791 FXUS66 KOTX 182348 AFDOTX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SPOKANE WA 345 PM PST Mon Feb 18 2013 .SYNOPSIS...The weather pattern will become increasingly active over the Pacific Northwest this week. Tonight into Tuesday, an upper level disturbance will bring a chance for accumulating snow to portions of southeast and east central Washington, as well as the Idaho Panhandle. Wednesday should be a relatively quiet, but cool day. Look for increasing clouds and the threat for some light snow on Thursday. A big storm will potentially bring strong winds to the region on Friday into Saturday along with heavy mountain snows. && .DISCUSSION... ...LOCALIZED SNOW ACCUMULATIONS TONIGHT AND TOMORROW IN SOUTHEAST WASHINGTON AND SOUTHERN IDAHO PANHANDLE...AND A THREAT OF WINTRY CONDITIONS RETURNING FRIDAY INTO EARLY SATURDAY... Tonight: An upper level trough will near the WA coast while a low level occluded front will push east of the Cascades. Cloud cover will continue to thicken and lower this evening while the best chances of precipitation will lie across the northern mountains as a weak upper level shortwave presses east along the Canadian border. Dry low levels will give way to areas of virga across the Columbia Basin this evening as seen by the increase in radar returns but lack of ground reports. But overnight as the front pushes across eastern Washington and easterly winds lighten up, anticipate a band of light snow to develop with minor accumulations of under an inch spanning from Blue mountains, Palouse, Spokane/Coeur d'Alene area into northern Idaho. Tuesday and Tuesday night: The upper level trough will continue to deepen as energy dives south into California. This will slow the eastward progression of the frontal boundary and stall it out near southeast Washington and southern Idaho panhandle. Meanwhile, the models show a weak upper level shortwave shooting from the parent low and across northeast Oregon to southeast Washington. The combination of the low level deformation band left by the decaying front and this upper level lift has the potential to lead to intense bands of snow that will persist through the day on Tuesday. Then by Tuesday night, the upper level trough axis shifts over the region enhancing the lift while low level wind flow shift northwest and enhance the upslope flow. Confidence is lowered by the placement of the these snow bands and specifics amounts. But after blending the NAM and GFS qpf amounts, have indicated the most likely area will stretch from the Palouse region into the central Idaho panhandle where 1 to 3 inches of snow are possible and south from the Blue mountains to the Camas prairie where 2 to 4+ inches are possible. Snow chances will decrease rapidly to the north and west in the wake of the upper trough axis. Temperatures will remain cooler than normal. /rfox. Wednesday and Thursday: Wednesday has the potential to be a relatively quiet weather day during an increasingly active stretch of weather. There is good agreement between the NAM, GFS and Canadian models that eastern Washington will be between systems on Wednesday with a weakening trough axis over western Montana and a series of storm systems over the northern Pacific. By Thursday, moisture will be on the increase. The medium range models are in fair agreement that a low amplitude shortwave will move through the Inland Northwest Thursday. This upper level system will not pack the same punch as storms expected Friday and into the weekend. Snow levels will be low enough for snow to the valley floors on Thursday and Thursday night, but at this time, accumulations appear to be light. /GKoch Friday through Saturday night: Models remain in good agreement with the depiction of a robust, wintry system moving into the region this period. Energy now coming off the Asian continent tracks into the Gulf of Alaska later this week, deepening toward the Pacific Northwest as a 140kt+ jet digs down its backside. The parent low occludes near the B.C. coast Friday morning. While a weaker disturbance moves east from the region early Friday, the warm front and accompanying subtropical moisture plume leading the strong system comes in from the west. Models still disagree over the precise timing. The GFS envelopes the region with modest isentropic lift and moisture earlier Friday, while the ECMWF holds it back until late Friday and offers a shorter period of isentropic lift, before the cold/occluded front passes Friday evening into Friday night. The forecast leans toward the slower ECMWF, which has had better continuity and carries some agreement from the GEM. Overall this means precipitation chances rising through the day Friday, peaking Friday evening with the incoming cold front and supporting mid-level trough, diffluence and jet dynamics, before waning overnight. As for snow levels and precipitation-type, some models are warmer today than this time yesterday. However this largely comes from the more amplified GFS. There is about a 2000 foot difference in snow levels between the ECMWF and GFS by Friday afternoon. This degrades confidence in the precise impacts a little. Yet when it comes to surface temperatures models are more consistent in showing highs in the mid to upper 30s across a large part of the