December 1st & 2nd Weather Briefing

Hello. My name is Douglas Weber. I'm a meteorologist here at the National Weather Service in Pendleton,
OR and I will be providing the special weather briefing for Saturday December 1st through
Sunday December 2nd. An upper level low pressure system continues
to spin off the pacific. Moisture transport across the area will continue for the next
several days. The image roughly indicates where the jet stream is oriented as of 11AM
Friday. As the low moves inland, the upper level jet will become more southerly over
our area Sunday morning, before pushing through Sunday afternoon.
So what does this pattern mean for us? Anytime we have a strong upper level jet over our
area winds have the potential to mix down. Keep in mind there are certain situations
when the strong winds aloft don’t mix down. There are several factors to consider when
looking to see if winds will reach the surface. Wind direction also favors some areas to receive
winds. For example, south winds usually favor stronger winds over the central and eastern
Oregon mountains, but they can also affect the lower elevations like the foothills of
the Blue Mountains. But as the upper level jet passes through with a cold front, the
west winds will typically pick up and favor the lower elevations.
Here is a 4 panel image for early Sunday morning. The black wind barbs point to the direction
the winds are coming from. As you can see they are pointing mostly to the south, which
indicates southerly winds. Per the previous slide, these winds will generally affect the
central Oregon and the mountains of northeast Oregon. The top left image indicates surface
winds. The upper right image indicates the boundary layer winds. And the lower left image
indicates roughly the 5,000 ft. winds. If it’s difficult to tell the wind speeds,
the grey areas are generally in the 15 to 25 mph range, green is roughly 25 to 35 mph,
and so on with the brighter colors. So what are the results of the previously
displayed information? Strong winds are anticipated across central and eastern Oregon. The Foothills
of the Blue Mountains are possible as well.
So what about the precipitation chances? The following slides will explain the upcoming
precipitation chances, prior to the strong winds.
This is a model image of the precipitation estimates over the Pacific Northwest from
10 pm Saturday through 4 am Sunday. As you can see northern California has a lot of precipitation
forecasted, but so does the interior Pacific Northwest.
The next slide will break down the 4 am through 10 am period for the east slopes of the Cascades
and the Kittitas Valley. This is an image at 4 am Sunday. What you
see here in blue is where the temperature at the surface is expected to be between 30
and 32 degrees. The purple image is where the surface temperature is expected to be
below 30 degrees. Basically, it is assumed a possible rain snow mix of precipitation
along the blue area, and a better chance of just snow along the purple area.
This slide is the estimated precipitation amounts. It’s tough to make out just how
much precipitation will fall, but the colors move from purple being the lightest estimates,
to red being the heaviest estimates. So for the area we are focused on it looks like a
tenth to 2 tenths of liquid, with slightly more in the southern area. But the southern
area is also closer to the warmer temperatures, and may likely be all rain.
Moving forward 3 hours to 7 am Sunday, coverage increases over the southwest Deschutes County,
but decreases slightly along the east slopes of the Washington Cascades.
Now looking at the precipitation estimates, it can be noted that the estimates are quite
high across the southwestern Deschutes County, and higher than the previous 3 hour estimates
over the Washington Cascades. Now estimates along the Washington Cascades are upwards
around a quarter of an inch, and the Deschutes estimates are over a half an inch.
By 10 am it can be noted that coverage still increases slightly over the Deschutes area,
but decreases slightly over the Washington Cascades, and the Kittitas Valley.
Liquid amounts decrease significantly by 10 am, and the Washington Cascade crest might
get upwards of a quarter of an inch, as well as the Deschutes area.
So again what does all this mean? While the precipitation estimates are quite high, the
majority of temperatures are very close to 32 degrees.
This suggests many options: First, pavement temperatures may be too warm, and the snow
may all melt on contact, since the air temperature is so close to melting. Second, it could be
a rain snow mix in a lot of places, which would significantly reduce the estimated snowfall
amounts. Third, the areas of all snow may only accumulate on colder surfaces such as
grass. Last, due to the abundant moisture, the snow could be very wet and pack down,
reducing the chance of getting large snow accumulations.
So what is the result of the previous slides? Although the snowfall amounts could vary significantly,
due to the high precipitation amounts forecasted along the Cascades, a Winter Storm Watch has
been issued for the east slopes of the Washington, and Oregon Cascades, as well as the Kittitas
Valley, for late Saturday through Sunday morning. High wind watches are in effect for: Wallowa
County, Southern Blue Mountains, Ochoco John Day Highlands & Basin, and the east slopes
of the Oregon Cascades, from late Saturday evening through Sunday morning. It is possible
that other wind highlights may be needed, but will wait for more data to decide.
Thank you for your participation in our weather briefing for the upcoming event. Always stay
tuned to your local forecast at www.weather.gov/pendlton.