Sunrise 6:03 AM   Sunset 9:22 PM Moonrise 2:37 AM   Moonset 4:14 PM

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FXUS65 KBOI 021605 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 1005 AM MDT Sun Jun 2 2024

DISCUSSION
A strong moisture surge, which is forecasted to qualify as a moderate atmospheric river, will spread precipitation into our area this evening. Precipitation chances over terrain and just upstream of terrain are near 100% until monday evening when the moisture moves past our area. Storm total rainfall forecasts are currently 1" in mountain valleys and 2" along ridges. A few summits could even see up to 3" of rain. Rainfall in terrain could be locally much higher as models Monday afternoon show banded terrain driven storms. This raises the confidence for flash flood potential as convection is supposed to kick off after the bulk of the steady rain has already fallen and near- surface soils are likely to be saturated. While the latest model consensus generally shows a decrease in the average QPF, some models show an increase. Our forecast so far has matches WPC guidance which has been on the higher end of the model range. Based on the strength of the moisture surge (~500 kg/m/s), the potential for convection in the afternoon, and the ENS indicating that models are still calling for record setting rain: current QPFs are likely fine until the afternoon forecast package.

This afternoon is still looking breezy, with gusts of 25-30 mph in the Magic Valley and in SE Oregon. Winds will be even gustier Monday afternoon, with gusts to 35-40 mph in the same area as a cold front approaches. Temps this afternoon will be a little colder than yesterday with the increasing cloud cover, and will be colder still on Monday with the rain. No major updates to the previous forecast until the afternoon package.

AVIATION
VFR today with deteriorating conditions tonight. Low confidence (20-40 percent chance) of rain reaching KBNO-KBKE-KMYL before 03/00Z, high confidence (80-90 percent chance) of MVFR conditions and mountain obscurations with rain developing across the north (KBKE-KMYL) between 03/00Z and 03/06Z. Chance of showers (30- 50 percent) impacting Treasure Valley terminals between 03/00Z- 03/06Z. Areas of LLWS developing after 03/02Z. Surface winds: SW-NW 5-15 kt increasing to 10-20 kt with gusts to 30 kt after 02/18Z. 10k ft MSL winds: W-SW 15-30 kt.

KBOI...VFR. W to NW winds 10 kt or less. Low confidence (30-40 percent chance) of rain reaching the terminal between 03/00-03/06Z, high confidence (85 percent chance) of rain and low VFR ceilings after 03/06Z.

HYDROLOGY
Current 24 hour precipitation forecasts on Monday show over 2 inches of rainfall on ridgetops and over 1 inch of rainfall in valleys in the Central ID and Wallowa Mountains, with nearly half an inch of rain in the Treasure Valley. Near- surface soil is slightly drier than is normal for this time of year, so there is some room for at least initial rainfall to be absorbed by the soil. However, the long period of steady rainfall on top of a decent snowpack could still lead to rapid river rises and instances of minor flooding in streams and creeks. For instance, Deadwood Summit SNOTEL's latest reading shows a snow depth of 35 inches at about 7000 feet with a snow water equivalent of about 13.8 inches of water. With snow levels likely around 10,000 feet for this storm, most precipitation will fall as rain in this twenty four hour period. This along with warm temperatures this last week will aid in snow melt.
Latest NOHRSC estimates show over 3 inches of water equivalent will melt from mountain snowpack.

Mainstem rivers currently are not forecast to reach flood stage but some smaller tributary creeks and streams could reach or exceed bankfull resulting in minor flooding. Mud and rock slides could be an issue in steep terrain as well. There is a Hydrologic Outlook in effect providing details as well. Use caution if traveling in the mountains on Monday and Tuesday, and avoid recreation near rivers and creeks in mountainous and steep terrain.

PREV DISCUSSION
SHORT TERM
Today through Tuesday night
Today looks to be the last mild weather day for the short term period, with slightly above normal temperatures and clear skies in the morning, and widespread rain in Eastern Oregon and in the West Central Mountains-Weiser River Valley by late tonight. A strong plume of moisture, known as an atmospheric river, will begin to move into the Pacific Northwest by late this evening, bringing record rainfall and the potential for flash flooding due to significant rain on snow melting. Zonal flow aloft will allow for significant moisture transport inland, leading to a prolonged stratiform rainfall event over Monday-early Tuesday.
Current model forecasts show high confidence in a significant and abnormal rainfall event for our region, with precipitable water values near 1 inch (99th percentile of values for this time of year). The Ensemble Forecast Index, or a measure of how abnormal the weather is for a time of year, shows Monday reaching the maximum QPF (record rainfall) over the Central ID Mountains which would mean we could see record breaking rainfall over much of that region. The WPC has put much of the Central ID Mountains in a slight risk of excessive rainfall, where the chance of flash flooding is around 15%.

There is a low chance of convection on Monday night after the initial wave of precipitation and a weak cold front passes, but instability will be weak so confidence in thunderstorms is low.
Any slow moving thunderstorms would greatly increase the flash flooding threat, especially in the mountains and near burn scars, so this potential will have to be monitored closely.

Some clearing on Tuesday morning could allow for fog and low stratus clouds to form in valleys, especially in areas with poor drainage where standing water will contribute to fog development. As the low pressure system with corresponding atmospheric river moves east, westerly winds will pick up on Monday afternoon. Winds will be strongest over southeastern Oregon, with gusts as high as 40-50 mph over ridgetops.
Temperatures will increase as clearing continues on Tuesday, with a ridge beginning to develop into the region, beginning a sharp warming trend for the rest of the week.

LONG TERM
Wednesday through Sunday
Models remain consistent in showing an upper ridge over the southern U.S. expanding northward and persisting into next weekend. The expansion of the ridge results in unseasonably hot temperatures developing by Thursday and lasting into the weekend. There remains a high probability (>60 percent chance) of high temperatures exceeding 90F in the lower valleys (including the Treasure Valley) and a low probability (less than 30 percent chance) of reaching 100F in the lower valleys (which would be near record territory). Models also remain consistent in showing enough moisture available for a slight chance (less than 30 percent)
of showers and afternoon/evening thunderstorms each day.

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

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