Special Weather Statement, NWS Pocatello, ID

Area Forecast Discussion
National Weather Service Pocatello ID
220 PM MDT Thu Jul 22 2021

...A FEW STRONG TO SEVERE THUNDERSTORMS WITH FREQUENT LIGHTNING,
HAIL, GUSTY WINDS, AND TORRENTIAL RAIN POSSIBLE THIS AFTERNOON...

.SHORT TERM...TODAY THROUGH SATURDAY NIGHT...

OVERVIEW: We kick off the afternoon with a persistent, synoptic-
scale high pressure dome situated over the 4-Corners region. A
favorable SW flow around the west/back side of the high has pumped
increased monsoon moisture northward into portions of our forecast
area, and this will help support one more day of afternoon/eve t-
storms. Meanwhile, a trough (connected to a low pressure center
crossing Canada) is lightly brushing across the nrn Rockies,
increasing regional winds. While this feature won't send any
organized cold front across the state of Idaho, it does signal an
important pattern change as it will serve to "flatten" the 4-Corners
ridge, introducing a drier, more zonal westerly flow across Idaho
and shunting monsoon moisture back to the south of our forecast area
for several days. This will generally support dry, mostly
clear/sunny conditions Fri/Sat, with afternoon min RH values
trending back below critical thresholds with poor overnight recovery
at upper slopes (see the FIRE WEATHER section below for more on core
fire wx concerns). One exception may be the Bear Lake area, where
several of the CAMs (especially the HRRR/NSSL/ARW) suggest a few
lingering isolated t-storms may be possible Fri afternoon. Blends
such as the NBM do not capture this and we're holding dry for now,
but low PoPs may need to be introduced here in future forecasts.
High temps today through Sat are forecast to top out in the upper
80s to mid 90s, continuing to run about 5 to 8 degrees above
climatological normals for late July. Overall forecast confidence in
the above trends and themes is high, with lowest confidence tied to
impact of cloud cover on destabilization this afternoon (more below)
and on whether isolated t-storms will indeed manage to develop over
the Bear Lake region Fri afternoon. The shift from SW to W flow
unfortunately will continue to bring wildfire smoke in across the
region given continuation of upstream fires in nrn CA, srn OR, and
central ID, with no significant signal for improvement. With fires
continuing in source regions for almost every flow we typically
experience this time of year, it's going to be hard to avoid the
smoke unless some of the fires can be extinguished. Continue to
refer to the latest monitoring data from Idaho DEQ for air quality
information. We continue to carry areas of smoke in the wx grids.

SEVERE WEATHER TODAY: The HREF ensemble consensus supports best t-
storm coverage (sct) across the srn/ern highlands (east of I-15 and
south of I-86) and initiation between 19-20z/1-2pm, with more
isolated cells also possible across the far ern Central Mntns and
Snake Plain early on (before 00z/6pm). Used a blend of NBM guidance
and the HREF consensus, lightly broadbrushed, for forecast PoPs.
Taking a look at core t-storm ingredients, this is where monsoon
moisture is indeed maximized. Lift will largely be orographically-
induced at initiation (with srn vestiges of the nrn Rockies trough
assisting as well on the broad-scale as our CWA is positioned near
the right-entrance region of the mid-level jet), with outflow
boundaries also coming into play after initial cells fire. Moderate
cloud cover early this afternoon continues to cast some uncertainty
on whether or not full heating/destabilization will materialize
everywhere, but not enough to change our forecast/messaging.
Assuming continued breaks in the clouds, localized pockets of fairly
impressive SBCAPE (instability) of 1,000-1,300 J/kg may be realized
per NAMNest/RAP forecast soundings, esp in the srn/ern highlands (a
bit less further NW). Combined with moderate effective bulk shear of
up to 20kts in the Snake Plain and 30kts into the srn/ern highlands
(higher in the raw 0-6km layer), the stronger cores may VERY
effectively/rapidly pulse up and could produce dime to quarter size
hail, esp (again) in the srn/ern highlands where CAPE profiles are
the fattest in the mid-levels along with decent mid-level lapse
rates. A lack of greater shear/instability should keep hail from
getting too much larger than this. On the wind front, somewhat
shallow "inverted-V" profiles with forecast DCAPE of 1,200-1,600
J/kg and sfc-3km lapse rates in excess of 10 degC/km (despite shear
and boundary layer winds a bit stronger than the ideal model)
support a pseudo dry-microburst regime, trending into a wet-
microburst regime across the srn/ern highlands given the stronger
instability (but more marginal DCAPE closer to 1,000 J/kg). In
either case, downdraft/outflow winds in excess of 50 MPH will be
possible. Fully expect we may see a severe cell or two this
afternoon as long as instability is sufficient...have added mention
of heavy rain, gusty winds, and small hail to the wx grids. Finally,
frequent lightning will be a concern both for those caught outdoors
and for new fire starts.

