571
FXUS66 KMFR 011209
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
509 AM PDT Wed May 1 2024

...New Aviation Section...


.AVIATION (12Z TAFs)...

VFR conditions are expected through the beginning half of this
TAF cycle. A weak front will move through today bringing rain
chances across the region. This added moisture will allow for
lower ceilings and MVFR conditions across much of the area today
including mountain obstructions. There is a possibility for IFR
conditions given the extra moisture around, but confidence on the
timing of these possible ceilings is low at this time. That said,
North Bend is the likely candidate here for IFR ceilings. We may
need to reevaluate for Roseburg and Medford, but at this time
confidence was higher to keep ceilings in MVFR.

-Guerrero

&&

.PREV DISCUSSION... /Issued 343 AM PDT Wed May 1 2024/

DISCUSSION...A transient, weak ridge over the area today will
result in dry and clearer weather, with mild afternoon
temperatures after a chilly morning. The pattern will remain
progressive, however, with a system bringing a return to wet and
cooler weather tonight into Thursday and another, stronger/wetter
system, Friday into Saturday. Unsettled, showery conditions
continue into this weekend.

Satellite imagery (nighttime microphysics RBG) show that skies
have generally cleared over the region and showers have tapered
off, though some residual, low-level cloudiness lingers in the
valleys of Douglas and Coos counties and in the southern edges of
Rogue and Illinois valleys. Clearer conditions will result in
another morning on the chillier side, with temperatures near or
below freezing in the majority of the forecast area (with the
exception of some of our western valleys and the coast, which will
see lows in the high 30s to low 40s). A Freeze Warning for parts
of the Illinois and Applegate valleys continues this morning to
account for the effect of sub-freezing temperatures in the area
on sensitive vegetation. After this cooler morning, temperatures
will be mild (warmer than yesterday) and skies generally clear.

The next system approaches later today, approaching the coast in
the evening and spreading inland eastwards overnight into early
Thursday morning, with lingering showery weather Thursday
afternoon. This will bring widespread precipitation to southwest
Oregon, though our northern CA counties will generally miss out on
anteing but light precipitation. Snow levels rise today with
warming temperatures and will generally stay high, so snow will be
generally restricted to mountainous areas above 5,000 feet.
Higher peaks in the Cascades will see 3-5 inches of wet snow and
Cascade passes less than an inch up to 2, and impacts are expected
to be minimal. Breezy but not overly strong westerly winds will
build over ridges and east of the Cascades.

As the driving trough moves east, another brief ridge will build
into the region Thursday evening into early Friday, providing
another dry break in the weather.
-CSP

PREVIOUS LONG TERM DISCUSSION...Issued 437 PM PDT Tue Apr 30
2024...The axis of a short wave upper ridge will be centered over
the area Friday morning with a surface warm front just offshore.
The upper ridge will be transient as an active jet over the north
Pacific allows another deep upper trough to carve out a closed low
over the Gulf of Alaska. Any warming that occurs Friday (compared
to Thursday) will be modest at best and where it stays sunniest
the longest (in NE California/East of the Cascades). These areas
probably have high temps 5-10F higher than on Thursday. But, west
of the Cascades, warming will be muted due to the increased cloud
cover, with temps topping out mostly in the 60s. As the warm front
pushes northward and an associated surface cold front well
offshore approaches the PacNW, rain chances increase along the
coast with rain becoming likely there (50-70% chance) by Friday
evening.

This frontal system will be fairly potent in terms of precipitation
Friday night into Saturday with good moisture transport into the
region and the lack of a significant downslope component that would
otherwise lead to rain shadowing. PoPs peak at 80-100% area wide.
Don`t get me wrong, there won`t be as much rainfall in the typically
drier areas as say, the coast, but it probably won`t be as large a
difference as usual. So, the front should be an efficient rain
producer for a larger part of the area. Widespread wetting and
beneficial rainfall is expected with preliminary amounts for the
coast in the 1.25-2.50 inch range and many areas inland receiving
0.50-1.25 of an inch. A little less rain is expected for the Klamath
Basin and over the deserts east of Winter Rim where 0.25-0.50 of an
inch appears to be the most likely amount. Snow levels will be up
above 7000 feet as precipitation arrives Friday night, so no
significant winter impacts are expected initially as precipitation
begins. Snow levels will lower to around 5000 feet late Friday
night/Saturday morning as the cold front moves through, so some snow
will accumulate over the mountains (mostly above 5500 feet). Several
inches (4-8") are possible up at Crater Lake, but down on the more
traveled roadways around Diamond Lake and also over Highway 140 near
Lake of the Woods, it`ll probably just be wet or slushy (especially
due to the time of year and warmer ground temps). Model wind fields
aren`t particularly impressive with this system, with most
suggesting peak wind gusts in the 40-50 mph range over usual spots
east of the Cascades (eastern Siskiyou/Modoc and up into
Klamath/Lake counties).

The cooler, wetter weather pattern with post-frontal showers will
continue Saturday afternoon and also Saturday night into Sunday as
the deep upper trough/closed low swings through the area, but
coverage of showers will lessen with time. While showers are
possible just about anywhere and at just about any time on Sunday,
much of the time for most areas will be rain-free. Highs on the
weekend in the west side valleys will be mostly in the 50s to around
60F. East side temps will top out in the upper 40s to mid 50s.

Models are showing moist onshore flow continuing into early next
week with renewed (and increased) chances for rain showers (50-70%)
along the coast and into the Umpqua and over to the Cascades
Monday/Tuesday. But, precip chances are lower (20-40%) across SE
sections.

We may get a reprieve from the active weather as the Climate
Prediction Center 8-14 day outlook favors a return toward more
"normal" conditions (temps/precip) for the second week of May.
-Spilde

AVIATION...01/06Z TAFs...Residual moisture will result in areas of
clouds lingering through the night over Umpqua Basin. Clouds may
fill in enough to produce brief IFR ceilings around KRBG around 12Z.
Clouds will also continue to bank up against the west and north
facing slopes of the Cascades and Siskiyous resulting in mountain
obscurations tonight.

Elsewhere, conditions will be VFR through the TAF period with
typical diurnal winds (light and variable at night and gusty west-
northwest in the afternoon). -Wright

MARINE...Updated 230 AM Wednesday, May 1, 2024...A weak surface low
will continue to develop over the Gulf of Alaska today. This low
will eventually move inland tomorrow evening, and is not expected to
be much of a concern for hazards over the waters, but we will see
rainfall (no thunder) starting Wednesday night. The next surface low
expected on Friday will have a stronger front associated with it,
and this may result in the next round of small craft advisory
conditions Friday afternoon through Saturday morning. This system
will be the start of multiple rounds of precipitation and building
west-northwesterly swell through the weekend into early next week.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Freeze Warning until 8 AM PDT this morning for ORZ024.

CA...None.

PACIFIC COASTAL WATERS...None.

&&

$$