Flooding in Oregon, May 20 to 22, 2020

Area Forecast Discussion
National Weather Service Pendleton OR
1046 AM PDT Wed May 20 2020

.SHORT TERM...Upper low centered over Idaho with a band of
moderate rain over northeast Oregon and southeast Washington.
This area of rain will be nearly stationary for the morning then
move slowly east during the afternoon and eventually into Idaho in
the evening. Fairly significant amounts of rain have occurred
overnight and additional rain today. Flood watches will be
upgraded to advisories and warnings for many locations along the
Blues and foothills into Wallowa county. Radar indicates a sharp
edge to the rain shield further to the west becoming partly cloudy
toward the Cascades. Temperatures will be cool under the heavy
cloud cover over eastern Oregon with 40s and 50s. Highs in the 60s
toward central Oregon. The low pressure is generating gusty winds
and expect 15-30 mph west winds today with gusts 30-40 mph some
areas.  94

.AVIATION...18z tafs. Light to moderate rain will continue over
PDT ALW PSC into the afternoon. Ceilings bkn-ovc 020-040. Rain
will end late afternoon with ceilings becoming bkn 030-060. Other
taf sites sct-bkn 060-120 today. Winds 15-30kt.

PREV DISCUSSION... /issued 258 AM PDT Wed May 20 2020/

SHORT TERM...Today through Friday night...Water vapor satellite
imagery and radar shows center of upper low over east central ID
this early morning. The low is currently wrapping moisture around
from the north over the forecast area, with a broad north/south
line of light to moderate showers from the Tri-Cities east. This
line of showers is expected to continue to impact the eastern
portion of the forecast area through the afternoon, with total
precipitation amounts in the 0.5 to 1.25 inch along the foothills
of the eastern mountains, and 1.25 to 2 inches along the crests of
the Blues and Wallowas. A flood watch is currently active over
these regions as there is a concern for rising rivers, mostly the
Imnaha and Grand Ronde rivers as well as streams east of the
Blues. There is a level of uncertainty for the impacts to the west
side of the Blue Mtns as bulk of QPF will fall on crest and
eastward. Though its worth stressing we are not looking at a
repeat of Feb devastating flood, there are some rises expected in
the Umatilla and Walla Walla Basins with the hydrograph at Gibbon
expected to reach just below action stage.

Through the rest of today, the upper low over ID will be forced
northeast into western MT this evening as the next upper level low
and surface cold front approaches from the NW. A weak ridge will
work its way between these systems, which will help push showers
east out of the forecast area overnight. Surface gradients will
also tighten over the forecast area today, with widespread
15-30mph westerly winds, and some locations seeing 20-40mph. Wind
advisories are active for the Columbia Gorge and Kittitas Valley
this afternoon. Overnight tonight, surface pressure gradients
will relax and winds becoming breezy in the Kittitas valley and
the Columbia Basin/Gorge. Precip will also trail briefly
overnight, with light upslope showers mainly over the Cascade
Crests.

Thursday, an upper low/trough and coupled surface cold front will
move over the forecast area. Light rain will be expected with the
initial passage of the frontal boundary, with showers developing
behind the front from lingering moisture, mainly in the
central/eastern OR mountains and the Cascades. This system will
also be fairly cold, lowering snow levels in the mountains down
btwn 3,250ft to 3,750ft by Thursday night. Snow accumulations in
the mountains will generally be less than an inch, except for the
peaks of the Cascades, Wallowas and Blues where 1-3 inches of snow
accumulations is possible.

Friday, upper level trough will exit the region to the southeast,
with northwest flow aloft setting up over the region and a upper
level ridge building over the eastern Pacific. Some upslope
mountain showers will continue in the early afternoon, however
moisture will be cutoff as ridge axis moves closer to the west
coast. By Friday night, showers will have dissipated over the
region. Lawhorn/82

LONG TERM...Saturday through Wednesday...An upper ridge will build
into the region on Saturday with a dry northwest flow aloft. Then on
Sunday another Pacific weather system will move into the northwest
with a cold front. This will bring a chance of showers.
Thunderstorms will not out of the question. Long range models
diverge from each other by Monday morning with the Canadian and
ECMWF showing an upper ridge while the GFS shows a weak trough over
the area. Will go with a national blend of the models during that
time with possible showers. Thunderstorms will be possible in the
mountains in eastern Wallowa County near Hells Canyon. After that a
subtle ridge will build to the west with a northwest flow aloft over
the forecast area through the remainder of the extended period. It
will be mostly dry, except for the possibility of showers over the
eastern mountains. Temperatures will be mostly in the 60s to mid 70s
in the lower elevations...which is near normal. 88

&&

.PRELIMINARY POINT TEMPS/POPS...
PDT  56  44  62  42 / 100  40  40  20
ALW  58  45  64  42 / 100  80  30  20
PSC  64  49  68  46 / 100   0  20  10
YKM  72  41  65  39 /   0  10  20  10
HRI  61  48  65  44 / 100  10  20  10
ELN  67  40  59  40 /  10  10  20  20
RDM  63  38  56  30 /   0  10  40  20
LGD  53  41  53  37 / 100  90  70  50
GCD  53  38  53  35 / 100  40  70  50
DLS  64  47  61  45 /  10  10  40  20

&&

.PDT WATCHES/WARNINGS/ADVISORIES...
OR...Flood Watch through Thursday morning for ORZ049-050-502-503-507.

     Wind Advisory until 8 PM PDT this evening for ORZ041.

WA...Flood Watch through Thursday morning for WAZ029-030.

     Wind Advisory until 8 PM PDT this evening for WAZ024-026.

&&

94