NWS Pocatello, ID wrote its last area forecast discussion before the first event at Aug 18, 2:42 AM MST, 8 hours 18 minutes earlier.
Area forecast discussion, Aug 18, 2:42 AM MST, 8 hours 18 minutes before
Area Forecast Discussion
National Weather Service Pocatello ID
342 AM MDT Tue Aug 18 2020
...HEAT, STRONG THUNDERSTORM, AND FIRE WEATHER HAZARDS CONTINUE OVER
THE NEXT SEVERAL DAYS ACROSS SOUTHEAST IDAHO...
.SHORT TERM...TODAY THROUGH THURSDAY NIGHT...
OVERVIEW: On the synoptic scale, we open this AM with a broad
ridge of high pres that remains centered over the UT/NV border
covering much of the wrn half of the US. A clockwise flow around
this high will continue to bring hot air northward along the
Pacific coast and then across our forecast area on a deep-layer
SW flow. This flow will also continue to gradually pump monsoon
moisture northward from the 4-Corners region, increasing the
threat of t-storms each day. This ridge will remain our dominant
player for the foreseeable future. If any trends are to be noted,
the ridge will weaken a bit between today and Thurs, and the top
half of the ridge in particular will diminish/"lay over" as broad
troughing builds inland across the PacNW. This trend, combined
with increased cloud cover due to the monsoon moisture, should
slightly moderate our temps as we head into mid/esp late week.
Continued heat, strong t-storms, and fire wx all remain concerns
over the short-term. Let's break each hazard down in more detail.
HEAT WAVE: Our long-duration heat wave continues across SE Idaho on
that warm SW flow. Mon we tied or broke record high temps all over
the region, and records are possible again today at most of our
climate sites. While confidence remains high on the GENERAL flavor
of the heat wave continuing, we start to introduce a bit more
uncertainty today in the form of cloud cover. Early AM satellite
shows quite a bit of cloud debris (with some sfc shower activity
as well) inbound from nrn NV/SW ID. NBM guidance (with support
from the NAMNest) suggests some of this may wash out as it moves
in and leave us party-mostly sunny before afternoon convection
fires, but confidence is low as some other models (like the RAP
and Superblend) are more pessimistic. In addition, high-res
guidance fires t-storms late this afternoon over the Cassia
County area, with potential for cumulonimbus anvils to stream NE
into the Snake Plain and mess with heating there. All things
considered, see no strong reason to change the forecast...just
keep in mind that more clouds may hold highs several degrees shy
of the forecast, while more sunshine will help potentially record
highs to again be achieved. Some of that cloud-related uncertainty
will continue tonight and Wed as well, all depending on how the
t-storm picture evolves. The HEAT ADVISORY remains in effect
through Wed eve. Concern continues over potentially poor overnight
temp recovery tonight/Wed AM, especially if clouds remain in
place to act as a blanket, and highs Wed are still only coming in
a couple degrees cooler than today. Remember, heat-related stress
on the human body is cumulative, and Wed (as Day 4 of our heat
wave) really doesn't look to offer much "real" relief. With some
weakening of the ridge, highs Thurs looks to back off a few
degrees (although still exceeding 90 for most lower-elevation
areas and pop centers), with better overnight recovery progged
for Thurs night. We encourage everyone to continue taking
precautions to avoid heat-related illness. NBM guidance continues
to perform brilliantly for our temp forecast, and within 1-2
degrees of statistical guidance and ensemble clusters. We did
nudge the forecast 1-2 degrees over NBM guidance for Thurs.
THUNDERSTORMS: With increasing monsoon moisture comes an increasing
risk for t-storms each day. NBM guidance was useless for PoPs, so as
usual we sided with the best consensus off of the HREF ensemble
members/HRRR/RAP for coverage/placement, and manually edited
varying blends of NAM/SREF/NBM guidance to light up the corridors
deemed favorable by these convection-allowing models (CAMs). The
main lifting mechanism will be orographic, so t-storms will really
favor the mntns. Today, initial development appears likely from
near Mackay/Challis NE along the MT border by 2-3 PM, with
coverage expanding a bit across the Central Mntns, Upper Snake
Plain, and Teton Valley/Big Holes after 3 PM. Coverage MAY reach
sct for some of these areas, esp the ern Central Mntns/Upper
Snake Highlands, prompting a Red Flag Warning (see the Fire
Weather section below). Some late-afternoon development may also
occur over the Cassia County region. Confidence is moderate today,
but then falls apart overnight as high-res models are all over
the place on potential for iso nocturnal showers/t-storms. Best
chance might stay north of the Snake Plain and our forecast
reflects this. Wed and Thurs afternoon/eve coverage expands
across all of our mntn ranges to high end iso or sct. While not
captured well by the models, experience suggests it's going to be
hard for a stray storm or at least convective outflow boundaries
to NOT escape into adjacent portions of the Snake Plain (and serve
as localized lifting mechanisms themselves), so we broadbrushed
low-end PoPs regionwide to cover this. PWATs will be on the
increase so storms may produce heavy downpours, but the sfc will
remain fairly dry all week keeping QPF in check. As far as fire
season is concerned, do NOT expect significant rainfall or any
attendant improvement to fuel moistures anywhere this week.
