851
FXUS65 KBOI 211602
AFDBOI

Area Forecast Discussion
National Weather Service Boise ID
1002 AM MDT Tue May 21 2024

.DISCUSSION...Skies are sunny this morning, with increasing
clouds expected this afternoon and especially this evening as a
strong upper level system moves in from the northwest.
Precipitation will move in this evening as well, first in
eastern Oregon and then spreading to the east and south late
tonight. Models are trending cooler with temperatures for Wed,
and earlier with the chance of thunderstorms. An update will be
out shortly with these changes.

&&

.AVIATION...VFR. Increasing and lowering clouds from north to south
today. Numerous showers after Wed/03Z in E Oregon, and after Wed/06Z
in SW Idaho creating low VFR and local MVFR in valleys. Areas of
IFR/LIFR in mountains obscuring terrain. Snow levels 6500-7500 ft
MSL through Wednesday morning, lowering to 4500-5000 after Thur/00Z.
Thunderstorms are possible on Wednesday. Surface winds: SW to NW 5-
15 kt local gusts to 25 kt east of KMUO today. Widespread gusty
winds to 30 kt on Wednesday. Winds aloft at 10k ft MSL: W-SW
10-20 kt.

KBOI...VFR today. Rain beginning around Wed/09-11Z, with a 20%
chance of brief MVFR ceilings. Winds: NW today shifting to SE around
sunset. Frontal passage tonight will switch winds to NW 10-20 kt
with gusts up to 30 kt.

&&

.PREV DISCUSSION...
SHORT TERM...Today through Thursday night...Dry and cool
north flow aloft this morning will transition to warmer west-
northwest as weak ridging builds over the Pacific Northwest
today. While temperatures will remain slightly below normal,
today will be the warmest day in the short term period. A cold,
vigorous trough will approach from western Canada late tonight,
spreading moisture southeastward overnight through early
Thursday. Precipitation is likely (70-100% chance) for the
majority of the forecast area Wednesday morning and/or
afternoon ahead and along the accompanying cold front.
Afternoon instability will ramp up as the relatively cold air
mass moves overhead and enough lift is generated with the front.
This will promote convective activity with a slight chance
(15%) of thunderstorms for late Wednesday morning/afternoon, as
the center of the low continues to track into northeast Oregon
and w-central Idaho.

In addition to shower activity, west and northwest winds will
become breezy to windy along and behind the cold front, with
gusts reaching 30-40 mph in east Oregon and far southwest/south-
central Idaho. Temperatures will cool to 10-15 degrees below
normal for Wednesday and Thursday from the colder air influence
and wet weather. In correlation, snow levels will rapidly lower
throughout the day Wednesday, allowing the potential for light
snow accumulations as low as 4500 ft MSL by Wednesday night.
There is a 60% chance or greater that elevations above 7000 ft
will receive at least 6 inches of total snow accumulation in the
central Idaho mountains Wednesday through Thursday afternoon.
While shower coverage will begin to decrease by Wednesday
evening, wrap-around moisture behind the low will maintain a
chance of showers (30-50%) over the southwest Idaho mountains
through Thursday afternoon. Breezy west-northwest winds will
continue Thursday afternoon/eve in south-central Idaho while
drier north-northwest flow aloft establishes over the forecast
area.

LONG TERM...Friday through Tuesday...Fair and mostly sunny
skies will start off the period and dominate throughout most of
the five day extended forecast. The exception, starting late
Friday through all of Saturday, will be a change to showery
conditions as a rapid influx of moist, unsettled atmospheric
determinants associated with a closed low pressure system speeds
through the region. The track of this system will direct most
of the activity across the northern thru northeastern portions
of the forecast area with the lion`s share dropping out across
Adams and Valley counties Saturday afternoon. Model
probabilities show 30 to 40 percent chances for precipitation,
with isolated areas in and near Valley county experiencing
chances between 55 to 75 percent. With the drop in snow levels
in the north (to between 5500 and 6500 ft) during this onslaught,
the higher elevations could see a dusting of snow. Breezy winds
are also expected on Saturday throughout the major valleys and
higher elevations in the south, especially around showers. By
Monday a relatively drier, more stable ridge will build into the
region allowing for sunnier skies and temperatures to warm up
once again to normals and slightly above.

&&

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

&&

$$

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DISCUSSION...SP
AVIATION.....DG
SHORT TERM...SH
LONG TERM....WH