FLASH FLOODING TODAY: NAMNest/RAP soundings support PWATs in the
0.75 to 1.0" range in the t-storm risk area, confirmed by the latest
SPC mesoanalysis and HREF mean forecast (both 0.8 to 1.2"), and
current sfc dewpoints check in at 50-55F. All of this is well above
average for our area and may support rainfall rates in excess of 0.5
inches per hour, a regime similar to what we saw Tue this week.
Storm motions will be decent taking the edge off the overall threat,
but we will still need to closely monitor for localized torrential
rainfall and flash flooding, particularly over burn scars, where
cells get "hung up" on terrain features, or where multiple t-storms
train over the same area. The overall threat is marginal/isolated
enough to preclude a Flash Flood Watch. Interestingly, quite the
moisture gradient exists across the CWA where PWATs quickly drop off
to less than 0.7" as you reach the nrn/wrn edges of the Snake Plain
and even less than 0.5" up toward Stanley, helping to support a Red
Flag Warning for low RH and gusty winds for two of our Central Mntn
zones! - KSmith

.LONG TERM...SUNDAY THROUGH NEXT THURSDAY...
Fairly zonal flow will continue Sun with slightly warming temps.
Starting Mon (but esp Tue onward), the srn high pressure dome
rebuilds/amplifies, and nudges NE to a position just SE of our CWA
again. A return to southerly flow on the back side of the ridge will
once again begin pumping increased monsoon moisture back into the
region, with daily chances of t-storms returning Tue onward. High
temps will also make a run back toward upper 90s/100 degrees Mon
afternoon, with ECMWF EFI values increasing to 0.6-0.7 (suggesting
an unusually warm event compared to the model climate). This signal
actually continues Tue, but with the potential for increased cloud
cover in association with the monsoon moisture, NBM MaxTs back off
several degrees with a similar nudge in GFS/EC ensemble meteograms,
so Mon is really looking like the hottest day and the one to watch.
Made no significant deviations from NBM guidance from Sun onward at
this juncture. - KSmith

&&

.AVIATION...
Isolated to scattered thunderstorms in play once again today over
East Idaho, with highest confidence for the Snake Plain terminals
and KDIJ. Less confidence for KSUN as drier air beginning to slide
into the region. Convection expected to last through about
sunset, then dissipate, leaving clearing skies and trend toward
warmer and drier for the weekend. Breezy conditions this afternoon
expected, also helped with thunderstorm outflows. Breezy to windy
conditions in place on Friday as well. - DMH

&&

.FIRE WEATHER...
Today should be the last of this round of monsoonal thunderstorms as
shortwave feature flattens ridge and shunts moisture out of the
region. Best thunderstorm coverage remains over eastern highlands,
and thus will keep Red Flag for scattered thunderstorms. Breezy
conditions continue through the evening, with the addition of gusty
erratic thunderstorm outflows. Red Flag will remain in place for
marginal to just above critical wind thresholds for central
mountains. As drier air begins filtering into the region Friday,
expect another round of breezy to windy conditions. Gusts look to
approach or exceed critical levels once again, so another Red Flag
Warning is likely, filtering into more areas as humidities drop
across the region. Expect hot dry and occasionally breezy conditions
to persist through the weekend into early next week. The next round
of monsoonal thunderstorms should arrive by Tuesday and then last
through mid week. - DMH

&&

.PIH WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 9 PM MDT this evening for IDZ411-413-427-
475-476.

&&

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