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FXUS63 KGID 261212 AFDGID

Area Forecast Discussion National Weather Service Hastings NE 712 AM CDT Fri Apr 26 2024

Aviation and Short Term Update

KEY MESSAGES

- We're suddenly mired in a VERY active weather pattern, with mainly the northeast 1/2 of our coverage area (CWA) in a Marginal-to-Slight Risk for a few severe storms today (mainly 11 AM-5 PM), and then following a brief break overnight into much of Saturday daytime, most of our CWA again goes under a threat for severe storms mid-late Saturday afternoon-overnight (overall-highest risk looks to be our southern half, and a few far southeast counties are now in an Enhanced Risk (level 3 of 5).

- Storms over the last 24 hours (and especially the last 10 hours) were VERY EFFICIENT "rainers", and at least localized flooding issues are likely ongoing within a swath across the heart of our Nebraska CWA that picked up a widespread 2.50-4.50+" of rain overnight. See separate HYDROLOGY section below for all further discussion of recent and upcoming heavy rain concerns.

- Beyond the Saturday afternoon-overnight severe storm threat, the overall pattern remains active, and although there are no truly "obvious" severe storm threats evident at this time, at least parts of the CWA carry thunderstorm chances Sunday daytime, and then again between Tuesday afternoon and Thursday.

- Temperature-wise: nothing very noteworthy to speak of by late- April into early-May standards, with highs mainly in the 60s/70s each day and lows mainly in the 40s each night (with some very limited 30s/50s).

- Chances for frost development late Sun night-Mon AM mainly in northern/western parts of the CWA have (at least for now)
backed off a bit from previous forecast (although this is still the overall-chilliest night in our 7-day.

UPDATE
Issued at 712 AM CDT Fri Apr 26 2024

- Quick "patchy fog" note: On top of everything else going on in today's forecast, we've recently seen the onset of at least patchy, mainly light fog (visibility mainly no worse than 1-2 miles) across various parts of our CWA especially over the last hour. As a result, have added generic "patchy fog" to the forecast for all but our southern- most KS counties through mid-morning.

DISCUSSION
Issued at 530 AM CDT Fri Apr 26 2024

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

- In the very short term (first 12 hours), although the threat certainly still exists, it appears the spatial/temporal threat for a few tornadoes mainly in our northern/northeastern counties today has perhaps diminished a bit (higher confidence that the MAIN tornado threat should focus east of our CWA, although it could be a VERY close call and mesoscale/radar trends will need monitored very closely.

- Concern is increasing for the Saturday mid-late afternoon into overnight severe storm threat, and have added more detail to our Hazardous Weather Outlook to better heighten awareness.

-- 7-DAY FORECAST OVERVIEW HIGHLIGHTS (chronological order, but "finer details" heavily weighted toward first 24-36 hours):

- CURRENT/RECENT WEATHER SCENE AS OF 430 AM: In short, while there was widespread thunderstorm activity overnight (especially in our Nebraska zones), heavy rain/flooding ending up being a much bigger story than actual severe weather, as our threat was muted by remaining north of the primary low-level instability axis (storms were more elevated above a cooler/more stable low layer). As of this writing, only showers and a few weak storms are ongoing in our eastern and also extreme western zones, although a strong 40-50KT low level jet (evident at 850 millibars) continues overhead. In the big picture, water vapor satellite imagery and short term model data confirm the heart of a powerful mid-upper level low churning over the NE/KS/CO border area, while a roughly 994-996 millibar surface low axis resides along a north- south axis through western NE/KS (just west of our CWA). As a result, early-AM breezes are mainly out of the east at 10-20 MPH (except more southeasterly in far southern/southeast zones where convective outflow has not been as big of a factor. Low temps this morning are on track to bottom out mainly low-mid 50s.

- TODAY-EARLY THIS EVENING: In the big picture, the heart of the upper level low will shift northeast to over north central/northeast NE by this evening, as the surface low track through mainly the northwest half of our CWA over the course of the day, deepening to around 990 millibars in the process. As this low migrates east-northeast, winds will gradually shift to southerly, then southwesterly, then westerly across our CWA Speed-wise, we are fortunately not expecting anything overly-strong with the non- thunderstorm/synoptic winds, with sustained speeds both ahead of and behind the passing Pacific cold front/weak dryline not expected to exceed 20-30 MPH and gusts mainly 35-40 MPH tops.

