NWS Medford, OR wrote its last area forecast discussion before the first event at May 20, 1:35 PM PST, 1 hour 38 minutes earlier.
Area forecast discussion, May 20, 1:35 PM PST, 1 hour 38 minutes before
Area Forecast Discussion
National Weather Service Medford OR
235 PM PDT Sat May 20 2023
.DISCUSSION...Another active afternoon is shaping up across
eastern portions of the forecast area. Daytime heating is already
initiating convection over the east side, particularly the Warner
Mtns and eastward, but satellite imagery is showing cumulus
developing in an arc from northern Klamath County down to Winter
Ridge and the aforementioned areas down to the Warners in Modoc
and near Lassen National Park. These areas are primed for
additional scattered thunderstorms (25-50% chance) this afternoon
with plenty of low-mid level moisture, 500-1000 J/KG MLCAPE, and
effective shear magnitudes in the 30-35 kt range. Forcing is
modest with weak short wave activity on the NW periphery of an
upper ridge over the Great Basin. But, it is sufficient to produce
some fairly healthy cells out there this afternoon. Yesterday,
there were just shy of 300 lightning flashes in Lake County alone,
50 or so in Klamath County and about 30 in Modoc County. A couple
of the stronger cells prompted severe thunderstorm warnings.
We'll probably have similar lightning output today. It pretty much
goes without saying that any of these storms will have the
ability to produce heavy rainfall, gusty winds and small hail.
But, the shear profiles are enough to sustain updrafts in some of
the stronger storms. This is most likely later this afternoon and
evening across northern Klamath/Lake counties, where the Storm
Prediction Center (SPC) has a marginal risk for severe storms.
This means that a couple of cells could contain large hail
(exceeding 1" in diameter) and damaging wind gusts (exceeding 58
mph). Be on the lookout for potential severe thunderstorm
warnings. If you're outdoors and you hear thunder, seek shelter in
an enclosed building, or, if that isn't available, in a hard-
topped vehicle with the windows closed to wait out the storm. Wait
at least 30 minutes until the last thunder is heard to resume
outdoor activities. Instability is also present across NorCal
(Trinity Alps) and northeastward across NE Siskiyou/SE Jackson
County this afternoon. Isolated thunderstorms (15-25% chance)
could develop in the mountains and then drift across the Shasta
Valley as they head toward southern Oregon before dissipating this
evening. We think they'll miss the Rogue Valley to the south and
east, but there is a small chance (~15%) that a cell moves near
or just south of Ashland after 5pm this evening. Cell motions
this afternoon should be mostly off to the NE due to SW flow
aloft.
Meanwhile, an upper level trough resides just offshore. This
trough will push inland tonight and serve to deepen the marine
layer west of the Cascades. We often talk about how the marine
layer won't be deep enough to get over the Umpqua Divide. Tonight,
it WILL be deep enough and low clouds will spill over into the
Rogue Valley by around sunrise, banking up against the Siskiyous.
There will likely be some fog at mid slope and ridge elevations
west of the Cascades/north of the Siskiyous along with patchy
drizzle, but a general stratus layer elsewhere. Other than the
drizzle, we aren't expecting much precip with this marine push,
but it will cool us down by about 5-10 degrees compared to today.
We have a high of 77F here in Medford on Sunday. The last time it
was that low for a high was on May 11, when the high was 78F. This
will end a 9-day stretch of highs in the upper 80s/low 90s. Breezy
conditions are expected Sunday afternoon/evening.
A surface cold front will push east of the Cascades on Sunday, and
the air mass will remain unstable ahead of it. This should
trigger another round of convection across portions of eastern
Klamath, Lake and Modoc counties in the afternoon, pushing
eastward to eastern Lake County by evening. SPC still has
portions of Lake County in a marginal risk for severe storms, so
we have added wording to the grids to account for possible gusty
winds, heavy rain and hail. Behind the front, we'll get a break
Sunday night into Monday with no precipitation expected as a
closed low sets up in northern Washington. It'll be cooler Sunday
night with lows in the 40s west of the Cascades, probably a good
night for opening windows (as long as there aren't any skunks).
