Heavy Snow in Oregon, February 19 to 20, 2019

Area Forecast Discussion
National Weather Service Pendleton OR
938 AM PST Tue Feb 19 2019

.UPDATE...An upper low over the western states has shifted slightly
east this morning allowing an offshore ridge to turn the flow aloft
over our area more northwesterly and somewhat warmer this morning
except in the Washington Columbia Basin where clear skies allowed
for good radiative cooling overnight. However, a strong frontal
system is poised to move into the area later today bringing
increasing chances of snow through the day. Most areas will get an
inch or less today though the mountains could get up to 2 inches.
The heavier precipitation with this system will pick up this
evening. Temperatures over much of the area remain in the lower to
mid 30s though the lower Columbia Basin is still in the upper teens
to mid 20s. Am expecting the lower Columbia Basin to warm to the
lower 30s this afternoon with the rest of the area just warming a
degree or two from current temperatures and generally in the mid to
upper 30s. These warmer temperatures will give some locations more
rain than snow and may lower snow amounts with the current system.
Most of the current warnings and advisories still look good though
the 4 to 5 inch snow values in the northern Oregon Blue Mountain
Foothills will probably be above 1500 feet, affecting Athena and
Milton-Freewater. Just made some temperature and cloud cover
adjustments. Forecast update already out. Perry

&&

.AVIATION...18Z TAFS...A storm system will move into the area this
afternoon through Wednesday morning. This will spread snow across
the area with lowering CIGS and VSBYS mainly toward evening and
overnight. Best potential for IFR will be at KALW...KPDT...
KRDM...and KBDN though all sites should see at least some MVR
conditions. Heaviest snow tonight will likely be at KALW. Winds will
be a bit gusty at KDLS this afternoon before diminishing this
evening. 78

&&

.PREV DISCUSSION... /issued 340 AM PST Tue Feb 19 2019/

SHORT TERM...Today through Wednesday night...Big temperatures
differences are across the forecast area this morning with most of
south central WA under high clouds and in the single digits and
teens, including the Kittitas and Yakima valleys.  Meanwhile the
remainder of the forecast area is mostly cloudy with stratus clouds
and temperatures in the teens and 20s.  WSR-88D has picked up on a
few weak returns over the Blues, and light snow (mainly flurries)
have been observed.  Clouds will increase today and snow will
develop as the day progresses.  Snow will develop along the Cascades
by late this morning, and the Blues and Wallowas will have some
scattered orographic snow showers during this time. After 1 PM, all
of the forecast area will have at least a slight chance of snow.  A
shortwave trough off the B.C. coast this afternoon will deepen as it
tracks across the Pacific NW tonight.  This is a rather vigorous low
pressure system aided by a strong 130+ knot jet along the backside of
the trough.  Winter storm warnings and winter weather advisories
have been posted for a large portion of the forecast area.

There have been several forecast challenges with the approaching
system.  The biggest challenge has been precipitation type.  Warm
air advection will occur ahead of the trough, and snow levels will
rise to only around 1000-1500 feet tonight for the western
half of the forecast area. Elevations at and below 1500 feet may
observe light freezing rain or sleet tonight mixed with snow, and
this was added to the forecast.  I will also mention the mixed
precipitation in the Winter Storm Warning for the Blue Mountain
Foothills. Surface temperatures along the foothills will be in the
mid 30s this afternoon and evening before lowering to around 30
degrees overnight, therefore it may be difficult for the Blue Mtn
Foothills to receive 3-6 inches of snow.  Will keep the warning but
lower amounts to 3-5 inches.  Ice accumulations in the Yakima Valley
and the Lower Columbia Basin should be light and not enough coverage
for any highlights, but a slight chance to chance will be mentioned
in the forecast.  Confidence is high that the northern Blue
Mountains will have substantial amounts of snow accumulation and
around a foot of snow from Meacham north to the Bluewood Ski Resort.
 The Grande Ronde Valley is under a Winter Weather Advisory.  This
looks reasonable but amounts could be close to 5 inches that would
meet warning criteria.  Based on the position of the low over
northeast Oregon Wednesday and Wednesday night, Wallowa County may
observe snow accumulations meeting warning (i.e. 5+ inches) and will
upgrade the advisory to a warning for this zone.  Winter weather
advisories for snow have also been issued for the southern Blues,
the John Day Basin and nearby mountains, and the east slopes of the
WA/OR Cascades.  Wister

