Freeze Warning 7, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
941 AM PST Mon Mar 4 2019

.AVIATION...For the 04/18Z TAFs...Stratus with IFR to MVFR
ceilings remains over the Umpqua basin (including KRBG) this
morning and will gradually recede from the edges through the
afternoon. Elsewhere, VFR conditions will continue through this
evening. Stratus will likely reform again in the Umpqua Basin
(including KRBG) again late tonight. It is also possible at KMFR,
but confidence is not high enough to put in the TAF just yet.
-Wright

&&

.PREV DISCUSSION... /Issued 930 AM PST Mon Mar 4 2019/

UPDATE...Valley fog is mostly burnt off in the Medford area, and
is expected to linger through much of the morning in the Grants
Pass and Roseburg areas. Sven

PREV DISCUSSION... /Issued 549 AM PST Mon Mar 4 2019/

DISCUSSION...There remains model agreement on the general
evolution of a complex split flow pattern during the next week,
but excellent agreement on only a shorter than normal portion of
the short term. The main update was to issue a Winter Storm Watch
for elevations above 5000 feet in Siskiyou County from the end of
the afternoon on Tuesday through Wednesday evening.

Freezing temperatures are present in Coos County this morning, and
may occur over a smaller portion of valley areas again on Tuesday
morning. The two immediate features of interest are a lingering
stationary boundary over southern portions of Siskiyou and Modoc
Counties, and a closed upper level low centered over Vancouver
Island. The radar indicates a band of what are expected to be
mainly morning snow showers from northeast Shasta County into
southern Modoc County. To the north, a few shortwave impulses
associated with the low will track inland will produce variable
amounts of mid and high level clouds. Otherwise, areas of fog and
freezing fog in southwest Oregon will be at their most extensive
coverage around sunrise and likely persist through mid-morning.

The complexity in the pattern increases on Tuesday with the key
approach of another notably wet but warmer closed low from far
offshore of California. The low will track across the area on
Wednesday and merge with the northern branch while a third low
moves from the Gulf of Alaska to near Vancouver Island.

Snow levels are likely to be around 4500 feet with the onset of
precipitation during Tuesday afternoon into the evening, then
rising slightly to around 5000 feet into Wednesday. There is some
uncertainty in both precipitation amounts and snow levels, but
high confidence that the broad region of substantial precipitation
amounts will extend across northern California into Siskiyou
County...and to a bit lesser extent into Curry and southern
Josephine Counties. Snow amounts above 5000 feet in Siskiyou
County are forecast in the range of 10 to 20 inches from late
Tuesday afternoon through Wednesday evening. The NAM is the
coldest model and suggests snow levels may be nearer to 4000 feet
at the start of the event (Tuesday afternoon and evening), then
rise modestly late Tuesday night. This lower snow level would
still be above Mt Shasta City and most of the Siskiyou County
portion of I-5, but it goes without saying that the lower the
snow level, the greater the impact. The other portions of our area
above 5000 feet...in Modoc County, the southern Oregon Cascades
and Siskiyou Mountains, and Klamath and Lake Counties will have a
lesser duration and accumulation of snow but may reach winter
weather advisory criteria. Amounts of 3 to 6 inches are forecast
to be common over the higher terrain while some snow may fall as
low as 4000 feet MSL. Hopefully, the next run of model data will
enable higher confidence in the Tuesday night through Wednesday
night snow level and precipitation amount forecasts.

The level of model agreement diminishes further beyond Wednesday
night, but snow levels will likely begin dropping Wednesday night
with a series of impulses interacting with the colder northern
branch upper low. This would bring periods of likely lighter
showers and snow showers at least Thursday into Friday night, and
possibly all the way through the weekend. Snow levels are forecast
to drop to around 2000 feet on Thursday and may reach 1000 feet
early Friday. It is simply too early to tell if there will be any
impact of snow at low elevations, The 06Z GFS is among the wetter
of solutions with a continued snowy forecast for Siskiyou County
on Thursday, and a lingering band of advisory level snowfall
across Siskiyou County into south central Oregon Thursday night with
only up to an inch of snow in southwest Oregon late Thursday
night.

MARINE...Updated 200 AM PST Monday, March 4, 2019...North winds
will continue over the waters through today before they weaken a
bit on Tuesday. Waves will remain steep as a short period swell
mixes with light wind waves. Winds strengthen again on Wednesday
as a low moves into the northern California coastline. As the
winds strengthen, they will turn northeast a bit and may reach
small craft thresholds over the outer waters. Eventually, a high
long period swell will move into the waters Sunday and next
Monday. -Smith

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...Winter Storm Watch from Tuesday afternoon through Wednesday
     evening above 5000 feet in the for CAZ080-082-083.

Pacific Coastal Waters...None.