Area Forecast Discussion
Issued by NWS Spokane, WA

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000 FXUS66 KOTX 190017 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 417 PM PST Tue Feb 18 2020 .SYNOPSIS... The remainder of the week will be dry with near average temperatures. We can expect a good deal of sunshine and relatively light winds through Friday. The arrival of a storm system over the weekend is expected to bring increasing chances for rain and snow, and gusty winds Sunday into Monday. && .DISCUSSION... Tonight through Wednesday: A weak upper wave and accompanying upper cold pool that extends west into the northern ID Panhandle continues to trigger some convective showers along its axis moving south through Benewah and Latah counties. Showers are expected to diminish later this afternoon. As the upper trough axis passes south into NRN OR-CNTRL ID, a stable northerly upper flow will follow as a sharp upper ridge begins to exert its influence and build over the Inland Northwest. Model guidance follows in relative lockstep presenting a quiet and dry air mass influencing eastern WA and northern ID for a brief time before the next upstream disturbance breaks into the region by weeks end. Northern Valleys will remain at risk for some patchy freezing fog but with the dryness across the region confidence is low as morning RH values hover around the low to mid 80 percent range. The Palouse and Columbia Basin could expect some Wednesday afternoon wind gusts varying between 12-18 mph, but winds will diminish quickly after sunset. The rest of the CWA will see relatively light winds under the dominant high pressure. Highs for the period will be mostly average in the upper 30s and low 40s. Overnight lows will be in the mid to high 20s for the Basin and mid to high teens for the Northern valleys and higher terrain. Thursday and Friday: Into Thursday morning, the risk for the development of morning lowland and valley fog and status does not abate under the stable umbrella of high pressure and mostly clear conditions. But a pattern change is in the works as the upper ridge begins to weaken and exit east across northern Idaho and then into western Montana. A southwest upper wind will then increase across eastern WA as a broad meridional upper trough, which has been staging over the eastern Pacific, splits the upper flow and forms a cut-off low off-shore the western US. The ensuing pattern change will in turn allow a resultant weak wave to track over B.C. and drag across northern WA. An increase in mid to high level clouds across the northern Cascade Crest will be the only likely outcome. As the wave quickly passes east and southeast, its exit will usher in a more zonal upper pattern ahead of the next weather producing system. /ayoung Friday night to Tuesday: the period begins with relatively quiet weather, then things become more active for the second half of the weekend; temperatures are projected to be near to slightly above seasonal norms. Friday night to Saturday the area sits in a westerly flow. A mid-level shortwave skims by the Canadian border while a stronger system approaches from the west. This will largely mean variable clouds, with some threat of mountain snow and a valley rain/snow mix within the Cascades. The remainder of the area stays largely dry until Saturday afternoon when a threat comes to north Idaho as the systems skim in. Between Saturday night and Sunday a more dynamic, organized frontal wave pushes through. The leading warm front comes in Saturday night and the trailing cold front passes sometime between Sunday morning and evening. The warm front carries a precipitation risk Saturday evening, with the Cascades having the highest potential. The broader threat comes with the cold front and upper trough couplet sometime between early Sunday morning and Sunday afternoon. In this regard with the most recent model runs there are timing disagreements. The EC/EC Ensemble/GEFS are 3-6 hours slower than the operational GFS, with the GFS showing peak precipitation earlier in the day and the others holding it off until later. There is still time for the models to come into better agreement in this respect. So stay tuned for fine-tuning. In the mountains largely expect snow, which could produce some impacts of moderate to heavy accumulations around the passes. In the valley there will be rain/snow potential, with snow more likely to mix in during any morning or night precipitation threat. Some accumulation still cannot be ruled out in the valleys too. The highest risk will be in the sheltered Cascade valleys and the northern mountain valleys, but even some areas along the rim of the Columbia Basin (including the Highway 2 corridor) and near the WA/ID border could see some accumulations if timing sets up right. There is a little bit CAPE mixed in during the afternoon which might support some embedded thunderstorms, though it is more apt to come out as locally heavier showers rather than produce lightning. That will also be monitored. Winds are expected to pick up through Sunday afternoon and evening on the backside of the system. I do not see them reaching advisory potential, but it is early and this could change. Early numbers suggest gust potential in the 20-30 mph range, especially in unsheltered areas and near the mountain peaks. From Sunday night into Tuesday the area transitions to a north- northwest flow, with the parent trough shifting through. The overall threat of precipitation starts to wane and retreat to the mountains Sunday night. On Monday with the unstable upper trough overhead I cannot entirely rule out some hit-and-miss pop-up showers in the instability over a broader area, similar to some recent graupel showers. Yet lapse rates do not look as unstable as yesterday. So the main risk will be in the mountains. Winds remain breezy from the west-northwest Monday, but speeds are expected to be much less than Sunday. By Monday night and Tuesday models show a ridge start to nose in and the main shower threat pushes to the ID Panhandle mountains and wanes. /Cote` && .AVIATION... 00Z TAFs: Widely scattered snow showers in the vicinity of Pullman, Lewiston, and St Maries will decay as the sun sets this evening. With temperature-dewpoint spreads between 10 and 20 degrees, a good deal of the precipitation being detected by radar is probably virga (sublimating before reaching the ground). By mid to late evening, mid-level subsidence under increasing high pressure will produce mainly clear skies and light winds overnight and into Wednesday. /GKoch && .PRELIMINARY POINT TEMPS/POPS... Spokane 19 41 20 43 24 45 / 0 0 0 0 0 0 Coeur d`Alene 15 40 18 43 21 44 / 0 0 0 0 0 0 Pullman 23 40 25 43 27 45 / 0 0 0 0 0 0 Lewiston 27 45 27 48 30 51 / 0 0 0 0 0 0 Colville 15 41 15 43 18 43 / 0 0 0 0 0 0 Sandpoint 16 36 17 40 20 40 / 0 0 0 0 0 0 Kellogg 18 38 18 40 21 41 / 0 0 0 0 0 0 Moses Lake 24 46 24 46 25 48 / 0 0 0 0 0 0 Wenatchee 25 42 24 42 25 45 / 0 0 0 0 0 0 Omak 22 39 21 40 22 42 / 0 0 0 0 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$

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