Area Forecast Discussion
Issued by NWS Pocatello, ID

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393
FXUS65 KPIH 202013
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
213 PM MDT Mon Mar 20 2017

.SHORT TERM...Latest surface analysis would have the surface low
closer to Bishop/Tonopah area than where the NAM12 would. Some
sunshine allowing for instability down Nevada way where as we have
been under some clouds for awhile. Potential could just be taking
a little longer. Will keep some isolated thunderstorm wording in
the forecast for the South Central Highlands and Lower Snake Plain
this evening. Snow levels have been rising, Horton Peak elevation
8800 feet near Galena is currently 40F. Any snow impacts tonight
should be mostly above the level of mountain passes. The low in
the Eastern Pacific moves closer to the coast Tuesday. Some
precipitation to overspread the Central Mountains, and by late
afternoon early evening another round of showers develops across
the South Central and Caribou Highlands, isolated thunderstorms
possible again. Snow levels do not fall until Wednesday morning.
By that time the low in the Pacific begins to take on a split
pattern, with the Stanley area maybe seeing a chance of showers
from the northern branch, while moisture from the southern part of
the split tracks diagonally from about Oakley to Driggs Wednesday
afternoon and night. The risk of rain fall will aggravate the
flooding threat particularly around Bear Lake. Although the
bigger rain threat may well wait until this coming weekend.
Otherwise the chance of precipitation late Wednesday night through
Friday night loos pretty minimal. RS

.LONG TERM...Friday through Monday. Upper ridge transitions east
across Idaho through the day Friday, with models bringing
precipitation into central mountains by late in the day. Main
shortwave digs well south into Saturday as upper trough begins to
transition through the PacNW, with precipitation shield extending
across almost all of the region. GFS and ECMWF in reasonable
agreement with timing and depth of the trough but ECMWF translates
heaviest precipitation through the region slightly quicker. ECMWF
also slightly colder than the GFS, but both keep Idaho in warm
sector until cold frontal passage late Saturday night or early
Sunday. Snow levels do not quite drop to valley floors but
reasonable accumulations likely at mid slope or higher for this
event. Leading shortwaves shift east and quite a bit of difference
in model solutions heading into Day 8. GFS keeps western US under
broad trough with next set of shortwaves diving through coastal
states into southern Nevada. ECMWF translates ridge across the
region with a following consolidated trough that splits similar to
the previous system but less amplified. Opted to maintain an
unsettled blend for rest of forecast period. DMH

&&

.AVIATION...Another round of showers starting to develop this
afternoon. Best instability for potential thunderstorms working
northeast out of Nevada. Best chance of thunderstorms would be
roughly 21Z-03Z, but likely to impact all but KSUN. Ceilings
expected to remain VFR but could see MVFR within convective
activity. Showers may continue overnight but low confidence in
timing and location at this time. Much stronger upper feature
impacts the region during the day Tuesday, and expect additional
precipitation to overspread the area by afternoon. DMH

&&

.HYDROLOGY...Emergency Management across the region have reported
mostly status quo or worsening conditions. Thus have continued all
areal flood advisories plus added a few more in particular trouble
spots. Area web cams and/or emergency management indicate snow pack
continues to ripen, and have added additional areas to areal flood
watch in response to these concerns. Will likely trim portions of
the flood watch in the Snake Plain at next update. With respect to
rivers, most gages showing modest rises in response to runoff.
Portneuf remains above flood at this time, so warning was continued
there. Bear River could reach flood stage by Wednesday night, so
issued watch for that region. Remainder of rises covered by blanket
river statement at this time. DMH

.PIH WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday morning for IDZ017>019-021>025-031-
032.

&&

$$



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