Pocatello, Idaho

• Winter Weather Advisory starts 6 PM MDT today and goes through 5 PM MST Sunday. Snow above 7000ft elevation in the Central Mountains.
• Moderate to heavy rain redevelops starting late tonight, continues into Sunday night.
• Cooling trend starting today, with warming returning to western areas Monday, other areas Tuesday.

There is currently moist, southwesterly flow aloft over southern Idaho with light showers currently, mainly over our the Central Mountains, western areas and the Island Park/Driggs areas. Currently, there is an upper level low centered just off the coasts of Washington and Oregon as part of an overall trough that extends much farther south to off the coast of California, where there is ample sub-tropical moisture available. This low/trough is bringing our current moisture and will drive the weather/precipitation today and Sunday, especially as this low/trough moves over our area late today into Sunday.

Precipitation for lower elevations today has dropped for the Snake Plain, southeastern areas (Bear Lake Region), and northern areas (Stanley and Challis) where a T to 0.04 is expected. The Sun Valley Region and Island Park region will receive around 0.05 to 0.15 today. This precipitation will likely have some impacts on Halloween activities. However, the bulk of the precipitation is still arriving later this evening through Sunday and lingering into Monday morning.

By late today into Sunday the low off the Washington coast begins to move and makes it way inland aided by an deepening trough, which will really picks up more Pacific moisture and transports it over southeastern Idaho. This more organized round of precipitation will begin in the Central Mountains/Wood River Valley/eastern Magic Valley as early as 6 PM today and then spread eastward throughout the night. The NBM model is still showing about an 90 to 100 percent chance of the entire CWA receiving at least 0.25 inch of precipitation with this system. Precipitation amounts are up about 0.15 from yesterday`s model round during day and down about 0.25 from last nights model run. It still seems likely that many will see 0.20 to 0.50 inches in the eastern Magic Valley and Challis area, 0.50 to 0.80 inches in the Snake River Plain and lower elevations of the Eastern Highlands and Central Mountains. For higher elevations (above 8000 feet) precipitation will be pushing 0.80 to 1.25 inches, especially for Sun Valley Region, Big Lost Highlands/Copper Basin, Lost River Range, and the Montana Divide as well as the Southern areas along the Utah border including the Wasatch mountains. Temperatures will still be slightly above normal today with highs in the 40s and 50s, but by Sunday our highs will drop into the mid and upper 30s and 40s. Snow impacts are likely at Galena Summit today into Sunday morning with the forecast still showing 3-7" of snow there. A winter weather advisory is still in effect for the Sun Valley Region and the Big Lost Highlands above 7000 feet for this evening through Sunday. Elsewhere, the bulk of our snow is expected to remain above most of the rest of our mountain passes, but that could change easily with temperature changes of just a few degrees. Even though snow levels will start high, convection can lower snow levels, which could drop snow levels to pass levels especially for areas like Emigration and Raynolds pass.

A few showers linger on Monday morning, but we will be drying out from west to east by that point and high pressure builds in on Monday. The breakdown of this upper level ridge is still in question. There remain four different clusters of solutions in this medium range period. It is about a 60-40 split that the ridge will remain on Wednesday, then flatter, zonal flow or weak troughing developing Thursday. Zonal flow makes timing of shortwave energy more difficult, so there is quite a bit of uncertainty in the forecast in the long-term.

East Idaho under the influence of wet low pressure system. Radar shows rain showers moving through the region. Precipitation timing for terminal forecasts heavily based on current radar trends mixed with high-res model blend favoring the HRRR. CIGS transition lower toward MVFR with occasional IFR overnight, then to IFR around/after sunrise as rain becomes more widespread. Other than an occasional shower induced gust 10-15kts, winds are expected to remain light. SUN winds should remain terrain driven as normal, but there is some question as to how much influence the precipitation will have. There is some guidance suggestion that the winds at SUN could turn south and remain south until surface frontal passage mid-day Sunday, at which time winds should become west or northwest.

A low moving through the great basin, to Idaho`s south, will bring rain and high elevation snow, with most of the wind on the southern side of the low. Expect low temperatures through Mon, high humidity through at least Tue, and some possible wind in the southern Sawtooth NF on Sun.

Winter Weather Advisory from 6 PM this evening to 5 PM MDT Sunday for IDZ073-074.