272
FXUS66 KPQR 290353 AAA
AFDPQR

Area Forecast Discussion...Updated
National Weather Service Portland OR
852 PM PDT Sun Apr 28 2024

.UPDATE...Made some updates to the forecast this afternoon and
evening, mostly to remove the thunder mention for this evening as the
00z KSLE sounding confirms a strong mid-level capping inversion
holding most cloud tops to around 10 kft...too shallow to achieve
charge separation. Not sure why the NBM was so intent on holding on
to thunder chances this afternoon and evening, as the 12z HREF showed
nearly 0 percent chances of lightning and SPC SREF calibrated thunder
didn`t show much more. Ironically, thunder chances may actually
improve overnight and into Monday morning as a pocket of sub-minus 30
deg C air at 500 mb swings across the forecast area, especially along
the coast where temps likely remain a touch milder overnight. This
cold pocket aloft will be accompanied by the left exit region of a
100 kt jet, which should also enhance lift early Monday morning. At
the very least, expect a significant uptick in shower activity early
Monday morning, with these showers moving into the Cascades while
snow levels lower from around 4000 ft to around 2500 ft.

While few changes to QPF were made, the NBM snow ratios of up to 17:1
for the Cascade passes Monday morning look a bit excessive. We nearly
halved these, given the strong late April sunshine as Monday
progresses and marginal temperatures anyway. Snow ratios of that
magnitude would be impressive for mid-winter, so it seems suspect
that they would occur during a late season event with marginal
temperatures. The resulting snow amounts are still enough to get us
into advisory criteria, especially above 3000-3500 feet where 3-8
inches can be expected. At elevations where it is snowing now
(generally 5500 ft and above), snow totals may exceed 8 inches as
they will see all snow from this point forward in the event. Latest
high-res models suggest shower activity will remain plentiful through
midday before becoming more spotty, so we extended the Winter Weather
Advisories through 2 PM Monday for all our high Cascades zones.

Aviation discussion has also been updated below.  Weagle

&&

.SYNOPSIS...Multiple colder weather systems to move through NW
Oregon and SW Washington tonight into Monday morning and again
Monday night into Tuesday, bringing continuing showers along
with chances of isolated thunderstorms. Snow levels drop tonight
down to 2500-3000 feet by tomorrow morning with snow at times
expected over the Cascades through Tuesday. Dry and warmer
weather returns Wednesday with unsettled weather returning late
this week into the weekend.

&&

.SHORT TERM...Sunday afternoon through Wednesday...Radar
imagery early Sunday afternoon shows showers are becoming more
widespread across NW Oregon and SW Washington as the next
weather system approaches the region. This upper trough and
associated surface front are slated to move through the region
late tonight into early Monday quickly followed by another
weather system Monday night into Tuesday. Showers are expected
to continue into Tuesday with these systems. The IVT associated
with these are very low (less than 250 kg/ms), so expecting
mostly showers instead of stratiform rain through Tuesday. A few
bands of moderate rain are possible ahead of and along each
front, with the Monday night through Tuesday front packing a
little more punch. Current QPF through 5pm Monday is 0.1-0.3
inch for the inland valleys and 0.4-1 inch for the coast and
mountains. Additional QPF through 5am Wednesday is 0.25-0.5 inch
for inland valleys and 0.3-1.25 inch for the coast and
mountains. For both systems, Lane county is expected to be on
the lower end of these estimates as the strongest portion of
each system remains north of Lane county. These are colder
systems as they originate from west of Alaska, and as such they
is forecast to bring colder air aloft into the region. Latest
observations indicate this colder air aloft is beginning to push
into the region this afternoon, with colder temperature
expected tomorrow into Tuesday. This will cause both
thunderstorm chances and lower snow levels.

First, as colder air moves in aloft, instability is forecast to
increase. This afternoon only has a weak chance of isolated
thunderstorms with CAPE values only expected to increase to
150-250 J/kg, mainly along and north of the Columbia River,
including the Portland/Vancouver metro area, and the north
Oregon coast and Cascades. There are better chances for isolated
thunderstorms across the region both tomorrow and Tuesday along
and behind each frontal passage with CAPE values increasing to
200-400 J/kg and 0-6 km shear of 45-55 kts. Heavy rain, small
hail, gusty winds, and lightning are the main impacts expected
with any strong shower or thunderstorm that forms.

Additionally, snow levels will begin dropping this evening into
Monday as the cold air continues moving into the region. Expect
snow levels to fall to 3500-4000 ft by this evening and
2500-3000 ft by tomorrow morning. As there will be a couple
decent shots of precipitation with the front, expecting snow
amounts of 2-8 inches above 2500 ft with up to 10-14 inches
above 5000 ft. Elevations down to 2000 ft could see a dusting to
an inch or so, especially in heavier showers. A Winter Weather
Advisory remains in effect for the Cascades through tomorrow
morning. Lighter snow is expected to continue tomorrow
afternoon through Tuesday, though since conditions will turn
to more scattered showers, there will likely not be much
widespread additional accumulation except for potentially
Tuesday morning with the second frontal passage.

