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FXUS63 KGID 271038 AFDGID

Area Forecast Discussion National Weather Service Hastings NE 538 AM CDT Sat Apr 27 2024

KEY MESSAGES

- Although a completely different "animal" than Friday's event (warm front driven instead of tightly-wound upper/surface low), here we go again on a severe weather threat, with particularly counties along/south of the NE/KS border of greatest concern this time around where an Enhanced Risk (level 3 of 5) exists from SPC.

- While all severe weather hazards are possible today-tonight within our coverage area (CWA), the main threat for very large hail and possible tornadoes should focus within the Enhanced Risk area in our south (a strong tornado even possible if a supercell tracks along the frontal zone).

- Although opted against a formal Flood Watch (see reasoning below in separate HYDROLOGY section), spotty flooding/possible flash flooding issues cannot be ruled out in addition to the severe storm threat.

- Although isolated/scattered storms will occur Sunday daytime, fortunately the track of the primary surface low pressure center (and attendant cold front) looks to track more south- through-east of our CWA (as opposed to directly across it such as Friday). As a result, we currently look to largely be spared a severe storm...although a few storms with up to around 1" size hail cannot be ruled out.

- Following a break in thunderstorm chances mainly Sun night-Mon night, various low-confidence chances return especially Tues- Thurs as an overall-active pattern continues.

- Temperature-wise: nothing unusual for late-April/early-May whatsoever, with highs most days somewhere in the 60s/70s and lows most nights in the 40s/50s. However, one of the chillier nights with more widespread mid-upper 30s is Sun night-Mon AM, and some frost cannot be ruled out especially north/west of the Tri Cities.

DISCUSSION
Issued at 537 AM CDT Sat Apr 27 2024

-- PRIMARY CHANGES TO THIS 7-DAY FORECAST VERSUS THE PREVIOUS ONE (issued Fri afternoon):

- Nothing major to speak of. The overall severe weather threat for this afternoon-tonight remains largely on track/unchanged, and (fortunately) it still looks like we'll be spared from much of a severe storm risk Sunday daytime (albeit possibly a close call mainly in our far east).

- In the less impactful department, models continue to offer varying ideas regarding temperatures both in the shorter and longer term, and based on a blend of various models/guidance, highs were actually lowered at least 2-6 degrees in most areas today and Sunday, and also for Thurs-Fri.

-- 7-DAY FORECAST OVERVIEW/HIGHLIGHTS (heavily focused on the first 24-36 hours):

- CURRENT/RECENT WEATHER SCENE AS OF 430 AM: As was expected, it's fortunately been a quiet/dry night in the wake of an active Friday that started with early-AM heavy rain and ended up with early-afternoon tornadoes generally along a path between the Ravenna-Elba-Wolbach areas in our northern CWA (please note that damage surveys will continue today and we ask your patience in awaiting these results). In the big picture of the mid-upper levels, water vapor satellite imagery and short term model data confirm that we are in a brief lull between strong upper level low pressure systems, with yesterday's disturbance now departed to over MN, while the next one is currently churning over the Four Corners region. At the surface, weak high pressure has settled in, which has allowed breezes to really lighten up and shift more northerly overnight (most places under 10 MPH at this time). Under mostly clear skies overall, low temps are on track to bottom out mainly upper 40s-low 50s.

- TODAY-TONIGHT: Needless to say, spent the vast majority of "mental attention" on these first 24 hours. In the big picture, the heart of the incoming upper low will reach eastern CO by early evening, while a surface low deepens to around 996-999 millibars over southwest KS. A potentially very sharp, west-southwest to east-northeast oriented warm front/stationary front will extend out from this low, initially remaining just south of our CWA in central KS much of today, before probably lifting north into mainly our KS zones late this afternoon-evening, and providing a focus for severe thunderstorm development.

