Area forecast discussion
National Weather Service Spokane Washington
443 am PDT Tue may 21 2013
Synopsis...
the mild Spring weather will come to an end. A strong cold
front will sweep through the region Tuesday through early
Wednesday with showers likely, a chance of thunderstorms, breezy
winds and cooler temperatures. Cool and showery conditions will
prevail through much of the week, but will improvement a bit for
the Holiday weekend.
&&
Discussion...
today and tonight: a strong cold front moves into the region,
with increasing rain and the potential for thunderstorms, with the
added threat of snow coming to some of the low lands late tonight
into Wednesday morning. This morning satellite imagery showed the
cold front advancing onto the Pacific northwest coast and radar
showed a swath of rain extending about 100-150 miles east of it.
Deep layer lift with the cold front and supporting jet stream
coming around the upper low is expected to sustain the precipitation.
So precipitation that is just coming into the Cascades is expected
to expand across central Washington through the morning and early
afternoon. More widely scattered showers are expected to develop
across eastern Washington and north Idaho this afternoon, before
the steadier band of precipitation comes here late this afternoon
and evening with the cold front. The center of the upper low and
linked strong vorticity Max comes onshore near northwest or this
evening, before swinging toward the central Washington/or border
overnight. This will sustain the deep layer lift and keep
precipitation chances high tonight across the eastern third of
Washington, northern Idaho and west to southwest across the
Okanogan through Cascades.
As for the thunderstorm potential, convective parameters indicate
increasing cape and unstable Li value through this morning and
afternoon. The highest cape and most unstable Li values and so
highest thunderstorm threat is projected to be from the Okanogan
Highlands into the northeast mountains and Idaho Panhandle.
However some low-grade cape and the high level total totals
instability parameter indicating values between 30-36 c back into
the Okanogan Valley through Waterville Plateau and Cascades, some
thunder cannot be ruled out here either. Lastly models indicate
some modest 0-6km shear across the region. Where instability is
best values are between 35-50kts. This support the potential for a
few organized and stronger thunderstorms, with a threat of hail
and strong winds. However is the cold front passes quicker this
potential will be limited.
Temperatures will vary greatly from the Idaho Panhandle to the
Cascades today. The incoming cold front and precipitation is
expected to keep temperatures across the western basin through
Cascades relatively cool and below normal. The eastern third of
Washington and northern Idaho, however, will see more sun and less
precipitation ahead of the cold front until later in the day. This
will hold temperatures closer to or slightly above normal.
Overnight into Wednesday morning colder air begins to punch in
with the center of the upper low. Across the Cascades through the
Palouse and Camas Prairie, models show 850 mb temperatures
dropping to between 0 and -2 c. This suggests snow levels dropping
to near 3000 feet across this region. So some snow remains in the
forecast, even around Pullman. Accumulations should be limited by
surface temperatures generally remaining above freezing. The
exception will be in the higher Cascades and potentially around
the Blue Mountains and higher Camas Prairie, including near
Winchester. /J. Cote'
Wednesday and Wednesday night: as we progress into the overnight
hours we will continue to see widespread rain for the inland northwest
and potentially moderate precipitation for parts of NE Washington
and the northern Idaho Panhandle especially in the higher
elevations. A major swing in temperatures will also be accompanied
by the strong cold front.
Mentioning the higher amounts of quantitative precipitation forecast we can base this off of not
only the front slowing its progression in the mentioned
areas...but also a strong jet will be supportive of higher
precip. Intensity. The key to all of this relating to potential
impacts of flooding will be the snow levels. For this forecast the
snow levels were only slightly adjusted but with current forecast
levels much of the precip that falls in the higher elevations will
fall as wet snow rather than rain and this will help minimize
the flooding impacts. If all precip were to fall as rain the
flooding potential for rivers and streams would be elevated due
to higher amounts of direct runoff...but given the current
scenario the snowfall will act as a storage mechanism for much of
the liquid and let it slowly melt and contribute to streams as
temperatures warm back up.
Other changes that were made to the forecast would be even further
increasing the pops for the NE part of Washington and the Idaho
Panhandle for Wednesday. Although pops were increased the over quantitative precipitation forecast
amounts were trimmed back a little especially for the east slopes
of the northern Cascades as new model runs show slightly lower
amounts. Quantitative precipitation forecast was also trimmed for NE part of the forecast
area...but remain high for the time frame which will still lead
to rises in the streams and rivers. All rivers minus the Okanogan
river near Tonasket is currently forecast to remain below flood
stage and the Okanogan will only reach minor flood stage. In
general this period will see temps drop significantly from the
previous day (nearly 30f in some areas) and some breezy conditions
as the front pushes through the area. /Fliehman
Thursday through Monday...unsettled and cool conditions will
continue through the extended period. Models are in very good
agreement in holding a deep upper level low right over or very
close to the forecast area through Friday...which is a high
confidence recipe for showery conditions just about everywhere in
the forecast area. The latest GFS and ec models both sense a
secondary deformation area transiting the region from south to
north on Friday which may enhance the density of showers on this
day especially across the northern tier zones.
For the upcoming Holiday weekend model consensus begins to break
down but both models depict a weakening of the coherence of this
upper low...an opening up and broadening trend into a general
trough over the region (gfs) or a migration of the upper low
center into southern Canada (ecmwf). Both solutions suggest a
continued unsettled and showery regime...but showers may not be as
prevalent as when the upper low was directly over the region. At
this time the weekend does not look like a wash-out...but none the
less iffy for outdoor activities...with guarded optimism that the
densest shower activity will occur over the mountains north and
east of the basin. Temperatures through the upcoming week will
start out well below normal but slowly add a degree or two each
day and top out around normal by next Sunday and Monday. /Fugazzi
&&
Aviation...
12z tafs: a cold front moves from the Cascades to Idaho trough
the next 12-18 hours, while the center of an upper low swings in
from the west/southwest tonight into Wednesday morning. The front
will bring an increasing area of rain near the western taf sites
this morning and early afternoon. Scattered showers will develop
further east this afternoon, while a steadier band of
precipitation is expected to come by late afternoon and evening
with the cold front. A chance of thunderstorms also exists
throughout much of the region, with the best chance for organized
activity north and east of kgeg. Storms may be locally strong with
gusty winds/small hail. Tonight the incoming upper low is expected
to keep precipitation chances high across eastern taf sites. Local
MVFR cigs are possible in the more persistent precipitation. Colder
air accompanying the upper low will allow some snow to mix in
toward Wednesday morning but surface temperatures are expected to
limit the accumulation threat below 3000 feet. /J. Cote'
&&
Preliminary point temps/pops...
Spokane 73 41 51 36 61 42 / 30 90 90 40 30 30
Coeur D'Alene 75 41 48 36 59 41 / 30 90 90 50 40 40
Pullman 72 38 51 35 60 41 / 10 80 80 30 40 40
Lewiston 80 44 59 41 66 46 / 30 60 70 30 30 40
Colville 76 45 55 37 66 41 / 50 90 90 60 50 40
Sandpoint 76 43 50 36 59 39 / 30 90 90 80 40 50
Kellogg 75 40 44 34 55 40 / 30 90 90 70 50 50
Moses Lake 71 43 62 40 68 44 / 70 50 30 30 30 30
Wenatchee 63 43 58 42 65 45 / 60 60 40 40 50 30
Omak 69 43 59 38 66 41 / 80 80 70 50 50 30
&&
Otx watches/warnings/advisories...
Idaho...none.
Washington...none.
&&
$$
