Area Forecast Discussion Issued by NWS Pocatello, ID
000
FXUS65 KPIH 102100
AFDPIH
Area Forecast Discussion
National Weather Service Pocatello ID
300 PM MDT Wed Apr 10 2024
.SHORT TERM...Today through Thursday Night.
Early afternoon satellite imagery shows a broad H5 ridge of high
pressure over the WRN CONUS which will continue to lead to warm
and dry conditions across CNTRL and ERN Idaho through Thursday.
Highs in the 40s/50s/60s today will increase around 5-10 degrees
for Thursday to the 50s/60s/70s as the ridge axis shifts directly
overhead. 15-20 kt 700 mb winds today supporting gusts to around
15-25 mph will increase later in the day Thursday and through the
overnight hours Friday to around 20 to 35 kts supporting wind
gusts to around 20-35 mph, especially in the mountains. Conditions
will continue to remain dry through Thursday evening as increased
cloud cover builds in out of the west as shallow moisture moves
atop the ridge. This will support some isolated shower chances
primarily in the higher terrain Thursday night as conditions
remain predominantly dry. MacKay
.LONG TERM...Friday through next Wednesday.
Well...for better or for worse...the long-term portion of the
forecast continues to feature a whole massive pile of active wx
for southeast Idaho. Over the last few days we`ve been discussing
a couple different potential scenarios for the evolution of our
next main trough/low pressure storm system for this weekend...hold
that thought, we`ll look at that momentarily. One thing that has
remained quite consistent in the forecast from model run to model
run for a couple days now though has been Friday, featuring a
weak, initial shortwave trough riding up the front/leading edge of
the main approaching trough. This feature...barely detectable in
height analysis and weakening with time...is negatively tilted,
which can be a synoptic red flag for potential strong to severe
t-storms. Despite the weakening nature of the shortwave, it should
still provide some broad ascent, and every mid-range to long-
range model IS in fact generating at least some isolated
shower/t-storm activity coincident with diurnal heating Friday
afternoon. A first look at NBM-projected SBCAPE (which we honestly
haven`t evaluated extensively yet but is based on multi-model,
ensemble-based, calibrated guidance) suggests a marginal but
adequate 200-400 J/kg SBCAPE (instability to support updrafts and
storm strength), while the NAMNest (which is often a bit
overzealous or at least falls near the upper-end of the guidance
envelope) advertises as much as 400- 900 J/kg SBCAPE...with a
sounding near Burley hinting at 30-35kts of both effective and
0-6km deep-layer bulk shear as well as very strong low-level lapse
rates. Bottom line...while we`ll need to continue to evaluate the
potential for strong t-storms as more guidance comes in...Fri
looks convective, with otherwise continued south flow ahead of the
main approaching trough continuing to support warm temps
averaging 15 degrees above normal for mid-April (highs in the
mid-60s to mid-70s for nearly all population centers).
Okay...now the main event. A main approaching trough of low
pressure will manufacture a solid-looking closed low along the
coast of CA by Sat. 500mb height cluster analysis Sat continues to
hint at the two possible scenarios we`ve been watching for this
low, with two clusters (about 45% of the ensemble membership)
supporting a slightly stronger/more anomalous closed low quite
disconnected from the remnant nrn branch of the trough that will
progress inland slower and bring a later pattern change to SE
Idaho, and two clusters (about 55% of the ensemble membership)
supporting a slightly more intact and progressive overall trough
(even with a closed low still advertised as the srn branch) that
brings cooler, wetter conditions inland to SE Idaho as much as
36-48 hours earlier. These ensemble percentages almost sound like
a flip-flop from this time yesterday when the first scenario was
more strongly favored, but we note that ALL clusters have trended
toward a much more similar timing/position solution for the closed
low compared to the differences we saw 24-48 hours ago...ALL
keeping the low well the SW of our CWA. Thus, confidence continues
to increase (and is now pretty high) that temps Sat will end up
remaining warm and almost identical to Fri with southerly flow in
place. Also, despite a first chance of precip Fri, upper-level
support associated with the low might remain far enough SW of our
region that Sat may end up leaning mostly dry for most of our
forecast area, and NBM PoPs have slowly been decreasing in line
with this conceptual expectation. Moving forward to Sun, we see
the same two scenarios, with the clusters supporting a more intact
overall trough and slightly weaker closed low behaving more
progressively with a trough axis near or already over SE Idaho
(60% support), and the clusters supporting a slightly stronger
closed low as the main player lagging back over central CA (40%
support) leading to YET ANOTHER very warm day, although perhaps
with an increasing trend in shower potential. The more progressive
solution would cool temps (and slow mid-elevation snowmelt) by
Sun with increasing showers, while the delayed solution would
support another warmer, drier day with continued snowmelt...thus
some potential implications here extending to the flood forecast
for the Portneuf River (more on that in the HYDROLOGY section
below). Interestingly, the 12z deterministic model suite seems to
be leaning toward the slower solution, despite the ensemble
percentages just mentioned. Thus, confidence remains low on
whether Sun will side with spring, or side with a pattern change
toward cooler, wetter wx.