LONG TERM...Thursday through next Wednesday...The region and
forecast area will remain under a cold and unsettled weather
pattern. On Thursday there will be some lingering snow showers over
the area, mainly over the eastern Mountains and Foothills of the
Blues. However, cannot rule out a few snow showers over the Lower
Columbia Basin as well. Additional snow amounts will be very light.
Then on Friday there will be a break in the weather with an upper
transient ridge moving across the area with dry conditions. The next
weather system will then move into the region and CWA on Friday
night and Saturday. Temperatures and thicknesses will be marginal to
support snow in the lower elevations and will mention a chance of
rain and snow, while the mountains, especially the Blue Mountains,
will receive more snow. It is too soon for any highlights at this
time for Friday night and/or Saturday but QPF amounts do suggest
that winter weather highlights may be needed eventually. Colder air
will drop down from the north on Saturday and Saturday night
changing any mixed precipitation to all snow showers, but the cold
air will not be significant. However, it will still be below normal
with highs mostly in the 30s in the lower elevations with 20s in the
mountains. This is about 10 degrees below normal for late February.
There will be another subtle break in the weather on Sunday night
and Monday before the next weather system brings more precipitation
to the forecast area on Tuesday with another push of colder air
dropping down from the north into the CWA. This colder air will
ensure all precipitation to be snow. QPF amounts do suggest the
possible need for more winter weather highlights again for next
Tuesday with the push of this colder air from the north and
significant moisture. There will also be considerable upslope snow
into the Blue Mountains and Foothills for Tuesday into Wednesday
with the cold front stalling over north central Oregon on Wednesday.
Could see significant upslope snow showers with significant
accumulations in the central and northeast mountains as well as some
snow showers over the Blue Mountain Foothills. However, model
differences by this time will be considerably different from each
other and will take a compromise between the models and keep it
colder than normal, but not a significant arctic push of cold air.
The last few instances when the models were indicating an arctic
push of cold air, it did not materialize and temperatures remained
warmer than what the models predicted a couple days in advance.
Winds will be mostly light through the extended period. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  36  28  31  22 /  50  90  90  20
ALW  36  29  32  22 /  50  90  90  20
PSC  32  28  33  23 /  20  50  60  10
YKM  30  24  34  22 /  30  30  30  20
HRI  34  31  34  23 /  30  60  60  20
ELN  34  21  34  18 /  30  40  30  20
RDM  39  26  36  20 /  20  60  70  50
LGD  34  27  33  22 /  50  90  90  30
GCD  33  27  34  20 /  30  80  80  30
DLS  41  32  40  26 /  40  60  60  30

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 1 PM this afternoon to 4 PM PST
     Wednesday for ORZ509.

     Winter Storm Warning from 4 PM this afternoon to 4 AM PST
     Thursday for ORZ050.

     Winter Storm Warning from 1 PM this afternoon to 10 PM PST
     Wednesday for ORZ502-507.

     Winter Weather Advisory from 1 PM this afternoon to 10 PM PST
     Wednesday for ORZ503.

     Winter Weather Advisory from 4 PM this afternoon to 10 PM PST
     Wednesday for ORZ049.

     Winter Weather Advisory from 4 PM this afternoon to 4 PM PST
     Wednesday for ORZ505-506.

WA...Winter Storm Warning from 1 PM this afternoon to 10 PM PST
     Wednesday for WAZ030.

     Winter Storm Warning from 1 PM this afternoon to 4 PM PST
     Wednesday for WAZ029.

     Winter Weather Advisory from 1 PM this afternoon to 1 PM PST
     Wednesday for WAZ520.

&&

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