Oregon and the John Day-Ochoco Basin, 0.10-0.30 of an inch
over lower elevations of the Basin, and 0.01-0.10 for Central
Oregon and the Yakima/Kittitas Valleys is anticipated during this
timeframe. Confidence in these rain amounts is high (70-80%) as
the NBM showcases a 70-100% chance of 1 inch or more over the
Cascades and northern Blue Mountains, a 50-80% chance of 0.50 inch
or more over the northern Blue Mountain foothills, and a 40-60%
chance of 0.25 of an inch or more across lower elevations of the
Basin, Central Oregon, John Day-Ochoco Basin, and the
Yakima/Kittitas Valleys. There is also a slight chance (10-20%) of
isolated thunderstorms developing late Monday morning and
afternoon over eastern Grant, southern Union, and Wallowa county.
The HREF does highlight low level shear of 45-55 knots, with most
unstable CAPE (MUCAPE) values of 100-200 J/kg. Timing also seems
to be a little earlier in the morning, which coincide with storms
developing more to our north and east through the afternoon. Heavy
rain and gusty winds will be the primary concern associated with
any developing storm cells, with a slight chance (<15%) of
hail. At this time, there is a low chance (<10%) of any storm
cells exhibiting severe characteristics.

The final concern through Tuesday resides with this ample moisture
and rainfall attributing to river rises on some area reaches.
This has resulted in the issuance of a Hydrologic Outlook as
periods of moderate to heavy rain are anticipated to occur -
especially over mountainous terrain. At this time, only the Naches
River near Naches and near Cliffdell is forecast to exceed action
(bankfull) stage, with the reach near Cliffdell reaching action
stage Monday morning and near Naches Monday afternoon. Confidence
in reaching action stage near Cliffdell is 49%, and near Naches is
12%. However, with the significant moisture arriving with this
system and enhanced precipitation expected over our mountain
zones, localized flooding and flashy rivers may be a result. Make
sure to have multiple ways of receiving pertinent weather and
hydrologic information over the next 48-60 hours. 75


.LONG TERM...Wednesday through Sunday...Models continue to struggle
with the strength and position of a developing upper level ridge
over the western US. WPC cluster analysis does show a westward trend
compared to yesterday with only 20% of members showing an axis near
the Continental Divide with 80% hovering around the OR/ID border.
This would certainly increase confidence in a warmer pattern for the
PacNW. However, models have decreased/slowed its amplification
keeping it more of a flat ridge through Thursday or Friday then
amplifying it quickly over the weekend. Overall, confidence has
increased (70-80%) for a more ridgy pattern, but confidence in
temperatures remains low to moderate (25-35%) as raw temperature
values range between 10-15 degrees. This range would be the
difference between daytime highs around 5 degrees above normal to 10-
15 degrees above normal. The ECMWF continues to be the warmer of the
models with temperatures 10-15 degrees above normal by the weekend.
Ensembles mimic the WPC cluster analysis, but GFS pushes the ridge
axis east quicker allowing for a better chance of afternoon/evening
convection over the mountains Sunday. Both models keep general S-SW
flow over the area from Thursday onward which will continue the
threat of afternoon/evening convection over the mountains. Given
consistency, confidence is moderate (40-50%) that convection will
develop in the afternoon/evening from Thursday onward, but will
certainly depend on where the ridge axis ends up.

In summary, daytime highs will be near normal Wednesday and above
normal through the rest of the period. Afternoon/evening
showers/thunderstorms should develop over the mountains beginning
Thursday and continuing through the weekend. Earle/81


&&

.AVIATION...06Z TAFS...VFR conditions and dry weather are expected overnight,
then -RA will overspread the area on Sunday. DLS will likely see
some MVFR conditions later Sunday afternoon/Sunday evening, and
MVFR coan`t be ruled out in other locations, especially if there
are periods of moderate or heavy rain, but confidence is low
(<30%) in the lower conditions for the other sites at this time.

Winds have increased at PDT and ALW to around 30 kts at PDT and 20
to 25 kts at ALW, but these winds should decrease overnight.
BDN and RDM will gust between 25 and 30 kts on Sunday. The other
TAF sites should be 10 kts or less though some local gustiness
can`t be ruled out, especially around heavier rain.


&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  67  53  66  46 /  60  80  70  10
ALW  70  55  67  49 /  80  90  90  20
PSC  70  58  72  54 /  80  70  60  10
YKM  66  50  67  45 /  70  60  30  20
HRI  70  56  70  50 /  60  70  50  10
ELN  62  48  63  45 /  80  60  50  20
RDM  66  52  63  44 /  40  50  30  10
LGD  67  53  63  43 /  70  90  80  10
GCD  68  53  66  44 /  70  80  70  10
DLS  67  56  66  52 /  80  80  40  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 8 AM to 11 PM PDT Monday for ORZ041-044-507-
     508.

WA...Wind Advisory from 8 AM to 11 PM PDT Monday for WAZ024-026>029-
     521.

&&

$$

SHORT TERM...75
LONG TERM....81
AVIATION...77