Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Spokane Washington
225 PM PDT Wednesday Apr 16 2014

Synopsis...
a weak wave will continue to bring more showers to the inland
northwest this afternoon and evening before tapering off overnight.
A stronger system will result in widespread rain Thursday and
Thursday night, with breezy conditions Friday. At this time the
Easter weekend looks to be mild and mainly dry. Showers will be
more prevalent for the first part of next week.




&&

Discussion...

Tonight: the stratiform showers from earlier today has now
transitioned to more of a convective regime as we move into the
afternoon hours. This has resulted in more spotty showers versus
the widespread nature we saw this morning. Some showers have
produced brief periods of light to moderate rain, but totals have
still remained light. As we continue into the afternoon and early
evening hours I expect the showers to move to the NE being confined
to mainly the NE mountains of Washington and the Panhandle by this
evening. With convective parameters weak, the showers will come to
an end rather quickly this evening as sun sets and the atmosphere
stabilizes. This evening into tonight will bring a brief break in
the action as we have some weak ridging in place. Although clouds
will remain firmly in place, the chances for precip diminishes.

Tonight features the mentioned break period as we await the
arrival of next system set to move in tomorrow. The precip looks
to move from west to east reaching the Cascades early Thursday
morning. This system looks to be much moister than the one today
leading to a much better chance of widespread precipitation
Thursday. Temperatures tonight will remain milder than last night
as cloud cover will limit radiative cooling. /Fliehman

Thursday to Friday night: precipitation comes to the inland northwest as
a strong frontal wave migrates across the region through Thursday
night. This is followed by more showery and breezy conditions
Friday.

First Thursday morning a warm front is draped across the region,
while the cold front and it supporting upper wave approach the
coast. Weak to moderate isentropic ascent in the 290-300k layer is
directed across the eastern third of Washington and north Idaho. Initially
condensation pressure deficits show the best saturation and
highest precipitation chances lay over northern mountains and
Cascades. However between 8 am-12 PM the atmosphere begins to
saturate further south. The system taps a subtropical moisture
plume, with pwats rising to 0.60-0.80 inches (or about 180-240% of
normal). This will lead to a gradual expansion of in precipitation
across the basin. Initially precipitation looks light, save for
near the Cascade crest and perhaps the northern mountains. The lc-
valley toward the Camas Prairie and possibly lower Shoshone County
may remain dry.

Then between later Thursday afternoon and Friday morning the cold
front moves from the Cascades to the western Montana, with the
supporting upper wave on its heels. This is when the highest
precipitation chances and amounts develop, with deep-layer lift
coming to the 850mb-500mb layer. There are lingering model
differences as to how quick the front and upper wave pass. This
has implications on how quickly precipitation decreases from the
west, I.E. Toward late evening or not until the later overnight
hours going into Friday morning. Either way much of the region
stands a pretty good chance of seeing wetting rains (over a tenth
of an inch). Some areas may pick up between 0.30 to 0.75 inches of
rain, especially northeast Washington and the Idaho Panhandle and the
Cascades.

There is a thunder potential with this system too. In the late
afternoon and evening hours models shows some uncapped cape across
the upper Columbia Basin through northeast Washington mountains, with some
negative dtheta/dz lapse rates. Overnight into Friday this shifts
to northeast Washington and north Idaho overnight. Confidence is low with
regard to the available potential instability. However a pocket of
-20 to -25c 500mb come across the region and the system has
impressive lift. So I added a slight chance of thunderstorms
across these areas.

Temperatures are expected to be held back by the clouds and
precipitation Thursday, with highs expected to be below normal.
Temperatures in some areas may struggle to get to the 50. Winds
will also increase through by Thursday afternoon and remain in the
breezy category through the night, thanks to the increased
gradient and mixing with the incoming system.

From Friday to Friday night a trough trailing the exiting system
sags across the northern rockies. With low-grade cape and
marginally negative Li values across northeast Washington and north Idaho in
the afternoon and very early evening, in conjunction with a core
of colder 500 mb temperatures around -28 to -30c), this will mean
the threat of isolated to scattered showers. The modest mixing and
still tight gradient behind the exiting low will also mean breezy
to locally windy conditions. Depending on how much rain falls
prior to this period, these winds could lead to some blowing dust.
The best threat will be out toward the deeper basin, where lighter
rainfall amounts are more likely with the Thursday system.
Otherwise look for a mix of sun and clouds, especially in the
afternoon. The overall shower threat and breezy conditions will
abate rapidly after sunset. I did add a few areas of patchy fog
overnight into early Friday morning across the sheltered mountain
valleys across northeast Washington and north Idaho. The recent precipitation
moistening the bl and low level high pressure stretching across
this region will bring this threat. Temperatures will remain
slightly below normal. /J. Cote'

Saturday through Wednesday...a ridge of high pressure will bring
dry conditions to the region on Saturday then a shortwave trough
will move across the northern tier with a chance of showers north
of Interstate 90 early on Sunday. For the start of the work week
the region will be in moist southwest flow aloft that will keep
unsettled weather in the forecast through Wednesday. Southerly
flow will allow temperatures to remain on the warm side of climo
through Monday, then an upper low will move over the region with
cooler and showery weather expected through midweek. /Kelch

&&

Aviation...
18z tafs: a weak weather system is moving across the eastern third
of Washington and the Idaho Panhandle this morning resulting in lowering
cigs and light rain across these areas. Cigs for the geg-sff-Coe
corridor have now dropped to MVFR, but are expected to rise back
to VFR this afternoon after the wave passes. Light stratiform rain
will become more convective this afternoon with rain showers
possible at the kgeg, ksff, kcoe, kpuw and klws taf sites. Moving
into this evening will bring an end to precip chances leaving
mostly cloudy to overcast VFR conditions for the region. Tomorrow
morning we will see the next wave start to enter the region from
west to east bringing lower cigs (likely MVFR for some) and a
higher chance for widespread precip. /Fliehman




&&

Preliminary point temps/pops...
Spokane 41 53 40 56 35 63 / 20 90 100 40 0 0
Coeur D'Alene 39 52 41 55 33 63 / 40 90 100 50 10 0
Pullman 41 58 41 53 34 63 / 20 60 100 30 0 0
Lewiston 43 64 45 59 38 69 / 20 40 90 20 0 0
Colville 38 56 41 61 32 68 / 30 90 100 30 10 10
Sandpoint 39 51 41 54 32 62 / 30 90 100 60 20 10
Kellogg 38 52 39 52 34 62 / 40 60 100 60 10 0
Moses Lake 46 61 42 64 36 69 / 10 60 50 10 0 10
Wenatchee 47 60 41 62 41 67 / 20 60 20 10 0 10
Omak 41 59 38 62 34 67 / 20 60 40 10 10 10

&&

Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&

$$