Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 222021

Area Forecast Discussion
National Weather Service Pocatello ID
221 PM MDT Thu Mar 22 2018

.SHORT TERM...Tonight through Saturday night. Rain on snowmelt
event continues in the short term. Deep trough off Pacific coast
pushing vigorous shortwave axis through coastal states toward ID
this afternoon. Weak boundary on the verge of entering western
counties of forecast area per radar and surface observations.
This boundary might be sufficient enough to support isolated
convection, even in absence of significant instability despite
brief clearing in dry slot ahead of it, with gusts approaching
50mph. True cold front lagging this feature by several hours, but
is still a concern both for gust potential and the temporary
increase in rainfall rates. Minor flooding already occurring
throughout Wood River Basin per spotter and media reports. If
stronger convection makes it as far north as this region, the
additional quick burst of rainfall could worsen the developing
situation. Already have an urban/small stream flood advisory in
place through 03Z, but will bear additional watching this
afternoon. Main shortwave axis shifts east overnight along with
main front, taking precipitation focus to regions east and
bringing snow levels down. Main trough remains off the coast
Friday with East Idaho between significant shortwaves for a
relative dry trend. Any lingering light precipitation over higher
elevation areas will likely be snow. Next feature pushing inland
and across the region during the day Saturday. Weak instability is
present, so isolated thunder not out of the question, and
especially east of I-15 during the afternoon. Breezy winds
possible through the Snake Plain. Otherwise temperatures warm just
enough to support mix of rain/snow in lower elevations, but
generally all snow above 6000 ft. Upper trough shifts inland
Saturday night with shortwave pushing across the region. Main
focus under west/southwest flow should be higher elevations
overnight Saturday night, with temperatures cold enough to support
snow at all elevations. DMH

.LONG TERM...Sunday through next Thursday. No major model variances
in the long-term periods among the GFS/ECMWF. Forecast shows gradual
drying from NW to SE Sunday night through Monday. All this happens
while models begin cutting off an upper low over the Desert
Southwest Monday night, while eastern Idaho comes under the
influence of upper ridging. This will force temperatures back into
the 50s by Wednesday and Thursday. Hinsberger


.AVIATION...Widespread rain across the Intermountain West will
influence ceilings and visibilities over the East Idaho terminals.
Good confidence we will see below VFR conditions, but less
confidence on how low it will go. Worst conditions will exist in the
proximity of the cold front as it passes through later this
afternoon/evening. Instability across the region supports a slight
chance of thunder, but not confident enough to include in forecast.


.HYDROLOGY...We continue under a warm, moist southwesterly flow,
thanks to the inland penetration of a Pacific atmospheric river.
Snowmelt and rainfall will contribute to excessive runoff into area
basins as well as sheet flooding. Hydrographs are beginning to
respond to the increased runoff, but none are anticipated to enter
flood stage just yet. Precipitation will begin to wane after the
passage of a cold front late this afternoon and into the evening.
Snow levels will fall behind the cold front, and inhibit further
snowmelt runoff. Hinsberger


Flood Watch through late tonight for IDZ018-019-022-023-025-031-


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