492
FXUS63 KGID 130203 CCA
AFDGID

Area Forecast Discussion...CORRECTED
National Weather Service Hastings NE
903 PM CDT Fri Jul 12 2024

...Key Messages and Short Term Update...

.KEY MESSAGES...

- Spotty thunderstorm chances (15-20%) late tonight into early
  Sat AM, but severe threat looks low (SPC has recently removed
  a formal Marginal Risk from our coverage area)

- Dangerous heat expected this weekend, with a Heat Advisory in
  effect from 1pm Saturday through 8pm Sunday.

- Widespread heat index values of 100-110 degrees on Saturday
  and Sunday. Heat index values over 100 possible across
  portions of the area on Monday.

- Cooler temperatures and precipitation chances (15-55%) return
  by the middle of next week.

&&

.UPDATE...
Issued at 830 PM CDT Fri Jul 12 2024

-- Some fresh thoughts regarding thunderstorm/fog potential
 overnight (and summary of forecast updates made since main
 "afternoon package" was issued earlier this afternoon):

- Thunderstorm potential:
Not surprisingly, SPC has recently removed their Marginal Risk
of severe storms from our entire coverage area (CWA) for the
rest of tonight on their latest update. Here at the local level,
we fully support this, as although there are a few stronger
storms ongoing well to our northwest in NW Neb at this time
(with more probably developing into that area yet this evening),
the vast majority of this activity is expected to focus at least
slightly off to our west as it rides south- southeastward into
early Sat AM. In addition, with warmer air aloft directly
overhead versus last night (700 millibar temps 12-14C), capping
overnight will be quite strong to any lower- level parcels. That
being said, latest RAP suggests that especially western parts of
our CWA could realize modest available elevated instability of
500+ J/kg late tonight into Sat AM (rooted mainly up around 700
millibars/10K ft. AGL), which could MAYBE support a few sneaky
stronger storms especially well after midnight to around 8-9 AM
Saturday (main hazards gusty winds to around 50 MPH and possible
weak heat bursts...lesser threat of smaller hail).

As a result of above reasoning:
Have "reworked" our overnight forecast a bit to remove all pre-
midnight thunderstorm chances, while expanding post-midnight
slight chances (20%) to include the majority of our CWA (except
a few far northeast/east-central counties which appear least
favored). For now, have not extended any formal thunderstorm
chance beyond 7 AM, and will defer to overnight shift to decide
if it`s warranted.


- Fog potential:
Much as was the case yesterday evening (for Friday AM), we are
not expecting any kind of widespread and/or dense fog situation
overnight into Sat AM, and most models/guidance indicate only
weak signals for even light fog. That being said, the very high
dewpoints/humidity levels are hard to ignore, with even late-
night dewpoints expected to drop no lower than mid-upper 60s
especially in north-northeast portions of our CWA. While winds
will not be truly calm (should be 5+ MPH most places, and there
could be some passing mid-high level clouds (both mitigating
factors to a more widespread/dense fog situation), the
aforementioned copious low level moisture, along with signals
of a weak warm front setting up late tonight near I-80, suggest
that counties especially along/north of I-80 are most favored to
see at least patchy light fog development (if not more "areas"
in nature especially in favored low-lying and valley areas, near
rivers etc.).


As a result of above reasoning:
Have at least introduced basic "patchy fog" potential to several
counties mainly along/north of I-80 mainly for the 3-9 AM time
frame. This also aligns well with latest 21Z SREF visibility
progs, which also highlights our northern counties for the
overall-highest (20-40%) chances for visibility dropping to 1 mile
or less a few hours either side of sunrise.

&&

.DISCUSSION...
Issued at 308 PM CDT Fri Jul 12 2024

This Afternoon and Overnight...

Highs this afternoon have climbed into the upper 80s to upper 90s
with heat index values in the mid to upper 90s. Portions of north
central Kansas will have heat index values top out around 100
degrees.

Thunderstorms are expected to develop over western Nebraska this
afternoon and move east into the area during the evening-overnight
hours. Overall thunderstorm coverage is expected to be fairly
isolated, with most HREF members showing only isolated coverage.
Thunderstorms will weaken as they move east with a loss of daytime
heating and increasing capping. Instability and shear will be
sufficient for a few strong to severe thunderstorms, capable of
producing damaging wind gusts and large hail. Scattered showers and
thunderstorms chances shift east overnight, ending by sunrise on
Saturday.

Saturday through Monday...

The biggest concern for the forecast period is the potentially
dangerous heat this weekend. A Heat Advisory has been issued for the
entire forecast area from 1pm Saturday through 8pm Sunday.

