453 FXUS66 KOTX 211744 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 945 AM PST Thu Feb 21 2013 .SYNOPSIS...An upper level disturbance will bring some light snow to the Idaho Panhandle and portions of Northeast Washington today. A much stronger storm will plow into the Pacific Northwest on Friday and Friday night bringing windy conditions and heavy snow to the mountains. Mountain snow showers will likely linger into Saturday afternoon. A break in the active weather is expected on Sunday, before another frontal system brings increasing chances for rain and snow to the region on Monday. && .DISCUSSION... Today and tonight: A well-defined vort max will accompany a very swift moving frontal passage this morning, with a few inches of snow expected in mainly the mtns of NE Wa and the Id Panhandle. Though the low- level flow backs briefly to the SW, there's not a whole lot of waa/implied isentropic ascent to produce any heavy snow threat. However, strengthening westerly upslope into the Id Panhandle will translate into a few inches of snow especially for elevations above 3000 feet. The beginning of a pattern change to a much wetter, snowier, and windy fcst will begin with the first initial onslaught of strengthening west winds Thurs Nt. This will help maintain the briefly decreasing light snow threat for the Id Panhandle Thur Nt. It's late Thurs Nt that this flow will rapidly back to the S/SW ahead of the strong cyclone moving down the BC coast that will begin to ramp up the deep and strong ascent over the warm front that will be near the Cascades by late Thurs Nt. bz Friday and Saturday: A potent winter storm system will impact the Pac NW bringing a mix of rain and snow to the valleys and potential for moderate to heavy snow in the mountain. The storm system will also bring breezy to gusty winds across a majority of the Basin...Palouse...West Plains and exposed ridgetops. Models are generally in good agreement with the large scale features but subtle differences do still exist with the related to the timing of rising snow levels Friday morning...and snow amounts within the convective post-frontal air mass Friday night and Saturday. The storm system of interest is currently visible on this evening's Water Vapor imagery as a sharp area of darkening off the western tip of the Aleutian Islands. This storm system will deepen quite a bit over the next 24-48 hrs and dive southeast into the Northwestern US. A rich fetch of 1.00" precipitable water will be driven toward the region ahead of a strong warm front leading to increasing precipitation Friday morning and afternoon as the moisture rises along the 285-300K isentropic surfaces. Precipitation will be at it highest intensity Friday afternoon into the early evening as lift becomes enhanced by the approaching cold front. Some clearing will be possible behind the front but the air mass will become increasingly unstable Friday night and Saturday within the cusp of the upper-level trough leading to continued mountain snow showers and hit or miss valley snow showers. Some of this activity will be heavy at times and the most pronounced orographics will focus over the Idaho Panhandle and along the immediate Cascade Crest. We will be closely watching for a Puget Sound Convergence Zone bringing burst of snow over the crest into locations like Plain, Lake Wenatchee, and Stehekin. Precipitation is expected to start off as snow for most lowlands north of I-90 and across the Palouse. As warmer air, associated with the subtropical moisture fetch, punches northward within a 40-50kt low level jet, snow levels will be rapidly rising near 3000 feet and most valleys will transition to rain or at least ...non-accumulating snow. Exactly when this transition occurs for locations along Hwy 2 and northward carries considerable uncertainty and could have large ramifications on valley snow amounts. Given the speeds within the LLJ and daytime timing...a non NAM solution was preferred which brings the transition into Spokane-CDA around late morning and toward the Canadian Border early afternoon. The main changes that we made to the previous forecast were to increase snow amounts in the Idaho Panhandle Mountains, including IDZ001. As a result, we have included this zone into the current watch which was already in place for the Central Panhandle Mtns. There is a strong possibility that winter highlights will be necessary for the Northeastern Mtns of WA, Cascade Crest, Camas Prairie, and perhaps the Okanogan Highlands but amounts/coverage were lower and did not warrant winter storm criteria...and we will cover this with the HWO at this time. We also expanded the watch for the Northern Blue Mtns through 11PM Sat night as post frontal orographic flow orients to the west/northwest during this time. Otherwise, inherited snow amounts were generally on track with the heaviest amounts focusing over the Central Panhandle Mtns and Blue Mtns. The storm system will also bring the likelihood for breezy to gusty winds with our confidence low whether the 40-50kt LLJ will be able to surface to valley floors. Typically...these winds remain just off the surface under such stable profiles and Bufkit data would agree with gusts largely in the 30 to 35 kt range. However, this is a rather strong cold front...vertically stacked from the surface to 500mb and this is worrisome. The track of the main surface low (southern Alberta) along with a 16-18mb pressure gradient expected between Portland and Kalispell is a common pattern for wind highlights, mainly advisory speeds, across the CWA and this is almost a certain. We will be taking closer look into the higher resolution guidance tomorrow to determine if stronger gusts will surface with the passage of the front between 00-06z Friday evening to assure advisory speeds is all we can expect. The weather will remain active following this storm system with at least three more storm systems through next week. /sb && .AVIATION... 18Z TAFS: A TROF is currently moving through the region. The eastern TAF sites can expect to have MVFR to IFR ceilings throughout the next 24 hours with periods of snow that will drop vis around MVFR. KEAT and KMWH can expect mid to high level clouds to continue with MVFR to VFR conditions. KLWS will be impacted by low to mid level clouds with periods of rain showers. && .PRELIMINARY POINT TEMPS/POPS... Spokane 37 31 42 29 40 24 / 80 50 100 90 30 20 Coeur d'Alene 36 31 41 29 38 25 / 90 70 100 100 40 30 Pullman 38 32 43 31 39 25 / 90 60 90 90 50 30 Lewiston 44 35 51 34 47 29 / 20 20 60 80 60 30 Colville 39 32 40 24 44 24 / 50 40 100 80 20 20 Sandpoint 33 31 40 29 38 25 / 100 80 100 100 50 30 Kellogg 36 31 40 29 37 26 / 100 100 90 100 70 40 Moses Lake 45 30 52 31 51 26 / 10 10 70 30 10 10 Wenatchee 41 31 47 30 46 26 / 10 10 80 30 10 10 Omak 36 28 41 22 41 20 / 10 10 90 30 10 10 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Winter Storm Watch from Friday afternoon through Saturday afternoon for Central Panhandle Mountains-Northern Panhandle. WA...Winter Storm Watch from Friday morning through Saturday evening for Northeast Blue Mountains. && $$