Area forecast discussion
National Weather Service Pendleton or
230 PM PDT Thursday may 23 2013
Short term...tonight through Sunday...instability is on the
increase this afternoon as surface heating combines with the cold
air aloft to create an unstable environment over the region. Expect
instability showers with possible thunderstorms to continue into the
evening then decrease around sunset and after. Other concern is a
band of precipitation rotating around the upper level low pressure.
This band is now over southwest Washington and northwest Oregon.
Model projections indicate that this will sweep into the western
portion of the forecast area later tonight then move across the
forecast area late tonight and early Friday. This will bring a more
steady precipitation to the forecast area. Snow levels will be
around 4000 to 5000 feet with several inches expected above these
elevations by Friday mainly in the western portion of the forecast
area. Don't expect accumulations in the populated areas along the
east slopes of the Cascades but mountain passes will be impacted.
Should be enough cloud cover overnight to mitigate the cool
temperatures and prevent freezing temperatures in the lower
elevations. Upper level low will lift a little northward on Friday
with best chance of showers focused over the northern portion of the
forecast area. Expect enough stable conditions Friday night that I
went with a mainly dry forecast. Saturday and Sunday will see
another weaker upper level disturbance impacting the region with
more showers and potential thunderstorms mainly on Sunday.
Temperatures will be slowly warming with daytime temperatures
reaching near normal by Sunday but continued cool overnight
temperatures.
Long term...Sunday night through Thursday. A disturbance is
expected to track across the two state area Sunday night and some
convection is possible mainly over our eastern zones. As the
disturbance moves east...a low pressure system is forecast to near
the pacnw early Monday. This should spread clouds and showers into
the two state area Monday and Tuesday. As the system drifts away...
moisture lingers over the region during the middle of the week and
the clouds and showers should persist. Another low pressure system
approaches Washington/or on Thursday and will spread additional moisture
into our area keeping the unsettled weather in the forecast.
Temperatures will remain near normal through the period.
&&
Aviation...a low pressure system is forecast to move inland and
increase clouds and showers across the area into tonight. MVFR
ceilings will continue at taf site kdls in and near showers.
Otherwise deteriorating ceilings can be expected at taf sites kykm
and krdm that will lower to areas of MVFR conditions after 22z.
After 02z the remainder of the taf sites should experience MVFR
ceilings in showers into Friday morning.
&&
Previous discussion... /issued 1058 am PDT Thursday may 23 2013/
Short term...today and tonight...a large upper level low pressure
system will remain over the region with unusually cold air aloft
associated with it. This cold air aloft combined with the surface
heating will create an unstable atmosphere during the afternoon and
evening resulting in showers and some isolated thunderstorms. This
has been the pattern the past couple of days and will be the case
again today. The instability showers will decrease around sunset.
Models are also suggesting an area of more steady precipitation
wrapping around the low pressure into central Oregon late today and
overnight. This could provide some overnight steady precipitation to
this portion of the forecast area not reliant on the unstable
conditions discussed previously. Temperatures will also remain cool
and below normal for this time of year with snow levels between 4000
and 5000 feet. Convective bursts of snow associated with todays
convection are possible which could result in snow levels below 4000
feet under convective cells. Not expecting the need for any freeze
warnings mainly due to anticipated cloud cover overnight.
However...if some prolonged clearing takes place overnight it is
possible for temperatures to spike down rapidly under this weather
pattern.
Aviation...a low pressure system is forecast to move inland and
increase clouds and showers across the area into tonight. MVFR
ceilings will continue at taf site kdls in and near showers.
Otherwise deteriorating ceilings can be expected at taf sites kykm
and krdm that will lower to areas of MVFR conditions after 22z.
After 02z the remainder of the taf sites should experience MVFR
ceilings in showers into Friday morning.
Previous discussion... /issued 155 am PDT Thursday may 23 2013/
Short term...today through Friday night...the models are in very
good agreement with the position and strength of a large upper low
pressure system over the region that has been providing significant
precipitation. The forecast models remain in good agreement through
tonight and then they begin to diverge from each other on Friday. In
any event they all keep an upper low or an upper trough over the
Pacific northwest through the short term period. This will keep a
cool and unsettled weather regime over the forecast area for the
short term forecast period. Today the center of the upper low is
expected to pivot back around to the west and then drop southward
west of the Cascades tonight and then eastward again. This will
bring a slug of moisture back westward and then southward and
eastward across southern portions of the forecast area. The southern
forecast area has seen the least amount of precipitation so far with
this system. There is a potential that these areas (central Oregon
and the ochoco-John Day highlands) could see more significant
precipitation beginning this afternoon and continuing through
tonight into Friday morning. The atmosphere is forecast to become
unstable through much of the forecast area from the eastern
mountains across through central Oregon and up along the Cascade
east slopes into south central and central Washington. Have decided
to include isolated thunderstorms in most areas of the County Warning Area for this
afternoon and this evening except the lower Columbia Basin and the
Blue Mountain foothills. The atmosphere becomes unstable again
Friday afternoon and although not as widespread have also included a
slight chance of thunderstorms again in much of the same areas
Friday afternoon and evening. The upper low becomes elongated from
southwest to northeast late Friday night and early Saturday
resulting in less organization of the overall system. The result
will be decreasing coverage of precipitation across the forecast
area Friday night. Temperatures will remain cooler than normal of
about 8 to 12 degrees across the area...especially daytime highs.
Freeze warnings are in effect this morning for the southern Blue
Mountain foothills and the Grande Ronde Valley. There may be some
snow in the Simcoe Highlands and in the upper Cascade east slopes
but at this time snow amounts will be under advisory criteria in
these areas. 88
Long term...Saturday through Thursday...the upper low over the area
will lift into southern Canada over the weekend with slightly lower
chances for showers. However...another low will drop southeastward into the
region for much of next week with increasing chances for
showers...with the best chance looking to be late Monday through
Tuesday. There could be an isolated thunderstorm or two through the
period but as of now it does not look like the activity will be too
significant. Temperatures will be around normal for this time of
year which equates to 70s for highs lower elevations and 50s and 60s
mountains.
&&
Preliminary point temps/pops...
PDT 42 62 39 68 / 30 50 20 10
alw 42 63 42 71 / 30 50 20 10
psc 43 67 40 74 / 20 40 20 10
ykm 42 63 42 69 / 20 40 20 10
hri 44 66 40 73 / 20 50 10 10
eln 38 60 40 65 / 30 40 20 10
rdm 38 56 32 64 / 70 30 10 10
lgd 35 55 35 64 / 40 60 20 20
gcd 34 54 33 65 / 40 50 10 20
dls 44 64 43 71 / 60 40 10 10
&&
PDT watches/warnings/advisories...
or...none.
Washington...none.
&&
Threat index
Friday : green
Saturday : green
Sunday : green
Green: none or limited need for watches, warnings, or advisories.
Yellow: a few watches, warnings, or advisories possible or in effect.
Red: numerous watches, warnings, or advisories possible or in effect.
For additional weather information, check our web site at...
www.Weather.Gov/Pendleton
$$
91/97/97