CWA, with some 40s to lower 50s in the Basin/Wenatchee area and L-C Valley. Tack on the potential PWATs around 0.45-0.55 inches where it is difficult to expect just snow starting around this level, suggests favoring higher snow levels. This lends a forecast with rain or mix of rain and snow below 2000-3000 feet, changing to snow through the evening. This means impacts during the day and at least the early commute time should be limited. All snow is expected generally above 3000 feet. With this snow, the biggest possible impacts at this time should be around the mountain zones and Camas Prairie, and secondarily over the Palouse/C'dA area and thirdly over the Spokane area and areas into the Basin along and north of Highway 2. The potential remains for moderate to heavy accumulations above 3000 feet starting around midday to early afternoon Friday, while light to moderate accumulations will threaten the other areas outlined above toward evening. The other aspect of the storm system will be winds. As a sharp shortwave and cold front slices through the region, mixing with higher levels will bring some breezy to windy conditions. They are expected to begin to increase through the late morning to afternoon hours Friday as the front approaches, but the strongest winds are expected Friday evening into Saturday morning. Some guidance is showing winds around the around the 900-850mb level (500-2500 feet AGL) between 30 and 50kts. How effectively this is mixed down to the surface remains to be seen during. Guidance shows mixing heights not really reaching well into this lay until after the precipitation has wound down later Friday evening and by then this region of winds has shifted out. Mountains, on the other hand, will still have the opportunity be directly impacted being more exposed. Blowing snow may not be a large impact outside of the mountains given that the snow looks to be more on the wet-side at this time. As the system continues to move east on late Friday night into Saturday a secondary low develops over northwest to north-central Montana. An inverted surface trough deepens and trails west through north Idaho and southwest into eastern Washington, until shortwave riding briefly builds in Saturday night. Moisture wraps around the exiting low into the aforementioned inverted trough, with conditionally unstable lapse rates around 7 C/km and pockets of CAPE with the mid-level trough. Combined with embedded vorticity lobes dropping through in the northwesterly flow, this will keep precipitation chances high across through the eastern third of Washington, as well as near the Cascades. Snow looks like the predominant weather type, with some rain or a rain/snow mix in the afternoon. Look for continued breezy to windy conditions, especially in the morning, before gradients and mixing slackens through the afternoon and into Saturday night./J. Cote' Sunday and Monday...The active pattern continues, with yet another wet weather system to affect the Inland Northwest early next week. The problem is with the timing. The features on this system look similar to the Friday/Saturday system, with perhaps a little less moisture. The track is similar, meaning that a good chance exists for precipitation at many locations, save for parts of the Columbia basin. However, with the extended models differing on timing that is wide as the state of Washington, confidence to focus one particular period for higher precipitation chances was not high. The one area that precipitation chances were increased slightly was over the Idaho panhandle late Sunday into Monday. If the faster GFS verifies, the panhandle will be in the post-frontal up-slope convective region (it is -37 degrees Celsius at 500 mb). But if the ECMWF verifies, the brunt of the storm will be affecting that region during that time frame. Lots of time to look at this and fine tune the details. ty && .AVIATION... 00Z TAFS: Low pressure digging off the coast will eject a weak upper level disturbance into eastern Washington and north Idaho tonight. A tough and low confidence set of TAFs especially for Spokane, Coeur D'Alene, Lewiston and Pullman. Steep lapse rates to almost 20 thousand feet will contribute to convective (or showery) bands of snow tonight and potentially into tomorrow. The NAM and GFS forecast bands of light to moderate snow showers spreading over southeast Washington and north Idaho beginning around 09z. Localized pavement accumulations will be possible and could impact the aforementioned airports. But given the hit and miss nature of convective snow showers, it is tough to get too specific with snow amounts, visibilities or ceilings. Frequent amendments may be needed from early Tuesday morning onward. /GKoch && .PRELIMINARY POINT TEMPS/POPS... Spokane 28 38 25 38 25 38 / 60 50 10 10 0 40 Coeur d'Alene 28 38 26 37 27 38 / 60 50 40 10 0 40 Pullman 30 40 28 39 27 41 / 60 60 60 20 0 30 Lewiston 32 45 32 45 30 45 / 50 60 60 10 10 20 Colville 26 41 26 41 24 40 / 60 20 10 10 10 50 Sandpoint 27 38 24 36 28 37 / 60 50 30 20 10 50 Kellogg 28 36 28 35 29 35 / 30 50 60 40 10 50 Moses Lake 26 42 24 44 26 44 / 20 10 0 10 0 20 Wenatchee 27 42 26 44 27 42 / 20 10 10 0 0 10 Omak 24 37 21 41 21 37 / 60 10 0 0 0 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$