Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Pendleton or
755 am PDT Monday Apr 21 2014

Update...the latest radar was showing showers developing over north
central Oregon and moving north and the zones were updated to
reflect this. Otherwise a low pressure system near the Washington/or coast
will continue to pump moisture into the two state area with
increasing showers expected into this afternoon. Otherwise the
remainder of the forecast appears on track for now.

&&

Previous disc... /issued 426 am PDT Monday Apr 21 2014/

Short term...today through Wednesday...a weak ridge of high
pressure will keep the forecast area dry this morning in most areas
with the exception being the Cascade east slopes. Precipitation is
expected to begin spilling over the Cascades today as the next
weather system approaches the County Warning Area. Moisture will spread eastward
this afternoon and this evening as this next system moves into the
Pacific northwest. A southwest flow will develop and pump moisture
northward into the County Warning Area. Precipitation amounts today though will be
very light as the main brunt of this system is not expected to
arrive until tonight and Tuesday. A surface low will develop over
south central Oregon this evening and then ride northeast overnight.
As it does moisture and precipitation will increase across the County Warning Area
from southwest to northeast...especially over the mountains. By
Tuesday morning precipitation will cover the entire County Warning Area with likely
to categorical probability of precipitation. This looks to be a fairly moist weather system
with all areas receiving a considerable amount of precipitation.
Snow levels will be moderately high so there is no threat of winter
weather except for the highest elevations. The precipitation will be
mainly stratiform through Tuesday morning. Then the upper trough
with colder air aloft will cross over the County Warning Area which will destabilize
the atmosphere and cause the precipitation to become more showery by
Tuesday afternoon. The flow aloft will then become more westerly
again which will cause downslope drying to take place in the valleys
to the Lee of the Cascades and the lower Columbia Basin. Will
therefore begin to lower probability of precipitation in these areas beginning late Tuesday.
Tuesday night orographic up slope showers will continue over the
mountains with a continued decrease over the lower Columbia Basin
and adjacent valleys...including central Oregon to the Lee of the
Cascades. A cooler northwest flow will develop on Wednesday which
will keep up slope rain and higher elevation snow showers going over
the mountains and just a slight chance of showers elsewhere.
Temperatures will cool down on both Tuesday and Wednesday as the
upper trough moves across the County Warning Area and brings in a cooler air mass.
Unsettled weather will continue into the extended forecast period
with more weather systems expected. 88

Long term...Wednesday night through Monday...a wet pattern will
continue through much of the long term period. A trough will have
moved off to The Rockies by Wednesday night. A weak ridge with a
moist westerly flow will rapidly move across the area Wednesday
night as another system approaches the coast. The warm front from
this system will move across the Cascades after midnight followed by
the trailing cold front Thursday afternoon and evening. There will
be a chance of rain for the entire area Wednesday night through
Thursday night. This system will set up an upper level trough along
the coast that will move across the Pacific northwest Friday and
Friday night. The unstable conditions with the trough will keep a
chance of rain showers Friday and Friday night though it will be
tapering off overnight. Saturday and Saturday night look as if there
will be a break between systems though I have kept a slight chance
of rain showers in the lower elevations and a chance of showers in
the mountains until confidence in the models improve. Models differ
with their handling of a front arriving Sunday through Monday. The
European model (ecmwf) has it arriving late Saturday night with showers through
Sunday evening then another disturbance arriving Monday. The 00z GFS
brings a weak front to the Cascades Saturday night that fizzles out
with little if any rain on our side of the Cascades then a stronger
system with a good moisture feed Sunday night through Monday that
rides northward through that time frame with rain confined mainly to
the Cascades and northern Blue Mountains. Given the general run to
run and model to model disagreement, have kept a slight chance of
rain in the lower elevations and a chance of rain in the mountains
Sunday through Monday. Temperatures will be in the 60s with 50s in
the mountains. Perry

Aviation...12z tafs...a front with broken to overcast clouds will
move into the area this morning with ceilings at 8000-10000 feet above ground level
this morning then steadily lowering to 2000-5000 feet after 00z.
Rain, moderate at times, will start around 22z at western taf sites
and around 06z at eastern taf sites and will continue through 12z.
Winds will generally remain below 15 kts though a few higher gusts
will be possible this afternoon, especially at kbdn and krdm. Perry

&&

Preliminary point temps/pops...
PDT 65 44 55 39 / 20 70 80 30
alw 68 47 56 42 / 20 50 80 40
psc 70 49 60 44 / 20 60 70 20
ykm 67 41 57 37 / 20 70 60 20
hri 69 46 58 41 / 20 70 80 20
eln 65 41 54 36 / 30 70 70 20
rdm 65 36 47 31 / 30 70 80 30
lgd 67 40 50 36 / 20 70 80 40
gcd 66 37 45 30 / 20 70 80 40
dls 68 47 55 43 / 30 70 80 30

&&

PDT watches/warnings/advisories...
or...none.
Washington...none.
&&

Threat index
today : green
Tuesday : green
Wednesday : 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

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