Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 270927
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
227 AM PDT Fri Apr 27 2018

.SYNOPSIS...
Warm and dry weather persists today with many valley locations
seeing high temperatures surging into the 80s. Expect an abrupt
pattern change with breezy, cooler and showery weather for the
weekend and into early next week. High temperatures will drop
around 20 degrees compared to Friday. The cool and unsettled
weather will ease by midweek as a ridge rebuilds over the Inland
Northwest with warmer and drier weather.

&&

.DISCUSSION...
Today through Sunday: With high pressure aloft, expect unseasonable
warm temperatures for one more day with afternoon readings 2 to 5
degrees warmer than yesterday, with many more low land spots
reaching or exceeding 80 under the relatively light north and easterly
winds.

Big changes arrive overnight as a cutoff low in the Pacific pivots
inland with a steady stream of moisture. The thermally induced
pressure trough, currently over western Oregon, shifts east and
tracks across eastern Washington overnight as a cold front. The
GFS and NAM seem to have slowed down its arrival. Winds will
switch to the west southwest during the night and become gusty
across the Columbia Basin and Palouse. Surface based instability
peaks over the Blue mountains and Camas Prairie by early evening,
although it will remain capped until the front`s passage and
limiting any convection potential. As the upper low arrives on the
Oregon coast, a deformation band reaches the Inland NW. The main
swath of precipitation pushes across central Washington late
tonight and shift northward on Saturday with precipitation amounts
of 0.2" to 0.4". Some mid level instability will accompany this
band although the thunder threat look meager overnight.

By Saturday night into Sunday, a second swath of precipitation
enhances across the Blue mountains, across extreme eastern Washington
and north Idaho. This round will bring 0.2" to 0.5" of
precipitation across much of the Idaho Panhandle. Snow levels will
lower to around 5k ft with light snow accumulations possible in
the higher elevations. The upper low looks to hang over the region
Saturday night into Sunday and trended toward more diurnally
driven showers, waning during the night and increasing by
afternoon. Gusty west to southwest winds will continue into
Saturday, tapering off Saturday night and returning by Sunday
afternoon. The more noticeable change will be temperatures where
daytime highs will struggle to reach the mid 50s to mid 60s this
weekend. This will be some 20 degrees cooler than those
experienced on Friday and below normal for late April.

The warm temperatures will continue the mountain snow melt today,
yet the weekend cool down should slow this process. Although the
ample precipitation will increase runoff. Higher flows are
expected to persist on area rivers and streams into early next
week. /rfox.

Sunday night and Monday...The cold upper level low will be slow
to exit the region through this period, however the exact timing
is uncertain. The GFS seems to be the slowest of the solutions and
takes the core of the low into southern Idaho by Monday afternoon
with plenty of moisture and some shallow instability wrapping
around its northern periphery. This would keep a high chance of
showers over most of the forecast area through this time.
Meanwhile the Canadian and to a lesser extent the EC models take
the low farther south and east and allow shortwave ridging to
build in from the northwest. This will lead to marked drying at
least over central Washington with residual showers hanging over
the eastern third of Washington and the ID Panhandle. Temperatures
will remain significantly cooler than the current warmth and many
locations will see high temperatures in the 50s with milder 60s in
the lee of the Cascades.

By Tuesday there is good agreement that the upper low will
eject east of the Continental Divide with swift northwest flow
heading across the BC/WA border. All models are showing a shearing
shortwave disturbance wafting into northern Washington and the
Idaho Panhandle on Tuesday with the EC surprisingly being the
wettest model. It is also the coldest and most unstable model with
500 mb temperatures dipping to -25c or cooler. This feature is
sufficient enough to hold onto high chances of showers over NE
Washington and the northern Panhandle but confidence in wet
conditions with NW flow is not good.

Wednesday through Thursday...The northwest flow begins to exit the
region and high pressure ridging builds over the region from the
west. This will deliver a warming and drying trend with high
temperatures surging into the upper 60s and 70s with dry relative
humidity values and light winds.  fx

&&

.AVIATION...
12Z TAFS: VFR conditions will continue at all TAF sites through
much of the period. Expect an increase of mid and high clouds
from the south and west by Friday evening. Winds will switch and
to the west to southwest overnight with higher gusts expected.
Cloud decks lower from the south overnight with showers likely at
KEAT, KMWH, KLWS and KPUW between 06z and 12z Saturday. /rfox.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        81  46  57  42  55  41 /   0  10  90  20  40  50
Coeur d`Alene  82  45  56  41  53  39 /   0   0  40  30  50  50
Pullman        80  45  56  42  53  40 /   0  40  70  20  40  40
Lewiston       86  51  61  46  60  45 /   0  70  70  30  40  40
Colville       85  47  61  44  57  42 /   0   0  60  40  60  60
Sandpoint      79  45  56  41  51  39 /   0  10  40  40  60  60
Kellogg        80  44  56  39  50  37 /   0   0  40  40  70  70
Moses Lake     86  48  65  43  64  43 /   0  70  60  10  10  10
Wenatchee      81  50  63  44  62  43 /   0  60  50  10  10  10
Omak           83  51  62  45  62  44 /   0   0  90  20  40  30

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.

&&

$$



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