639
FXUS66 KOTX 220932
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
232 AM PDT Sun Sep 22 2024

.SYNOPSIS...

Temperatures will be chilly Sunday morning, promoting frost for
many areas and freezing temperatures in the mountain valleys of
northeast Washington. Yet afternoon will be mild. There is a
small chance of light mountain showers in the north this afternoon
and tonight. A ridge of high pressure will deliver above normal
temperatures early in the week. A more active pattern is expected
toward midweek and beyond, with the potential for breezy
conditions and showers. Temperatures return to near normal by
Thursday.

&&

.DISCUSSION...

...Warm early this week then midweek pattern change...

Today through Monday night: This morning will start off cool to
cold over portions of northeast WA and the ID Panhandle an even
some the Palouse. As of 2AM there were several areas reporting
lower to mid 30s in these areas, while relatively warmer readings
of 40s and 50s were found elsewhere to the west and south . Frost
and freeze warnings remain in place in the those colder regions
through 8 AM this morning. Clouds are starting to increase over
the Cascades now and will continue to increase over the rest of
the area through the day.

This week starts somewhat seasonal with very limited precipitation
chances. We start today off with a zonal flow, with a disturbance
(now pushing onto West Coast) moving by. The jet stream stays
north of the Canadian border and thus so does the brunt of the
lift and moisture. Then tonight into Monday a low pressure trough
starts deepening into the Gulf of AK and as a result a ridge
begins to amplify over the eastern Pacific and a warm front lifts
into the Pacific Northwest before that ridge builds over us. Rain
chances come to the Cascade crest this afternoon and expand to
the immediate Canadian border late this afternoon into tonight and
linger there into Monday. The remainder of the CWA is generally
expected to stay dry. Skies will become partly to mostly cloudy
for today into Monday morning, before clouds erode into Monday
afternoon the ridge of high pressure starts to come in. I cannot
entirely rule out some passing sprinkles tonight into early Monday
outside of the mountains, but the risk is very low. Winds will
increase a bit this afternoon and evening near the Cascades into
the Columbia Basin. On average with winds near 5-10 mph with gusts
to 15 mph will be possible, except near the Cascade gaps where
they will be locally higher this evening with gusts near 20-25 mph
possible. Highs will be in the 70s to low 80s through Monday,
with the more in the way of those 80s near the lee of the Cascade
and deeper basin Monday. Lows tonight will be a milder than the
past two nights, with mostly 40s to mid-50s. There will still be
some 30s over the sheltered valleys of NE WA and ID. /Solveig

Tuesday and Wednesday: We start with warm temperatures and
relatively quiet weather, then return to broader precipitation
chances, maybe some thunderstorms, and breezy/gusty winds. Tuesday
a trough will still be deepening into the eastern Pacific and
pushing toward the West Coast, with the ridge over the Pacific
Northwest continuing to amplify and shift toward the Continental
Divide. Clouds will be limited Tuesday save for some passing
higher clouds. Tuesday night into Wednesday morning moisture
starts to feed in ahead of the approaching low pressure system,
leading to increasing clouds. By early Wednesday the threat of
precipitation returns to the Cascades. Then Wednesday afternoon
and especially Wednesday night the chances expand east as the cold
front and supporting upper trough move in. I did increase PoPs a
bit above the NBM over the east third of WA/north ID in that
18-00Z time frame Wednesday. Instability along ahead of the front
will bring the chance of embedded thunderstorms, with the best
chances Wednesday afternoon and evening near the mountains. I
cannot entirely rule out some coming up through the eastern third
of WA and lower ID too, but confidence is lower with models
currently depicting chances less than 10 percent there. The
overall best chances of wetting rains (>=0.10 inches) will remain
near the Cascade crest and along the immediate Canadian border.

Winds will be the other notable feature of this system. Tuesday
they start off relatively light, but will be easterly ahead of the
approaching system. Then heading through Wednesday winds turn
southwesterly again and increase for afternoon and evening. By
then speeds of 10-20 mph will be possible, with gusts of 20-30
mph. The higher of that range is expected over the Upper Columbia
Basin to the Waterville Plateau. Strictly speaking afternoon RH
values are not expected be at critical values, yet indices
indicate a hot, dry and windy potential. So this will be something
for the fire weather community and other with concerns to
monitor, with the increased potential for spread with any fire
starts, regardless of precise RH numbers.

