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FXUS63 KGID 250931 AFDGID

Area Forecast Discussion National Weather Service Hastings NE 431 AM CDT Thu Apr 25 2024

KEY MESSAGES

- Elevated thunderstorms with marginally severe hail threat possible this morning near and south of the KS/NE state line.

- Next round of thunderstorms, which models are trending more intense with, expected to develop over and/or move into SW portions of the forecast area this evening, then continue NE overnight. Most widespread threat will be large hail, but damaging wind gusts and tornadoes will also be possible.

- Center of the upper low will only move slowly E/NE on Friday, which could keep the potential for renewed strong-severe thunderstorm development possible through the daytime. Areas N and E of the Tri-Cities appear most at risk.

- Expect a general lull in precipitation chances Friday night into at least the first half of the daytime Saturday.
Thunderstorm chances will increase late Saturday afternoon into the evening, with more severe weather possible.

- Active weather continues into Sun, though details become more uncertain due to previous days' convection. Should finally dry out early next week as temperatures remain mild in the 70s/80s

DISCUSSION
Issued at 415 AM CDT Thu Apr 25 2024

Very busy forecast this morning as there appears to be some important trends in model data that would suggest greater risk of severe, potentially significant, not only this evening, but into Friday and possibly even Saturday, as well. Will take it one round at a time below, then wrap up with total precipitation expectations, and a brief look at general pattern next week.

Morning Thunderstorms...
Hi-res guidance has remained consistent in showing a NW to SE band of elevated convection (which the EC has had virtually every run for days) developing and expanding right through the forecast area over the next few hours. If anything, model guidance QPF has increased a bit, suggesting greater coverage and perhaps more intense activity. Very steep mid level lapse rates and effective shear magnitudes of 35-45kt will support the potential for large hail, and perhaps even a few transient elevated supercells, for areas mainly along and S of the NE/KS state line through mid to late AM. However, model progged MUCAPE has come in a bit lower than 24 hours ago which could help limited the overall magnitude of the large hail threat. So think nickel to half dollar size hail will be main concern. CAM guidance remains consistent that brunt of organized activity will shift SE into central/eastern KS for the aftn hours, though can't rule out some additional iso to widely scat showers or weak storms at just about any today. More widespread sensible weather this afternoon may turn out to be drizzle.

Thunderstorms This Evening-Overnight...
Will state up front that some of the recent CAM guidance has trended towards initial development CLOSER to the SW fringes of the forecast area, which means earlier arrival and thus potentially more significant threat. 00Z/06Z HRRR runs would fall into this camp, as well as some HRRR members. Instead of what appeared to be an after 03Z, and frankly more like after 06Z, timing has now trended more towards the 23Z-02Z time frame, at least for areas like Furnas, Phillips, and Rooks Cos. Still looks like timing for Tri-Cities will be well after sunset as convective intensity begins a downward trend in intensity...just not sure how QUICKLY the downward trend will ensue, however.
Probably most concerning with the most recent model runs is the signal for scattered very intense supercells to develop near a sfc triple point in GLDs forecast area, and remain at least semi- discrete for longer as they quickly track NE into far SW CWA during what is climatologically an uptick in probabilities for tornadoes as LCLs lower and the low level jet ramps up. Any particular scenario/evolution is far from certain, though, as AM convection could both help and hurt later convection, and there will be some mid level capping to contend with. The AM convection will likely reinforce the warm front and serve as a limit to the NEward extent of surface based t-storm and legit tornado threat. Again, some of the guidance brings the triple point slightly further N/NE by 00Z, which at least brings into play the potential for a supercell tornado into a few of our far SW zones. Models still have a tendency to "pinch off" the warm sector S of the state line, and even Bunkers RM vectors would take the supercells more N/NE and ACROSS the warm front and less so eastward ALONG the warm front. So most plausible evolution would be for convection bec increasingly elevated late evening into the overnight and pose primarily a large hail threat (up to around golf balls) considering continuation of steep lapse rates (near or above 8 C/km), strong shear (50+kts) and moderate instability. Main severe threat should conclude by ~3AM Fri.
Mesoanalysis will be key to communicating afternoon and evening trends, but from what I'm seeing, one could make the argument for expanding the Enhance Risk (level 3 of 5) into portions of the forecast area SW of the Tri-Cities.