One of various uncertainties with the ultimate magnitude of our severe threat is just HOW FAR this frontal boundary can push into our southern CWA If it happens to remain closer to I-70, our threat for truly surface-based and potentially tornadic storms would be muted, but if it happens to lift closer to the NE border then the currently-advertised Enhanced Risk area (including 10% "hatched" tornado probs) could stand a better chance of coming to fruition. Leaning heavily on higher res/short term model solutions (namely HRRR), the general expectations for today tonight are as follows:

1) This morning-early afternoon (including temperatures): The vast majority of our CWA likely remains dry/storm-free, although a few spotty storms cannot be ruled out mainly in our KS zones along the far northern fringes of an initially mainly elevated instability axis. If anything models have trended a bit cloudier/cooler north of the main frontal boundary today, as we currently have large areas of lower stratus approaching both our northern and southern CWA, and expect fairly extensive to gradually take over north of the front as the day wears on. As a result, high temps were lowered 2-5 degrees most areas, and we are now calling for a only low 60s north/west, mid 60s central, but then a higher likelihood for 70s to even near-80 in our KS zones in closer proximity to the warm front:

2) Mid-late afternoon into this evening: Between 2-6 PM, we expect initial (and probably very rapid)
development of storms along/just north of the frontal zone in our KS zones, which could quickly become severe as either transient supercells or possibly congeal into a line fairly quickly. Either way, assuming forecast mixed-layer CAPE/instability of around 2000 J/kg is realized in the presence of strong deep layer wind shear of at least 50-60KT, things could get active very quickly. At least initially, the most intense storms should remain tied to the frontal zone, while weaker activity (but still with mainly a hail threat) will lift farther north into our Neb zones on the cool side of the front.
Please note that most areas especially north of I-80 could easily make it through most of the afternoon with little to no activity whatsoever.

Following the initial round of afternoon storms, there MIGHT be a relative lull before another uptick in convective intensity/coverage occurs mainly 7-10 PM as the low level jet increases and the surface boundary tries to lift a little farther north, but still likely remaining in our KS zones.
During this time frame, an increase in low level shear/helicity MIGHT lead to an increasing tornado threat, with the HRRR suggesting that an intense supercell (or two) could try "latching onto" the front and tracking across mainly parts of our KS zones. Meanwhile, farther north, more widespread (and not as intense storms) will overspread much of our Nebraska zones, with the primary threat likely being hail and localized heavy rain. If anything, am starting to think that counties especially north of I-80 may not see much of a severe storm threat at all, as even elevated instability should largely remain under 1000 J/kg (would not be surprised if SPC perhaps pushes the various severe risk areas a bit farther south in later Day 1 outlooks?).

3) Late evening-overnight: Although mainly a transient hail/heavy rain situation could persist well past midnight, expect the MAIN sever threat to wane by around midnight, as the aforementioned frontal zone starts sinking southward again as the main surface low shifts to in/very near our extreme southern CWA by sunrise Sunday.

- SUNDAY DAYTIME: Barring a major change in model trends, we are fortunately NOT expecting a repeat of Friday's tornadic activity in our CWA, as instead of the main surface low pressure center tracking directly over us (and putting much of our area in the warm sector), it is instead expected to track just barely along and mainly east of our far southern and eastern CWA borders (generally from central KS to the NE/IA border over the course of the day. This should keep the vast majority of our CWA in the cool sector of this system, in a regime marked by breezy west to northwest winds. That being said, this surface low track will need monitored VERY closely, as any westward shift could at least briefly bring southerly winds into especially our far southeast CWA around mid-day, which could perhaps result in a sneaky/brief tornadic threat. However, the more likely scenario is that we see mainly just isolated/scattered mainly weak storms, but with perhaps just enough elevated instability to pop a few stronger updrafts with nickel-quarter sized hail potential. The bottom line: we agree with SPC keeping the main severe risk categories just east our CWA Temperature wise, highs were lowered a good 3-6 degrees most areas given the overall-cooler expectations, ranging from only mid 50s northwest to upper 50s/low 60s central, and mid-upper 60s far southeast.

- SUNDAY NIGHT: Assuming skies clear efficiently, light winds will result in what will likely be the chilliest night of the next week, with widespread mid-upper 30s and MAYBE some frost development especially north/west of the Tri Cities.

- MONDAY-MON NIGHT: Although not "guarantee dry", our forecast remains dry as we reside in between systems.

