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Area forecast discussion
National Weather Service Medford or
255 PM PST Thu Nov 21 2019

Short term...today through Monday morning...mundane weather in
the near term results from a ridge that stretches across the
coast. Divergent flow can be observed in infrared satellite well
offshore. This is resulting from a low that is splitting to the
the north and south of the high pressure over our area. Most of
the clouds offshore can be seen getting sucked into the portion of
the low that split to the north, but off the coast of central
California a portion of the flow can be seen directed to the
south. This south flow is being caused by an upper level low
spinning at the southern most part of Nevada. This splitting low
will lead to a flattening of the ridge over our area. This will
mark a change from predominantly meridional flow to a more zonal
flow pattern. The zonal flow will allow for more complex weather
systems to enter the southern Oregon area.

The first of these complex weather systems will attempt to move
into southern Oregon Sunday morning. As this time frame draws
closer an upper level (ul) jet will link energy that is building
near Alaska and drive it onto the West Coast. The first of this
energy will come into the area behind a front advancing from the
northwest in the form of a short wave trough, backed by an ul jet.
On this the deterministic models agree. Where the models differ
is on the strength of the ul jet and where it's positioned. The
ECMWF, the model blended towards for qpf, is less bullish in its
placement of the ul jet. The GFS and NAM have the jet displaced
farther south, meaning that the region favorable for upper level
divergence is not over southern Oregon and the moisture is driven
onshore more quickly; which would occur to our north, near
Portland. The reason for choosing to favor the European model (ecmwf) stems from
its continuity between runs and ensemble members, something the
GFS/gefs has lacked. The European model (ecmwf) shows southern Oregon in the right
exit region of the ul jet, promoting large scale vertical ascent.
The ul jet is also not as far south and east, nor as strong. This
change in ul jet placement and strength means that the moisture
will move onshore farther south from the Portland area to the
northern portion of our County Warning Area.



Current thoughts are for this to not bring areas of extensive
precipitation. Despite favoring the ecmwf's more southerly track,
it looks as though only Northern Lake County, along with Coos and
Douglas counties are most likely to receive precipitation. The
current estimate is for places of higher elevation along the
Cascades and in Douglas County to receive around 0.15 inches of
precipitation and places at lower elevation to measure around 0.05
to 0.10 inches. This will also mark the timing of when snow
levels will begin to steadily drop. Through Sunday evening snow
levels will remain above the passes, but some snow accumulation
will begin to take place by early Monday morning near and north of
Crater Lake.

Mid-morning on Monday Marks when snow levels and snow accumulation
may begin to affect passes, followed by the approach of a strong
surface low discussed in more depth in the long term discussion
section below. -Miles

Long term...Monday, November 25th through Thanksgiving day,
November 28th...here's what we know will occur: the weather pattern
for one of the most-traveled weeks of the year, the lead-up to and
after Thanksgiving, will change dramatically, especially considering
the mostly dry and tranquil weather we've had for more than a month.
Since we'll be having relatively calm and quiet weather prior to
this change, now is a good time to prepare for what is expected to
be a colder, wetter and snowier period with at least some expected
wintry travel impacts. This means stocking your vehicle's emergency
travel kits, checking the vehicle's tires/chains, and knowing Road
conditions before you go. Also, make sure to check and double check
current forecasts from reliable sources for updates.

With that said, these systems coming in are likely to bring
some beneficial rainfall and accumulating snow in the mountains that
has been severely lacking since mid-October. There is still a fairly
large amount of uncertainty in the forecast, especially with respect
to a more potent disturbance expected to dig southeastward from the
Gulf of Alaska Monday night that will move onshore Tuesday. Before
we get to that, we expect the jet stream across the North Pacific to
strengthen (~165 kt). On Monday, we'll be in a moist onshore flow
behind an initial front that will lower the snow levels to the
higher passes - around 3500 to 4500 feet (highest in the south).
Precipitation though will be mostly confined to areas from the
Cascades/Siskiyous north and west due to the west-northwest flow
aloft. Very preliminary snow amounts through Monday night will be
1-3 inches for the Cascades, highest north of Crater Lake.

Meanwhile, the potent shortwave will move quickly southeastward
from the Gulf of Alaska Monday night, buckling the strong jet.
Increased baroclinity offshore will likely result in cyclogenesis
near 130w by Tuesday morning. The operational runs of the European model (ecmwf) and
the GFS both show a deepening surface low moving onshore Tuesday,
but are vastly different with the GFS burying the low farther south
near San Francisco Bay and the stronger European model (ecmwf) bringing it in near or
just north of Cape Mendocino. The implications here are large since
if the GFS is correct, our area would largely get missed with the
most significant precipitation heading south into California. The
ECMWF, meanwhile, would imply a sizable snowstorm for the mountains
above 2500 feet Tuesday into Tuesday night followed by additional
snow showers reaching valley floors west of the Cascades by
Wednesday morning. The GFS solution, while plausible, appears to be
a dry outlier when compared to the most recent 12z gefs runs and
also the European model (ecmwf) ensembles. Since there is still such a wide spread in
the guidance and it is still close to a week away, we have leaned
more heavily on the nbm, which isn't quite as extreme a solution as
the 12z ECMWF, but still brings moderate rain/snow to the area
Tuesday into Wednesday. OH, and it also shows some light snow
accumulations on the valley floors west of the Cascades Wednesday
morning! Keep in mind though that plenty can change between now
and then, so look for additional updates.

The cold upper trough should remain over the area through the end of
the week, so expect below normal temperatures. But, after the main
low moves in on Wednesday, precipitation should become more sparse
and showery. -Spilde



&&

Aviation...for the 21/18z tafs...IFR cigs in the Umpqua basin
and Christmas Valley/Alkali Lake area will gradually improve to VFR
with in the next hour or so. Elsewhere, VFR prevails and will do so
through the taf period. However, low clouds will likely return to
the Umpqua basin overnight and possibly to the rest of the valleys
west of Cascades. Confidence is lower for the Rogue Valley given the
drying that has occurred. However, winds will be lighter, dewpoints
will be slightly higher, and it's that time of year for fog in the
valleys so this will need to be evaluated further for future taf
issuance's. /Br-y

&&

Marine...updated 130 PM PST Thursday 21 November 2019...after a
brief period of improved conditions today, seas will build late
tonight into Friday. While winds will remain below advisory criteria
through at least Saturday, persistent long period northwest swell
(10-13 ft at 15-17 seconds) will produce steep seas through the
weekend. Seas will likely remain high and steep well into next week
as northwest swell persists. A cold front is likely on Sunday with a
series of disturbances to follow. /Br-y

&&

Mfr watches/warnings/advisories...
or...beach hazards statement from 10 PM PST this evening through late
tonight for orz021-022.

California...none.

Pacific coastal waters...Small Craft Advisory from 4 am Friday to 4 am PST Sunday for
pzz350-356-370-376.

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