Area Forecast Discussion
Issued by NWS Pendleton, OR

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FXUS66 KPDT 270355
AFDPDT

Area Forecast Discussion
National Weather Service Pendleton OR
855 PM PDT Wed Jul 26 2017

.SHORT TERM...Tonight through Saturday...Sat WV loop shows upper
low moving into nw Nevada at mid evening. Still looks like a
closed circulation and 00z model initialization confirms this as
so. Movement to the northeast over the next 12 hours will continue
to bring moisture into eastern Oregon mainly east of John Day to
LaGrande line. Daytime heating, of course, has ended and
thunderstorms are weakening. Will eliminate the chance for
thunderstorms to the west of this line and allow the Red Flag
Warning to expire at 11 pm. 00Z NAM shows upper low opening into a
wave and moving into SW ID Thursday morning. Thus the POPs on
Thursday will be considerably lower across the southern tier of
counties compared to today. A more westerly flow will bring
increasing winds to areas near the Cascade east slopes including
the Gorge and Kittitas valley, but below advisory criterion.
Slight ridging will begin on Friday and continue Saturday for
continued dry conditions and above normal temperatures.

&&

.AVIATION...06z TAFs...As upper low moves east away from TAF
sites, VFR conditions will prevail with SKC at DLS,BDN,RDM,YKM.
SCT-BKN150 will gradually become SKC at PSC, ALW, AND PDT after
12z. DLS will have G25kts after 18z.



&&

.PREV DISCUSSION... /issued 242 PM PDT Wed Jul 26 2017/

SHORT TERM...Tonight through Saturday...An upper low remains over
northern California at this time and is circulating moisture and
disturbances around and through our area. Instability from the low
and daytime heating is starting to generate some thunderstorms over
the higher terrain of the eastern Oregon mountains and the Cascades
of central oregon. Expect that this will continue through the
evening with isolated to scattered thunderstorm coverage. The number
of thunderstorms should decrease but probably not end overnight.
Have kept a slight chance of showers and thunderstorms over the
eastern Oregon mountains through the night and into early tomorrow
morning. Precipitable water amounts are over an inch (which is well
above normal), so believe storms will have rainfall amounts up to a
quarter inch, perhaps up to a half inch with the stronger cells. The
storms may have gusty outflow winds as well. The combination of
lightning and the possibility of outflow winds has lead to the
issuance of a Red Flag Warning for Central Oregon and the eastern
Oregon mountains. See the Fire Weather Discussion below for
additional details. Late tonight the low will weaken and open up
into a wave. It will get absorbed into the main upper flow and move
northeast just to the south and east of our area. Despite it`s
weakened state, have kept a slight chance of showers and
thunderstorms over the higher terrain of the eastern Oregon
mountains south of the Blue Mountains. Elsewhere in the area, the
main impact of the low passage will be breezy 10 to 20 mph winds in
the Columbia Gorge and Kittitas Valley as the pressure gradient
pulls a weak marine flow into the area. By tomorrow evening the
remnants of the low will be over the Idaho panhandle and aside from
a few showers over the Wallowa mountains, the impact of that low
will be over. By Friday, a ridge over the desert southwest will try
to expand north but will be countered by a large low in the Gulf of
Alaska. This will keep a dry west to slightly southwest flow over
the area Friday through Saturday and fair and sunny weather is
expected. Temperatures will be a few degrees cooler tomorrow with
highs in the lower to mid 90s in the Columbia Basin and in the mid
80s to lower 90s elsewhere. Friday will be a couple of degrees
cooler then Saturday will warm back to the mid 80s to mid 90s. Perry

LONG TERM...Saturday night through Wednesday...Models in good
agreement in building an upper level ridge over the Great Basin
through the period. This will result in southwest flow becoming more
westerly across our forecast area as the weekend gives way to the
workweek. There are differences in how each model handles a weak
disturbance riding over the ridge Tuesday. GFS is stronger with this
system and produces an area of convection over Oregon while ECMWF is
much weaker and drier with no convection.  At this time, prefer the
drier ECMWF solution as it seems to be the more consistent from run
to run.  In addition, more members of the GFS Ensemble show a weak
open trough which lines up better with the EMWF`s solution. In
summary, outside a few possible buildups over the mountains on
Tuesday, we should be dry and hot through the extended period.  Earle

AVIATION...00Z TAFs...VFR conditions can be expected for the next
24 hours. Mid/High level clouds will stream over the area tonight as
a system in northern California moves eastward. BDN/RDM may see a
thunderstorm before the evening is done. However, thunderstorm
coverage is expected to remain scattered so will use VCTS for now.
Winds will be generally less than 10 kts, except KDLS where winds
will be 10 to 15 kts with a few gusts to 25 kts.  Winds are expected
to decrease overnight, but increase to 10 to 15 kts Thursday
afternoon.  Earle

FIRE WEATHER...In the past hour, thunderstorms have developed east
of Prairie City and over southwest Deschutes County. Visible
satellite shows the towering cumulus across east central Oregon that
will soon become buildups over the next few hours. The Red Flag
Warnings (RFW) currently in effect appear on track based on the HRRR
composite reflectivity through midnight. For Oregon Fire Zones 611
and 640 that cover the southern Deschutes National Forest and the
Prineville District, the RFWs are in effect through 11 PM.  For
Oregon Fire Zones 642, 644 and 645 that cover Grant County, the
southern Blue Mountains, the Grande Ronde Valley, and Wallowa
County, the RFWs are in effect until 9 AM Thursday. Thunderstorms
will decrease in coverage after midnight, and there may only be
isolated thunderstorms around sunrise.  The upcoming shifts may
decide to cancel the warnings if storms have dissipated and are no
longer a threat.

A dry southwest flow will remain over our eastern Washington and
north central Oregon zones through Thursday.  An onshore flow west
of the Cascades will allow some marine air to filter through the
Columbia River Gorge where winds will increase to around 15-25 mph
during the afternoon.  Winds will also increase to around 10-20 mph
along the I-84 corridor to as far east as Arlington or Boardman and
south toward Condon around sunset.  The relative humidity will range
from 18-30 percent. The combination of winds and RHs will not be
enough to warrant a RFW but will be highlighted in the forecast.
Wister

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  64  93  59  90 /  10  10   0   0
ALW  69  95  64  93 /  10   0   0   0
PSC  64  96  60  94 /   0   0   0   0
YKM  64  94  60  94 /   0   0   0   0
HRI  66  94  61  93 /   0   0   0   0
ELN  61  88  58  89 /   0   0   0   0
RDM  53  90  51  91 /  30   0   0   0
LGD  56  92  54  92 /  20  10  10  10
GCD  59  92  58  93 /  20  10  10  10
DLS  65  88  61  89 /   0   0   0   0

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 9 AM PDT Thursday for ORZ642-644-645.

     Red Flag Warning until 11 PM PDT this evening for ORZ611-640.

WA...None.
&&

$$

79/81/79



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