The southerly flow into our region and the ridge itself will
promote warm temperatures, with highs in the 80s to low 90s. This
will be some 10-15 degrees above normal. Tuesday night lows will
be in the 50s to near 60s, with some localized 40s in the
northeast valleys. At this point most locations Tuesday and
Wednesday will be similarly warm. However the increased onshore
flow will bring a relative cool down near the Cascades by
Wednesday. If the system timing changes or cloud cover changes
those highs temperatures may need to be adjusted up or down.
/Solveig

Thursday to Saturday: The region remains in a progressive
pattern. The midweek trough shifts east Thursday. The westerly
flow and minor impulses will keep some shower chances alive around
the mountains, but amounts look light if anything. The rest of
the area will be dry.

Then the next frontal wave moves into the region between Thursday
night and Friday, expanding precipitation chances again. Models
do diverge over how much precipitation may come with this system.
The higher probabilities remain around the Cascades and northern
mountains. Yet elsewhere models show probabilities for measurable
precipitation as low at 10 percent of as high as 50 percent.
Roughly 65% of the ensembles lean toward the drier solution
outside the mountains, with the other 35% wetter. Either way, the
best chance of wetting rains remains around the Cascades and
precipitation amounts elsewhere are not expected to be that much,
if anything falls. By Saturday model diverge some but still
generally show a more zonal flow again, with the higher
precipitation risk retreating to the mountains.

Precipitation will largely fall as rain, but some high mountain
snow is expected in the Cascades and maybe some the higher
mountains near the Canadian border. Ensembles show about 10-30%
chance of >=1 inch of snow with the Friday system near the Cascade
crest. For most that may not be impactful but if anyone is in the
high mountains that is something to be monitoring.

Also look for breezy conditions in the afternoon through this
period, but especially Thursday. Winds are not expected to be as
strong as Wednesday, but there could be still some gusts near
15-20 mph, with the higher range near the Waterville Plateau.
Temperatures will also be notably, compared to temperatures found
on Tuesday and Wednesday. Highs will be some 10 degrees cooler on
Thursday than Wednesday, with many area seeing 70s. Overnight lows
will be in the 40s and low 50s through Friday morning, then 30s
and 40s toward next Saturday including the potential for more
freezing temperatures in sheltered valleys. /Solveig

&&

.AVIATION...
12Z TAFS: VFR conditions. Bands of mid and high clouds will move
over the region, thickening this afternoon into tonight. Some
showers possible in the Cascades late this afternoon into tonight.
Gusty winds to 15kts or so near Cascade gaps into the Columbia
Basin by late afternoon to early evening, including in vcnty of
EAT, MWH and GEG.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence
for VFR conditions at all TAF sites. Low confidence sprinkles at
TAF sites overnight.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        75  50  78  51  85  57 /   0  10   0   0   0   0
Coeur d`Alene  73  49  75  51  82  56 /   0  10   0   0   0   0
Pullman        71  47  73  49  82  55 /   0   0   0   0   0   0
Lewiston       78  56  80  56  87  62 /   0   0   0   0   0   0
Colville       73  39  78  42  83  45 /  10  10   0   0   0   0
Sandpoint      68  45  73  47  79  52 /  10  10   0   0   0   0
Kellogg        69  49  71  53  81  60 /   0  10   0   0   0   0
Moses Lake     79  52  81  52  86  55 /  10  10   0   0   0   0
Wenatchee      79  60  83  59  85  62 /  10  10   0   0   0   0
Omak           78  54  84  56  86  59 /  10  10   0   0   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Frost Advisory until 8 AM PDT this morning for Central Panhandle
     Mountains-Idaho Palouse-Northern Panhandle.

WA...Frost Advisory until 8 AM PDT this morning for Washington
     Palouse.

     Freeze Warning until 8 AM PDT this morning for Northeast
     Mountains.

&&

$$