Daytime temperatures are forecast to remain fairly steady in the
mid to upper 50s for the lowlands and 30s to low 40s for the
higher terrain. Nighttime temperatures will cool a bit each day,
with morning lows tomorrow down to the low 40s for the lowlands
then potentially falling to the upper 30s for Tuesday and
Wednesday mornings. May need to keep an eye out for frost
potential Wednesday morning if clouds clear out enough for
radiative cooling, especially in the outlying valley areas and
the Coast Range. Could also see some locally breezy winds at
times with each frontal passage with brief periods of gusts up
to 20-25 mph, locally up to 30 mph along the coast and 40-45 mph
over the high Cascades.

By Wednesday, ensemble guidance agreement continues in increase
in dry weather returning as ridging builds, though there is
uncertainty on whether this dry period will last beyond
Wednesday. More details on that below in the Long Term
discussion. However, temperatures are forecast to warm back near
seasonal normals for late April with NBM indicating 70-90%
probability of temperatures rising back into the 60s. -HEC

.LONG TERM...Thursday through next Sunday...Forecast confidence
continues to drop off dramatically after the midweek period as
guidance struggles with how to handle multiple disturbances
presently over the west and central north Pacific. As longwave
troughing exits the western US, it remains uncertain whether the
next weaker system will maintain enough momentum to bring rain to SW
Washington and NW Oregon. Latest NBM guidance basically shows a
50/50 chance of some light precipitation Thursday, then suggests a
drier period of uncertain length. For Friday, 12z LREF grand
ensemble shows a fairly normal distribution of 850 mb temps between
0 deg C and +10 deg C, with about 50% of members falling between +4
and +7 deg C.

The majority of EC/GFS/CMC ensemble members want to bring troughing
back to the Pac NW by next weekend, with cool/unsettled weather
returning. However this is far from certain. While 20% of NBM
members suggest high temps Friday will fail to reach 65 def F in the
Willamette Valley, another 20% suggest highs will reach 80 deg F for
the PDX metro. For Saturday, 20-30% of members suggest the
Willamette Valley will reach the 70s, while 10% suggest another day
reaching 80 degrees for the PDX metro. While our forecast is much
closer to seasonal norms than the warmer members, with cooler/wetter
wx returning by the weekend, the warmer members illustrate how there
is still potential for upper ridging to hold on longer than what the
majority of members and ensemble means suggest.  Weagle

&&

.AVIATION...Moist onshore flow with strong westerly flow aloft
through tonight. Light rain showers this evening, then expect an
increase in shower activity after 10Z Monday as an upper level
disturbance moves across the region. Widespread VFR expected
through at least 10Z Monday, then guidance suggests around a
20-40% chance of MVFR conditions through Monday morning. With the
aforementioned weather disturbance comes an increased chance for
thunderstorms (15-25%), especially for the north Oregon coast,
after 08Z Monday. Southwest winds expected through the TAF period
with gusts up to 20 kts.

Notes: The ASOS at KTTD is only available via the dial in option.
The AWOS at KMMV is still out of commission, but a tentative work
around has been implemented. There is no estimated time of
full repair and restoration for either of these locations.

PDX AND APPROACHES...Predominately VFR expected through the period
with scattered rain showers. With heavier showers possible
between 10-19Z Monday, there is a 20-40% chance of conditions
deteriorating to MVFR at times. Slight chance, around 10-20%, for
thunderstorms, with highest probabilities during Monday
afternoon. West to southwest winds expected around 8-12 kt. /DH

&&

.MARINE...Active weather is expected to continue into midweek. A
surface front moving through the waters this evening through early
Monday will bring another round of elevated winds along with
steep and choppy seas through midday Monday. Generally westerly
winds with gusts up to 25 kt and seas of 6 to 9 ft at 10 to 13
seconds through Monday. A Small Craft Advisory remains in effect
through Monday afternoon.

A series of weak fronts will move through the waters Monday night
into Tuesday and again late Tuesday into Wednesday, bringing
elevated winds and seas at times. Some models are suggesting that
small craft conditions could develop with each frontal passage.
Will continue to monitor the situation as it develops. -HEC

&&

.PQR WATCHES/WARNINGS/ADVISORIES...

OR...Winter Weather Advisory until 2 PM PDT Monday for ORZ126>128.

WA...Winter Weather Advisory until 2 PM PDT Monday for WAZ211.

PZ...Small Craft Advisory until 11 PM PDT Tuesday for PZZ210-251>253-
     271>273.
&&

$$

www.weather.gov/portland

Interact with us via social media:
www.facebook.com/NWSPortland
www.twitter.com/NWSPortland