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