Obviously the convective/thunderstorm trends are then our primary concern. Leaning heavily on short term/higher-res models such as HRRR/RAP, there appears to have been a SLIGHT increase in the timing of the passage of the main Pacific cold front as it sweeps into our CWA from southwest to northeast today. with even northern/northeast counties expected to see winds turn more due-southerly by early afternoon and then southwesterly by mid afternoon. Of course, this wind shift will be key to any tornado threat, as in these kind of "cold core" severe setups, typically the tornado threat disappears once winds shift to southwesterly and low-level shear diminishes. That being said, especially our far northern/northeast counties will need watched very closely especially 11 AM-4 PM for any signs of rotation with potential low-topped supercells, as sometimes storms near the strong surface low can produce with little notice. Overall though, currently agree with SPC keeping the more concerning Enhanced Risk and 10% tornado probs slightly east of our CWA Whether or not any tornadoes materialize within our CWA, there is a better chance that a few isolated storms will carry mainly a severe hail threat (PERHAPS up to around golf ball size if any supercells can fully tap into the projected 1500-2000 J/kg mixed layer CAPE in the presence of strong 40-50KT deep layer shear.
By around 5-6 PM, we will likely be "in the clear" from any severe storm threat, although a few cores with small hail could persist.

Potential limiting factors to truly surface-based (and tornadic)
storm potential today will be how quickly/slowly the current expansive deck of low stratus clouds might clear out. Although it might only take a small amount of heating/clearing to pop a few robust storms, the longer low clouds stick around, the lower our surface-based severe threat might be. That being said, an elevated storm threat is still very real, and even at this early hour we are seeing redevelopment of a few stronger storms JUST west of our CWA where steeper mid level lapse rates are arriving.

Temp-wise, blended several short term model sets together to derive highs...aimed from low-mid 60s northwest to low-mid 70s east-southeast (any upper 70s mainly Mitchell County KS area).

- THIS EVENING-OVERNIGHT: In the wake of the departing system, we are looking at a dry and storm-free overnight. Winds will gradually lighten and turn more northerly than westerly, and skies will become no worse than partly cloudy if not mostly clear. Low temps aimed mid-upper 40s most areas.

- SATURDAY DAYTIME-OVERNIGHT: Just don't have much time to dive in deeply here, but here are the main things:

- Most of the daytime hours should be dry/storm free as the next powerful upper low approaches from Colorado.

- Sometime especially 4-7 PM, isolated to scattered storms will likely erupt along/north of a warm front lifting into our our southern zones, potentially becoming severe quickly and offering a threat for hail/damaging winds and perhaps even a few tornadoes in an environment projected to feature 2000+ J/kg of mixed-layer CAPE and very strong deep layer shear of 50+KT.

- The primary area of concern looks to be counties south of I-80 down into our KS zones, where the Slight/Enhanced Risk categories reside. This severe threat could last well into the night, but probably peak prior to 1 AM.

- Temp-wise, highs Saturday could vary more sharply from north- to-south than our current forecast projects (depending on exactly where the frontal zone ends up), but for now aiming from mid 60s north-northwest to mid 70s and a few 80s far south-southeast. Lows Sat night mainly 40s west/50s east.

- SUNDAY-SUNDAY NIGHT: At least for now, not expecting a severe storm threat Sunday, but at least a few stronger storms with mainly a hail threat cannot be ruled out, and any slowing of the upper system could bring at least a Marginal Risk back west into parts of our CWA
Most areas should be dry Sunday-Sunday night, and IF low temps trend any colder then perhaps some limited frost could develop in far west-northwest counties such as Dawson/Sherman/Valley (the counties overall least susceptible to frost impacts in our CWA).

- MONDAY-MONDAY NIGHT: Although not a guarantee to stay this way, this is currently projected as a dry 24 hours CWA-wide with no notable disturbances passing through the mainly zonal (west-east) flow aloft.

