Area Forecast Discussion
Issued by NWS Boise, ID
Issued by NWS Boise, ID
844 FXUS65 KBOI 152028 AFDBOI Area Forecast Discussion National Weather Service Boise ID 228 PM MDT Sun Sep 15 2024 .SHORT TERM...Tonight through Tuesday night... KEY POINTS *This afternoon scattered showers in southeast Oregon, thunderstorms in southeast Oregon and south-central Idaho with heavy rain and strong winds (30-40 mph gusts) the main concern. *Heavy rain in showers and thunderstorms could bring localized flooding on Monday afternoon-evening, mainly in steep drainages and burn scars. *Temperatures will cool, with highs about 10-20 degrees below normal by Tuesday. A deep low becomes close on the PacNW coast today, and begins to move directly south of our area. This will bring scattered showers to SE OR and isolated thunderstorms to SE OR and S-central ID. Instability is marginal, so if storms develop gusts to 30-40 mph are in the upper end of what would be possible in outflows. Winds will also be breezy tonight in the Lower Snake Plain, with gusts of 20-30 mph. Cloud cover increases to mostly overcast heading into Monday. With the low south of us Monday afternoon and evening, moisture flow and favorable dynamics will bring a stark increase in precipitation chances. While scattered shower coverage decreases in SE OR, coverage in SW Idaho increases to widespread. Decent instability over the West Central Mountains is supportive of thunderstorm development, with most convective allowing models showing a cluster of storms developing Monday afternoon. Gusty winds and heavy rain are again the main concern with storms. Heavy rain over burn scars, especially those in steep terrain, poses a localized flash flood risk. This begins a period of widespread rainfall that lasts through Tuesday evening. Updated rainfall accumulations by the end of Tuesday are 0.5-0.8 inches of rain in mountainous areas, up to 1 inch on ridges, and 0.3-0.5 inches in lower elevations. This continues a trend of accumulation forecasts decreasing, though as the event begins to move into range of convective allowing models we have increasing confidence in our totals. There is still room for the forecast to come down, given the downward trend each run, but unless the position of the low changes major rainfall updates are unlikely. This system will bring a cooling trend as well as increases to surface relative humidities. Highs in the Snake Plain on Tuesday are in the 60s, generally about 15 degrees below normal in the latest forecast. Breezy conditions are expected each afternoon, especially Tuesday as the low exits eastward and another fills in quickly behind it for the long term forecast. .LONG TERM...Wednesday through Sunday...An upper level low will move south and east across northern CA and the Great Basin Wednesday and Thursday. The bulk of the moisture associated with this system is expected to remain to our south. However, there is a chance of showers, especially across southern areas closest to the low on Wednesday. Highest pops Wednesday (around 60 percent) will be near the Nevada border and generally 15-40 percent chance elsewhere. Instability is marginal with this system, but enough to justify a slight chance (less than 20 percent) of thunderstorms mainly near the Nevada border. Models generally favor a drying trend from west to east as the upper low moves east Thursday followed by a weak ridge on Friday. Temperatures will remain below normal but trend slightly warmer each day. Forecast confidence is low next weekend but models are showing a weak trough impacting the area in NW flow resulting in low pops (15-30 percent) with northern areas having the best chance of precipitation. Temperatures remain 5 to 10 degrees below normal. && .AVIATION...VFR and mostly cloudy. Scattered showers in SE Oregon along with a slight chance of afternoon thunderstorms. Showers and thunderstorms spreading into SW Idaho after Sun/22Z. Showers continue overnight in SE Oregon. Surface winds: NW 5-15 kt with gusts up to 25 kt in the Snake Plain this evening. Winds Calm to NW 5-10 kt early Tuesday morning. Winds at 10kft: S 10-20 kt, becoming W 10-20 kt around Mon/06Z. KBOI...VFR. A 30 percent chance of showers after Mon/06Z. Surface winds: NW 5-10 kt this afternoon, increasing to 10-15 kt with gusts to 22 kt this evening. && .BOI WATCHES/WARNINGS/ADVISORIES... ID...None. OR...None. && $$ www.weather.gov/Boise Interact with us via social media: www.facebook.com/NWSBoise www.twitter.com/NWSBoise SHORT TERM...JM LONG TERM....TL AVIATION.....JM