667 FXUS66 KOTX 141238 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 422 AM PST Thu Feb 14 2013 .SYNOPSIS... A weak warm front will cross the region today delivering a small chance of light precipitation...mainly to the Cascades and Idaho Panhandle. High pressure will return to the region on Friday bringing dry conditions and possibly some fog. A low pressure system will bring slightly cooler and more unsettled conditions beginning this weekend. A stronger disturbance is expected to move into the region late Monday and Tuesday resulting in the threat of snow for many valley locations. Cool and unsettled weather will continue through the middle of next week. && .DISCUSSION... Today through Friday...This should be a quiet period as upper level ridge become reestablished over the PacNW. Water vapor loops are placing the ridge axis around 132w...which is in good agreement with where model guidance places it. Despite the presence of the ridge...there is a weak warm front riding over its eastern flank. While the models are placing this mid-level moisture fairly well (SW BC into northwestern WA), they differ quite a bit on the resultant QPF fields. The western Washington radars are depicting some light precipitation over the southern tip of Vancouver Island and into the Olympics...but little if anything is occurring south of there. The NAM says precipitation should be occurring to the mouth of the Columbia River and even into the southern Washington Cascades. This is far too wet and widespread. The GFS is doing a little better...but it too spreads precipitation too far east. Looks like the winner is the ECMWF and its precipitation forecast will be followed for today. The front shows up fairly well on the 285-295k isentropic plane and suggests the front will track across the northern Washington Cascades by late morning. This should spread light snow into the Cascades...but not sure it will make it far past the crest as its expected to weaken rapidly with time...while NW mid-level flow will lead to at least some downslope drying. Also working against the front will be a fairly dry air mass currently in the lee of the Cascades. As of 2am...the dewpoint spread in Wenatchee was 14 degrees with a similar spread reported in Stehekin. So it looks like the only threat of precipitation this morning will occur near the Cascades and possibly into the Okanogan Highlands. While we can't entirely rule out precipitation in the Okanogan and Wenatchee Valleys...it would be very light and consist of flurries or sprinkles. By afternoon the front is expected to move to the Washington/Idaho border...but by that point the isentropic ascent and front is quite weak. Whether or not the front will contain enough moisture/ascent for precipitation will be the big question. The SREF and GFS would say yes...while the ECMWF and surprisingly the NAM say no. Based on the current state of the front...we will hedge toward the drier solutions...but keep a small chance of light rain/snow over the Idaho Panhandle and the NE corner of Washington. QPF amounts if they occur will be less than .05 inches. For tonight and Friday the upper level ridge will embark on a slow eastward migration. This will dry out the area with low-level inversions likely forming. The NAM shows the strongest moist inversions over the northern Columbia Basin and Okanogan Valley. Fog will be a distinct possibility beginning late tonight and continuing into early Friday afternoon. Temperatures could struggle to escape the 30s unlike the past several days where 40s and even some lower 50s have been commonplace. fx Friday night through Monday...Models are in good agreement during this portion of the forecast with an upper level trough swinging through the region on Saturday...followed by a short wave ridge late on Sunday...followed by another trough approaching late on Monday. High pressure will shift east Friday night allowing an upper level trough to sweep across the region on Saturday sending a cold front across the area. Models show fairly decent lift reaching the Cascades around noon on Saturday...reaching Eastern Washington and the Idaho Panhandle Saturday afternoon...and finally southeast Washington and the Lewiston area Saturday evening. Models also agree of pre-frontal 850mb winds out of the southwest at 10-20 knots which will result in downslope flow off the Cascades...while providing orographic lift into the Cascade crest and mountains of northeast Washington and North Idaho. Good agreement exists between the NAM, GFS, GEM, and ECMWF of one to two tenths of liquid precipitation for the Northeast Washington Mountains and Idaho Panhandle...with up to a tenth for the Spokane area and Palouse, with little to no measurable rain in the immediate lee of the Cascades such as Wenatchee, Moses Lake, and Omak. Snow levels when most of the precipitation falls (afternoon and evening) will be around 3000 feet near the Canadian border and 4000 feet for southern areas (Blue Mountains and Camas Prairie). Thus valley accumulations are not expected with the front passage...with generally 1-3 inches for the mountains. Snow levels crash down to the valley floors overnight on Saturday behind the front but drier air and downslope northwest flow off the Cascades should quickly be ending the precipitation threat except for lingering upslope snow showers over the Camas Prairie and Idaho Panhandle into Sunday. A short wave ridge will move across the region Sunday night with the next trough approaching late Monday. JW Monday night through Wednesday...A low off the Pacific Coast will continue to move through the region by way of the Oregon Coast. The models are handling the location and timing of this system well. The GFS is moving the moisture associated with Low further North and pushing it through quicker than the ECMWF as the system moves through the region on Tuesday. With a few more model runs a clearer picture should develop. A rain snow mix is expected in the lower elevations and will transition to snow as temperatures cool through the night. Elevations around and above 3000 feet will receive snow. Precipitation amounts are expected to be light as the bulk of moisture associated with the Low is passing South of the Pacific NW. Remnants of the system will continue to impact the Idaho Central Panhandle throughout Wednesday while Eastern Washington begins to be affected by a ridging pattern. Temperatures will be around seasonal normals for this period. /JDC && .AVIATION... 12Z TAFS: Hi and mid level clouds quickly streaming in from the west which should mitigate the chances for fog at any of the TAF sites this morning. Although there still is a small chance of it forming near GEG SFF COE and PUW through sunrise based on very small dewpoint spreads...the risk wasn't great enough to place in forecast. For most of the day...the main trend will be a thickening and lowering cloud trend as a warm front moves into E WA later today. Cigs should remain well in the VFR category. That notion could change overnight as the mid-level cigs track out of the region and strong high pressure builds in. The result will be strengthening inversions and the psblty of IFR conditions due to fog/stratus. Confidence isn't high and the MOS is split on whether it will form aft 06z Fri. Although we are fairly confident it will form somewhere over the Inland NW...the confidence isn't quite as high that it will impact one of the fcst sites. MOS guidance is split on whether it will form or not...but lately any ridge has resulted in a rapidly forming stratus deck and we will continue with this trend. Best chances...as always this time of year will be for GEG. fx && .PRELIMINARY POINT TEMPS/POPS... Spokane 38 27 40 30 42 28 / 10 0 0 0 50 50 Coeur d'Alene 39 28 40 29 42 29 / 10 10 0 0 80 70 Pullman 42 31 47 32 46 30 / 10 10 0 0 50 60 Lewiston 51 35 52 36 52 35 / 0 0 0 0 20 60 Colville 41 26 39 30 43 30 / 20 0 0 10 60 50 Sandpoint 38 28 38 29 40 30 / 30 20 0 10 90 70 Kellogg 39 29 42 29 43 30 / 20 20 0 0 70 80 Moses Lake 48 28 45 29 47 28 / 10 0 0 0 20 10 Wenatchee 44 30 44 31 46 30 / 10 0 0 0 20 10 Omak 38 23 34 28 40 25 / 10 0 0 10 20 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$