Sunnyslope, WA Weather

Key Messages

- Active weather pattern with chances for precipitation increasing on Monday and chances continuing throughout the work week.

- Snow levels lowering to between 3,000 and 4,500 feet with snow in the mountains by Tuesday night and continuing through Thursday. Winter travel over mountain passes in the Cascades to Sherman Pass in north-central WA will be possible.

Synopsis

The weather pattern turns more active next week with a pair of low pressure systems impacting the region. The first weather system moves across Sunday night through Monday night and is expected to bring light precipitation, especially across southeast WA and into the ID Panhandle. A second, colder weather system for Tuesday night through Thursday night will drop snow levels to as low as 3000 feet over the Cascades and north-central WA and down to around 4500 feet in the ID Panhandle. Valley rain and mountain snow is expected with the potential for travel impacts over mountain passes.

Discussion

The weather pattern will undergo changes this week as winter finally begins to rear its head in the mountains over the Inland Northwest. A stalled out occluded front over the Cascades is already providing a taste with snow observed on the Stevens Pass WSDOT camera. Temperatures over the pass are sitting right around freezing with snow melting on contact with the road surface. This initial band of precipitation is associated with a tightly wound, vertically stacked are of low pressure spinning just off of the Washington coastline. This will be the first low pressure system that impacts the region. Models depict a second, nearly identical looking low pressure system that takes the place of this current one in the middle of the week. It will be an active week with the main challenge being snow levels and snow accumulations in the mountains.

Rest of today through Tuesday: The low pressure system will spin offshore over the next 12-15 hours. A potent shortwave disturbance rounding the low will act to back the flow pattern over the Northwest turning more southerly into tonight. This will have the effect of entraining drier air over the northern Cascades with precipitaiton decreasing through the morning hours on Monday before the low tracks east roughly along the OR/WA border. The shortwave round the low at this same time will supply a healthy dose of dynamics that will fuel frontogenesis lower in the atmosphere. The associated deformation band will strengthen and swing through from south to northeast across the area Monday afternoon/evening. Snow levels only drop to between 4500 to 5500 feet with this band and will bring mainly valley rain and high mountain snow. Best dynamics will occur across southeast WA into the Idaho Panhandle and generally miss our highest mountain passes of Washington Pass in the North Cascades and Sherman Pass. Little impacts from precipitation is expected and will bring much rainfall. A modest cold pool of -23 to -25 C at 500 mb over northeast OR into the southern ID Panhandle will destabilize mid levels for a 10-20% chance of thunderstorms primarily in the Northeast Blue Mountains to the Camas Prairie. Wrap around moisture that feeds into the low pushing into western Montana will continue to result in showers across extreme eastern WA and into the ID Panhandle Monday night into Tuesday.

Tuesday night through Thursday: This is the part of the forecast period when the second low pressure system will deepen along the west coast of WA. Reinforcing cold air dives south with the low and will push across the Cascades by Tuesday evening. This will cool off thermal profiles for snow levels to drop further down to 3,000 feet. A shortwave rounding at this time will ramp up precipitaiton over the Cascades, along with moist upslope on the western slopes that enhances precipitaiton over the Cascade crest. Mid levels also steepen quite a bit with the passage of shortwave and frontal band. Strong westerly flow will bring with it the potential for a convergence zone and the increasing instability will increase the risk for bursts of heavier snow into Wednesday morning. The Cascades mountains, and the crest in particular, will easily see the potential for 3 to 8 inches of snow, and near a foot of snow isn't of the question where the convergence zone sets up. The limiting factor for snow accumulation over Washington Pass and Stevens Pass will how quickly snow levels fall and temperatures cool. Road temperatures will start out warm, but heavier intensity snow will have a chance to overcome this. The NBM is showing fairly low probability of seeing 6 inches of snow Tuesday night into Wednesday morning for these passes, but snow covered roads and winter travel will definitely be in play and potential risk for travelers. Sherman Pass is also expected to receive snow Wednesday morning with the frontal band with around 2 to 4 inches expected.

Thursday night through Saturday: Drier air moves over the region Thursday night into Friday and this is expected to result in a decreasing trend of precipitation. The closed upper level low pressure system becomes cut off and remains in close proximity just west of the PNW. There is less confidence in the forecast details this far out in the period. Temperatures will be near normal and snow levels remaining right around 3,000 feet. There will continue to be a risk of disturbances rounding the low with precipitation at times that would bring valley rain and mountain snow. /SVH

Aviation

18Z TAFS: VFR conditions are expected at all terminals through the forecast period. As a low pressure approaches the Washington coast, winds will mostly be out of the northeast and east. Mid and high level clouds are expanding across the region. Chances for rain increase from west to east beginning around 00Z for EAT, and around 15Z for MWH. Showers may begin earlier for EAT as radar echoes are increasing around the terminal but no rain has been reported. Current high- res models show chances for rain at GEG, SFF, COE, PUW, and LWS increasing right around the 18Z mark, with around a 15-20 percent chance of precipitation starting around 16Z. Not confident enough to put in as PROB30 but worth mentioning if rains move in earlier than that. Chances for low cloud coverage increase with rain chances right around the end of the TAF period as well.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions at all TAF sites. Highest impact alternate scenario would be EAT seeing rain earlier than 00Z, which would result in an amendment. Otherwise, moderate confidence in timing of precipitation. /AS

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Confidence descriptors: Low - Less than a 30 percent chance Moderate - 30 to 70 percent chance High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports, please refer to the Aviation Dashboard on our webpage: https:/www.weather.gov/otx/avndashboard