Scattered Thunderstorms Daytime Friday...
Believe there will be another decr/lull in convection Fri AM, perhaps aided by advancing mid level dry slot. Both the sfc and mid level low center are forecast to become stacked over the Sandhills by 18Z. Mid-upper low will then take on increasingly negative tilt as it moves NE into the Siouxland region Fri PM.
Arrival of the mid-level dry slot may contribute to some breaks in the cloud cover and actually aid in some sfc-based instability, perhaps as early as ~midday owing to close proximity to pool of cold mid level temps. In fact, models now indicate a lobe of near -20C at H5 tracking right through the CWA during the daytime. Arrival of these cooler mid level temps and strong ascent, in conjunction with aforementioned sfc heating, could lead to rapid convective development as early as 16Z-18Z, roughly near the Hwy 281 corridor. Backed SErly sfc flow beneath a northern punch of a strong mid level jet (60-70kts at H5) will provide ample deep layer shear for supercells, which when combined with low freezing levels and steep lapse rates, could lead to some very large hail. High ambient vorticity and backed sfc flow amidst 40-50kt of Srly flow at H85 will lead to some tornado potential. Ultimately, the threat for the GID CWA will come down to timing - the earlier the development, the greater the risk/threat. Given it's a highly dynamic system, tend to favor earlier rather than later development, thus why I think the potential for significant severe weather may be incr for areas along/E of Hwy 281 Fri aftn. Additional low-topped supercells directly beneath the mid level cold core (high shear, lower CAPE) could also present an isolated tornado threat, perhaps back as far NW as Ord. Overall, this feels like it has some similarities to the event that produced the highly photogenic Greeley County tornado last spring which also produced due to early and further W dev. A potential failure mode to this scenario would be for widespread convection tonight to sweep E/NE, taking with it the primary instability axis to our E. However, even if this occurs, hard to completely remove the threat anytime there's -20C above you at H5 during the daytime hours. Should see a more distinct lull in precipitation Fri evening through at least early Sat aftn.

Saturday PM into Sunday Thunderstorm Chances...
Already murky details in the short term become even murkier by Sat, though it seems like there's been a bit of a trend N and W with the instability axis Sat aftn/eve, which could bring sfc- based convection more into play for at least far E/SE zones.
This subtle trend is reflected in the latest SPC Day 3 Outlook which brings a Slight Risk (level 2 of 5) into areas along and SE of a line from Phillipsburg to York, and an Enhanced Risk (level 3 of 5) right up to the Mitchell County line. By late afternoon, expect a sfc low/triple point to set up somewhere over central KS, with a stationary/warm front extending NE, and a dry line extending to the S. In terms of significant severe potential, the question will be how far W and N does the warm sector, and associated risk for sfc-based supercells, become established. Is the triple point back towards Hays/Russell/Plainville...or is it more towards Salina and Great Bend. Fairly small difference in the grand scheme of things, but key to specific threats. NBM highs are now creeping into the lower 80s far SE zones, further suggesting that NWward trend.
Within the warm sector, all modes of significant severe will be fairly likely (Day 3 Enhanced is not thrown out like candy in a parade) beginning late afternoon and continuing into the eve.
Further back to the NW, still think scat to widespread convection will develop in these areas as heights fall and ascent incr ahead of next strong shortwave disturbance. While the threat for sfc-based supercells (and thus tornadoes) will be lower in this area, threat for large hail will persist owing to magnitude of the CAPE/shear/lapse rate parameter space.

Actual sfc/upper disturbance doesn't kick through the Plains until Sunday. Similar the Fri, there could be a significant gradient in sensible weather from cool/cloudy NW to much warmer and more unstable to the SE. Exactly where this zones sets up remains pretty uncertain, but if anything, there's been a wobble to the NW in 00Z EC guidance. Mid level lapse rates should finally see a significant decr to closer to 7 C/km, so instability won't be quite as large. Strong shear will still be in place, though. Would like to see more model and run continuity before really latching onto idea of continued severe threat into Sun as conceptually speaking, seems most likely that primary instability axis would favor areas to the SE near I-35.
Any wrap around showers should exit/end Sunday night.

Total Precipitation Amounts...
Deterministic QPF has edged higher, likely owing to greater potential for stronger/deeper convection tonight and Sat eve/night. Most areas still seem primed for a solid 0.75-1.5", though it's worth pointing out that a sharp drop-off in amounts is not too far to the S/SW in central KS. Hopefully, this doesn't creep into Rooks/Osborne Counties. As always with convection, there will be winners and losers. Still think with locally higher amounts that flooding concerns are low since it's been dry as of late and there is time between each round.

Latest ensembles indicate mild temperatures for next week, and mainly dry conditions for at least the early part of the week.
Various rain/storm chances return for mid to late week, but signal for anything more specific is just too messy attm.



AVIATION /06Z TAFS THROUGH 06Z FRIDAY/
Issued at 1250 AM CDT Thu Apr 25 2024

Expect significant deteriorations to aviation conditions this period, particularly during the day today.

Isolated showers early this morning are forecast to increase in coverage and intensity (to thunderstorms) by around dawn, and move NW to SE. Appears greatest potential for strong storms and perhaps some hail will remain S of the terminals. This first wave should exit by mid to late AM, but in it's wake, models show quick lowering of CIGs to MVFR and even IFR by around midday. Seems quite likely even further deterioration to IFR or near LIFR will occur during the aftn, and possibly be accompanied by minor VSBY reductions and drizzle or light rain showers. The next round of thunderstorms is forecast to develop during the late evening or overnight Thu, but exact timing and coverage in the area of the terminals remains uncertain. Thus, have handled this with a prevailing showers and only VCTS for thunder. Storms tonight could be severe with mainly a large hail threat. IFR to LIFR CIGs will likely prevail beyond 06Z Fri.
Confidence: Medium.

Winds will be on the incr out of the SE, becoming brzy around or shortly after sunrise. Sustained winds of 15-20kt and gusts of 30-35kt are likely for majority of the daytime hrs, and even into Thu eve. Confidence: High.

GID WATCHES/WARNINGS/ADVISORIES
NE...None.
KS...None.

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