Temperatures will increase across the area on Saturday, with highs
in the mid 90s (north) to mid 100s (south). Heat index values of 100-
105 degrees are expected across most of the area on Saturday. Lows
Saturday night are expected to remain in the 70s for most areas,
providing limited relief from heat overnight. The hottest day of the
forecast period will be on Sunday, as highs soar into the upper 90s
(north) to mid 100s (south-southwest). Heat index values of 105 to
110 degrees are expected across the area.

An approaching trough will bring a cold front into the area on
Monday. The exact timing of this front is still uncertain, however
areas behind the cold front will see cooler temperatures with highs
in the low 90s. Areas that remain south of the front through the
late afternoon hours will see another day with highs in the upper
90s to mid 100s and heat index values over 100 degrees. Areas north
of I-80 are the most likely areas to see the cold front pass through
by the mid afternoon hours. Areas along and south of the NE/KS
border are the most likely to not see the cold frontal passage until
the evening hours. Heat headlines will likely be needed for portions
of the area on Monday, though it will depend on the timing of the
front. Mostly dry weather is expected this weekend, though an
isolated thunderstorm may impact northern portions of the area
Sunday night.

Overall, potentially dangerous heat is expected across the area this
weekend and into Monday. Heat index values over 100 degrees are
expected across the area on Saturday and Sunday, and a portion of
the area on Monday. Those with outdoor activities this weekend will
want to take precautions to avoid heat related illnesses.


Monday night onwards...

Thunderstorm chances (15-30%) return to the area Monday evening-
overnight. Cooler temperatures are expected on Tuesday, with highs
in the 80s across north central Nebraska and in the low 90s across
north central Kansas.

A welcome break from the heat will arrive by the middle of the week.
Highs in the upper 70s to low 80s are expected across the area on
Wednesday and Thursday. Scattered precipitation chances (15-55%)
continue across the area Tuesday through Thursday, with the highest
chances (35-55%) occurring Tuesday night.

&&

.AVIATION /00Z TAFS THROUGH 00Z SUNDAY/...
Issued at 722 PM CDT Fri Jul 12 2024

For KGRI/KEAR Airports:
- General overview:
Unless some "sneaky" sub-VFR visibility and/or ceiling manages
to develop late overnight in light fog, confidence is high in
VFR conditions throughout. There is also an outside chance for a
passing, high-based shower/weak thunderstorm at some point late
overnight into early Sat morning, but this is deemed a low
probability (only 10-20%). Winds should not be a significant
issue throughout, with sustained speeds mainly at-or-below 10KT
and direction mainly somewhere between east-southeasterly and
southerly.

- Ceiling/visibility/precipitation/thunderstorm details:
Starting with light fog potential:
While nearly all models/guidance are currently very pessimistic
that much of any fog will develop overnight (possibly slightly
more favored 25+ miles north of KGRI/KEAR), am a bit concerned
that some "sneaky" sub-VFR visibility could indeed materialize
given such high humidity levels in the presence of light south-
southeast breezes. As a result, will at least "hint" at this
potential with a low-end VFR "6SM BR" focused 09-13Z. Outside of
any light fog potential, however, confidence is high in VFR
with only varying degrees of passing mid-high level clouds.

As for rain/thunderstorm potential:
A few higher-res models suggest that especially the 09-13Z time
frame could also bring the potential for a few very spotty,
high-based showers or non-severe storms. However, with this
chance deemed no higher than 10-20% have omitted from TAF
inclusion.

- Winds (including weak LLWS potential):
No significant issues foreseen here, as sustained speeds
through the period should average near-to-below 10KT, although
direction will fluctuate somewhat between east-southeasterly
(mainly overnight) and more southerly (mainly Saturday daytime).


Regarding weak low level wind shear (LLWS) potential:
It appears some fairly weak LLWS could occur tonight mainly
06-10Z as winds within the lowest 1,500 ft. AGL increase to
around 30KT out of the south-southwest. However, with shear
magnitude between the surface and this level currently expected
to top out closer to 25KT than 30+KT, have opted against formal
TAF inclusion.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Heat Advisory from 1 PM Saturday to 8 PM CDT Sunday for
     NEZ039>041-046>049-060>064-072>077-082>087.
KS...Heat Advisory from 1 PM Saturday to 8 PM CDT Sunday for
     KSZ005>007-017>019.

&&

$$

UPDATE...Pfannkuch
DISCUSSION...Davis
AVIATION...Pfannkuch