277
FXUS65 KPIH 200937
AFDPIH

Area Forecast Discussion
National Weather Service Pocatello ID
337 AM MDT Mon May 20 2024

.SHORT TERM...Today through Tuesday night.
Upper trof drops over Idaho today and main short term impacts will
be showers and and isolated thunderstorms in the afternoons
mainly both today and Tuesday as the trof works through.
Temperatures will remain very cool with a frost advisory this
morning with lows in the mid to upper 30s in the Magic Valley and
Snake River Plain. It will likely be another marginal case Tuesday
morning but will issue nothing right now. Highs will again be
around 10 degrees cooler than normal today with perhaps a couple
degrees warmer on Tuesday but still well below normal once again.
Highs today mainly 40s to lower 50s mountains and 50s valleys.
Snow levels 6 to 7 thousand feet but not expecting any impacts
with light accumulations well above 7 thousand feet.
GK

.LONG TERM...Wednesday through next Monday.
The midweek forecast period continues to look active with seasonably
cold temperatures as a trough overhead earlier in the week departs
east into Wyoming Tuesday night with another one moving in right
behind for Wednesday and Thursday. In between these two systems, a
transitory ridge of high pressure building in late Tuesday night
into early Wednesday morning will support drier conditions ahead of
increasing precipitation chances throughout the day Wednesday NW to
SE. A mix of rain/high elevation snow showers, and thunderstorms
will continue into Thursday as a H5 low tracks directly over the
CNTRL Mountains, SW Montana, and NW Wyoming.

As a result of this track over NRN portions of our CWA, look for the
highest precipitation totals in the CNTRL Mountains and along the MT
border, including along and near I-15 east into Wyoming. The latest
48-hour NBM QPF/rainfall forecast shows around 0.20-0.50" in these
valleys and 0.35-1.00" in these mountains with lighter amounts less
than 0.20" elsewhere in and around the Magic Valley. The NBM carries
generally the most liquid in comparison to the ECMWF/GFS at this
time with the placement of the low being the main determining factor
for where those heavier amounts end up developing. With respect to
snow, best chances for accumulations will remain confined above 6500-
7000` with the potential for light snow in places like Ketchum,
Stanley, Island Park, and across higher elevation mountain valleys
along the Wyoming border. Moderate snow accumulations are also
expected above 8000-8500` in that 5-10" range across the CNTRL
Mountains and along the Wyoming/Montana borders.

As the Wednesday/Thursday system departs NE into Wyoming and Montana
Thursday night into Friday, drier conditions will return regionwide
and drier, zonal flow fills in behind the exiting low. A H5 ridge of
high pressure still offshore in the Pacific will keep our favorable
storm track going into the weekend with another H5 low on the way
for Saturday. The difference between this low and the midweek low
will be its further northerly track amongst the latest ensembles and
deterministic models which means conditions would remain drier and
temperatures would not see as much of a drop versus a further
southerly track. That system Saturday will also be quicker moving in
nature as dry conditions return for Sunday and Monday with a H5
ridge moving overhead from the Pacific. This will support the return
of a warming trend for early next week with highs back in the
60s/70s and maybe even some low 80s across our lowest elevations.

High temperatures before that warmup however will be coldest on
Thursday with highs in the 40s/50s while every other day through the
weekend will be in the 50s/60s. Will also have to monitor the
potential for frost/freeze products Thursday and Friday mornings
across the Snake Plain given colder overnight lows in the 20s/30s.
In addition, winds will also remain breezy through the weekend,
peaking each afternoon with gusts up to around 30-40 mph. MacKay

&&

.AVIATION...For the 12Z TAFS through 12Z Tuesday.
Predominant VFR conditions will continue for Monday as our next
upper-level trough shifts overhead. This will support scattered
showers and isolated thunderstorms developing later today that will
continue across ERN Idaho into early Tuesday morning. The HREF model
probability of thunder shows a 20-40% chance at all terminals, again
highlighting those isolated chances for convection this afternoon
and evening regionwide. Winds today will be lighter in comparison to
yesterday, peaking around 10-15 kts with gusts to 15-25 kts. CIGS
will be partly to mostly cloudy through the TAF period, generally in
that 6-12k feet range with the potential for lower CIGS/VIS at times
associated with any shower or thunderstorm activity. MacKay

&&

.FIRE WEATHER...
Unsettled and seasonably cool conditions will continue through the
work week as a series of upper-levels troughs pass through the Gem
State for Monday/Tuesday and again on Wednesday/Thursday. These
systems will support a mix of rain/high elevation snow showers and
thunderstorms at times each day. That midweek storm system at this
time looks to provide the most widespread precipitation chances with
an emphasis on FWZ 410, 411, 413, 422, and especially 475/476 given
this low pressure system tracking directly overhead these zones with
drier conditions favored in FWZ 425 and 427. In addition to cooler
temperatures, winds will remain elevated given this active pattern
with the strongest winds expected on Wednesday/Thursday/Saturday.

A third, quick moving trough remains on track to pass through on
Saturday supporting another round of isolated showers/storms with
increasing confidence surrounding a return to warmer and drier
weather to round out the weekend into early next week. This will be
courtesy of a ridge of high pressure that was out in the Pacific
helping guide these troughs to our region finally move onshore and
bringing with it a return to seasonably warm conditions. Highs will
be in the 40s/50s/60s this week with lows in the 20s/30s/40s ahead
of warmer weather on the way to kick off next week. MacKay

&&

.HYDROLOGY...A flood warning continues for the Portneuf river in
Pocatello.  A flood advisory continues along the Portneuf river at
Topaz. Water levels are slowly receding for the Portneuf river due
to the snowpack diminishing and slightly overall cooler
temperatures.  River levels for most other big rivers look to
remain the same this week, if not slightly less, due to overall
slightly cooler temperatures expected this week. However, warmer
temperatures, perhaps the warmest of the year, look to return the
week of May 27th, which should bring increased runoff for most
other big rivers.
TW

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Frost Advisory until 9 AM MDT this morning for IDZ051>055.

&&

$$