But, it'll be chilly again with lows near freezing in the usual
areas of northern Klamath/Lake counties. Lows in the upper 30s are
expected in Klamath Falls, which may come as a bit of a shock to
the system given the recent milder weather, but that's right
around normal for this time of year.
The closed low will wobble across the PacNW and into SW Canada
Monday into Tuesday with another shortwave trough coming
southeastward and onshore into California. This will keep
temperatures on the cool side compared to the recent warmth, but
still at least a few degrees above normal for most. By Tuesday,
models are showing SW flow aloft with some instability across
NorCal and into south-central Oregon. We have introduced a slight
chance (15-25% chance) of showers/thunderstorms then, mainly
across NorCal, but also up to around Klamath Falls/Lakeview.
The general consensus in the guidance from mid-late next week is
to be under the influence of an upper level trough, the axis of
which should be near the Cascades. Some modest warming will occur
west of the Cascades Wednesday and Thursday, but instability and a
chance of showers/isolated thunderstorms will persist in NorCal
and over the East Side, where temperatures will be largely
unchanged day to day. Shower chances expand back toward the
Cascades and perhaps even west of the Cascades Friday into next
weekend as another closed low comes southeastward from the Gulf of
Alaska and settles along the British Columbia coast. It remains to
be seen just how much influence this will have on our weather, but
it does increase the odds for upper troughing in the PacNW and
along the West Coast with the mean ridge position expected to
remain out near 140W. As such, as we approach June, it doesn't
look like any major heat waves are in store. -Spilde
&&
.AVIATION...20/18Z TAFs...Marine stratus/fog persists over the coast
and into inland Coos and Douglas Counties, including into the Umpqua
Basin (Roseburg).
Guidance suggests that the stratus from coastal Coos into inland
Coos and Douglas Counties will persist through the day, including in
the Umpqua Basin/Roseburg. Ceilings should lift some as diurnal
heating kicks off some breezier winds, but coastal MVFR to IFR
conditions (north of Cape Blanco) are expected to remain through the
day, lowering back to IFR/possible LIFR later tonight.
Inland areas and coastal Curry County (including Brookings) will
fair better, with stratus hanging around but lifting to VFR, before
ceilings likely lower again tonight as the marine layer pushes
inland again. Tonight we're expecting it to pull further inland than
last night/this morning, with lower stratus advecting into the
Illinois and north Rogue Valleys (including Medford), creating
possible MVFR ceilings, though visibilities should stay high.
Elsewhere, VFR conditions persist through the TAF period. There is a
chance for thunderstorms, over and eastwards of the Cascades
mountains and through Siskiyou and Modoc Counties this afternoon,
including a 20-25% chance for isolated thunderstorms near Klamath
Falls. The majority of guidance suggests thunderstorms will form to
the east and north of the area (25-40% chance in Lake and NE Klamath
Counties for any one local area to see a thunderstorm, increasing
towards northeast Lake County), so have elected to leave it out of
the TAF at this time, but we'll be watching closely to see if this
changes. -CSP
&&
.MARINE...Updated 235 PM PDT Saturday, May 20, 2023...North winds
will continue to increase this weekend. Steep seas will continue
south of Cape Blanco through Saturday night. Steep seas are then
expected to extend across the northern waters Sunday while gales and
very steep seas develop south of Cape Blanco Sunday night as the
thermal trough builds in strength. The gales and steep to very steep
hazardous seas are then likely to persist through Tuesday evening as
the thermal trough lingers. -Miles/Smith
&&
.MFR WATCHES/WARNINGS/ADVISORIES...
OR...CA...None.
PACIFIC COASTAL WATERS...Gale Watch from Sunday morning through
Monday evening for PZZ356-376.
Small Craft Advisory from 11 AM Sunday to 11 PM PDT Monday for
PZZ350.
Small Craft Advisory until 11 AM PDT Sunday for PZZ356-376.
Small Craft Advisory from 8 PM this evening to 11 PM PDT Monday
for PZZ370.
&&
MAS/MCB/CSP