Area forecast discussion...updated
National Weather Service Pendleton or
1035 am PDT Sat may 18 2013
Updated aviation discussion
Short term...update...a weak Pacific system will move across the
region today and tonight. Will see clouds increase and thicken
across the area this morning into early afternoon. Also isolated to
scattered showers will spread across the area, with the best chances
across central and east-central Oregon. Also expect scattered
showers over the mountains with locally numerous showers along the
Cascade crest. May see enough instability late this afternoon into
early this evening for stray thunderstorms over the southern Oregon
Blue Mountains, the John Day basin and the John Day-ochoco
Highlands. Winds will be breezy to locally windy across the area
through today into this evening. Updates this morning included
adding mention of isolated thunderstorms in the areas listed above,
as well as making adjustments to the other forecast parameters based
on current trends. 90
Long term...previous discussion...Monday morning a large middle/upper
level trough moves southeast along the southeast alaskan archipelago and
comes into phase with a frontal zone moving east near 45n/135w in
the southern jetstream. During the day on Monday these two systems
merge with the southern system pulling moisture northward into the
eastern flank of the middle/upper trough moving southeast near Queen
Charlotte island.
Tuesday the large upper trough transforms into a large middle/upper
level low. The low continues moving southeast with the center of the
low approaching the coast just west of the Columbia River by late
Tuesday afternoon. This will cause a southwest flow aloft to back
more to the south over the forecast area and transport moisture and
instability into the region. The cold upper low will usher in a
cooling trend and periods of unsettled showery weather, as the upper
low center moves over the forecast area and persists through Friday.
The GFS and European model (ecmwf) model runs show better agreement for Tuesday
through Friday in terms of the placement of the upper low. Daytime
high temperatures will be slightly cooler than normal for Tuesday
through Friday.
Temperature lapse rates from 5000 feet to 20000 feet in the middle levels
of the atmosphere of seven degrees celsius or larger suggest that
there may be enough marginal instability, combined with very strong
dynamic lift and vertical wind shear of 40 to 60 knots from the
surface to 20000 ft, which would favor development of a few isolated
thunderstorms during the daytime on Wednesday, especially in the
afternoon and early evening over the mountains. But will refrain
from introducing a slight chance of thunderstorms on Wednesday since
this far out confidence is low with respect to areal coverage of
thunderstorm potential.
&&
Aviation...18z tafs. Scattered showers may briefly cause ceilings to
lower to near MVFR this afternoon and evening. Otherwise...VFR
conditions will prevail for the next 24 hours. Ceilings will range from
4-8000 feet above ground level...although there will be breaks in the cloud cover
overnight. Sustained winds of 10-20 knots can be expected in most
areas. Wister
&&
Preliminary point temps/pops...
PDT 66 45 68 42 / 20 20 10 0
alw 69 51 69 46 / 20 20 10 0
psc 74 50 74 44 / 20 10 10 0
ykm 70 48 72 45 / 20 10 10 0
hri 72 49 72 43 / 20 20 10 0
eln 68 47 69 42 / 20 10 10 0
rdm 63 35 63 29 / 30 20 10 0
lgd 62 45 62 39 / 20 20 20 0
gcd 65 43 63 38 / 30 30 10 10
dls 68 50 68 44 / 20 20 10 0
&&
PDT watches/warnings/advisories...
or...none.
Washington...none.
&&
Threat index
today : green
Sunday : green
Monday : 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
$$
90/99/85
