Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 211945

Area Forecast Discussion
National Weather Service Pocatello ID
145 PM MDT Wed Mar 21 2018

.SHORT TERM...Tonight through Friday night. Main story of the
forecast continues to be upcoming significantly wet period. Upper
low continues to sag south along PacNW coast, with base of
amplified trough well off CA coast and associated moisture stream
ahead of it. Northern low still expected to act as kicker feature,
sending moisture streaming into East Idaho as early as late
tonight. Radar already showing light showers over northern NV/UT,
reaching just into southern ID this afternoon. Main batch of
moisture still located over central/northern CA but moving into
western NV this afternoon. Models have diverged slightly on where
these moisture streams shift, and how much QPF is expected as the
trough shifts toward the coast. GFS significantly wetter than the
NAM, but both a little west of current radar returns. Both shift
band of heavier precip across the central mountains overnight, but
GFS is much faster with this feature and again significantly
wetter. Given current upstream radar returns, and with support
from high-res runs, gave much further weight to NAM. Result is a
lowering of QPF overnight across the central mtns, holding on to
slightly higher values further south. With next system ejecting
into the region during the day Thursday, more consolidated
moisture stream moves into the region. Models generally agree on
Thursday daytime having more widespread precipitation and greater
QPF. In addition, some weak instability appears to be present so
have added Thunder mention in the 21z-03z time frame. Main region
of moisture shifts east overnight Thursday night, along with cold
front allowing snow levels to fall back to valley floors in the
central mtns. Overall QPF values now through Thursday night still
support areal Flood Watch over higher elevation zones. Given the
late start in the rainfall and the recent model shifts, opted to
NOT upgrade to a warning at this time despite threat of continued
snowmelt at lower elevations. See Hydrology segment below.

Friday and Friday night still appearing to be a relatively quiet
period. Next segment of upper low digs toward PacNW coast, but
East Idaho between major associated shortwave features and in
much drier air mass. Thus only expecting a few light showers, at
best, in central mtns and along WY/UT border regions. DMH


.LONG TERM...Saturday through next Wednesday. Models are
advertising similar solutions for the weekend with a closed low
circulation developing over OR/NV Sunday afternoon. This will keep
precip over much of the area through Monday evening. The upper
low cuts off and drops southward into the Desert Southwest by
Tuesday morning while most of Idaho undergoes a drying trend.
Precip for Monday will likely be confined to the Southeastern
Highlands around the periphery of the upper low. Stability
profiles show a very slight chance of thunder, but not confident
enough to include in forecast. Temperatures look to be rather cool
over the weekend, and warm up gradually by the middle of next
week. Hinsberger


.AVIATION...Extensive moisture is beginning to advance into the
Great Basin where atmospheric river conditions are beginning to ramp
up. Isolated showers are currently seen on radar across southeast
Idaho, but none at the terminals as yet. Ceiling has lowered to MVFR
at KSUN at issuance time, but was low VFR just prior. Models are
suggesting an initial wave of precip across the Southeastern
Highlands and possibly the Snake Plain this afternoon. The main
onslaught of moisture will enter the picture around 2 am and work
its way west to east by around sunrise. This will likely bring MVFR
or IFR ceilings to most of the area terminals. Hinsberger


.HYDROLOGY...Atmospheric river conditions coupled with a strong low-
level warm air advection will increase runoff potential through
tomorrow afternoon. Snowmelt and QPF-driven runoff will make its way
into lowlands and will likely cause sheet flooding as well as
increasing flows in rivers and streams. Of note, the Portneuf River
in Pocatello, and the Bear River near the Utah border will be near
bankfull conditions by Friday. Precip rates will taper off Friday
night, and snowmelt will be inhibited by cooling temperatures Friday
night into Saturday. Hinsberger


Flood Watch from 6 PM MDT this evening through late Thursday
night for IDZ018-031-032.

Flood Watch from midnight MDT tonight through late Thursday
night for IDZ019-022-023-025.


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