Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 210918

Area Forecast Discussion
National Weather Service Pocatello ID
320 AM MDT Wed Mar 21 2018

.SHORT TERM...Today through Friday night. Early morning satellite
imagery was showing a low digging south along the Canadian coast
while a second low remained positioned well off the NRN California
coast with associated sub-tropical moisture spreading inland across
California and the WRN Great Basin.  Regional radar returns were
also showing scattered showers advancing across NW Nevada toward SRN
Idaho. Numerical models spread this moisture ENE across the SRN
mountains this morning while trailing energy and moisture spread
into the CNTRL mountains this afternoon. Thus have nudged the
precipitation potential upwards some in these areas for today. Temps
warm some today but even given that and the added moisture, the
National Operational Hydrologic Remote Sensing Center Snow Melt
model is not getting overly excited about snowmelt production today.
The onslaught really gets rolling Thursday morning and continues
through the day as the low off the NRN California coast ejects
inland focusing an atmospheric river on the CNTRL Idaho mountains
which gradually spreads east across the forecast area and into the
SE mountains by Thursday night. All in all, somewhere between 1 and
2 inches of liquid precipitation is expected to fall in the Wood
River region and 0.75 to 1.5 inches across the SE mountains by
Friday morning with elevated snow levels. All of which supports the
potential for flooding in the aforementioned areas and an Areal
Flood Watch continues for those regions through Friday morning.
Upwards of 5 to 10 inches of snow fall is also anticipated above
8500 feet in the Wood River region late tonight and Thursday morning
with lots of melting below that level. We catch a breather Friday as
the Canadian low begins to setup off the NW coast with attending
moisture and ejecting disturbances sweeping inland across SRN Idaho
Saturday morning for the start of the next round of precipitation.
Daytime temps cool some Friday but perhaps not enough to shut down
any ongoing flooding that may have begun Thursday-Thursday night.

.LONG TERM...Saturday Through Next Wednesday.
We open the weekend with a large ridge of high pressure slowly
shifting east over central/eastern portions of the US, with a wet,
well-stacked trough (all the way into the upper-levels of the
atmosphere) moving onshore across the Pacific NW. This trough slows
down early this upcoming week as a low pressure center closes off
over the Four Corners region Mon/Tues. As this occurs, some weak
ridging looks to work in across our forecast area, resulting in a
drying and warming trend over the course of the week.

Coming down to the surface and digging into the details, Sat and Sun
appear to be the wettest days of the long-term period as the trough
marches onshore and into SE Idaho. In particular, The Island Park
region, mtns surrounding the Teton Valley, Caribou Highlands, and
mntns SE of a line from Idaho Falls to Pocatello to Malad may pick
up the highest totals from this system, and we carry the highest
PoPs in this area, particularly Sat. A rain/snow mix is possible Sat
AM in the lower Snake Plain/eastern Magic Valley, but temps cool to
support all snow for the rest of the system. First call on QPF is
0.10-0.25 inches, supporting snow of a dusting at lower elevations
to 3-5 inches in the mntns, again particular over the eastern CWA.
We will, of course, closely monitor model trends in the coming days,
but with good agreement between the GFS/ECMWF on the progression of
this system, forecast confidence is moderate at this time.

The EC is quicker than the GFS to close off a surface low south of
our area Mon, and thus is slower to end precip, but overall we
expect a drying trend west to east over the course of the day Mon.
Ridging creeping in from the west looks to keep the forecast dry
Monday night right through Tues and Wed, with high temps improving
about 5 degrees each day on winds turning into the W-SW. 50s are
possible again by Wed at lower elevations. - KSmith/Huston

.AVIATION...Model guidance advertises a wave of moisture well ahead
of the approaching Pacific trough crossing SE Idaho from mid-morning
into early afternoon from west to east today. Additional showers
then look to move in this eve and overnight, becoming heavier as we
go beyond the end of the 12Z TAF period into Thurs. KPIH/KIDA/KBYI
look to largely remain VFR today, although brief periods of MVFR due
to lower cigs are possible during shower activity. KSUN will be the
first terminal to see more significant precip moving in ahead of the
main event Wed night into Thurs AM, and IFR ceilings will be
possible. KDIJ may fall somewhere in the middle, with a trend toward
MVFR/IFF ceilings as we head into Wed night/Thurs AM. Expect cigs at
all terminals to deteriorate to MVFR and then IFR Thurs AM/early
afternoon as widespread rain arrives. - KSmith/Huston

.HYDROLOGY...Models continue to heat things up through Thursday as
the mountain snowpack undergoes a rapid melt and runoff. This is
causing some concerns for flooding in low-lying areas and small
creeks. In addition to snowmelt, a significant amount of QPF will
fall over southeast Idaho as an atmospheric river makes landfall and
penetrates inland. Hinsberger/Huston

Flood Watch from Wednesday evening through late Thursday night
for IDZ018-031-032.

Flood Watch from late Wednesday night through late Thursday
night for IDZ019-022-023-025.

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