Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 300215
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
715 PM PDT Mon May 29 2017

.SHORT TERM...Short wave moving into northern California initiated
scattered showers and thunderstorms this afternoon and early
evening along the Oregon Cascades and points east from Bend to
Madras to The Dalles. Satellite and radar trends indicate this
activity will continue through the evening and may last into part
of the overnight period. Skies will be partly to mostly cloudy in
this area. From the John Day highlands northward through the
Columbia basin expect mainly clear skies overnight. With a warm
airmass over the region low temperatures will be in the 50s to
lower 60s. A larger upper level trough and cold front offshore
will move through the Pacific northwest on Tuesday. This will
bring increasing clouds during the day with showers and isolated
thunderstorms in the afternoon and evening. The upper level trough
will result in much cooler temperatures on Wednesday with 60s and
70s. 94

&&

.PREV DISCUSSION... /issued 430 PM PDT Mon May 29 2017/

SHORT TERM...Tonight through Wednesday night...The upper ridge over
the region will begin to shift east tonight in advance of an
approaching Pacific system. A small, weakening system off the
northern California coast has been sending moisture north into
central Oregon. This moisture, plus instability due to daytime
heating, will cause isolated showers and thunderstorms over central
and north-central Oregon this evening. Elsewhere pop up
showers/storms will be possible early in the evening over the
mountains. The moisture flowing from California will allow a few
showers to linger along the Cascade east slopes overnight through
Tuesday morning. The main Pacific system currently approaching the
PACNW will push into the region Tuesday and Tuesday evening. This
system will initially cause a push of marine air into the region
Tuesday, followed by a cold front Tuesday evening. The marine push
will be felt first in the Columbia Gorge where breezy to windy
conditions with cooling temps can be expected. The cooling and
increasing winds will be felt elsewhere from late afternoon through
the evening. Isolated to scattered showers and thunderstorms will
develop due to daytime heating Tuesday afternoon. These storms look
to become somewhat consolidated Tuesday evening as the upper level
portion of the system moves through the area. Shear will run in the
45 to 50 kt range over north-central Oregon and south-central
Washington. There will be the potential for locally strong
thunderstorms in these areas. An upper level trough will linger over
the region late Tuesday night through Wednesday night. Thus expect
cooler conditions with varying chances of showers over the higher
terrain. Afternoon and evening thunderstorms may be possible
Wednesday over our eastern most zones. 90

LONG TERM...Thursday through Monday...The extended period will
start off with a lingering upper level trough over the Pacific NW.
This will keep chances of showers and a slight chance of
aftn/evening thunderstorms in the forecast...mainly over the eastern
mountains. Portions of the Lower Columbia Basin and surrounding
valleys look to stay mostly dry. Temperatures will be close to the
seasonal average...reaching the 70s to lower 80s Thursday afternoon.
On Friday and Saturday conditions are expected to be mainly dry,
under partly cloudy skies as a low amplitude upper ridge moves
overhead. Temperatures will see gradual warming each day...with
highs between 75 to 85...except 60s to low 70s mountains. Overnight
lows will be in the 40s to lower 50s. For Sunday and Monday the
latest model guidance is having a tough time handling the
strength/amplitude of the next trough. The latest guidance has flip-
flopped since yesterday...with the 12z GFS now showing a closed
upper low over the CWA by Sunday night/Monday. Meanwhile the latest
ECMWF brings the trough quickly through to the north...with a
building ridge by Monday. Either solution would tend to bring some
breezier westerly winds by Sunday. For now...with large amounts of
continued uncertainty...decided to take a blend of the latest
available guidance for the official forecast. Did lean toward
climatology...which would favor a drier solution. Therefore, the
official forecast has chances of showers over the eastern mountains
in the Sunday/Monday timeframe...with other locations staying mainly
dry. Also took a blend for the temepratures...which gave mid-70s to
lower 80s for highs...except mid-60s to lower 70s mountains.
Overnight lows will continue in the 40s to around 50. 77

AVIATION...00z TAFS...VFR conditions are forecast to prevail over
the next 24 hours for much of the forecast area. Scattered showers
and isolated t`storms are moving through Central and N. Central
Oregon at this time. Expect VCTS to continue through about 30/02-03z
at KBDN and KRDM...as the t`storms move north toward KDLS they are
expected to weaken...therefore am just expecting VCSH at KDLS after
about 30/03z. The remaining taf sites are forecast to remain dry,
with just sct mid and high clouds through the evening and overnight
hours. On Tuesday, additional CB/TCU will likely develop during the
afternoon hours...with sct showers and possible t`storms developing
after 30/21z over Central OR then moving NE toward or just after
31/00z. Away from any t`storms winds will be light, AOB 12 kts this
evening and tonight. Westerly winds begin to increase late Tuesday
afternoon...and will reach 15-25kts at KDLS after 31/19z. 77

FIRE WEATHER...The first fire weather highlight of the season will
be issued today with a Fire Weather Watch in effect for Tuesday
afternoon and evening for the grasslands of Central Oregon and the
Lower Columbia Basin.  It has been about 12 days since this area has
seen any appreciable amounts of rainfall, and the fine fuels are
responding quickly to the prolonged dry conditions as well as the
low humidity the past several days.  The watch will be for
thunderstorms producing abundant lightning.  Gusty winds are also
expected in this area Tuesday which could contribute to escaped fires
that may develop. Confidence is high that Central Oregon and the
Oregon portion of the Columbia Basin will have frequent lightning
strikes with thunderstorms Tuesday. Confidence is moderate for areas
farther north.  The decision on whether to upgrade or remove the
watch will be made by Tuesday morning.  Wister

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  58  91  53  75 /   0  10  40  20
ALW  63  92  58  78 /   0  10  20  20
PSC  58  96  58  81 /   0  10  30  10
YKM  60  93  56  82 /  10  10  40  10
HRI  57  92  56  80 /   0  10  40  10
ELN  57  87  56  76 /  10  10  50  10
RDM  51  83  43  73 /  20  30  20  20
LGD  51  91  56  72 /  10  10  20  30
GCD  52  91  53  73 /  10  10  20  30
DLS  63  76  54  78 /  20  20  20  10

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for ORZ640-641.

WA...Fire Weather Watch from Tuesday afternoon through Tuesday
     evening for WAZ641-675.

&&

$$

94



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