458 
FXUS63 KGID 040028
AFDGID

AREA FORECAST DISCUSSION
National Weather Service Hastings NE
728 PM CDT Thu Aug 3 2023

...Aviation Update...

.DISCUSSION...(This evening through Thursday)
Issued at 434 PM CDT Thu Aug 3 2023

Key Messages:

* Threat for strong-severe storms will increases as we get later
  into the evening/overnight hours, with the best chances focused
  near/south of the NE-KS state line. Damaging wind gusts near 70
  MPH and very heavy rainfall rates will be the primary threats.

* Periodic thunderstorm chances remain in the forecast all the way
  through this weekend and into next week...with the best overall
  chances looking to be Friday evening/night and again Saturday
  afternoon/night. Large hail, damaging winds, and heavy rain
  would be the primary threats. 

* In the wake of Saturday's frontal passage, cooler temperatures
  settle in for Sunday into the start of the new work week...with
  forecast highs Sun/Mon in the mid 70s-low 80s. Sunday is also
  looking to be breezy/windy, with northwesterly sustained speeds
  of 20-25 MPH (and higher gusts) possible. 

Currently through tonight...

Upper air and satellite data show generally zonal flow in place  
across the area this afternoon...set up between a departing 
shortwave disturbance moving off to the east and a more organized 
area of low pressure making its way into western WY. The departing 
shortwave disturbance helped drive a cluster of thunderstorms which 
passed east through the CWA through the early-mid morning hours...a 
few of which were on the strong/severe side. This activity waned in 
coverage and intensity with time...and outside of a few isolated 
showers/storms trying to develop across northern portions of the 
area here at mid-afternoon, it's been pretty quiet since then. At
the surface, the pattern across the CWA remains weak, resulting 
in light/variable to generally easterly winds. The main frontal 
boundary is draped across far southern KS...with a weaker boundary
extending northward through western NE/High Plains. 

Main focus going into this evening and tonight continues to lie
with thunderstorm development off to our west along any 
lingering frontal boundary and over higher terrain, as shortwave 
energy associated with that upper low over WY swings into the 
region. There are still some uncertainties with the exact 
details...but the threat for strong/severe storms remains in place
through tonight, especially over SSWrn portions of the CWA. As 
far as model trends go...there was a bit of a slow-down in the 
timing of the main cluster of activity moving off the higher 
terrain...but we'll have to keep an eye on activity developing a 
little further east along a lingering weak boundary. General 
consensus with timing suggests the main push into the CWA may be 
closer to 07-08Z (vs say 05-06Z). Compared to 24hrs ago, recent 
model runs have shown a slight southward shift in the location of 
the main instability axis/frontal boundaries...and thus in the 
general track of strongest storm potential. It had roughly been 
near the NE/KS state line...more recent runs showing it closer to 
I-70. Not a large change mileage-wise...but it impacts how much of
the CWA may be affected by the 'worst of it' and potentially 
decreases overall precip chances the further north you go. Hard to
have a high degree of confidence/grab on to any one particular 
model or run...as there was plenty of struggle with the activity 
that ended up occurring this morning. 

With models showing a that slight southward shift of the main 
instability axis...values over our CWA may be more modest in 
nature...and deeper layer shear remains around 30kts. While large 
hail is not out of the question, the greater threat continues to
be with damaging straight-line winds, as a more mature complex 
is expected to be in place by the time it gets this far east. 
Gusts near 70 MPH are not out of the question. Another concern is 
with the potential for very heavy rainfall rates, plenty of 
moisture remains/PWATs over 1.5"...and expect efficient rain-
makers with the potential to drop 1+" of rain in a short amount of
time. Considered issuing a Flood Watch for WSW portions of the 
CWA...but with the fairly progressive nature expected of this 
activity, felt the threat for significant amounts/flooding will be
on the more isolated side. 

Friday and Saturday...
 
As we get into Friday, main question is with PoPs...with how
quickly morning activity wanes/moves out of the area and then 
timing with the next round of storms. Kinda sound like a broken 
record, but Friday looks like it'll be similar to the past couple 
of days...once the morning activity moves out generally dry 
conditions are expected into the afternoon hours...and main change
to the forecast for Friday was to adjust PoPs to that thinking. 
Expecting gradually diminishing sky cover from west-east (with 
some uncertainty with its timing) and overall light easterly 
winds. Forecast high temperatures are similar to today, with low-
mid 80s across NE and mid 80s-near 90 across KS. 

