252
FXUS63 KGID 010941
AFDGID

Area Forecast Discussion
National Weather Service Hastings NE
441 AM CDT Mon Jul 1 2024

.KEY MESSAGES...

- Scattered to widespread thunderstorms this morning with
  locally heavy rain - quick 2-3" - possible.

- Mainly dry this afternoon and becoming hot and steamy for
  areas along and south of the state line. A Heat Advisory is in
  effect for heat indices up to 105-107F in these areas.

- Another round of thunderstorms expected this evening, some of
  which could be severe and pose risks for all severe hazards

- Cold front should push just far enough south on Tuesday to
  keep the really steamy heat out of our area. However, the
  front stalls out and a series of disturbances will keep off
  and on thunderstorm chances going through Wednesday night.

- 4th of July has trended drier for the afternoon and evening in
  latest model runs. Expect highs in the 80s to near 90F.

&&

.DISCUSSION...
Issued at 420 AM CDT Mon Jul 1 2024

Interesting/complex forecast for today, and honestly one that
reminds me more of late May/early June than early July. First
off this morning...a seasonably strong LLJ/WAA within broad
SWrly upper flow, to the NW of a strong ridge in TX, is leading
to fairly widespread elevated thunderstorms development early
this morning. This activity has 1000-1500 J/KG of MUCAPE and
40-45kt of effective to work with for perhaps some small to
marginally severe hail in the strongest cores. However, think
heavy rain will be the main concern given fairly deep warm cloud
depths and PWATs already in excess of 1.8" per recent SPC MesoA.
The NW to SE band of convection should make steady progress E/NE
through the morning, but localized training of hvy cores and
just the overall longitudinal width could promote enough
duration to produce some areas of hvy rainfall. 00Z HREF and
more recent HRRR runs suggest some 2-3.5" amounts are possible,
which seems more than reasonable given aforementioned PWATs
running between the 90th percentile and max moving avg per DDC
and TOP sounding climo. Now my confidence on where exactly
these higher end amounts are more likely is low, however, given
disparities in location from one run to the next. With the
orientation of the veering LLJ as it should be, would think
along and S of I-80 have the greatest risk. Recent mainly-dry
7-10 days and peak growing season should keep risk isolated.

Expect much of this elevated activity to shift E towards the MO
River by around midday, leaving lingering stratus and strong
SErly low level flow. Strong insolation and deep mixing will
erode the stratus from SW to NE, but just exactly how quickly
this process occurs leaves some uncertainty in high temps, esp
from around the Tri-Cities N and E. HRRR/RAP trends suggest it
could be 20-22Z before these areas break into sunshine, so have
trended highs a bit lower. Still appears a warm front will be
able to make progress into SW/S zones, though, by early-mid
afternoon, with intense heating and lingering moist boundary
layer expected to support highs near 100F and heat indices up to
around 102-107F. Made no changes to the existing Heat Advisory.

By late afternoon (~21Z), expect a sfc low/triple point to set
up near KMCK, with the aforementioned warm front - separating
hot/deeply mixed air and veered sfc flow to the S from strongly
unstable and backed low level flow to the N - arcing E/SE
across south central NE. A sfc trough will extend to the N/NW
towards the Black Hills to another sfc low along a SEward moving
cold front. Still expect warm, capping mid level air (13-14C at
H7) to prevent convection through at least 21Z, but CAMs remain
insistent that subtle mid level height falls/cooling, along with
persistent low level convergence invof sfc low/triple point,
will be enough to break the cap around 22-23Z, most likely in
the area from around MCK to LBF. Once convection forms, there
will be strong instability (MLCAPE 2000-3000 J/kg) and
unseasonably strong deep layer shear (40-50kt from 0-6km) to
work with - which should support supercell structures with very
large hail (golf ball to maybe tennis ball?) and strong
downburst wind gusts. Still think the tornado threat is elevated
by early July standards as 0-1km and 0-3km shear is quite high,
owing to strong veering atop backed sfc flow along and just N of
the warm front. This shear is forecast to only incr in the
typically favored 00-03Z time frame (diurnally lowering LCLs)
thanks to incr LLJ. Also, deep layer shear vectors are largely W
to E and parallel to the warm front, suggesting potentially
optimal residence time for supercells to interact with the
vorticity rich low levels invof of the front. With that said,
very hot/deeply mixed air with high T/Td spreads and inverted-V
low level profiles will be in close proximity to the S and would
think some of this air would get entrained and eventually
support increasingly strong downdrafts and upscale growth with
time mid to late evening. So the window for tornadoes is
probably quite limited in time and space. Should the upscale
development occur, then a transition to damaging wind gusts,
perhaps in excess of 70 MPH, would be favored and potentially
sustained through late eve and eastward progression thanks to
aforementioned incr LLJ. I continue to think that a localized
upgrade to Enhanced Risk (level 3 of 5) is possible, though the
current Slight Risk (level 2 of 5) seems most prudent at this
time given inherent uncertainties associated with effects from
AM convection and anticipated capping. CAMs take the primary
threat E of the area by around midnight.

