Area Forecast Discussion
Issued by NWS Spokane, WA

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FXUS66 KOTX 291308
AFDOTX

Area Forecast Discussion
National Weather Service Spokane WA
608 AM PDT Fri Jul 29 2016

.SYNOPSIS...
Friday will be hot, with a subtle increase in afternoon winds. A
dry cold front passes Saturday, creating breezy conditions and an
increased risk for spreading grass fires. Another breezy cold
front will be possible on Monday. Temperatures will be cooler
Sunday into next week, with an occasional threat for showers and
thunderstorms in and around the northern mountains.


&&

.DISCUSSION...
600 AM UPDATE: added FWZ 674 (East Washington Palouse and Spokane
area) to the Fire Weather Watch for Saturday afternoon and
evening.

Today through Saturday: A slight chance of showers and
thunderstorms will linger near the Canadian border mountains over
the next couple days. However the bigger story will be the hot,
dry and breezy conditions and potential fire weather concerns.

Early today a low-amplitude ridge over the area will promote dry
and mostly clear conditions. Later this afternoon and evening the
next weak shortwave slides toward the Cascades and northern
mountains. Interacting with the areas of SBCAPE developing in the
afternoon heating across these areas, the threat of isolated to
widely scattered showers and thunderstorms will increase. With
PWATs remaining at about 150 percent of normal any storms that
develop will have the potential for some locally heavy downpours.
After dark the primary risk will wane. However lingering elevated
instability and the still passing shortwave will keep an isolated
shower/t-storm risk through the overnight around the far northeast
WA/north ID mountains near the Canadian border. As for
temperatures, today is expected to be the peak of this stretch of
hot weather, with most areas in the 90s and some lower 100s near
Wenatchee into the Columbia Basin and the L-C Valley.

Saturday a more robust/sharper shortwave pivots east-southeast
across British Columbia and cold front starts to push in, with a
surface low deepening across NE WA/NRN ID. The main lift lingers
near the Canadian border. Model shows only isolated pockets of
instability near the Pasayten Wilderness and northern Pend Oreille
into Boundary county. There could be some isolated showers and
thunderstorms in these areas. However drier air starts in with
PWATs dropping to around 100-120% of normal. So this will limit
the risk. On average the day looks dry. Afternoon temperatures
will be about 3-7 degrees cooler than Friday, but this still
leaves temperatures above normal.

* Fire Weather: the other issue to be monitored both today and
  Saturday will be related to fire weather concerns due to
  increasing winds and low RH values. Thursday afternoon relative
  humidity values bottomed out in the teens and 20s over basin and
  valley areas. Minimum RH values are expected to drop further
  today and Saturday, with the lowest on Saturday as drier air
  invades behind the cold front. The slight cool down will limit
  how much those values drop. Yet as a whole RH values are
  expected to have little trouble falling to or below critical
  thresholds. The main question mark is the winds: including
  timing and speeds. Once again this morning look for some locally
  breezy conditions down the Okanogan Valley and near the Cascade
  gaps. They abate through the mid to late morning, until they
  start to increase again through the Cascade gaps late afternoon
  into evening. They may be close to critical levels near
  Ellensburg but right now it appears too brief to warrant a
  highlight today. This will be monitored. Saturday, however,
  models have maintained or slightly increased speeds in the
  afternoon. So the ongoing Fire Weather Watches remain justified.
  Additionally with the morning fire weather forecast package FWZ
  674 (East Washington Palouse and Spokane Area) will be added,
  supported by that aforementioned increase in speeds. /J. Cote`

Saturday night through Monday: The deep upper level low is
expected to depart the region as it pushes into the Prairie
Provinces by Sunday afternoon. The atmosphere will stabilize as
the 500 mb level cold pool departs and some shortwave ridging
moves in ahead of the next upper level low. Residual moisture and
some elevated instability could keep the threat of showers over
the far NE portion of our forecast area for Sunday afternoon but
the odds are small. The main story for the Sunday and Monday
period will be how dry the post-cold frontal air mass will be.
Afternoon dewpoints both Sunday and Monday afternoons will likely
drop into the mid 20s to mid 30s which is the lowest they have
been in over a month. These dewpoints will combine with near
normal high temperatures resulting in rather low relative humidity
values in the teens and possibly even some localized single
digits. This combined with ample sunshine for most of the area and
moderate winds should result in some good fuel drying across the
region.

Monday night into Tuesday: this is where the forecast could get
somewhat interesting as the next low moves into the area. Model
agreement is improving with all models bringing a 562-565dm 500 mb
low into the north Cascades by Tuesday morning. This is quite
impressive for late July. The low is expected to drift NE into SW
Alberta by afternoon. The forcing ahead of the low will be rather
impressive with a good chance of precipitation expected across the
northern third of the forecast area. Given the strength of the
low, we cannot rule out nocturnal thunderstorms near the Cascades
late Monday night/Tuesday morning spreading across the remainder
of north Washington and the northern Idaho Panhandle by afternoon.
The story elsewhere will be focused around winds. The model
consensus is the peak 850 mb winds will occur Tuesday morning with
speeds around 30-35 mph. Whether or not these can mix down to the
ground is questionable since its not the favorable time of day.
However we would expect to still see some 15-25 mph winds on the
ground over the Columbia Basin, Palouse, and Spokane area by
sunrise and perhaps some stronger winds over the mountains.
Despite the breezy conditions we will likely see moderating fuel
conditions as high temperatures drop into the 70s to lower 80s and
humidity values climb well into the 20s or higher.

For Tuesday night and Wednesday the low is expected to make a hasty
retreat with drying and subtle warming returning. Plenty of
sunshine is expected and the resultant daytime heating could
trigger some showers and thunderstorms over the northern
mountains. fx

Wednesday night through Friday: Models in general agreement with
the overall pattern in the far extended forecast. A low pressure
system will slide down the BC coast and in turn place the Inland
Northwest in southwest flow ahead of it. Forecast soundings show a
lot of dry air aloft and PWATs are relatively low so I don`t
expect much in the way of precipitation. 850 mb temperatures warm
back to near 20C by Thursday and Friday which will bring high
temperatures back to seasonal normals in the mid 80s to low 90s.
/Kalin


&&

.AVIATION...
12Z TAFS: VFR conditions expected at all TAF sites. Isolated
thunderstorms will be possible near the Canadian border late
this afternoon and evening. Drainage winds will be stronger
this evening at KEAT with gusts to between 20-25 kts possible.
/J. Cote`


&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        95  66  91  59  80  58 /   0   0   0   0   0   0
Coeur d`Alene  94  63  90  56  80  55 /  10   0   0   0   0   0
Pullman        94  59  89  51  79  51 /   0   0   0   0   0   0
Lewiston      102  69  97  62  87  61 /   0   0   0   0   0   0
Colville       96  61  93  54  81  52 /   0   0  10   0  10   0
Sandpoint      90  59  88  52  77  50 /  10   0  10   0  10   0
Kellogg        92  62  88  52  77  51 /  10   0  10   0   0   0
Moses Lake     99  63  94  58  86  57 /   0   0   0   0   0   0
Wenatchee      99  67  92  62  85  62 /   0   0   0   0   0   0
Omak           99  65  94  60  85  59 /   0   0   0   0   0   0

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...Fire Weather Watch from Saturday afternoon through Saturday
     evening for East Washington Central Cascade Valleys (Zone
     677)-East Washington Northern Columbia Basin (Zone 673)-
     East Washington Palouse and Spokane Area (Zone 674)-East
     Washington South Central Cascade Valleys (Zone 676).

&&

$$


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