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FXUS65 KPIH 232006
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
206 PM MDT Wed May 23 2018

.SHORT TERM...The low pressure disturbance is moving into Montana
this afternoon. The main area of precipitation is roughly from
Howe to Spencer, Osgood, and Driggs with radar showing a
decreasing area the past hour or two. Isolated thunderstorms have
developed between Hailey and Howe, also between Oakley and
Rockland where the sun has broken through for some afternoon
heating. By Thursday morning we will see showers linger in the
Central Mountains and along the Montana line as a ridge of high
pressure builds quickly ahead of the next round of weather.
Southerly winds will push temperatures back to the 70s and lower
80s Thursday and Friday. By Saturday, the next closed low pressure
disturbance will cross California into the Great Basin. Current
models are similar in timing, but this disturbance looks a bit
removed from the upper level flow pattern and may stall a bit over
Nevada and Utah. RS


.LONG TERM...Sat night through next Wed night. By Memorial Day (Mon)
the eastern Pacific upper level longwave trough has slid farther
north, leaving much of the U.S. West under weak and variable flow.
Thus there is low confidence in the forecast starting Memorial Day.
Such a pattern does allow subtropical moisture this far north
easily. This would mean mild overnight lows, and if the wind is from
the south, warmer than normal highs. Certainly any subtropical
moisture will mean also unstable air and a threat of afternoon
thunderstorms and rain showers. The lack of any organized front
should keep wind light, outside of thunderstorms. The ECMWF and the
GFS both have a closed low moving through Great Basin in different
locations, with the ECMWF sometimes stationary. But on Mon night
this closed low lifts directly over the forecast area heading nearly
northward. So by late night Mon/Tue morning, the precipitation
decreases significantly from what we will be receiving prior to this
point. Showers and thunderstorms do continue though for Tue
afternoon/evening. By Wed afternoon, a will organized upper level
trough is moving into western Idaho, bringing showers and
thunderstorms with it. Through it all, the GFS has considerably more
precipitation, compared to the ECMWF. Have attempted to compromise
between these two product suites, but even the location of heaviest
rainfall is at times nowhere near what the other guidance says. So
painted with a very broad brush. Messick

&&

.FIRE WEATHER...Low pressure over the region will produce scattered
showers and thunderstorms through Thursday. Gusty erratic winds and
small hail are likely near thunderstorms. May see widespread
coverage today across much of northeast Idaho along the upper Snake
Highlands and perhaps into the upper Snake Plain. Friday may be dry
before another system arrives for the weekend.

Valle

&&

.AVIATION...Abundant rainfall today continues through the early
evening. It has brought with it some marginal VFR at two airdromes,
where it has been raining since last night, KIDA and KDIJ. Skies
will clear from west to east, and so KIDA will improve earlier. KDIJ
is likely not to clear much at all, as it already has had close to
half an inch of precipitation since 23/06Z. Expect stratus and BR to
be a problem there tonight. Vertical height-time cross sections have
the increase in surface humidity around 24/12Z, and the NAM output
reduces visibility to the 1SM range, and it all seems plausible.
Elsewhere, TSRA should develop this afternoon, but HRRR guidance
does not present any sort of boundary that develops and pushes
through. This could give some accuracy in timing, but without it, am
pretty much limited to VCTS remarks so that the airdromes are not
shut down for the entire afternoon and early evening. KBYI is the
least affected, and may receive no rain this afternoon. Mid- to low
level moisture appears to continue at KSUN, and so the CIG will
continue during the overnight and into Thu afternoon. TSRA activity
for Thu afternoon/evening should be much less at KSUN and the three
valley airdromes, but actually higher at KDIJ as skies will be
clearer and more heating generates strong instability. Messick

&&

.HYDROLOGY...The past 24 hours there is a pretty good size area
that includes Rexburg, Ashton, Ririe, Driggs, and Antelope Flat
that received .75 to 1.40 inches of rain. a Flood Advisory was
issued for the Teton River at St. Anthony and a Flood Watch was
issued for the henry's Fork River near Rexburg. It may take a few
hours before the impact of rain and runoff is felt. There is some
uncertainty about how much rain is yet to fall through the
afternoon hours. Some smaller creeks and streams could run swift
tonight and Thursday. Total observed rainfall in the area of the
Big Wood at Hailey has been much less than anticipated and impacts
should be minimal. RS

&&

.PIH WATCHES/WARNINGS/ADVISORIES...None.

&&

$$
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