Area Forecast Discussion
Issued by NWS Pocatello, ID

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FXUS65 KPIH 211938

Area Forecast Discussion
National Weather Service Pocatello ID
138 PM MDT Fri Jul 21 2017

.SHORT TERM...Tonight through Mon afternoon. Stronger westerly
wind aloft should finally put an end to the pesky isolated
convection that peaked yesterday with some reports of large hail,
at least through the weekend anyway. The unstable and moist air
mass will be shunted in central and northern Utah without a chance
to infiltrate this far north, except possibly the southern
highlands, leaving a great weekend to see the Blue Angels at
KIDA. But late Sunday night/Monday is the start of a radical
change to the forecast. The surge of unstable subtropical air will
return from the west around the Four Corners high. So until late
Mon night, expect mainly dry thunderstorms, possibly at the end of
this period transitioning to a mixture of wet and dry.
Temperatures are warming up over the weekend and will likely peak
on Mon afternoon before the thick cloudiness arrives. Messick

.LONG TERM...Mon night to next Fri night. The big change occurs
right at the beginning of this period. The precipitable water goes
above 1 inch late Mon night; this brings a threat of heavy
rainfall and flash flooding, plus an increase in lightning
activity. There is no let-up in this amount of moisture until Wed
night. The thicker cloud cover drops temperatures as much as 10 to
15 deg F from Mon afternoon. Overnight lows will be very mild. Do
not expect much in the wind department, outside of the
thunderstorm activity. Admittedly the ECMWF is drier, but the GFS
has been unusually consistent with this extended period. Messick

Generally zonal flow today will give way to slight, gradual ridging
Saturday across the region, with high pressure firmly in control
both days. High confidence in VFR conditions at all terminals with
no forecast concerns. Combined with highs in the 80s to near 90 at
all our major airports...two gorgeous days for aviation! Next chance
for monsoonal moisture to creep back north across far southern Idaho
comes Sun afternoon with perhaps VCTS not far from KBYI, but
potential is very marginal at this time. - KSmith/Messick

Dry westerly flow will be in place through the weekend as SE Idaho
remains sandwiched between disturbances passing eastward along the
Canadian border and a building ridge of high pressure over the
Colorado Plateau. Mainly light winds expected Saturday, but some
increase in winds Sunday, particularly across Zones 425 and 410
Sunday afternoon/early evening, with SW winds gusting to around 25
mph. Min RH will be below 15% with a Haines index of 6 across this
region -- so fire danger is elevated but not quite critical due to
winds being below Red Flag thresholds. Early next week, a Pacific
trough swings through the Pacific Northwest. Southwesterly flow
ahead of it will mean an impressive surge of moisture will flow into
the region, peaking on Tuesday. As a result, precipitation chances
increase substantially. Expect isolated dry thunderstorms developing
Late Monday across western zones, with numerous showers and
thunderstorms expected Tuesday all areas. Thunderstorms should
transition to more of a `wet` variety due to precipitable water
values increasing to 200-250 percent of normal by Tuesday. Locally
heavy rain will be possible. Showers and isolated thunderstorms
linger Wednesday before a dry high pressure ridge builds back into
the area to close out next week. AD/Messick


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