Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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934 FXUS66 KOTX 020013 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 513 PM PDT Sat Jun 1 2024 .SYNOPSIS... A dynamic system pushes across the region Sunday into Monday, bringing widespread rain, followed by gusty winds, and the potential for thunderstorms on Monday. Next week will see a warming and drying trend. Confidence is increasing for temperatures warming into the 80s and 90s. && .DISCUSSION... Tonight: In the wake of a weak shortwave passage from this afternoon, some light showers will continue through the early evening over the Idaho Panhandle. A westerly pressure gradient will allow areas across the lee of the Cascades into the western Columbia Basin to see breezy winds through the evening with gusts 25 to 35 mph. Sunday and Monday: A strong late Spring storm remains on track to impact the Pacific Northwest late Sunday morning through Monday. Models have trended quicker with the onset of precipitation compared to this time yesterday as a low pressure system approaches the British Columbia coast. Aided by a rather strong jet with speeds of 140 to 160 knots at its core, this system will transport an atmospheric river into the region. Precipitable water values 175 to 200% of normal combined with strong upper level dynamics will deliver moderate to heavy rainfall across much of the region late Sunday morning through Monday afternoon. By late Monday morning and afternoon, the focus will shift to widespread breezy to gusty winds and the potential for thunderstorms across eastern Washington and north Idaho. RAIN: Precipitation totals through Monday evening will be between 1.50 to 3 inches for the Cascade crest. East of the crest, strong westerly flow will work against significant accumulations, but could see between 0.05 to 0.25 inches with areas such as Moses Lake and Wenatchee showing a 75% chance of 0.10 inches of rain from 11 AM Sunday to 11 AM Monday. When increased to 0.25 inches this potential decreases to 25-30% for the Moses Lake area and 40% for Wenatchee. Areas further north such as Omak and the Methow Valley are showing a 60-70% chance for 0.25 inches. As the system moves into far eastern Washington and north Idaho, precipitation amounts will re-enhance with around a 80% chance for above 0.50 inches of rain for Colville, Spokane, Pullman, and Lewiston. Additionally, there is a 35 to 45 % chance for these areas to see above 0.75 inches. Across the valleys of Idaho Panhandle, there is nearly a 100% chance for over 0.50 inches, including Coeur d`Alene and Sandpoint. There is a 80% chance for over 0.75 inches at these locations and a 45% chance for over an inch. Although this rain will be mostly beneficial across the area given low river flows and dry soils, there is an area of concern over central Idaho where the heaviest rainfall is forecast. Smaller creeks and streams could see quick rises, particularly in Lewis and Shoshone counties. WINDS & THUNDERSTORMS: Winds will be breezy at the onset of precipitation on Sunday through Monday morning, but will ramp up late Monday morning and afternoon as the front moves across the region. This will allow strong gusts to mix down the surface with sustained 25-30 mph and gusts of up to 40-50 mph from the lee of the Cascades across the Waterville Plateau, into the Moses Lake Area, Spokane Area, Palouse, and lower Asotin and Garfield County. Weak afternoon instability (MUCAPE 200-500 J/kg) combined with the strong forcing and 30-45 knots of 0-6km shear could support thunderstorm development across eastern Washington and north Idaho Monday afternoon. The instability will be the limiting factor for strong thunderstorm development, but any thunderstorms could locally enhance antecedent surface winds with potential for minor wind damage to small trees and branches. Winds will also result in hazardous cross winds along north-south highways and loose objects being pushed around. /vmt Tuesday to Saturday: The Inland NW will transition to a drier and warmer pattern through this period. Tuesday a zonal flow under a strong jet will keep some clouds and shower chances alive around the Cascades, the eastern third of WA and ID. Overall the best precipitation chances will be in the mountains. Then chances wane through the evening and overnight. It will remain breezy through the day the day, with gusts near 20-30 mph, strongest near the Cascades to the Upper Columbia Basin and West Plains, before gradually waning heading into the late evening to overnight. Wednesday to Friday will be drier with a ridge building in with drier air. Winds will still remain slightly breezy Wednesday but overall will be less than earlier in the week. Some upper impulses slip by the southeast CWA later Friday, but for now the main shower chances with that remains just south of our CWA until Friday night and Saturday. Going into Friday night and Saturday some of those impulses move into the region. These bring shower and isolated t-storm chances to the Blues, Camas Prairie, L-C Valley, Palouse and Panhandle Mountains Friday night. Confidence is low, but at least some risk is there. The more notable feature will be warming temperatures. Temperatures Tuesday remain below normal, then warm closer to normal Wednesday. Then Thursday into Saturday temperatures warm above normal in a more notable way. While there are is some disagreement on precise numbers, maybe areas are forecast to warm to between 85-95 degrees, with the warmer readings in the lee of lee-side zones, deeper Columbia Basin and L-C Valley. However guidance even has between a 20-40% chance of reaching near over the Palouse and Spokane area by the end of the week too. So I am saying there is a chance, with that chance higher toward downtown Spokane. The heat risk rises to a moderate level by Friday and Saturday, though temperatures may dip slightly Saturday with more moisture and clouds in the region. This means some precautions may be needed to stay cool. /Solveig && HYDROLOGY... An Areal Flood Watch has been issued for Lewis and Shoshone Counties in Idaho. Heavy rainfall forecast over the Clearwater Mountains will result in increased snowmelt and bring sharp rises to small creeks and streams in these areas. Additionally, the amount of water with a little bit of snow left to be melted in the highest elevations of the Cascades will result in a steep rise on the Stehekin River. For the rest of the forecast area, rain will be largely beneficial due to antecedent dry soils and low base river flows. Flooding is not anticipated at this time on larger rovers but will need to be monitored. Another potential impact will be for rock slides on steeper slopes in the Cascades and across the Idaho Panhandle. && .AVIATION... 00Z TAFS: General low pressure over the area will allow weather disturbances to move across the aviation area. Generally light shower and spotty weak short lived thunderstorm activity for the rest of this afternoon/evening with the expectation that a wetter system will impact the aviation area bringing a more significant rainfall for the most part Sunday night/Monday morning, however some increased rainfall has potential to occur as early as Sunday afternoon. Expectation is some sort of MVFR/IFR conditions may occur with the more significant rainfall after 21Z Sunday with VFR prevailing today for the most part with exception of short lived MVFR conditions near the spotty decreasing shower/weak thunderstorm activity. /Pelatti FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in VFR conditions and thunderstorms remaining north of TAF sites. ----------------------- 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 49 65 51 63 43 66 / 10 50 100 90 20 30 Coeur d`Alene 48 63 51 59 43 61 / 10 60 100 100 30 40 Pullman 48 64 50 59 42 63 / 10 50 100 90 20 30 Lewiston 54 72 58 69 50 73 / 10 50 100 100 10 20 Colville 42 65 44 62 36 63 / 30 70 100 100 30 40 Sandpoint 47 61 49 56 42 57 / 20 60 100 100 60 60 Kellogg 51 62 51 55 45 58 / 20 60 100 100 50 60 Moses Lake 50 68 51 69 47 74 / 10 60 80 50 10 10 Wenatchee 53 66 53 64 49 68 / 0 70 80 50 20 30 Omak 48 67 49 68 44 71 / 10 60 90 60 10 20 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...Flood Watch from Sunday afternoon through Monday evening for Central Panhandle Mountains-Lewis and Southern Nez Perce Counties. WA...None. && $$