River Flood Watch 1, NWS Spokane, WA

Area Forecast Discussion
National Weather Service Spokane WA
249 PM PST Sun Feb 19 2017

.SYNOPSIS...
The weather pattern will remain unsettled and stormy through
early this week. A pair of mild and wetter weather systems are
expected Monday and Tuesday, leading to moderate precipitation,
higher snow levels and renewed concerns of flooding. Cooler and
showery weather is forecast for Wednesday and continuing for the
rest of the work.

&&

.DISCUSSION...

...A RETURN TO WETTER WEATHER WILL INCREASE THE POTENTIAL FOR
MORE FLOODING MONDAY THROUGH WEDNESDAY...

Tonight: Scattered to numerous showers have developed across
portions of eastern WA and north ID this afternoon as the base of
the upper level trough moved through. A thunderstorm developed
along the WA/ID border south of Pullman and moved east northeast
and produced some pea sized graupel. This wave will continue to
track to the northeast through the late afternoon hours. This
evening our focus will turn towards the Cascades and adjacent
valleys into the Waterville Plateau as a wave moves up from the
south. The lowest elevations should see little to no snow
accumulation while the crest of the Cascades could see 2-3 inches.
Then overnight the focus will turn towards northeast WA and the
northern ID panhandle. The hi res ARW and NMM have been putting
some heftier qpf amounts across the mts of north Idaho north of
Sandpoint. Have increased our forecast to 2-5 inches in the
mountains. Other locations will see a brief break in precipitation
before the next weather system moves into our area by Monday
morning. Kept with the idea of some patchy valley fog overnight
for NE WA and N ID, have extended it further south into Spokane
and Kootenai counties. There is also a chance of some patchy fog
for the Wenatchee valley. Overnight lows will generally range from
25 to 32 with above freezing temps for portions of the WA Palouse
and LC valley. /Nisbet

A moist southwest flow will take aim on the Inland NW for the start of
the work week as two weather disturbances pushes through the
region. Even though the main moisture plume remains to the south
in California, a secondary moisture plum will accompany the
passage of two warm fronts that will not only warm
temperatures/raise snow levels but also bring moderate rains to
the lower elevations. The GFS and ECMWF seem surprisingly in
agreement on timing, placement and range of qpf.

On Monday, a deep surface low will track from the Oregon coast into
western Washington with a strong warm front lifting northward
across eastern Washington into north Idaho. Anticipate
precipitation spreading in from south to north through the day.
The initial precipitation type may be snow in most locations aside
from the LC and the deep Basin, but low level southerly flow will
increase temperatures and raise snow levels to above 3k ft by
late in the day. Anticipate the first round to produce light to
moderate rains, and taper off Monday evening as the warm front
weakens and lifts to the north.

A secondary surface low, deeper than the first, will follow a similar
track and send a secondary warm front through the region late
Monday night into Tuesday with moderate precipitation likely. The
surface low will track across the northcentral Washington Tuesday
afternoon, dragging a cold front across the region followed by
breezy westerly winds. Cold air advection will increase Tuesday
night with a decrease of showers and temperatures dropping to
below freezing in most locations. On Wednesday, a broad upper
level trough will be over the region with orographic showers
possible across the most of the higher elevations. Temperatures
will cool back into the 30s for cool day.

Mountain snow: Snow levels will remain near 4k ft across the northern
tier mountains and the Cascades. Given the amount of
precipitation, there is the potential for significant snowfall in
the higher mountains. In fact some Cascade valleys, like the
Methow Valley, may see light snow accumulations through much of
Monday.

Flood potential: These two rounds of precipitation will raise concerns
on renewed flooding where the snow pack remains below 3k ft,
primarily across the Columbia Basin, across the Spokane/Coeur
d'Alene area, into the northern valleys and into the central Idaho
panhandle. The low elevation snow pack may be ripe enough for
additional melting, increasing runoff and ponding of water and
increased flow in many of the small streams and creeks. River
forecasts indicate the mainstem rivers rising by Wednesday with a
few reaching bankfull from the Little Spokane, to the Palouse and
St. Joe into Wednesday. Will issue an areal flood watch for the
eastern third of Washington and much of north Idaho where the
melting of the low level snow pack may increase risks of flooding.
/rfox.

Wednesday night through through Sunday...Unsettled weather will
continue this period as we gradually transition from moist
southwest flow to a chillier northwest flow orientation. The focus
for the precipitation during this period won't be very strong nor
will it be very wet as there is no sign of any impending
atmospheric rivers. It looks like the best chance of precipitation
will occur on Wednesday night and Thursday as most of the model
solutions drop a weakening shortwave trough through the Inland NW
via British Columbia and NW Washington. The GFS and EC take the
bulk of the shortwave energy through western Washington before
shearing it through the Cascades and southward down the coast and
into Oregon. Yesterdays runs of the Canadian didn't follow this
evolution and instead brought a much deeper upper level low into
eastern Washington. The Canadian is now more in line with the
GFS/EC solutions so confidence is higher that any resultant
precipitation will be relatively light. Most will be focused near
the Cascade Crest as well as over the Idaho Panhandle. Instability
associated with the remnants of the trough and 500 mb temps of
-36c or colder will result in the threat of showers persisting
through Thursday. By Friday a stronger shortwave trough will drop
out of BC and eventually off the Washington coast. The trough will
continue to drop southward on Saturday taking aim on the northern
California Coast. 500 mb temps will remain chilly so the
instability potential looks good at least on Friday resulting in
more diurnal showers. The track of the low will also allow another
surge of cold air to move in from the north. Temperatures through
the period will remain cooler than normal and could bottom out by
the end of the week into the weekend. Highs through the period
will only be in the 30s with lows potentially in the teens to
middle 20s. fx

&&

.AVIATION...
18Z TAFS: A short wave disturbance will move north through the
forecast area today...with occasional -SHRA at all TAF sites. Have
added tempo thunder to the KPUW and KLWS tafs from 20-23Z. They
could see thunder with graupel possible. There will be a break
this evening and then more widespread -ra/sn will move into the
region aft 12Z. IFR/MVFR cigs and vis is expected for most sites
through the overnight hours and into Monday morning.

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        30  41  34  45  28  37 /  30 100  90 100  60  30
Coeur d'Alene  30  40  34  45  28  38 /  30 100 100 100  60  40
Pullman        35  46  38  47  30  39 /  30 100 100 100  60  30
Lewiston       36  51  40  52  32  45 /  30 100 100  80  40  30
Colville       31  37  35  44  27  40 /  60 100 100 100  40  30
Sandpoint      30  37  33  42  29  38 /  40 100 100 100  70  40
Kellogg        31  38  35  43  28  36 /  50 100 100 100  80  50
Moses Lake     29  45  34  48  27  40 /  20 100  80  80  10  10
Wenatchee      29  39  33  46  29  40 /  50 100  80  80  10  10
Omak           29  36  32  44  25  39 /  70 100  90  80  10  20

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...Flood Watch from Monday afternoon through Wednesday afternoon
     for Central Panhandle Mountains-Coeur d'Alene Area-Idaho
     Palouse-Northern Panhandle.

WA...Flood Watch from Monday afternoon through Wednesday afternoon
     for Moses Lake Area-Northeast Mountains-Okanogan Highlands-
     Okanogan Valley-Spokane Area-Upper Columbia Basin-
     Waterville Plateau.

&&