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000
FXUS66 KOTX 182158
AFDOTX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SPOKANE WA
258 PM PDT Sat May 18 2013

.SYNOPSIS...
Scattered showers and isolated thunderstorms will develop again
this afternoon and evening. The greatest threat of thunder will
occur over the northeast corner of Washington and much of the
Idaho Panhandle. Precipitation chances will diminish a bit on
Sunday and by Monday most locations will be dry. This drying will
come to an abrupt end by Tuesday afternoon and into Wednesday as a
a very cool and wet upper level low parks itself over the Inland
Northwest. Steady, moderate precipitation will be possible
especially over northeast Washington and the northern Panhandle.


&&

.DISCUSSION...
Tonight and Sunday...An upper level trough along the Washington
and Oregon coast this afternoon will move inland tonight. Energy
from this system will split with one piece crossing the East
Slopes of the Cascades and moving into the Okanogan Highlands this
evening and then stalling in this area overnight. A combination of
this wave passing through this evening combined with low level
instability with surface based CAPE values of 100-300 J/KG should
support scattered showers over the Okanogan Highlands. A stray
lightning strike is also possible.  These showers should begin to
die down overnight with the loss of daytime heating but with the
aforementioned wave stalling isolated showers may persist
overnight. Meanwhile drier air along the East Slopes of the
Cascades will keep showers isolated this evening despite the wave
passage. The southern piece will move south of the area into
Oregon. With the splitting nature this is not expected to produce
too much nocturnal shower activity but isolated coverage is
possible. Elsewhere mainly isolated showers through early this
evening before falling apart with the loss of daytime heating. On
Sunday a short wave ridge will be moving towards the Cascades but
residual moisture and daytime heating will result in another round
of showers mainly over north Idaho and northeast Washington. Also
just enough instability may be present over the northern mountains
with CAPE values as high as 400 J/KG for isolated afternoon
thunderstorms. JW


Sunday evening through Wednesday evening...Looks like this period
will begin on a dry and warm note as an upper level ridge
temporarily builds over the Inland NW. Residual instability could
still trigger some showers or even a thunderstorm early over the
extreme northeast Washington and the northern Idaho but the odds
aren`t great as warmer air aloft begins to cap the depth of the
potential convection. Precipitation chances look like they will
diminish even further on Monday as the 500 mb ridge axis shifts
toward the WA/ID border...with plenty of dry air moving into the
850-500 mb layer. Although there still could be enough instability
for some cumulus clouds over the northern mountains...most
locations should see mostly sunny conditions and temperatures
returning to near normal or slightly warmer than normal levels.

Just how long the ridge will retain its grip on the region is the
tough question to answer. All model solutions gradually replace
the ridge with a very deep and unusually cool upper level low
sometime on Tuesday as it dives into the region from the
northwest. The ECMWF takes the center of the the central
Washington coast by late Tuesday afternoon and subsequently shoves
a strong cold front through the Cascades and into the central
Columbia Basin. Meanwhile the GFS and GEM models are at least 6-12
hrs slower. The latest ECMWF is a fast outlier...however it has
some support from its ensemble. The timing of the front will have
a big impact on high temperatures for Tuesday. Suffice it to
say...the eastern third of the forecast area looks like it will
see significant warming compared to Monday with readings surging
into the mid 70s to lower 80s...whereas the western areas will
have a tougher time reaching values that warm as clouds and the
threat of precipitation moves in ahead of the cold front. Another
issue will be threat of deep convection and possibly some strong
thunderstorms as the heat and significant moisture advection
coincides with moderate upper level divergence ahead of the upper
level jet. The situation is somewhat similar to what we saw last
Monday...however the upper level forcing looks weaker with the
core of the 500 mb energy diving south of our forecast area. Of
all solutions...the NAM looks most favorable for big thunderstorms
with SBCapes reaching 500-800 j/kg with little if any CIN over NE
WA and N ID. The GFS meanwhile is much less impressive. Hopefully
there will be more resolution as the event nears.

Once the front passes and the core of the low shifts into the
region...we will be faced with much cooler and wetter
weather...and this pattern will persist for quite some time as the
low takes up residence over the area. In all cases...it appears
the front will move quite slowly and bring widespread
precipitation...including locally heavy precipitation with the
greatest threat over the northern third of Washington and into the
Idaho Panhandle. How much impact this will have on area rivers
remains questionable as the snow levels will fall precipitously
minimizing much of the snow melt contribution. The rivers most
susceptible to the significant rises will be those over north
Idaho....especially considering some model solutions are placing
the heaviest precipitation to our east and north. fx

Wednesday night through Saturday...Models remain in agreement with
the large scale pattern depicting an upper level low centered
somewhere over the Pacific Northwest during the extended forecast
period. The finer detains of the exact track of the vorticity lobes
have yet to come into sharper focus. Confidence is high that we will
see lots of clouds and cooler than normal daytime temperatures, but
exactly where, when and how much precipitation will fall across the
Inland Northwest will have to wait until the event nears. PoPs
continue the above climo trend while temps will remain below
seasonal normals. /Kelch

&&

.AVIATION...
18z TAFS: An upper level trough and weak disturbances embedded
within will aid in generating showers especially over the
mountains.  Isolated thunderstorms will also be possible mainly over
the mountains north of the Columbia Basin from 20z today through 03z
this evening. Westerly flow in the mid levels will lead to downslope
flow off the Cascades with dry conditions persisting at KEAT/KMWH.
CIGS are expected to be primarily VFR...with brief MVFR conditions
possible around KSFF/KCOE through 20z.  JW


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        45  66  46  72  52  75 /  20  20  10   0   0  20
Coeur d`Alene  45  65  44  71  47  75 /  20  30  20   0   0  30
Pullman        43  63  42  71  47  76 /  20  20  10   0   0  20
Lewiston       49  68  49  77  53  83 /  10  20  10   0   0  20
Colville       45  72  43  78  45  79 /  30  20  20  10  10  40
Sandpoint      43  65  42  71  44  75 /  30  50  30  10  10  30
Kellogg        44  60  46  69  50  74 /  20  40  30  10  10  20
Moses Lake     47  76  46  80  50  79 /  10  10   0   0   0  20
Wenatchee      49  73  49  78  53  74 /  10   0   0   0   0  20
Omak           45  74  43  77  46  76 /  20  10  10   0   0  30

&&

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

$$