Whether our pattern change arrives in earnest by Sun, or by Mon,
all models advertise yet another organized closed low pressure
system dropping out of Canada toward nrn Idaho, with cluster
analysis suggesting uncertainty in how quickly this system
arrives. At this time, it doesn`t look like there will be much of
any chance for a break (MAYBE Mon night?), with NBM guidance and
all long-range models showing widespread rain/snow across the
region on Tue. High temps will have already started cooling Mon,
but we take another 10 degree hit by Tue with highs only in the
40s to near 50. By this time the western half of the nation is
firmly enveloped in massive long-wave troughing, and there are
some indications that Tue`s low either slows down and lingers in
our area, or is replaced by yet another deepening low in the
region, either of which would keep cooler, wetter wx going into
Wed and likely beyond. We note that deterministic models have some
wildly conflicting solutions by mid- week as far as details are
concerned, but chances are pretty high in that cooler, wetter,
unsettled wx dominating for most of next week. By the way, breezy
conditions...first out of the south Fri/Sat and then more out of
the west thereafter...are expected each day throughout the
forecast, although so far holding below advisory criteria. -
KSmith
&&
.AVIATION...For the 18Z TAFS through 18Z Thursday.
A ridge of high pressure has finally taken hold across the nrn
Rockies and will reign as the dominant synoptic wx enforcer both
today and Thu, supporting high forecast confidence in dry, mostly
sunny/clear, VFR conditions for all terminals. Modest winds
checking in at 10kts or less today may trend just a touch higher
out of the WSW for Thursday, and sfc conditions will remain too
dry for any fog or low stratus concerns. We do carry some passing
high clouds in the TAFs over much of the period (generally FEW180)
based largely on the HRRR (which tends to pick up on high-level
cirrus the best and even perhaps over-advertises these shields)
but with some hints of support from upstream satellite
observations over the PacNW too. That said, no real impact is
expected for aviation for the next 48 hours, and there is perhaps
a 40% chance that SKC will be favored over much in the way of high
clouds. Looking ahead, an approaching shortwave trough will more
noticeably increase winds by Friday afternoon and support at least
isolated showers and thunderstorms for some terminals. - KSmith
&&
.HYDROLOGY...
As has been discussed for several days now, the Portneuf River
continues to be our only river of concern for flooding at this
early stage of our spring flood season, with very warm temps
averaging 15 degrees above normal by Fri and Sat supporting
increasing snowmelt, especially at mid-elevations in the basin.
Our latest collaborated forecast with the Northwest River Forecast
Center continues to bring the river back into minor flood stage
in the Pocatello area by early Friday morning, and then to
moderate flood stage by early Sunday morning. This potential crest
above moderate flood stage has PERHAPS trended SLIGHTLY higher in
the forecast over the past 48 hours...increasing from about 10.0
feet to about 10.3 feet...but otherwise no significant changes or
trends are noted, and resultant impacts should be approximately
the same with lowland flooding along the river, flooding into
Sacajawea Park and other trailheads in western Pocatello, and
significant flooding of agricultural land in the Blackrock area
and also down in Inkom near the confluence of the Portneuf River
and Marsh Creek. A FLOOD WARNING remains in effect until further
notice. A bit further upriver near Lava Hot Springs, the forecast
for the Portneuf River at Topaz continues to carry bankfull
conditions (action stage) with the river perhaps getting close to
that minor flood threshold. As discussed in the LONG TERM section
above, the evolution of this weekend`s approaching low pressure
system will have SOME impact on the river forecast...the more
progressive solution with the low arriving by Sun will bring
cooler temps earlier slowing snowmelt, while the delayed solution
with the low arriving Mon will extend snowmelt and may eventually
support SLIGHTLY heavier QPF, although overall QPF amounts in
either scenario look pretty modest at this juncture (with no
response on the ECMWF Extreme Forecast Index for QPF amounts),
thus runoff from snowmelt continues to be the driving factor.
Cooler and wetter wx is expected next week, and it`s uncertain
exactly how this will affect the river forecast as cooler temps
will slow snowmelt, but actively falling QPF could keep river
flows up. Stay tuned. REMEMBER... despite recent warm air
temps...the water in all of our rivers, creeks, and streams
largely originates from melting mountain snow and is dangerously
cold this time of year. Be especially careful to keep children
away from the water. - KSmith
&&
.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$