SEVERE WEATHER: Running some NAMNest/RAP point soundings, today's
environment will be characterized by very dry and well-mixed low-
levels with "inverted-V" T/Td profiles, 0-3km lapse rates in
excess of 10 degC/km, and DCAPE (downdraft instability) over
1,500 J/kg at lower elevations which is fairly impressive. This
environment screams dry microburst/outflow winds from high-based
storms, meeting pretty much all of our local checklist items.
Convective temps will run close to or slightly above the forecast,
certainly speaking to how marginal/isolated development should be
today outside the mntns (and how more cloud cover than expected
could mess things up). Limiting factors for an organized severe wx
threat will be marginal MUCAPE (sfc-based) around 500 J/kg for
most areas, effective shear generally running less than 25kts, and
lack of a more organized regional source of lift. Focusing in up
across the ern Central Mntns and MT border region, projected
MUCAPE values may approach 1,000 J/kg with better deep-layer
shear of 30+kts (owing to stronger flow aloft supporting
potentially rotating/more organized updrafts) and more attainable
convective temps. Toss in the mntns as a lifting mechanism, and
it's reasonable to see why SPC has highlighted this zone in a
Marginal Risk of severe wx. Thus, storms today will need to be
monitored for primary risks of lightning and strong outflow winds
in excess of 55 MPH, with isolated severe t-storms possible. The
risks for hail or tornadoes are lower, although the better
instability/shear could support hail in the stronger cores across
the Marginal Risk area. Looking ahead to Wed, despite continued
increases in moisture, a pseudo dry-microburst regime remains in
place but with better deep-layer shear (potentially 30-40kts
supporting better updraft maintenance/storm organization) and
SBCAPE exceeding 1,000 J/kg everywhere (assuming cloud cover is
not more extensive than forecast). This may support a few longer-
lasting multi-cell to borderline supercell structures with primary
threats of damaging winds/lightning (especially given PWATs
approaching 0.90") and some hail in the stronger cores. SPC has
the nrn/ern CWA in a Marginal Risk, and simulated reflectivity
off the CAMs looks more robust. Somewhat limited lift may be a
relative limiting factor Wed along with any cloud cover
suppressing full heating/destabilization. All in all, Wed COULD
be the better severe wx day to watch. - KSmith
.LONG TERM...FRIDAY THROUGH NEXT TUESDAY...
By Fri AM, a shortwave trough is expected to rotate NE across WA
into wrn Canada, and while this won't REALLY impact us directly,
it will further flatten the top of the ridge and provide more of
a westerly flow for Fri/Sat. This will shunt the effects of the
persistent ridge to the south and cut off the monsoonal flow.
Expect dry conditions with steady temps topping out in the upper
80s/lower 90s. The ridge tries to restrengthen a bit by Sun/Mon
as another potential shortwave rides over it across the nrn
Rockies, potentially supporting a few showers/t-storms by Mon,
again likely favoring the mntns. Model agreement regarding this
shortwave isn't great, so confidence on precip is low at this
juncture. No significant temp relief is in sight. - KSmith
&&
.AVIATION...
Showers and storms are possible across central and eastern Idaho
today. We kept with the previous trend of VCTS at BYI, IDA, SUN
and DIJ from mid afternoon into the overnight hours. It's possible
we may need to eventually add it for PIH. The main risk with be
strong outflow winds at or above 40kts. - Keyes
&&
.FIRE WEATHER...
Monsoon moisture continues to increase across central and eastern
Idaho. We are still THINKING there will be scattered storms
across the central mountains, and now extending into areas along
the Montana border including Monida Pass and Island Park. There
are a couple of things that could make for some last minute
changes during the day though. There is a lot of cloud debris
moving in from the west which COULD help prevent more widespread
thunderstorms vs. showers and just isolated storms. The second is
the models trending toward not much storm development across
portions of Zones 422 and 475. What this basically means is
confidence isn't the highest, but enough to add Zone 411 and keep
the central mountains going for the RED FLAG WARNING. It is also
possible that 427 could overachieve and see more storm coverage,
but confidence is definitely not high enough to add them to the
warning. We are looking for a mix of wet and dry storms, with
gusts possibly 50 MPH or higher. For Wednesday and Thursday, we
may see a repeat across those same areas...but with the potential
of everyone seeing a better chance of at least isolated storms
tomorrow if not both days. At the moment, if we need to add
another headline...the folks on the day shift will handle that.
Otherwise, hot conditions will remain although we should cool down
a few degrees overall. - Keyes
&&
.PIH WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 9 PM MDT Wednesday for IDZ051>059-068-070-
075.
Red Flag Warning from noon today to 10 PM MDT this evening for
IDZ411-422-475-476.
&&