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000
FXUS65 KPIH 112040
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
240 PM MDT Thu Apr 11 2024

.SHORT TERM...Today through Friday Night.
High pressure remains in full control once again this afternoon
leading to mostly sunny skies and seasonably warm temperatures as
mid-level and upper-level clouds pass overtop the ridge. Highs in
the 50s/60s and low 70s around the Magic Valley today will
increase for Friday into the 60s/70s with upper 50s in some our
high elevation mountain valleys as a ridge axis of high pressure
shifts east into the CNTRL CONUS. Winds will remain light today
courtesy of 20-25 kt 700 mb winds with gusts staying at or below
25 mph but will slightly increase for Friday to around 25-35 kts
supporting gusts to around 20-35 mph as a shortwave trough lifts
north out of NV/UT.

Overnight tonight, that aforementioned shortwave trough will
begin to lift north supporting isolated showers overnight
transitioning to isolated to scattered showers and thunderstorms
in the afternoon. The latest 12Z HREF model shows 400-700 J/kg of
SBCAPE and 20-35 kts of 0-6 km shear which with a slight uptick in
PWATs to around 0.50 inches will support isolated to scattered
thunderstorm development. With an emphasis on the stronger shear
profiles in WRN Idaho and the CNTRL Mountains, this is where we
are expecting stronger storms which is line with the latest HREF
probability of thunder showing a 40-70% chance of thunderstorms in
this area. Heading further SE, those chances decrease to around a
10-30% chance supported by a weaker shear profile. The SPC Day 2
Outlook has outlined our entire CWA in a general thunderstorm risk
and included a MARGINAL RISK for severe thunderstorms across WRN
Idaho supported by the over 500 J/kg of MUCAPE and 40+ kt
effective bulk shear for those areas. This should support the
potential for storms to near severe criteria but again will be
focused just west of our region.

As this shortwave trough departs north into Montana Friday night,
dry conditions will return regionwide as a drier airmass fills in
behind for Saturday. Overnight lows both tonight and Friday night
will be noticeably warmer in the 30s to mid 40s courtesy of broad
southerly flow. MacKay

.LONG TERM...Saturday through next Thursday.
500mb height clusters continue to converge on a "later" solution
for this weekend`s low pressure system moving inland from the
Pacific. Thus, confidence is high that our warmest temps Fri will
continue Sat (60s and 70s) with NBM-led PoPs continuing to trend
down as the approaching low remains a bit too far to our SW. EOFs
suggest forecast uncertainty by Sun is strongly tied to timing
differences in the evolution of the low, yet even by Sun, ALL
clusters now maintain a low that is positioned far enough SW of
our forecast area to hold on to milder temps and southerly
flow...even as cloud and shower potential increases. While there
is hope for Sun, sorry... there isn`t for Mon as all clusters
reflect the "axis" of the low over or east of the CWA, ending our
taste of spring. Cooler, wetter, unsettled wx remains on tap for
much of next week with multiple low pressure systems at play under
broad longwave troughing...high temps by Wed may only reach the
upper 30s to mid-40s right down into the Snake Plain! Every day
Sat onward looks breezy, but interestingly, QPF amounts continue
to look pretty modest (less than 0.50" over the entire period).
Perhaps the most useful question...when can we end the madness? By
Thu, one ensemble cluster offers a scenario where troughing is
firmly moving east and ridging may start to build back into ID,
but it only consists of 14% of the membership. By Fri, a bit more
hope...two clusters (55% of the membership) suggest increasing
influence from ridging, while another two (45% of the ensemble
members) keep us under troughing. For what it`s worth...while
there are significant differences from model to model...all of the
12z long-range deterministic models keep some sort of low
pressure system in our area by next Fri. - KSmith

&&

.AVIATION...For the 18Z TAFS through 18Z Friday.
High pressure will continue to support low-impact, VFR conditions
for aviation through early afternoon Fri, with just FEW to SCT
high clouds spilling across the region ahead of our next storm
system. The only potential fly in the ointment is some weak ascent
from modest mid-level warm air advection near 700mb potentially
supporting some elevated clouds or showers from after sunset
tonight through midday Fri, reflected in simulated reflectivity on
3 out of 5 of the HREF members. Confidence is low in whether
we`ll achieve clouds, virga, or bonafide showers, but we did toss
a few clouds around 7,000 feet into the TAFs for now and will
continue to monitor. Even if showers develop, their elevated
nature, weak instability, and a fairly dry low-level airmass offer
virtually no risk of anything other than VFR cigs/vsbys. An
approaching shortwave may support isolated showers/t-storms Fri
afternoon with the best chance at KBYI and KSUN...have introduced
VCTS so far for KSUN in their longer TAF starting at 21z/3pm Fri.
- KSmith

&&

.HYDROLOGY...
A FLOOD WARNING continues for the Portneuf River in the Pocatello
area, where the latest forecast has continued to trend toward a
faster/sooner rise, now exceeding minor flood stage by tonight and
moderate flood stage by Saturday afternoon. The crest has also
continued to trend slightly higher, now 10.7 feet by Mon AM. A new
FLOOD ADVISORY has also been issued upriver from Chesterfield
down through Lava Hot Springs to Inkom, where the river is
expected to be at bankfull soon and potentially approaching minor
flood stage Sun afternoon. Outside the Portneuf, significant
snowmelt across the south-central highlands will also result in
some smaller dams/ reservoirs and creeks/streams running high or
filling. We remain in close contact with local emergency
management officials in the event that any problems develop and
require alerts. Even with plenty of unsettled wx in the forecast
over the next 7 days, snowmelt and runoff due to warm temps
continue to be the primary driver of the flood risk, with overall
pretty modest QPF forecast through the end of next week (less than
0.50" total). - KSmith

&&

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

&&

$$