In the upper levels, models show that upper level system currently 
over western WY making a little eastward progress on Friday, with 
another round of thunderstorms expected to develop across the High
Plains during the mid-late afternoon hours. This activity is once
again expected to push east across the region, driving our next 
storm chances Friday evening into the overnight hours. The entire 
CWA is included in the SPC Day 2 Marginal Risk area...can't rule 
out a few strong/marginally severe storms, but overall forcing 
looks to be a bit weaker.

Saturday and Saturday night, that main upper level system
continues to gradually push east across the Nrn/Central Plains, 
and will be bringing along a stronger surface cold front. Even 
though it's just 2 days away...there are plenty of finer details 
to iron out, mainly with the timing of the front and just how much
activity develops along it. Front speeds up at all, storms become
less of a concern...slows down, more of the CWA will be at risk 
for strong/severe storms. Forecast highs for Saturday are in the 
mid 80s-near 90...but will be driven by the timing of that front.

Sunday and on...

Behind this upper level system/cold front passage on Saturday, 
periodic, scattered, lower-chance PoPs remain in the forecast
Sunday on into the middle of the work week. Confidence in the 
timing/location/other details of any particular disturbance and 
resultant precip chances are pretty low at this point.
 
Of more note is what this front leaves in its wake...overall cooler 
temperatures, and for Sunday, gusty NW winds. Normal highs this
time of year are mid 80s-low 90s...and forecast highs for 
Sun/Mon are in the mid 70s-low 80s, with a gradual increase back 
into the 80s/low 90s by Wednesday. Sunday looks to be breezy/windy
behind that front, forecast currently has NW winds with sustained
speeds around 20-25 MPH (and higher gusts)...with lighter winds 
into the new work week, turning more southerly each day. 

&&

.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Saturday)
Issued at 728 PM CDT Thu Aug 3 2023

- General overview (including winds):
Another tricky TAF period, with overall-high confidence in VFR
ceiling/visibility and rain/storm-free conditions through the
first several and last several hours, but with the middle portion
of the period (especially 08-18Z) frought with the potential for
sub-VFR ceiling/visibility and passing thunderstorms (initially),
followed by lighter/stratiform rain. Unfortunately, there is again
considerable uncertainty regarding just HOW FAR below VFR
categories could fall. As for winds, outside of any possible 
thunderstorm outflow (which of course could at least briefly 
greatly enhance speeds/cause sharp directional shifts), this 
should be a fairly mundane period with sustained speeds largely 
under 10KT and direction mainly easterly to southeasterly. 

- Ceiling/visibility details:
High confidence in VFR through at least the first 7-9 hours
(before onset of any lower ceilings/light fog and possible
convection) and final 6-7 hours (after any morning/early afternoon
lower ceilings raise to VFR and/or scatter out. Unfortunately,
what happens in between (especially 08-18Z time frame) is a bit of
a mystery at this point, as much will depend on how aggressively
rain and thunderstorm-outflow "disrupts" the ambient environment. A
lesser influence of convection will likely equal greater/longer 
concentration of sub-VFR ceiling/visibility (in at least light 
fog), while a more pronounced coverage of convection (or longer 
duration) could easily keep the low levels "mixy" enough to 
disrupt or at least delay onset of sub-VFR ceiling/visibility. As 
a result, TAFs largely reflect a typical "middle ground" scenario 
for now, calling for prevailing MVFR ceiling and/or visibility 
through mainly the 08-18Z time frame, but acknowledging with 
scattered IFR cloud groups that worse prevailing conditions are 
possible. 

- Precipitation/thunderstorm details: 
Pretty high confidence in rain/thunderstorm-free through at least
08Z and after 16-17Z. However, as is the case with
ceiling/visibility, what happens in between features above-average
uncertainty even at this closer time range. The main challenge is
trying to determine whether at least a semi-organized (and
possibly strong) storm complex rolls in from the west overnight
and more solidly impacts KGRI/KEAR with gusty winds (perhaps
40+KT), brief heavy rain and possibly small hail, or whether the
main push of most intense activity remains at least 50+ miles
west-through-south, with only minor impacts at the terminals
(latest model trends seem to be leaning more this direction).
While confidence in how intense convection might be is the main
uncertainty, confidence is a bit higher in timing, with most all
activity likely to focus between 08-17Z (starting as thunderstorms
but likely ending as lighter showers or stratiform rain). Given
the inherent uncertainty in convective coverage/intensity, have
maintained a mix of generic "vicinity" groups for now (both
VCTS/VCSH), and will defer to later issuances to introduce
possible higher-confidence TEMPO/prevailing groups. 

&&

.GID WATCHES/WARNINGS/ADVISORIES...
NE...NONE.
KS...NONE.
&&

$$

DISCUSSION...ADP
AVIATION...Pfannkuch