- TUESDAY-THURSDAY: While most places will surely be dry most of the time, we at least catch "glancing blows" from a few mid-upper level disturbances passing by mainly just to our north, with the potential for a few surface fronts to also get shoved southward through our area. As a result, the Tues afternoon-Thursday timeframe is currently littered with various rain/thunderstorm chances, but being of lower confidence none of these chances (PoPs) are currently more than 20-40%. Probably a non-zero threat for a few strong/severe storms at times, but details WAY too murky to get into yet.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/
Issued at 712 AM CDT Fri Apr 26 2024

UP FRONT NOTE REGARDING KEAR: Will continue taking KEAR TAF with "AMD NOT SKED" (amendments not scheduled) until routine weather observation transmission is restored there.

- General overview for KGRI/KEAR: This is a somewhat complex period, as a strong low pressure system passes through the region from southwest to northeast, accompanied by several aviation concerns including: sub-VFR ceilings and visibility, showers and thunderstorms, breezy to moderately-windy winds and a gradual directional shift from east-southeasterly, to southerly, to southwesterly, to westerly and eventually northwesterly.

- Ceiling/visibility/rainfall/thunderstorm details: Starting off right away this morning, these first few hours will feature the overall lowest ceilings and visibility of the period, with LIFR ceiling and IFR/MVFR visibility prevailing in light fog (and probably some drizzle). By late morning and into the early afternoon, ceiling is expected to gradually improve to MVFR and then low-end VFR, with VFR then expected to prevail through the remainder of the period.

As for rain shower/thunderstorm potential, at least spotty (generally isolated to perhaps scattered) coverage of convection is likely especially during the 15-00Z time frame, and this is being handled with generic "vicinity thunderstorm" (VCTS) groups as confidence in any particular prevailing time frame is low.
Odds favor any thunderstorms being non-severe, but a brief period of hail cannot be ruled out.

- Wind details (not accounting for any convective influences): For the majority of the first 18 hours, sustained speeds will average 15-25KT/gusts 20-30KT, with sustained speeds easing under 15KT by around 03/04Z, and then the lightest winds of the period (under 10KT) prevailing after 08/09Z. Direction-wise, passing low pressure will result in a gradual/general shift from east-southeasterly this morning, to southerly mid-morning to early afternoon, to southwesterly mid afternoon, to westerly late afternoon-evening and eventually northwesterly late tonight.

HYDROLOGY
Issued at 530 AM CDT Fri Apr 26 2024

Needless to say, this low pressure system has turned into WAY more of a rain-maker than its recent predecessors!

- LOOKING BACK: Based on radar estimation and various gauge measurements, roughly 75% of our CWA received AT LEAST 1" of much-needed rain in the last 24 hours, including a widespread 1-3" yesterday morning mainly along and slightly south of the KS border, and then a more impressive, roughly 40-50 mile wide swath roughly centered along an Elwood-Kearney-Grand Island-Osceola line that saw a widespread 2.50-4.50" (very localized higher) during this past evening-overnight.

Obviously parts of the aforementioned swath through the heart of our Nebraska CWA saw a little TOO MUCH rain...and as of this writing several Flood Advisories and a few Flash Flood Warnings are in effect. Reports of flood impacts have been few so far (typical of this time of day), but obviously soils were quite dry across the vast majority of our CWA heading into this given what had been a dry April, so runoff/flooding was likely muted to some degree.

Unfortunately, roughly 25% of our CWA fell short (in some places well short) of 1" over the last 24 hours, particularly large parts of the following counties: Adams, Webster, Nuckolls, Thayer, Rooks, Osborne.

- LOOKING AHEAD: Fortunately, rainfall over these next 12 hours or so should be less heavy/concentrated, although could see some spots especially north of I-80 perhaps pick up another 0.50-1.00".

Starting this evening, our entire CWA then enters a solid 18-24 lull in rain chances, which should allow most lingering flooding issues to subside pretty fully.

However, we will need to closely monitor rainfall trends with our next main round slated for Saturday mid-late afternoon into the overnight hours, as this system looks like it could easily drop another 1-2+" in various areas, including perhaps some of the same locations that just saw heavy rain. Given the aforementioned 18-24 hour break in any rain whatsoever "in between" the current and upcoming system, am not necessarily sure if an official Flood Watch is warranted for Sat afternoon- overnight, but it will probably need to be considered on later shifts.

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

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