Sunrise 6:30 AM   Sunset 8:52 PM Moonrise 3:51 AM   Moonset 3:56 PM

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FXUS65 KBOI 040304 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 904 PM MDT Fri May 3 2024

DISCUSSION
Tonight, the forecast area will experience warm and dry conditions as an upper-level ridge persists overhead. Late tonight, this ridge will shift eastward as the next upper-level low approaches from the west.

By Saturday, a well-developed upper-level low pressure system will impact the region, bringing initially windy conditions followed by heavy rainfall. Winds will pick up after sunrise on Saturday, starting from the east and then shifting to the northwest behind a cold front in the afternoon and evening.
Along the I-84 corridor from Twin Falls to Baker City, and across higher open terrain in southeast Oregon, sustained winds of 25-35 mph with gusts of 40-50 mph are expected ahead of the front, as detailed in the current wind advisory.

Furthermore, several high-resolution models indicate the potential for stronger thunderstorms ahead of the cold front across south- central Idaho on Saturday afternoon. With forecasted surface temperatures in the low 70s, along with 50 knots of shear and 200 J/kg of CAPE, there could be a few stronger thunderstorms from the Idaho/Nevada border through the Upper Treasure Valley and Western Magic Valley.

A substantial precipitation band will form along the cold front preceding the low, spreading from southeast Oregon early Saturday to southwest Idaho by late afternoon. Models are in good agreement with a 60-80% probability of receiving 0.50 inches or more of liquid across much of the area, and a 20-40% chance of exceeding 1.00 inch. Snow levels are expected to remain elevated above 6500 feet, then falling to around 4500 feet by Sunday afternoon.

The current forecast captures the precipitation well, and thunderstorms have been added to the forecast for Saturday afternoon. An update will be released shortly.

AVIATION
VFR tonight. Widespread rain moving into SE OR by late Saturday morning and into SW ID late afternoon. Isolated thunderstorms developing across SW ID Saturday afternoon and early evening. MVFR/IFR conditions and mountain obscuration in precipitation. Snow levels 7-9kft MSL, lowering to 3-4kft SE OR and 4-6kft SW ID by Sunday morning. Areas of LLWS developing overnight into Saturday morning, especially across the Treasure Valley and portions of SE OR, with winds S-SE 35-45 kt at 2000 ft AGL. Surface winds: S-SE 10-25 kt, with gusts 25-40 kt on Saturday. Winds becoming W-NW 10-20 kt Saturday evening behind a cold front. Winds aloft at 10kft MSL: S 20-35 kt.

KBOI...Increasing mid to high clouds overnight. Rain developing late Saturday afternoon and continuing overnight. MVFR conditions developing in rain. LLWS tonight until surface winds increase. SE surface winds increasing tonight into Saturday to 15-25 kt with gusts 25-35 kt. Winds becoming NW 10-15 kt Saturday evening.

Sunday Outlook...Showers continuing on Sunday, especially in the mountains. Areas of MVFR/IFR conditions and mountain obscuration.
Snow levels 3-6kft, lowest in SE Oregon in the morning. Surface winds: W 10-20 kt with gusts 20-30 kt.

PREV DISCUSSION
SHORT TERM
Tonight through Sunday night
Dry, warmer, and increasingly windy weather overnight and Saturday ahead of an approaching north Pacific cold front. Southeast through southwest winds will increase in southern CWA zones Saturday, reaching 30 mph with gusts to 45 mph, enough for a Wind Advisory in the Upper Treasure Valley and western Magic Valley, and the southern half of Harney County. These winds will help boost temps to near 70 degrees Saturday in the southern Idaho valleys, and 60s in the rest of Idaho, but only 50s and 60s in Oregon as rain will begin there around midday. A deep upper low in the north Pacific will come inland along the OR/CA border late Saturday then due east along the southern OR border Saturday night and southern Idaho border Sunday. This is an ideal track for pcpn in our CWA
The rain will be moderate to heavy late Saturday through Saturday night as the cold front moves eastward across our CWA
Totals will be .50 to 1.25 inch in the mountains, and .30 to 60 inch in the valleys. Snow level will drop sharply behind the cold front all the way to valley floors in Oregon, and 3500 to 4200 feet MSL in western Idaho Sunday morning. But by then most of the pcpn will be ending so snow totals will only be a couple inches, except above 7000 feet where a foot may fall. Behind the cold front Sunday's high temps will be some 20 degrees lower than on Saturday, only in the 30s and 40s in the mountains and 40s to lower 50s in the valleys. Sunday will also be windy but not as windy as Saturday, and from the west or northwest.
Lighter rain and snow showers will continue Sunday night as moisture wraps around the eastward-moving upper low. Thunderstorms were taken out of the forecast, except for a 15 to 20 percent chance in southern Twin Falls County Sunday afternoon.

LONG TERM
Monday through Friday
Northwest flow aloft will prevail as a deep low moves eastward out of the area. Embedded in the flow, another trough will move into the area late Monday and early Tuesday. As this trough moves into already moist and cold air, it will extend the chances (50-70%) of precip through the early parts of the long term. With snow levels still between 5000-6000 ft MSL, a chance for mtn valley mixed precip will exist, while mtns continue to favor snow. At the same time, Monday afternoon instability supports a slight chance of thunderstorms in the W-Central Mtns of Idaho and the ID/NV border. Precipitation chances will taper off between Tuesday and Wednesday as the trough exits the area, but a slight chance for showers will remain over central Idaho through Thursday morning.

Models continue to agree on a period of calmer weather kicking off on Thursday, as a ridge of high pressure builds north of our area.
The exact position of the high varies, but the forecast remains the same: warmer, drier, calmer by the end of next week. Temps will rise from 10-15 degrees below normal on Tuesday to near normal on Thursday/Friday.

BOI WATCHES/WARNINGS/ADVISORIES
ID...Wind Advisory from 9 AM to 6 PM MDT Saturday IDZ014-016.
OR...Wind Advisory from 8 AM to 5 PM PDT Saturday ORZ061.

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