textproduct: Boise

DISCUSSION

The Pacific cold front is currently pushing east across southwest Idaho bringing wind gusts up to 50 mph, heavy rain/snow and several lightning strikes. This area has moved through much of the Treasure Valley over the last hour with reports of heavy rain, small hail and wind gusts up to 40 mph. Winds in the foothills have been a bit higher with gusts reported as high as 56 mph near Horseshoe Bend. Webcams at Bogus Basin at 6000 feet showed the rain changed over to snow for about an inch of wet snowfall. The frontal band will continue to track east, reaching the western Magic Valley and Sawtooth Mountains around 930 pm. Despite the recent precipitation, surface winds will be elevated enough in the lower valleys to keep fog from forming tonight.

While the cold front will bring a temporary drop in snow levels to around 5500 to 6500 feet, the air mass behind it is not significantly colder. Consequently, temperatures on Sunday will remain 5 to 10 degrees above seasonal normals. There will be a brief break in the precipitation tonight with more widespread showers developing late Sunday morning into the afternoon across the central Idaho mountains. No updates were needed to the current forecast for this evening.

AVIATION

Mainly VFR outside of precip. Rain and snow will continue overnight into Sunday over the central Idaho mountains. Light rain showers will increase across lower elevations on Sunday. Snow levels 5-6kft MSL. VFR/MVFR in rain, and IFR/LIFR in snow. Mtns obscured in precip. Areas of LLWS overnight, especially across SE Oregon. Surface winds: SE-SW 10-20 kt with gusts to 20-30 kt. Locally higher over open terrain and highlands. Winds aloft at 10kft MSL: SW 25-40 kt, except up to 55 kt across SE Oregon through Sunday morning.

KBOI...VFR. A 50% chance of light rain showers Sunday afternoon into Sunday night. Surface winds: SE 10-20 kt with gusts to 25 kt, decreasing to 5-15 kt Sunday afternoon.

PREV DISCUSSION

SHORT TERM...Tonight through Monday night...A deep trough will remain positioned off the West Coast before transitioning inland on Monday. This will keep our region under a mild southwest through Sunday, followed by a slight cool down on Monday. While precipitation development will be focused in the mountains through the period, embedded shortwave troughs will bring a chance of rain to lower elevations tonight and again late Sunday. These waves will also act to enhance precipitation in the mountains.

The first wave that will push through the region tonight has the potential to pack a small punch for early January. Convective parameters aren't much, but the overlap of instability and upper dynamics is enough to support a slight chance of thunderstorms (~15%) this evening from far eastern Baker County though the w-central Idaho mtns and Camas Prairie. The 19Z and 20Z HRRR solutions are the most prolific and the only ones to show a broken line forming across all of SW Idaho this evening, to include the lower Snake Plain. For now have expanded thunderstorm coverage across the w-central ID and Boise mtns, while leaving mention out of the Snake Plain which has a 5-10% chance. Heavier showers or thunderstorms could bring gusty winds to 50 mph.

Snow levels are 5.5-7kft MSL this evening, edging down to 5-6kft on Sunday. Elevations above 6kft MSL will see moderate to heavy snow totals of 6-12 inches through Sunday night, while up to 3 inches is possible in a narrow range below 6kft MSL. Liquid totals in the mtns will fall between 0.5-1" with up to 0.20" in the valleys. The Snake Plain will see its signature shadowing signal for much of the period with the higher end of rainfall amounts (0.1-0.2") in the foothills and across the western Magic Valley through Sunday night.

Monday-Monday night will bring a slight drop in temperatures and snow levels as passage of a third wave shifts flow to the west. Mountain valleys could pick up light snow accumulations with snow levels of 3.5-4.5kft MSL while valleys see a 20-30% chance of showers. High temperatures will come down from Sunday's near records at some sites (10-15 degrees above normal), to 5-10 degrees above normal.

LONG TERM...Tuesday through Saturday...Precipitation chances Tuesday morning will remain elevated in the Central Idaho Mountains, as the progressing shortwave departs our area. However, Throughout the day Tuesday, A trough will dig down from the Gulf of Alaska, with a reinforcing shortwave trailing not too far behind it. This will drive increased precipitation and cooling temperatures through the long term. By early Wednesday morning, chances of precipitation in the West-Central/Boise Mountains will be at 80% or greater, with 30- 70% elsewhere. The best overlap of upper-level dynamics and moisture looks to come late Tuesday into Wednesday, where the terrain of the West- Central/Boise mountains will likely see the highest snowfall totals/rates. Higher elevation mountain valleys will see between 4-8 inches of snowfall Tuesday afternoon through Wednesday afternoon. The highest peaks have a 60-75% chance of seeing over 10 inches of snowfall over the same time frame. While Wednesday will give way to the most snowfall over a given period, snow showers will be over the higher terrain throughout the majority of the period.

While that big long paragraph was mostly focused on the mountain zones, weather will still exist outside of those zones during the period. Although in general, locations below 4000 ft MSL will see rain starting out Wednesday. However, the trailing shortwave mentioned above will not only act to aid in upper level support of precipitation, but it will also swing our flow to be north- northwesterly...allowing for some cold air to advect in. This, in turn, will lower snow levels through Wednesday night, with snow levels hitting valley floors come Thursday morning. While precipitation chances will also be on the decline at this point, as the trough progresses east, precipitation that does develop will be in the form of a snow/wintry mix across the entire area. Daytime highs come Thursday will be in the range from 34-40 throughout the Snake River Plain...So while we may see some accumulation on grassy/elevated surface Thursday morning, and accumulation will likely melt off later that day. Snow flurries may stick around for Friday morning in lower elevation valleys as well. Temperatures will start out above normal on Tuesday, before reaching near normal Thursday and beyond. Guidance hints at a dry pattern setting back in to end the week, with some uncertainty on the timing of a building ridge.

AVIATION...Generally VFR. Precipitation and clouds increasing this afternoon across the area. Isolated lightning over SW- Idaho this afternoon.Tapering off by around Sun/06Z, except showers continuing over the cntrl Idaho Mountains. VFR/MVFR in rain showers. IFR/LIFR in snow showers. mtns obscured in precip. Areas of LLWS overnight. Snow Levels: 5.5-7.5 kft MSL, lowering to 5-6 kft MSL overnight. Surface winds: E-S 10-20 kt with gusts to 20-30 kt. Locally higher over open terrain and highlands. Winds aloft at 10kft MSL: SW 20-30 in SW-Idaho, SW 30-50 in E-Oregon.

KBOI...VFR. Intermittent rain showers from between Sat/23z and Sun/04z. A low chance (<10%) of lightning in that same period. Surface winds: SE 10-20 kt with gusts as high as 35 kt this afternoon. Decreasing to SE 5-15 kt with gusts as high as 20 kt after Sun/06z.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None. OR...None.