Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Spokane Washington
258 am PDT Thursday Apr 24 2014

Synopsis...
occasional precipitation chances, breezy conditions and relatively
cool temperatures dominate the weather pattern. The next organized
storm system will bring periods of rain Thursday morning,
followed by a threat of scattered showers and thunderstorms
Thursday afternoon. Showers will remain a threat into the weekend,
before a relative lull comes for the start of the new work week.




&&

Discussion...

..potential for a few strong thunderstorms Thursday afternoon...

Today and tonight: the inland northwest transitions from a stable
to a convective precipitation regime, before the threat of showers
wanes after dark. A warm front and its stable precipitation will
continue to shift into northeast Washington and the Idaho Panhandle this
morning. Yet an occluding cold front and accompanying mid-level
shortwave will be coming east of the Cascades between 15-18z (8-11
am), before advancing east-northeast into north Idaho between 18-00z
(11 am-5 pm) and eventually out through Montana tonight. This will keep
the precipitation threat high over the eastern third of Washington and
north Idaho through the day, before starting to dissipate after about
00-03z (5-8 pm). Precipitation turns more showery over central Washington
westward to the Cascades before dying out with the loss of daytime
heating. Overall for this morning models place the higher
precipitation amounts across the eastern third of Washington and the Idaho
Panhandle. By afternoon models place the higher quantitative precipitation forecast amounts over
northeast Washington and north Idaho Panhandle, along and north of a line
from Davenport to Kellogg. However all areas will have a risk for
periods of locally heavy rain with instability and a thunderstorm
threat.

So yes, in addition to the generalized precipitation threat, there
will be potential for thunderstorms and some of these may be
strong. From near midday through early evening models show
convective instability blossoming. A swath of 200-500 j/kg of
cape and Lis between -1 and -3c across the upper Columbia Basin
around 18z (11am) shift toward the eastern third of Washington and north
Idaho between 21-00z (2-5pm). A potential limiting factor in the
coverage and strength of storms will 0-6km bulk shear. This is
favorable early today, with values between 25-40kts. However
toward midday this more favorable shear shifts south, mainly
impacting far southeast Washington and the central Panhandle. Still this
is enough to be concerned that more organized storms are
possible, especially in this area. With a low level jet around 25-35kts
feeding into southeast Washington and the central Panhandle and the
aforementioned instability, any organized storms will be capable
of strong winds, some hail and heavy downpours. With all these
factors I increased the thunderstorm chances from slight to chance
this afternoon across east-central and southeast Washington and the
central and southern Panhandle. The best threat appears between
21-00z (2 to 5 pm).

Aside from the precipitation threat the passing front will
increase the pressure gradient and mixing, leading to breezy and
locally gusty winds. Temperatures are expected to be near or
slightly below average. /J. Cote'

Friday through Sunday...the weather over the inland northwest will
continue to be subject to the whims of a deep upper level trough
which is expected to hang around through the weekend. Despite its
presence...the weather isn't expected to be as wet as what we will
experience today and tonight as there are no signs of a sub-
tropical moisture intrusion.

For Thursday night and Friday the models are in good agreement
that the 500 mb trough axis will shift onto the coast which
should transition the mid-level flow to a southerly orientation.
Just how much precipitation this will result in isn't
certain...but there quite a bit of model consensus that much of
the region will be dry...at least through the late morning hours.
The exception could be over the southeast corner of Washington and the adjacent
portions of the Idaho Panhandle. This is the location where
moisture from todays system could remain fixed as the negatively
tilted shortwave pushes through to the northeast. Whether it
remains fixed there or farther south through Friday morning isn't
certain but there is a good chance that it will work its way back
to the north during the afternoon as the mid-level flow backs
toward the south. This will spread a high threat of stratiform
precipitation across the Idaho Panhandle south of i90 with a
lesser chance north. Across the remainder of the forecast area we
will likely see diurnal heating lead to convection as lapse rates
fall in advance of the 500 mb cold pool. Unstable conditions are
not expected to be deep enough to trigger thunderstorms...but some
could near the Cascades as 500 mb temps drop to near -30c late in
the day.

For Friday night and into early Saturday trof and 500 mb cold
pool will shear apart as the longwave pattern is undercut by a
strong upper level jet cutting through the desert SW. This will
still leave the stratiform rain shield fixed over the Idaho
Panhandle and the western portions of Montana. The chances for
measurable precipitation will decrease steadily from east to west
across the eastern half of Washington. By afternoon the approach
of a minor shortwave trough from the southwest could wrap some of
the Idaho moisture into the NE corner of Washington. Precipitation
totals from Friday night into Saturday could be appreciable over
portions of the Panhandle with readings nearing a half inch or
slightly more over SW facing slopes. Snow levels will generally be
above 4500 feet...so Mullan Pass could see some appreciable snow.

For Sunday the base of the trough is expected to retreat to the
north and it will gradually be undercut by moist west-southwest
flow. This will result in an increasing precipitation threat
beginning Sunday morning over SC Washington and gradually
spreading northeast across the remainder of the region during the
afternoon and evening. This could be the last batch of widespread
precipitation as the extended models are all hinting at an
amplifying ridge beginning to develop by Monday and persisting
into Tuesday and beyond. Fx

Tuesday night through thursday: models have come into better
agreement concerning the middle of next week placing the inland northwest
in a Dry Ridge pattern. By Tuesday night the ridge looks to be
firmly in place pushing the storm track to our north and leaving
our region in a stable, dry pattern. With the ridge overhead we
can expect clearing skies, warming temps and light winds. The
biggest question of this time frame currently is how high the
temperatures get throughout the daytime. Right now the models
have pegged next Thursday as the warmest day, but disagree on how
warm. With the strength of the ridge, the region will likely
approach the warmest temps we have seen so far this year. An easy
way to sum up this period would be...very pleasant. /Fliehman

&&

Aviation...
06z tafs: steady rainfall continues to build northward through the
Columbia Basin with most terminals expecting pcpn by 08z. Cigs
will gradually lower and pcpn bcmg moderate temporarily as a cold
front presses through btwn 15-18z. The frontal passage will mark
the end of the steady rain and promote incr winds. A brief break
is possible however a destabilizing atmosphere will lead to widely
scattered -shra through late aftn with a good potential for isolated
thunderstorms. NAM/GFS indicate enough instability setting up btwn
mwh-geg-Coe to include vcts with the 06z issuance. Any storms will
have the potential for small hail and strong wind gusts. /Sb

&&

Preliminary point temps/pops...
Spokane 56 36 54 37 55 36 / 100 60 30 30 20 20
Coeur D'Alene 53 35 54 37 54 34 / 100 70 40 40 40 20
Pullman 55 37 51 37 53 35 / 100 50 50 50 20 20
Lewiston 61 43 55 42 58 40 / 100 50 60 50 10 20
Colville 59 33 60 36 61 34 / 100 20 40 20 50 40
Sandpoint 50 34 54 37 53 34 / 100 80 40 70 80 40
Kellogg 51 34 52 36 49 34 / 100 70 60 80 70 30
Moses Lake 64 38 61 38 63 40 / 70 10 20 10 0 30
Wenatchee 62 42 61 41 63 43 / 60 10 30 10 0 30
Omak 62 34 61 35 63 36 / 100 10 20 10 0 30

&&

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

$$