-- Discussion containing changed information from previous version are highlighted. --
469
FXUS65 KBOI 040304
AFDBOI

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

.DISCUSSION...
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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.
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&&

.AVIATION...
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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.
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&&

.PREV DISCUSSION...
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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.
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&&

.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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DISCUSSION...JDS
AVIATION.....ST
SHORT TERM...LC
LONG TERM....JM