Sunrise 6:34 AM   Sunset 8:52 PM Moonrise 3:08 AM   Moonset 1:22 PM

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FXUS65 KBOI 021553 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 953 AM MDT Thu May 2 2024

DISCUSSION
Today, another shortwave is sweeping through the region, maintaining the risk of showers and thunderstorms. This morning, it has already reached central Oregon and is slated to progress east-southeast across the forecast area until this evening. Observations indicate snowfall at around 4000 feet in Burns, Oregon, with mainly rain at Drinkwater Pass (3500 feet).
While snow is visibly accumulating, it seems to be less than a quarter-inch deep, causing slushy conditions on the roads.

High-resolution models from this morning consistently depict convective showers and thunderstorms forming later today across Southeast Oregon and along the Nevada/Idaho border. While significant Convective Available Potential Energy (CAPE) is present in the area, there's also expected to be partial clearing and sufficient forcing from the shortwave to trigger more showers and thunderstorms in the afternoon. Some of these showers might be intense enough to produce lightning strikes and graupel. Light snowfall could occur above 4500 feet. The forecast has been updated accordingly to include the possibility of thunderstorms this afternoon.

Looking ahead, a more notable low-pressure system is anticipated on Saturday, potentially bringing heavy rainfall to the forecast area. Further details will be provided in the afternoon forecast package.

AVIATION
A mix of overcast VFR and MVFR/IFR in rain and snow showers. Local LIFR possible in snow showers above ~4000 ft MSL snow levels. Mountain obscuration in precipitation and low clouds.
Isolated thunderstorm development expected this afternoon mostly west and south of the Snake Plain, although a stray storm could wander into the Magic Valley. Showers will dissipate by Fri/06z.

Surface Winds: W 10-20 kt with gusts to 35 kt in terrain west of the Owyhee Mtns, otherwise S-SE 5-15 kt in the Snake River Valley, becoming W-NW 5-15 kts after Thu/23z. Winds aloft at 10kft MSL: W-NW 20-30 kt south of the Snake Plain, variable 5-10 kt north of the Snake Plain.

KBOI...VFR with scattered/broken clouds 6000-9000 ft AGL.
Showers are moving in this morning near KBOI, with a 30% chance of rain showers hitting terminal between Thu/16Z-Thu/22Z, and again Fri/04Z-06Z. There's a low chance (<10% chance) in a thunderstorm near the terminal Thursday afternoon, but most of the thunderstorm potential exists well south and west of the area. Surface winds: SE 5-15 kt this morning, shifting to NW 5-15 kt around Thu/22z.

Weekend Outlook...Friday will be mostly VFR for the area, but conditions become unsettled going into Sat/Sun. As the next system moves in Saturday, skies become overcast. Precipitation moves in west to east through Saturday, as mostly rain with snow levels at 6000-7000 ft MSL. The core of a colder airmass moves overhead on Sunday, dropping snow levels to ~5000 ft MSL with consistent rain/snow through the day (60-80% chance of precip area wide).
Surface winds increase significantly Saturday morning and stay elevated through Sunday. SE 15-25 kt with gusts 25-35 kt Saturday afternoon, W 5-15 kt with gusts to 25 kt Sunday.

PREV DISCUSSION
SHORT TERM
Today through Saturday night
Precipitation will expand across the area today as a compact low tracks from the Oregon coast into SW Idaho. An initial shield of rain/snow will focus oriented along a NW-SE warm front that lines up over SE Oregon and far SW Idaho this morning. This area of more stratiform precipitation isn't expected to extend beyond the I-84 corridor this morning before dissipating. Snow levels this morning will range between 3500-4500 feet and accumulations of of to 2 inches above 4500 feet are possible through this morning. Showers will become more widespread this afternoon as the upper low center passes overhead. Snow levels will rise to between 4500-6000 feet and any accumulation will be limited to the higher mtn peaks. While the probability of a thunderstorm isn't zero, instability parameters are marginal, and less impressive than yesterday, so have opted to keep mention of thunder out of the forecast. Flow aloft will bring wind gusts to 30 mph across open and elevated terrain near the northern NV border this afternoon otherwise we'll see lighter winds than yesterday. Friday brings a break in the active weather as dry and mild conditions accompany a transiting shortwave ridge.

The next storm will take the form of a deep closed that will push into SW Oregon by Saturday night. Precipitation will form along the cold front at the leading edge of the low, spreading across SE Oregon early Saturday and into SW Idaho through the evening. There is good agreement among the deterministic models on the track of the low and development of precipitation along the cold front. While differences in amounts are notable they are all fairly robust and support a 70-90% chance of 0.10" or more of liquid across much of the area. Winds will ramp up on Saturday as well, first out of the east as air is pulled into the deep surface low, then shifting to the NW Saturday night behind the cold front. This will bring sustained winds of 20-30 and gusts of 30-40 ahead of the front on Saturday along the I-84 corridor from Twin Falls to Baker City as well as the over higher open terrain of SE Oregon. Warmer air ahead of the low will raise snow levels to 7-9kft MSL for the bulk of the precipitation through Saturday evening.

LONG TERM
Sunday through Thursday
A quick moving upper level low will move inland on Sunday, bringing a very cold air mass along with widespread precipitation to the region.
Temperatures will be about 10-20 degrees below normal through Monday as the low center moves to our southwest and then lifts out of the southeast by late Monday. Cold northwest flow will build in over the region, keeping similar weather for the area through Tuesday. High terrain will see the heaviest precipitation, with anywhere from 10-12 inches of snow anticipated above about 7000 feet by Tuesday morning. Mountain valleys are currently forecast to see about 1-2 inches by Tuesday morning, but there is a chance that forecast snowfall totals will increase as we get closer to the day. Current model blends show about a 5% chance of the Snake River Valley seeing snowfall. However, based on the depth of the upper level low and the forecast trend from the past few nights, there is medium confidence from this forecaster (50% chance) in at least a trace of snowfall in most locations on Monday and Tuesday morning.
Snow levels will be around 4000-5000 feet through Monday, with the potential for lowering to valley floors each night through Tuesday.

As this low tracks to our east, cold northwest flow will keep temperatures below normal and a chance for showers each day through the long term period. The best chance for precipitation will continue to be over high terrain (40-50% chance) on Wednesday and Thursday, with low chances over the Snake River Valley (10-20% chance). Model agreement remains high through Wednesday with regards to the storm track, but after Thursday, there's differences in the deterministic solutions, but better agreement in the ensemble clusters. A few members show another deepening low over the region on Thursday, although not quite as strong as the low this weekend. The majority of members show agreement in a ridge building in by the end of the week.
Therefore, forecast uncertainty is high after Wednesday.

BOI WATCHES/WARNINGS/ADVISORIES
ID...None.
OR...None.

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