The cold front should make slow, but steady progress to the SE
Mon night into Tue AM, such that by Tue aftn it is largely
through the entire forecast area and any Heat Advisory level
heat indices are shunted SE into central/eastern KS. In fact,
most of our Neb. counties should remain in the 80s, with any 90s
highs limited to to mainly along/S of Hwy 36. This will probably
keep the primary severe threat for the next round of aftn/eve
tstms to the SE of the area, as well. As such, far SE zones are
just barely clipped by the most recent SPC Day 2 outlook, with
the broader Marginal Risk area to account for rouge midday-ish
strong storm, or two.

Front should make far enough progression to provide a lull in
shwr/storm chances late Tue night thru most of Wed aftn.
However, consensus is that yet another round of convection will
develop N to S from the Black Hills into E CO late Wed aftn,
then shift E in the zonal upper flow Wed eve/night. Latest SREF
shows 20-40% probabilities for sufficient instability/shear for
severe hail/wind into Wed night, esp for areas W thru S of the
Tri-Cities, in closer proximity to stalled boundary. Scattered
shwrs/storms could continue much of the night and into Thursday
AM - 4th of July - thanks to another cold front and upper
disturbance. Fortunately, it appears models are converging
towards a consensus that the system timing would be favorable
for decr rain chances into the afternoon and evening - good news
for area celebrations and fireworks shows. The "cherry on the
top" could be much lower/more comfortable dew points in the 50s
to lower 60s advecting in behind the front in time for the
evening. High temps late in the week should favor mainly 80s.

&&

.AVIATION /06Z TAFS THROUGH 06Z TUESDAY/...
Issued at 1235 AM CDT Mon Jul 1 2024

For KGRI/KEAR Airports:

Significant weather: a lot going on this TAF period...one or two
rounds of thunderstorms, IFR to perhaps even LIFR CIGs,
potentially MVFR VSBYs and drizzle, and breezy SE winds.

Expect incr stratus next few hrs and arrival of at least IFR
CIGs. Some upstream obs suggest LIFR CIGs will be possible.
Coincident with the incr stratus will be incr chances for
scattered to widespread elevated convection, some of which could
contain small to marginally severe hail. The main time frame for
convection looks to be 08-13Z. IFR/LIFR status, and perhaps
even some light drizzle and MVFR VSBYs, will likely (60-70%)
persist through the remainder of the morning hours. Stratus
should lift and scatter out sometime between 19-22Z. The next
concern will be potential for a new round of scattered strong to
severe thunderstorms Mon eve, most probable in the 01-04Z time
frame. Have started with a VCTS group, for now, and will let
later forecasts hone in on prevailing conditions as we get
closer. Large hail and damaging winds may accompany this activity.

Winds will be primarily out of the SE and brzy, esp during the
daylight hours, at 15-20kt sustained, and gusting 25-30kt.

Confidence: High with the general evolution of impacts and non-
thunderstorm winds, medium on specific timing of thunderstorms
and flight category changes.

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for NEZ072-082-083.
KS...Heat Advisory from 1 PM this afternoon to 8 PM CDT this
     evening for KSZ005>007-017>019.

&&

$$

DISCUSSION...Thies
AVIATION...Thies