180
FXUS66 KPQR 020427 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
924 PM PDT Wed May 1 2024

.UPDATE...Made some updates this evening, mainly to the QPF and snow
levels tonight into Thursday. Incoming shortwave trough is having
some success latching on to deeper moisture, with total precipitable
water (TPW) values near 1 inch as far east as 127W. The weak surface
low appears to be developing a deformation band on its north side,
which appears to be aiming for Mount Hood based on latest KRTX/KLGX
Doppler Radar imagery. This will likely enhance precipitation on the
north side of the low track, where precip would otherwise be lighter
due to lacking orographics. With 00z HREF 25th percentile QPF largely
in the 0.50-0.75 inch range over the Marion/Linn/Lane Cascades and
the above-mentioned banded precip heading for the Mount Hood area,
feel the confidence is there to issue a marginal Winter Weather
Advisory for our Oregon Cascades tonight through Thursday morning.
This advisory will be for elevations above 4000-4500 ft, reflecting
the slightly warmer air associated with this system.

Aviation and marine sections have also been updated below.  Weagle

&&

.SYNOPSIS...Another system will spread relatively light rain
across the area this evening into Thursday morning, with another
round of light snow for the Cascade passes. Heavier rain arrives
Friday into Saturday, but do not expect any widespread
hydrological concerns at this time. Cool, wet, unsettled weather
likely continues through the middle of next week.

&&

.SHORT TERM...Through Saturday Night...Visible satellite shows
overcast skies across most of northwest Oregon and southwest
Washington this afternoon as high level cloud cover streams over
the region ahead of the next system, helping to hold daytime
highs down in the mid to upper 50s again across the area.
Speaking of the next system, the latest in a series of shortwave
troughs is apparent west of Vancouver Island on water vapor
imagery, with a warm front extending southeastward towards the
Oregon coastal waters from an attendant surface low. Light rain
associated with this front will spread across the area later
this evening from northwest to southeast and continue overnight
as the upper wave crosses overhead. QPF amounts remain rather
modest with this system ranging from a half to a quarter inch
across most of the area through Thursday morning, with generally
less than two tenths along and north of a Hood River-Portland-
Astoria line. The heaviest precipitation amounts will be
focused along the central Oregon Casacdes which could see
0.75-1.00" through Thursday morning. Snow levels remain around
4000 feet, with roughly 2-4 inches of fresh snow expected at
pass levels and locally higher amounts above the passes. Expect
another brief dry spell Thursday afternoon into early Friday as
transient high pressure builds in behind the departing system.
This should allow high temps to climb into the 60s for most
inland locations on Thursday.

The more active weather arrives Friday afternoon as a much
deeper trough drops out of the Gulf of Alaska and sends a
stronger front across the area Friday into early Saturday.
Expect heavier rainfall to accompany this system as IVT values
briefly max out around 250-400 kg/ms Friday evening, with
precipitable water values around 1 inch running two standard
deviations above normal by early May standards. QPF values
remain on track from the previous forecast, bringing 1.00-1.50
inches through Saturday afternoon in the interior lowlands,
1.5-2.0" along the coast and in the mountains, and locally
upwards of 2.5-3.0" in a few favored spots. Fortunately, the
system looks to move through rather quickly with rain tapering
off to showers by Saturday evening as the low dives south
towards the CA border. So despite the impressive QPF amounts
for this time of year, this should help to limit hydro concerns
across the area as HEFS probabilistic guidance keeps the chance
of even reaching action stage below 5 percent on all area
rivers. That said, the system will still bear watching into the
weekend given that area reservoirs are close to full and snow
levels will be rising above 6000 feet. If nothing else, could
certainly see some local ponding of water in prone areas from
late Friday into early Saturday. /CB

.LONG TERM...Sunday through Wednesday...Expect an active, cool,
and showery pattern to persist into the middle of next week as
WPC ensemble clusters continue to show general agreement on
some semblance of upper level troughing remaining over the
Pacific Northwest through Wednesday. Do not see any signals for
particularly notable QPF in any one period beyond Saturday, but
area rivers will likely continue to run high for this time of
year as episodic bouts of rain continue across the region.
Temperatures will continue to run a bit below normal, with
daytime highs struggling to get out of the 50s through the end
of the period. /CB


&&

.AVIATION...The next warm front is just offshore, and is expected
to properly reach the shore imminently around 07z Thu, with
stronger precipitation at the southern terminals. Precipitation
has already begun are most southern terminals, with northern ones
soon to follow. Ceilings will begin to lower within an hour or
two, with inland terminals falling to high end MVFR ceilings. KAST
should remain low end VFR as the majority of rain passes to the
south, but KONP sees a 70% chance of ceilings deteriorating to IFR
thresholds between 7-12z Thu. Other southern terminals (KSLE,
KEUG) only see around a 25% chance of dropping to IFR ceilings.

Around 14z Thu, the front pushes through, and ceilings will
slowly improve back to VFR over the following few hours. Winds
will be fairly light at almost all terminals throughout the TAF
period, with the exception of KONP, which could see some southerly
gusts up to 20kt accompanying the frontal passage. The next front
looks to be shortly after the end of the current TAF period,
bringing more rain and lowered cigs. /JLiu

Notes: The ASOS at KTTD is only available via the dial in option.
There is no estimated time of full repair and restoration for
this location.

PDX AND APPROACHES...VFR conditions will quickly turn over to high
end MVFR with the frontal passage around 07z Thu. Model guidance
shows around 40% chance of MVFR cigs, but there will be frequent
periods of intermittent MVFR, and high end MVFR cigs were put in
the TAF as a result between 07-14Z Thu. Afterwards, conditions
return to low end VFR ceilings. /JLiu

&&

.MARINE...With an incoming warm front, southerly winds will shift
more easterly, and gusts continue to just barely meet Small Craft
Advisory criteria until the front passes around 12z. Could see
higher wind speeds through the coastal gaps and Columbia River Bar
early Thursday morning but will be marginal for small craft
speeds. Seas will not be impacted by the winds, and in fact could
see lower wind waves due to the offshore winds.

The next impactful event arrives early Friday. This low pressure
system and associated front appear to be much more robust -
though there is ample time for it to change. Seas will build and
winds will increase. Not seeing gale force winds, or combined seas
exceeding 10 ft though. -Muessle/JLiu

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 11 AM PDT Thursday for ORZ126>128.
WA...None.
PZ...Small Craft Advisory until 5 AM PDT Thursday for PZZ271-272.

&&

$$

www.weather.gov/portland

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