Spokane forecast discussion

Key Messages

into Friday morning. Periods of moderate rainfall expected. There is a 50 to 90 percent chance for at least a quarter of an inch of rain in the Spokane Area, Palouse, and across the Idaho Panhandle (see weather story graphic on our webpage: www.weather.gov/Spokane)

- Dry and windy Friday Okanogan Valley/Highlands, Waterville Plateau, and Upper Columbia Basin leading to elevated fire weather conditions, potential for patchy blowing dust, and isolated tree damage.

- Cooler temperatures Friday into next week.

Synopsis

Rain will increase over the eastern third of Washington into North Idaho by Thursday evening and continue into early Friday. Central Washington will miss out on the rain, which combined with dry and windy conditions on Friday will lead to elevated fire weather conditions and the potential for patchy blowing dust. Cooler weather is expected Friday, through the weekend, and into the following week.

Discussion

Thursday afternoon through Friday: Active weather pattern is in store over the next 24-36 hours. There are a couple of areas worth monitoring on the synoptic scale that will bring both a period of wet weather and dry/windy weather conditions. The first area is associated with a deep closed low pressure system currently situated over the state of California. This low pressure system is draw northward a deep fetch of moisture off of the Gulf of California across the Southwest. There is a robust sub-tropical jet streak on the eastern flank of the low with speeds of up to around 95-100 kts at 250 mb. The sub-tropical jet will supply referencing moisture into the region and some dynamics aloft, but the bulk of the dynamics for our region will come via a cold front being supplied by the Polar Jet and northern branch of the storm track.

The cold front will push across the Cascade Mountains after midnight tonight and strong frontogenesis will provide the forcing needed to squeeze out a good amount of moisture being advected in. The final part of the equation is the instability. Mid levels of the atmosphere will be moderately unstable with lapse rates of around 6.5 C/km. This will bring a convective element to the precipitation, especially this evening and then become less steep overnight as the regime transitions from showers to stratiform precipitation along the front. Clouds will increase this afternoon with an increasing chance for showers. A few of these showers may also include short- lived thunderstorms where there is a 10% chance over the Northeast Blue Mountains, L-C Valley, and Camas Prairie. Chances for rain increases to a near certainty by 8:00 to 10:00 PM generally along a line from Ritzville to Colville and points eastward. There's around a 70% chance of at least a quarter of an inch of rain and a 10-30% chance for spots receiving an inch or more of rain. This higher end of more than inch will more likely occur where we see bursts of moderate to locally heavy rain under convectively enhanced cells.

Impacts with the rain will be mainly beneficial. There is potential for water to pond in low lying areas, especially for any areas where leaves and debris hasn't been cleared around drains. Seeing that much of the area is currently under a severe drought currently, this rain will be very beneficial.

Along with that northern branch of the storm track and cold front will be gusty north winds down the Okanogan Valley and a punch of much drier air. Water vapor satellite shows a lot of drying that is occurring in subsidence region on the backside of the advancing cold front. This punch of dry air will push across the Cascades and into eastern Washington early in the morning on Friday (after 4:00 AM). This will go along with strong cold air advection and a packing of the isobars with a northerly gradient across north-central WA. Gusty north winds with gust up to 40-50 mph is expected down the Okanogan Valley, across the Waterville Plateau, and into the western portion of the Columbia Basin. A Windy Advisory remains in effect beginning a midnight and continues through 3:00 PM on Friday. Dew points will crash into the teens and potentially into the single digits along ridgetops for this portion of the forecast area as well. Winds will peak in the morning and then gradually decrease through the afternoon. There is a minor risk for fire spread in dry grasses and also for patchy blowing dust.

Friday night through Thursday: There is high confidence for temperatures to cool to right around normal with highs in the 50s. Low temperatures Friday night will be chilly cooling into the 20s across much of the region; the more sheltered northern valleys look to cool down into the lower 20s. Winds will be weakening overnight Friday and the dry air in place will promote strong radiational cooling at the surface. Doesn't look as cold heading through next week, but a lot of that will be dependent on the synoptic pattern and that still looks uncertain. About 60% of the model ensemble members supports an upper level low dropping south along the BC coast and ejecting across the region by late Sunday into early next week. This leading scenario would bring precipitation to much of the region, but overall accumulations would be light. Then there is around 40% of the model ensembles that has the low further off to the west with ridging of higher pressure to more dominate the region. This slightly lower probability of possible outcomes would support a milder and dry forecast in the extended. /SVH

Aviation

00Z TAFS: Showers are currently stretching across the Columbia Basin. TAF sites currently impacted are KLWS-KPUW- KMWH. Showers will continue to move north into the KGEG-KSFF- KCOE area. Elevated instability does bring the potential for embedded moderate rain intensities which could briefly lower visibility to MVFR before 03z. After 03z, probabilities increase with time reaching 50% at KPUW/KLWS between 04-07z and 20-30% KGEG-KCOE closer to 09z. VFR skies are expected across Central WA for KOMK/KEAT/KMWH/KCQV. Northwest winds will increase across the Cascades after 03z and become gusty in the evening. Stronger northerly winds will develop in the Okanogan Valley between 07-12Z and spread toward KMWH. Local gusts around 45 mph expected at KOMK.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in VFR conditions through 20z Thursday. Greatest uncertainty is timing the initial cluster of rain and associated impacts (gusty outflow winds / lower visibility from moderate rain intensities). Models show a wide range of timing and track of this feature which could come in as far west as KMWH or east around KLWS/KPUW. Once rain settles in, the main uncertainty comes with precise ceilings developing after 05z for KGEG/KSFF/KCOE/KPUW/KLWS. There is 10-20% chance for IFR conditions to develop for several hours. /SVH

-----------------------

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

Preliminary Point Temps/Pops

Spokane        42  57  32  55  35  57 /  90  10   0   0   0   0 
Coeur d'Alene  43  56  33  55  34  57 /  90  30   0   0   0   0 
Pullman        40  52  28  54  32  58 / 100  30   0   0   0   0 
Lewiston       47  57  35  58  36  62 / 100  40   0   0   0   0 
Colville       36  59  22  57  26  57 /  60   0   0   0   0  10 
Sandpoint      44  55  31  53  31  54 /  90  30  10   0   0   0 
Kellogg        43  50  32  53  34  57 / 100  60   0   0   0   0 
Moses Lake     39  63  28  57  31  59 /  10   0   0   0   0  10 
Wenatchee      42  61  37  55  39  56 /   0   0   0   0  10  10 
Omak           41  60  30  54  33  57 /  10   0   0   0  10  30 

Otx Watches/Warnings/Advisories

WA...Wind Advisory from midnight tonight to 3 PM PDT Friday for Okanogan Highlands-Okanogan Valley-Upper Columbia Basin- Waterville Plateau. ID...None.