Frost and Freeze in Oregon, June 7, 2021

Area Forecast Discussion
National Weather Service Medford OR
802 PM PDT Sun Jun 6 2021

.DISCUSSION...Temperatures continue to lower across the forecast area,
but the cooling trend has slowed some. 1 to 5 degrees of cooling
separates this evening compared to yesterday evening. It was
breezy to windy again this afternoon, with highest winds east of
the Cascades and in the Shasta Valley. The highest gust was 42 mph
in Modoc County. Humidity has trended higher in most areas east
of the Cascades, and the Red Flag Warning will expire in about an
hour, and this is reasonable.

Tonight will be another cold night, and we expect freezing
temperatures in Klamath Falls and some other East Side locations.
Please see the previous forecast discussion for more details on
this cooling trend and the chances of rain arriving this
week. Keene

&&

.AVIATION...For the 07/00Z TAF Cycle...Along the coast and over the
coastal waters...VFR conditions will prevail initially with widely
scattered showers. Areas of MVFR cigs will develop Sunday evening
with some obscuration of higher terrain and continuing widely
scattered showers, mostly from Cape Blanco north.  The lower
conditions will persist into Monday morning, improving to VFR again
by Monday afternoon.  Over the remainder of the area...VFR
conditions will prevail through Monday, but there will be some
obscuration of higher terrain.  There will also be some isolated
showers over the Umpqua Basin and the adjacent Coast Range and
Cascades. Winds will be strong and gusty Sunday afternoon and
evening and again Monday afternoon and evening, especially east of
the Cascades.

&&

.MARINE...Updated 800 PM PDT Sunday 6 June 2021...Steep northwest
swell dominated seas 8 to 9 feet at 11 to 12 seconds will build
to 9 to 11 feet at 11 seconds by sunrise Monday, then subside to
8 feet by Monday afternoon.  A series of weak fronts will move
onshore tonight through Thursday. A stronger front will move onshore
Friday, which may bring small craft advisory south winds to the
area.

&&

.FIRE WEATHER...Updated 8 PM PDT Sunday, 6 June 2021...Today's Red
Flag Warning will expire at 9pm this evening. After another night
of moderate to good recoveries tonight, the pressure gradient is
expected to shift a bit Monday to the south, and as a result,
gusty winds are expected yet again across the Modoc (zone 285) and
portions of southeastern Lake County (zone 625). Monday afternoon
winds and daytime humidities are expected to be very similar to
those of today, although some model guidance has hinted that
humidities may improve significantly by tomorrow. Past experience
and analogs suggest that it will take a bit more time for the dry
air to moderate, so despite somewhat lower confidence, the Fire
Weather Watch for these areas has been upgraded to a Red Flag
Warning.

The cooling trend will continue into Tuesday and Wednesday, with
daytime highs expected to be 5 to 10, or even 15 degrees below
normal for early to mid June. Afternoon winds will remain breezy,
but daytime humidities should considerably improve, preventing the
necessity for additional fire weather concerns.

Precipitation chances will increase through this week as an upper
level trough sets up just offshore and several fronts pass through
the region. Shower chances will begin along the coast as early as
tonight or tomorrow morning, but will spread inland to portions of
the Cascades by Tuesday. Showers are then expected to spread to the
East Side by Wednesday, along with a slight chance for thunderstorms
in the afternoon as the offshore trough finally moves inland.
Thunderstorm chances are favored from Mt. Shasta, along the Cascades
and eastward.

A brief period of ridging passes through region on Thursday,
producing slightly warmer and drier conditions that afternoon into
the evening, then a potentially more significant trough is possible
Friday into Saturday. This system could be more akin to a wet season
system than one we typically see in mid June, and would likely
result in gusty winds across the area, with wetting rains possible,
especially along and west of the Cascades. Models are beginning to
consolidate around a Friday night to Saturday arrival, and continue
to slightly strengthen the system in terms of precipitation and
winds. However, given that this is June and the system remains six
to seven days out, confidence in this scenario is low. That being
said, it does remain in the realm of possibilities, and bares
watching over the next several forecast cycles. -BPN