- TUESDAY-FRIDAY: Spent VERY little time looking at these periods, but the bottom line is that periodic chances for rain/thunderstorms remain in the forecast as various upper level disturbances (of overall weaker intensity than these last few) pass through the region. A few surface frontal passages are also evident mainly around the Tues/Thurs time frames. One or more rounds of strong to at least marginally-severe storms probably cannot be ruled out between Tues-Thurs, but details are very murky at this time.

AVIATION /06Z TAFS THROUGH 06Z SUNDAY/
Issued at 120 AM CDT Sat Apr 27 2024

- General overview for KGRI/KEAR: While there is high confidence in VFR conditions through at least the first 6-9 hours and high confidence in dry/thunderstorm-free weather through at least the first 15 hours, anything beyond those respective timeframes brings increasing chances for sub-VFR ceiling and thunderstorms. Aside from any possible thunderstorm-related enhancement, winds will not be a major issue, although gusts around 25+KT will be likely by later in the period especially Saturday evening.

- Ceiling/visibility/precipitation (thunderstorm) details: Starting with ceiling trends: confidence in VFR and mainly clear skies these first 6-9 hours, although a few fleeting patches of MVFR stratus could pass through. However, already by as early as 15-18Z chances for MVFR ceiling will start to increase, but just how quickly this happens is still in question. For now, have introduced scattered MVFR mention at 15Z, but do not indicate prevailing MVFR ceiling until 18Z (stay tuned for adjustments here). Once showers/thunderstorms arrive by late afternoon/early evening, in theory there should be a better chance of a more solid/persistent MVFR ceiling (perhaps even IFR at some point?), but have seen many instances where convective activity keeps the low levels "mixy" and at least delays onset of sub-VFR ceiling.
Again something to watch.

Shower/thunderstorm trends (strong storm potential): Other than maybe a few rogue sprinkles before then, confidence is high in dry/thunderstorm-free weather until roughly 23Z. From that point onward, fairly widespread thunderstorm activity could persist at least in the general area through the remainder of the period. Although KGRI/KEAR will reside slightly north of the main severe storm threat area, storms could easily pose at least a small hail/gusty wind threat. Given these convective chances are still well beyond the first 12 hours, have simply maintained a generic "vicinity thunderstorm" VCTS for now.

- Wind details (not accounting for possible convective influences late in the period): The very lightest winds of the period (mainly at-or-below 10KT)
will occur these first 6-9 hours as direction shifts from westerly to northerly. Then, during the daytime hours and into the evening, winds will gradually increase out of the northeast, with sustained speeds commonly 15-20KT/gusts around 25+KT.

HYDROLOGY
Issued at 537 AM CDT Sat Apr 27 2024

- REGARDING HEAVY RAIN POTENTIAL THIS AFTERNOON-OVERNIGHT:

Fortunately from a flooding hazard perspective, the vast majority of our coverage area (CWA) picked up less than 0.50" of rain during the daytime-early evening Friday. As a result, for those various parts of our CWA that picked up anywhere from 2-5" of rain during the Thursday evening-Fri AM event, they will end up having roughly 24 hours (or more) of "drying time" before the next round(s) of storms moves in this afternoon into tonight.

As is typically the case with convective rains, confidence is is not overly-high regarding how this next round will unfold amount-wise, as much will depend on how efficient rain might be able to develop and possibly "train" to some degree along and especially north of the frontal zone that will likely remain quasi-stationary near and south of the NE/KS border. However, consensus of various models and a look at HRRR probability products (including 24-hour PMM) suggest that MOST of our CWA will probably not exceed 1-2" (although localized higher exceptions are certainly possible).

Considered joining OAX on a Flood Watch mainly for those parts of our Nebraska CWA that received the most widespread 3-5" amounts Thurs night-Fri AM, but at least "in theory" expect the heaviest rain this afternoon-tonight should fall at least slightly south of the main swath from the other night...hopefully over a handful of counties especially south of I-80 that really "missed out" over the last few days. Also, as we saw the other night most areas that had only 1-3" had few flooding impacts. For these reasons, along with national guidance from WPC keeping their initial Day 1 Excessive Rainfall Outlook category as only Marginal for our CWA, have opted against a formal Flood Watch for this afternoon-overnight, but while acknowledging in our Hazardous Weather Outlook (HWOGID) that at least localized flooding/flash flooding cannot be ruled out.

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

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