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000
FXUS66 KPDT 170315
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
815 PM PDT Tue Apr 16 2024

.EVENING UPDATE...Winds are starting to dwindle down for good
after a couple days of breezy conditions. A Canadian low with a
jet max overhead will swing eastward while leaving us under cold
NW flow aloft. Attention then shifts to the potential for an
overnight freeze tonight, primarily for the Yakima and Kittitas
Valleys. Some pockets in the Basin could also see subfreezing lows
by early tomorrow morning.

Gusty conditions have kept temps elevated despite strong cold air
advection coming in through the aforementioned Canadian low, but
dew points are awfully dry this evening due to this ongoing NW/N
flow aloft, so temps have the potential to really crater
overnight. Seeing dew points in the teens and 20s tonight, and as
soon as winds really slacken, temps could fall down towards
freezing. Only complication is that some guidance is stubborn as
to how long even moderately breezy conditions last. As a result,
opted to not issue a Freeze Warning for any eligible areas, as
confidence is just too low on the winds abating enough for temps
to fall. Did advertise the potential via social media, however.

Overall forecast changes were minimal. Made slight adjustments to
temps and winds, and added in PoPs across the northern end of our
Basin as radar has picked up on some light shower activity, likely
due to the jet max overhead. Evans/74

&&

.AVIATION...06z TAFs...VFR conditions expected for the next 24
hours. Winds will continue to die down and clouds will decrease by
tomorrow morning, leaving us with light NW to N winds and mostly
clear skies by Wednesday morning. Evans/74

&&

PREV DISCUSSION... /issued 303 PM PDT Tue Apr 16 2024/

SHORT TERM...Today through Thursday Night...

Key Messages:

1. Breezy conditions through this evening.

2. Cold temperatures tonight into Wednesday morning.

Current radar and visible satellite imagery showing spotty, light
returns across the Snoqualmie Pass area and along the west slopes of
the Cascades as ribbons of high level cirrus clouds pass through
Central Oregon, with mid-level clouds present through the northern
Blue Mountains and foothills. This is in response to the upper level
trough continuing to drop through Washington and into northeastern
Oregon later tonight. This synoptic feature has attributed to
tightening pressure gradients along the Cascades and northwest flow
aloft to provide breezy conditions through the Kittitas Valley.
Pressure gradients from Seattle to Pasco are currently 4.2 mb as
gusts of 32 mph are occurring at Ellensburg.
While the NAM is initializing the best, the RAP, SREF, and the GFS
all show continued gradient tightening through 4 PM before slowly
slackening through the remainder of the evening. The NAM is
advertising a 1.7 mb increase in the pressure gradient between
Seattle and Pasco from now until 4 PM, and keeping it above 5 mb
until 8 PM. These values correlate to wind gusts between 40-50 mph
as the peak nears 6 mb around 4PM - indicative of gusts near 50 mph.
The tight gradient along the Cascades has also brought breezy
conditions along the Eastern Gorge and the northern Blue Mountain
foothills of Oregon, but gusts are expected to stay sub-advisory and
between 30-40 mph. Confidence in these wind gusts is moderate to
high (70-80%) as guidance is in agreement on gradient enhancement,
and the NBM showcases a 61% chance of Ellensburg experiencing
gusts of 47 mph or greater before 8 PM.

Clouds will slowly clear west to east across the region this
evening as northwest flow aloft continues to pipe in colder air.
These clear skies will allow effective radiational cooling to
occur, which will drop temperatures quickly. However, the areas
that are expected to clear out the fastest, the Yakima/Kittitas
Valleys and the Eastern Gorge, will still exhibit breezy winds as
the pressure gradients slowly slacken. This will keep the lower
levels of the atmosphere mixed, slowing down the efficiency of
cooling. Broken mid-to high level clouds have been lingering
across the northern Blue Mountain and foothills through the day
today, which is expected to increase this evening and overnight as
the incoming trough`s axis drops into eastern Wallowa county.
This should keep mid-level clouds present across the northern
Blue foothills which will inhibit effective radiational cooling.
Both of the above mentioned processes should keep temperatures
above freezing over these areas, with the exception of protected
areas over the Yakima and Kittitas Valleys - where winds will have
a tough time mixing to lower levels. Confidence in these areas
not experiencing widespread freezing temperatures is moderate to
high (60-80%), as the HREF advertises a 20% and 0% chance of
temperatures of 32 or less at Ellensburg and Yakima respectively.
However, the NBM does hint at slightly better chances of
temperatures reaching 32 degrees or less Wednesday morning, with a
65% and 44% chance for Ellensburg and Yakima respectively. The
primary reason for these varying probabilities relates to winds,
in that the HREF has initialized better in terms of wind speeds
and gusts than the NBM, which has trended lower than what has been
observed through the day. This means that the NBM suggests mixing
will not be as efficient, leading to more enhanced radiational
cooling and overall cooler temperatures expected - which is why
the HREF probabilities are preferred.

Dry conditions continue through the remainder of the period as the
upper level trough broadens over the northern CONUS and a weak
upper level ridge nears the coast late Thursday. Temperatures will
slowly warm through Thursday, with highs bumping up 2 to 6
degrees Tuesday to Wednesday and another 3 to 7 degrees Wednesday
to Thursday. This warming trend will eventually bring highs into
the low to mid-60s across the Basin on Thursday, which is near
average values for this time of year. Morning low temperatures
will continue to stay 2 to 6 degrees below normal as clear skies
Wednesday and Thursday will keep efficient overnight cooling
occurring with morning temperatures staying in the low to mid-30s
through the Basin. 75

LONG TERM...Friday through Tuesday...Models in fair agreement
through the period. However, they do differ in their timing of an
upper level ridge moving through the area early in the period. By
early next week, general southwest to west flow is expected to
develop as an upper trough/low spins over the Gulf of Alaska.
Overall, ECMWF ensemble mean is drier and warmer for next week while
the GFS ensemble mean is a little wetter and cooler.

A weak shortwave behind the exiting ridge should spread some light
precipitation over the Washington Cascades Saturday into early
Sunday. There is a moderate chance (40-50%) we`ll see 24 hour QPF
totals of 0.10 inch along the crest, but these values drop off
quickly from west to east. Another weak shortwave is expected to
move through the general southwest flow producing some showers over
our northeast mountains and cascades Monday afternoon/evening and
possibly again Tuesday afternoon/evening. However, the signal is
pretty weak (15-20%) at this time. Overall, most locations will
remain dry.

Daytime highs are expected to remain near to above normal through
the period. Earle/81

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  34  56  32  61 /  10   0   0   0
ALW  35  58  36  64 /  10   0   0   0
PSC  37  63  37  66 /   0   0   0   0
YKM  31  61  34  64 /  10   0   0   0
HRI  34  62  34  66 /  10   0   0   0
ELN  32  58  33  61 /  10   0   0   0
RDM  25  53  26  61 /   0   0   0   0
LGD  29  51  28  58 /  10  10   0   0
GCD  28  52  28  60 /   0   0   0   0
DLS  36  62  38  66 /  10   0   0   0

&&

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

$$

SHORT TERM...74
LONG TERM....81
AVIATION...74