&&

.PREV DISCUSSION... /Issued 305 PM PDT Sun Jun 6 2021/

DISCUSSION...This afternoon's satellite imagery reveals a
vigorous low pressure system off the Washington coast supported
by a 130-140kt upper jet that will move onshore there this
evening. Our forecast area is on the southern fringes of this
disturbance and while some light showers are possible along the
coast and into the Umpqua Valley tonight, that won't really be the
main impact. The main effect will be the gusty winds this
afternoon into this evening. Satellite imagery also shows plenty
of standing wave clouds, indicative of the stronger winds aloft.
Gusts of 25-40 mph are expected for most areas, highest from the
Cascades eastward. The strong winds along with low relative
humidity over the east side are still resulting in some fire
weather concerns there, so please be sure to check out the fire
weather section below for the details.

Once the sun goes down this evening, winds should drop off again,
much like they did last night. Since the air mass is a bit colder
than yesterday, think temperatures will drop to freezing or below
in the Klamath Falls, Tulelake, Lakeview areas toward morning.
As a result, we have upgraded the Freeze Watch to a Freeze
Warning there. Models are showing some high cloud cover moving in
toward morning, but these should stay mostly to the north and
west and remain thin enough for the freezing temps.

Monday into Wednesday, the upper jet will buckle and this will
allow an upper trough to set up just off the WA/OR coast, which
will eventually move onshore Wednesday night. Short wave
disturbances will swing through and could generate some light
showers, especially from the coast to the Cascades Monday into
Tuesday, but precipitation amounts are likely to be light. We'll
remain quite cool for early June with high temperatures averaging
about 10 degrees below normal. This puts highs only in the upper
60s to near 70 here in Medford. We'll have to continue to monitor
for freeze potential east of the Cascades. As the trough pivots
toward the area on Wednesday, instability increases a bit with the
chance of showers expanding to areas east of the Cascades.
Instability parameters aren't overly impressive, but are enough
(model CAPE of ~250 J/KG and 850 computed LI of approx -2) to
include a slight chance of thunderstorms in an area from around Mt
Shasta, up the Cascades and over to the east side. Really don't
see much more than isolated storms with this, but we'll continue
to refine this part of the forecast in the next few shifts as we
get closer.

It appears we'll get a break between systems Wednesday night into
Thursday as short wave ridging moves in. So, probably dry and a
bit milder then. Long range guidance continues to show a deep,
anomalous, closed low forming out around 48N and 135W in the NE
Pacific by Friday. This will send a more moist frontal system
toward the west coast than we have seen in a long time. Ensembles
are showing Integrated Vapor Transport (IVT) of >250kg/m/s with
some members nudging up to around 500 at Brookings. Details are
still uncertain, like just how far inland the front can get and
just how much precipitation will occur, but much of the area
should expect increasing precipitation chances Friday into the
weekend, with wetting rains possible, especially from the Cascades
west.

Beyond that, shortwave interaction with this closed low will
ultimately determine how long it will spin out there in the
Pacific and when it eventually ejects ENEward into the westerlies.
Some ensemble members show the low ejecting across the area
rather quickly (Sun into Monday). Others show some retrogression,
which would allow the ridge to the east to return (at least
somewhat). The 12z deterministic run of the ECMWF seems to fall
into that camp and would prevent additional precipitation from
reaching east of the Cascades. While the deterministic ECMWF still
lies within the envelope of the guidance, it is still an outlier
with around a 10% chance of occurrence. There are still many
scenarios of which are yet to be determined. So stay tuned.
-Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 9 PM PDT this evening for ORZ624-625.
     Freeze Warning from 3 AM to 8 AM PDT Monday for ORZ029>031.
     Red Flag Warning from 2 PM to 8 PM PDT Monday for ORZ625.

CA...Red Flag Warning until 9 PM PDT this evening for CAZ285.
     Freeze Warning from 3 AM to 8 AM PDT Monday for CAZ084.
     Red Flag Warning from 2 PM to 8 PM PDT Monday for CAZ285.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT Monday for
     PZZ350-356-370-376.