887
FXUS66 KPQR 021719 AAA
AFDPQR

Area Forecast Discussion...UPDATED
National Weather Service Portland OR
1019 AM PDT Thu May 2 2024

Updated aviation and marine discussion.

.SYNOPSIS...
Another system will bring rain and Cascade snow
(above 4000 ft) this morning. Expect conditions to dry with
some clearing skies by this afternoon. Heavier rain arrives
Friday into Saturday, but we are not expecting any widespread
hydrological concerns at this time. Cool, wet, unsettled
weather likely continues through the middle of next week.

&&

.SHORT TERM...Now through Saturday Night...Mesoanalysis as of
245 AM PDT depicts a 1013 mb low pressure system centered off
the north Oregon coast. This low is currently progressing
southeast, bringing a band of light to moderate rainfall across
northwest Oregon and southwest Washington. Based on webcams,
snow is beginning to fall in the Cascades. The very marginal
Winter Weather Advisory remains in effect for the North Oregon
Cascades, and the Marion, Linn, and Lane County Cascades through
11 AM PDT today. Expect around 2-6 inches of snow this morning
above 4000-4500 feet, with heavier snow above 5000-5500 feet.

For everyone else, the heaviest rain amounts this morning will
be south of a line extending from Astoria-Portland-Hood River.
Locations such as the central Oregon coast, central Oregon Coast
Range, and central/south Willamette Valley are forecast to
receive an additional 0.25-0.40 inch of QPF this morning. Along
and north of the Astoria-Portland-Hood River line, precip
amounts will generally be less than two tenths of an inch. The
bulk of the precipitation will end no later than mid-morning
today (8-10 AM) as this system quickly exits and weakens to the
southeast. Expect predominately dry conditions by this afternoon
with a bit of sunshine as transient high pressure builds aloft.
This will allow today`s afternoon high temps to climb into the
60s for most inland locations.

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. There have been
minimal changes to the QPF forecast, with 1-1.5 inches for the
interior lowlands, and 1.5-2 inches for the coast and mountains
from late Friday morning to Saturday afternoon. Winter weather
impacts are not expected for the Cascade passes on Friday as
snow levels will be rising above 7000 feet.

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 OR/CA border. Snow levels will drop
back down to 3500-4500 feet by late Saturday morning, returning
snow to the Cascade passes. The latest NBM suggests a 40-60%
chance of 6 inches of snowfall or greater in a 24 hr period
ending 5 AM Sunday. Another Winter Weather Advisory is starting
to look more likely for Saturday.

Despite the impressive QPF amounts for this time of year, the
fast progression of this system should help limit hydro concerns
across the area. We could see a slight uptick in river levels,
however, HEFS probabilistic guidance still keeps the chance of
reaching action stage below 5-10% 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 6500-7000 feet on Friday. If nothing else, we could certainly
see some local ponding of water in prone areas from late Friday
into early Saturday.        -Alviz/CB


.LONG TERM...Saturday 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
Wednesday. Will note that clusters do start to show a high
pressure ridge building over the NE Pacific on Wednesday. If
this ridge ends up moving eastward, then we could potentially
see a pattern shift towards drier and warmer weather by late
next week. -Alviz/CB

&&

.AVIATION...The front continues to push eastward with post-frontal
showers and cumulus forming behind it. The main concern today is
CIGs, though in reality they will have minimal impact. Looking at
widespread VFR CIGs, with some lower level scattered clouds below
1,000 ft AGL through around 20Z. Those areas most impacted will be
along the coast. Some portions of the Willamette Valley are still
seeing IFR CIGs, but based off of satellite imagery, it looks like
it should dissipate soon as winds increase. One thing to note
though is that the northerly winds around KUAO and KSLE may keep
the stubborn stratus in place. After 06Z Friday, the lower levels
of the atmosphere moisten with radiational cooling. Models are
suggesting a low dewpoint spread in the Willamette Valley and the
central Oregon Coast. Based on high resolution models, there are
increasing chances for MVFR to even IFR conditions after 12Z
Friday.

The next frontal system arrives after this next TAF package, but
with the onset, because it is a cooler front, will see strong
winds, especially along the coast and through the Columbia River
Gorge.

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...Mainly VFR with scattered IFR clouds through
20Z Thursday. Will see minimal changes today aside from a slow
wind shift from the north to the south through the day. The main
concern is whether or not the clouds will lower overnight. Based
on some model outputs, the dewpoint depression falls to near zero
after 09Z Friday, while others keep conditions dry in the lower
atmosphere. Cannot rule out low stratus though as there is a 30%
chance of IFR CIGs and VIS after 12Z Friday. -Muessle

&&

.MARINE...Low pressure and associated front continue to shift
eastward today. The low will continue to weaken today as will
winds. High pressure builds offshore resulting in northerly wind
developing later in the day.

The next impactful event arrives early Friday. This low pressure
system and associated front appear to be much more robust, so have
issued a Small Craft Advisory for the outer zones, PZZZ
271,272,273, from Friday morning through early evening with gusts
20 to 30 kt. Lower forecast confidence for the inner zones with a
20-50 percent chance for gusts above 21 kt. Will see which way
models trend to decide on issuing an advisory or not.

Seas starting out around 4 ft this morning but looks to remain
below 10 ft through the weekend and early next week. -MH

&&

.PQR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 11 AM PDT this morning for
     ORZ126-127.
     Winter Weather Advisory above 4500 feet until 11 AM PDT this
     morning for ORZ128.
WA...None.
PZ...Small Craft Advisory from 8 AM to 8 PM PDT Friday for
     PZZ271>273.
&&

$$

www.weather.gov/portland

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