Marine Weather and Tide Forecast for Lewiston, ID

SYNOPSIS
Scattered showers and isolated thunderstorms will continue over portions of Central and Northeast Washington through Wednesday evening. Warm and windy Thursday will lead to elevated to critical fire weather concerns. Bigger changes arrive on Friday and continue into the weekend as a weather system delivers much cooler temperatures, gusty winds, and rain showers for much of the region.

DISCUSSION
Tonight into Thursday: A mid level wave over the Cascades this afternoon will shift into eastern Washington and north Idaho tonight. This will result in scattered showers and isolated thunderstorms over the Cascades and northern mountains with a 20 percent chance of showers developing over the Upper Columbia Basin into the Spokane/Coeur d'Alene area. With limited instability with CAPE of 300-600 J/KG the coverage of storms are expected to remain isolated and dissipate around sunset. A dry sub cloud layer initially will support enhanced downdrafts producing gusty outflow winds with several CAM's models showing 30-40 MPH gusts. With precipitable water values increasing to 125-150 percent of normal brief downpours are also expected with storms. Yet with dry fuels new fire starts are possible, especially with any strikes that occur outside of the rain cores. The remnants of this convection manifest into an area of elevated moisture/instability that tracks across Northeast Washington and the Idaho Panhandle overnight with isolated showers. Thursday afternoon a drier southwest flow develops, although lingering moisture and instability over the northern mountains will result in a 20 percent chance of showers and thunderstorms. The southwest flow ahead of a large upper trough will contribute to warm and breezy winds on Thursday, with elevated to critical fire weather conditions across the Wenatchee area, Okanogan Valley, Columbia Basin, West Plains, and Palouse. A Red Flag Warning is in effect from noon to 9 PM PDT Friday.

Friday through Wednesday: A large upper low will drop into the region providing significantly cooler temperatures, breezy winds, showers, and isolated thunderstorms. There is general model agreement with the global ensembles of the low dropping southeast to near Vancouver Island on Friday, then over southern Washington and Oregon Saturday, southern Idaho Sunday, and then start to pull away on Monday. Despite better agreement with the low track, there remains significant model differences with the precise coverage and amounts of precipitation. In general, the northern mountains, ID Panhandle and eastern third of Washington are likely to receive the highest amounts due to higher terrain, and/or closer proximity to the low track as a band of wrap-around moisture develops on the north and west side of the low track.
Below are the latest NBM low end (10th percentile), mean (model average), and high end (90th percentile) amounts for selected cities from 5 AM Friday to 5 AM Sunday

City Low End Model Average High End Wenatchee 0.00 0.09 0.29 Omak 0.03 0.33 0.92 Moses Lake 0.00 0.02 0.14 Colville 0.19 0.60 1.08 Spokane 0.03 0.25 0.57 Lewiston 0.03 0.19 0.50 Sandpoint 0.15 0.62 1.23 Kellogg 0.10 0.42 0.89

But as the low shifts east into Montana, there remains potential for band of wrap around precipitation on the back side of the low, especially over Eastern Washington and the Idaho Panhandle. Same as above, here are the 10th, mean, and 90th percentile precipitation totals from the NBM from 5 AM Sunday to 5 AM Tuesday

City Low End Model Average High End Wenatchee 0.00 0.04 0.08 Omak 0.00 0.10 0.40 Moses Lake 0.00 0.04 0.13 Colville 0.00 0.32 0.90 Spokane 0.00 0.22 0.83 Lewiston 0.00 0.18 0.68 Sandpoint 0.00 0.60 1.48 Kellogg 0.01 0.63 1.63

Temperatures begin to moderate on Tuesday and Wednesday as drier conditions develop. Yet lingering moisture and instability over the northern mountains will keep the chance of showers alive. JW

AVIATION
18Z TAFs: As of 1730z isolated showers and thunderstorms were already developing along the East Slopes of the Cascades. A shortwave will move into the region Wednesday afternoon/evening supporting expanding convection into the northern mountains as well. As the cells travel off the higher terrain, gusty outflow winds will be possible within the lower elevations of Central WA. Greatest concern will be in proximity of KEAT, KMWH, KOMK, and KCQV 21-03z. These sites come with a 10-20% chance for new cells to develop along the outflow winds.
Some hi-res models hint at lesser impacts at KGEG-KSFF 03-06z but still some impacts from decaying outflow winds and a small threat (~15%) for showers. A bulk of the shower and t-storm threat will retreat into North Idaho 6-12z.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in VFR conditions at all terminals through the forecast period.
Greatest uncertainty is coverage of showers and t-storms over Central and Northern WA and how outflow winds will spread into the Basin. Some hi-res models showing brief gusts of 35kts with this activity and high-based cells developing between KEAT-KGEG.
JW

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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

Preliminary Point Temps/PoPs
Spokane 57 86 53 69 48 67 / 20 10 30 60 80 40 Coeur d'Alene 59 84 55 67 49 65 / 20 10 30 70 80 50 Pullman 54 82 50 67 45 64 / 0 10 40 60 50 20 Lewiston 62 90 60 74 53 71 / 0 10 40 70 70 30 Colville 53 86 50 70 43 69 / 20 20 20 70 90 80 Sandpoint 56 83 52 64 47 63 / 20 20 30 80 90 70 Kellogg 56 82 54 64 48 62 / 20 10 40 70 70 70 Moses Lake 60 88 54 76 48 72 / 20 0 10 20 30 10 Wenatchee 64 85 59 74 54 71 / 20 0 20 50 20 10 Omak 59 89 58 75 50 73 / 20 10 20 60 80 50

OTX WATCHES/WARNINGS/ADVISORIES
WA...Red Flag Warning from noon to 9 PM PDT Thursday for Colville Reservation (Zone 702)-Eastern Columbia Basin -Palouse - Spokane Area (Zone 708)-Foothills of Central Washington Cascades (Zone 705)-Lower Palouse -Snake River (Zone 709)-Okanogan Valley (Zone 703)-Waterville Plateau (Zone 706)-Western Columbia Basin (Zone 707).
ID...None.