985 FXUS66 KOTX 062146 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 246 PM PDT Sat Apr 6 2013 .SYNOPSIS... Cool, breezy, and showery weather will occur through the weekend...with significant snow accumulations likely for the Cascade passes. There may even be some thunderstorms this evening over the Idaho Panhandle and far eastern Washington. The next break in the active weather pattern will likely occur on Tuesday, but showery and breezy weather will probably return on Wednesday and Thursday. && .DISCUSSION... Tonight: The forecast remains on track as far as the convective pcpn coverage and winds, as well as snow amnts, and we shouldn't need any major changes for this evening. The biggest player as far as pcpn is the vort max/short wave trough lifting N/NE across Nrn Wa, which is still expected to move into Srn BC shortly. We earlier expanded the isolated thunder threat a bit west into the Cascades, closer to the upper vort max and its accompanying cold pool aloft with -30c at 500mb. These cool temps aloft, combined with peak insolation/afternoon heating on the back side of the vort max, should combine to produce moderately steep lapse rates which will play a role in the shower/thunder threat the next couple hours. Inspection of all soundings show a solid thunder threat closer to the BC border based on CAPE lyr depth and LFC to EL temps, but becoming very marginal just to the south. We lowered snow levels to take into account the possible graupel/soft hail (often reported as snow showers). Accumulations will be very brief, if at all with these showers, and isolated. Wind will be the primary concern with these showers given the lack of veering with height, with 850-700mb winds SW at around 40kt. Transfer to the sfc of these winds should be efficient. Local gusts to 40 mph seem possible...especially on exposed ridges across the north Id Panhandle into early evening. For late tonight: We'll be focusing on snow amnts and snow highlights for locations closer to the Cascades...affecting mainly the passes. See rest of discussion below... Sunday... *Precipitation: A 998-1003mb sfc low will track northeast from southwest Washington Sunday morning towards the Northeast corner of Washington in the afternoon. Strong isentropic ascent on the north side of this low track will result in a band of moderate rain and mountain snow. There is good agreement between the GFS, ECMWF, GEM models of a wide swath of a quarter to a half inch of liquid precipitation north of Interstate 90 with generally a tenth to a quarter inch south of I-90. *Mountain Snow: Moderate snowfall above 3000 feet Sunday morning along the East Slopes of the Cascades will make for difficult travel over the mountain passes. This includes Stevens, Blewett, and Loup Loup. A winter weather advisory has been issued. Mountain snow will spread across the northern Washington and North Idaho Panhandle Mountains in the late morning and afternoon. This could result in difficult travel as well but with later timing arrival snow may have a hard time accumulating on roadways given time of year. Sherman Pass however given high elevation of around 5500 feet will have the best chance of accumulating snowfall in the afternoon. *Wind: The surface low track will be favorable for an increase in winds on the Palouse, Blues, Ritzville, and Pomeroy areas Sunday afternoon as the surface low tracks north of these areas and 850mb winds increase to 25-35 knots. This will favor windy conditions with much lighter winds north of these areas. *Thunderstorms: Models show marginal instability for thunderstorms Sunday afternoon around Moses Lake, Spokane, Coeur D'Alene, and Pullman areas. If the rain band lifts north of these areas in the afternoon and sun breaks materialize, an isolated thunderstorm is possible. However with low confidence in thunderstorms due to expected cloud cover left out of forecast. Sunday night through Monday night...The surface and upper low will rapidly weaken and fill Sunday night into Monday as the upper level jet drops south into California. However a cold pool will linger with 500mb temperatures around -28C triggering instability showers. A left over deformation zone will still be something to watch however that may provide an area of enhanced precipitation. Previous GFS runs were much more aggressive with this...with the new 12z GFS/ECMWF model runs keeping this area over southern British Columbia extending back towards the Northern WA Cascades. Given model variability with this feature did not make any big forecast changes. A short wave ridge will move over the area Monday night for dry weather after the instability showers wane. JW Tuesday through Friday: This period will start off under a ridge of high pressure across the Pacific northwest...sandwiched between an anomalously deep and cold trof over the four corners region of the US and a long wave and relatively stationary circulation over the mainland of Alaska. During the week the trof to our east will migrate across the central US allowing the ridge to break down. The flow over the Pac NW becomes zonal as the enormous Alaskan circulation very slowly fills and drifts down the panhandle. This regime will usher a series of disturbances out of the base of the trof in our general direction. Timing of these disturbances late in the week is very difficult to pin down although confidence is high temperatures will remain slightly below normal during this period. Precipitation chances looks most likely Wednesday night before off and on chances through the rest of the work week. Updates to the forecast in this time frame were primarily to downplay the chance of precipitation Tuesday and then bump up precipitation later in the week. /AB && .AVIATION... 18Z TAFs: A pair of storm systems will progress through the region today and tonight. The air mass east of a line from KOMK to KGEG will become unstable with scattered to numerous showers and isolated thunderstorms. A break period will ensue after sunset until around 08Z-10Z when a warm front from the next incoming system will begin to spread rain from south to north...mainly threatening the KEAT...KLWS...KPUW and KMWH TAF sites...with the best chance for MVFR ceilings before 12Z at KEAT where upslope into the Cascades will enhance cloud cover. && .PRELIMINARY POINT TEMPS/POPS... Spokane 36 47 34 50 34 54 / 20 100 50 30 20 10 Coeur d'Alene 35 46 33 50 31 54 / 20 100 60 40 20 10 Pullman 38 50 34 50 33 53 / 50 100 50 50 20 10 Lewiston 44 55 39 55 36 58 / 50 70 50 40 20 10 Colville 35 50 33 55 30 58 / 30 100 70 40 20 10 Sandpoint 35 43 32 47 30 52 / 30 100 70 40 20 10 Kellogg 37 44 31 43 30 50 / 40 100 70 60 30 10 Moses Lake 40 54 35 59 33 61 / 50 100 50 20 10 0 Wenatchee 39 50 38 56 37 59 / 60 90 40 20 10 0 Omak 35 48 33 58 32 60 / 10 90 60 30 10 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Winter Weather Advisory from 2 AM to Noon PDT Sunday for East Slopes Northern Cascades. && $$