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FXUS63 KLBF 031944 AFDLBF

Area Forecast Discussion National Weather Service North Platte NE 244 PM CDT Fri May 3 2024

KEY MESSAGES

- Strong to severe thunderstorm development is expected Friday afternoon and evening with damaging wind gusts and locally heavy rainfall the main concerns.

- Another episode of severe weather appear increasingly likely for Monday, but confidence in impacts in the local area is low at this time.

- Cooler temperatures are likely for Day 5 (Tuesday) and beyond.

SHORT TERM /THROUGH TONIGHT/
Issued at 243 PM CDT Fri May 3 2024

Immediate concern focuses on the threat for severe thunderstorms this afternoon and evening across much of western Nebraska.

As of 19z, convection was developing along a stationary boundary draped from the southern Panhandle into north central Nebraska or roughly just north of a SNY to just south of a VTN line. Ahead of this feature, strong southerly flow was driving low-level convergence in immediate proximity to the boundary. Moisture in the warm sector is marginal at best with dew points reading the upper 30s to low 40s as air temperatures read the upper 60s to lower 70s south of Highway 2. This will greatly limit instability with MLCAPE values only progged to approach 1000 j/kg for areas south of Highway 23. Further north, values will likely remain 750 j/kg or less with rapid decrease with northward extent. Even so, forcing should be fairly strong with the frontal boundary progressing through the area. At the same time, flow aloft remains fairly strong with h5 flow exceeding 40 knots. With southerly surface flow, this is producing moderate 0-6km BWD values of 40 to 50 knots. Forecast soundings show inverted-v profiles ahead of the main line of convection but with a lack of drier air aloft, max theta-e values are marginal only falling to around minus 10 degC. At the same time, DCAPE values are on the lower end as well with most locations reporting values of less than 750 j/kg. That said, flow off the surface immediately following the frontal boundary will respond accordingly as a strengthening cold pool drives this south quickly this evening. These elevated winds will likely be efficiently mixed to the surface by the convection. This will support at least a concern for isolated damaging wind gusts in the local area. While hail cannot be ruled out, believe this threat is fairly low given lack of greater instability aloft.

As forcing translates east along the boundary, increasing moist advection will help increase coverage of thunderstorms on approach to southwest and eventually central Nebraska later this evening.
Coverage should blossom as a result with a growing concern of locally heavy rain. While PWATs remain low, less than 0.75", rain rates exceeding 0.50"/hour with potential for training storms should lead to a smaller area of QPF likely exceeding 1.00" from southeastern Lincoln/Frontier Counties up through central Nebraska to include southern Custer County. These areas have recently seen fairly heavy rainfall events as departures from normal values exceed 150% and even approaching 300% for some of these locations. This has helped drive county based Flash Flood Guidance to around 1.5"/2.0" for the 1 hour/3 hour values respectively. Latest HRRR guidance shows Probability Matched Mean values of 3 hour QPF exceeding 1.25" for many of the mentioned areas and even peaking around 2.0". This suggests some hydrology concerns so this will need monitored as the evening progresses. Believe incoming dry air behind the front and the main source of lift should clear the area by Midnight Friday night though lingering stratiform precipitation could last into the early morning hours Saturday. This later departure of clouds and precipitation should help keep overnight lows across our east on the milder side with its implications on any frost threat covered at the start of the long term discussion below.

LONG TERM /SATURDAY THROUGH THURSDAY/
Issued at 243 PM CDT Fri May 3 2024

Saturday and Sunday...Departing trough to the northeast will allow mid-level heights to rise as shortwave ridging builds in its wake.
This will promote dry conditions for Saturday as surface high pressure settles through the area. Forecast lows for Saturday morning do not line up with latest guidance on vulnerable vegetation therefore thinking no frost headlines are needed at this time.
Supporting this thought is steady northerly winds with potential for lingering low stratus across our eastern zones. Admittedly, the residual cloud cover behind a progressive cool front tends to be overstated in NWP guidance. Even with the more pessimistic guidance though we see steady enough flow in the low-levels to prevent full boundary layer decoupling with surface winds remaining in the 10 to 15 mph range through sunrise. Further west where the setup for radiational cooling will be more favorable, lows will again fall below the freezing mark. That said, the areas remain outside the latest guidance for susceptible vegetation therefore no Frost or Freeze headlines are expected. All that said, lows will range from the upper 20s west to upper 30s east. Daytime highs will climb into the low to middle 60s with light and variable winds generally 15 mph or less through the daytime. Ridge breakdown begins Sunday as the next shortwave trough approaches the central Rockies late in the day. With developing surface low to the west and departing high pressure to the east, a west to east pressure gradient will compress locally and help drive some increasing southerly winds late Saturday into early Sunday. This will promote increasing moisture advection into the area. This will also favor a fairly stubborn low stratus deck across our western zones that may struggle to clear out until later in the day. As a result, cool temperatures will remain stubborn with afternoon highs remaining in the low to middle 60s.
Winds will be on the stronger side for areas west of Highway 61 where gusts may climb into the 35 to 45 mph range with 25 to 35 generally expected elsewhere. Can't rule out a few light rain showers west of Highway 83 in the area of greatest moisture advection but given the general lack of convergence and limited WAA, believe this potential is fairly low with minimal if any impacts expected.

Monday...A well advertised storm system will traverse the Plains, bringing with it the potential for a fairly robust severe weather episode across the region. Low-level southerly flow will persist early into the day helping increase surface moisture as a dryline tightens from western South Dakota down through western Texas.
Aloft, the aforementioned shortwave disturbance will begin to take a negative tilt as it ejects onto the Plains early in the day. A concentrated area of significant height falls in concert with strengthening upper-level divergence will help foster the development of an elongated area of low pressure stretching from southern Colorado up through eastern Montana. Within the plume of richer low-level moisture, the combination of mild afternoon highs and steep mid-level lapse rates will lead to increasing instability within a strongly sheared environment. For now, greatest confidence in severe weather remains east and southeast of the local area where all hazards will be possible. Closer to the local area, expecting the greatest threat to favor large hail and damaging wind gusts.
That said, the progressive cold front overtaking the dryline will encounter modest instability in our eastern zones so the prospect for severe weather affecting the local area cannot be ruled out. The latest SPC Day 5 severe weather outlook highlights areas east of Highway 83 in the 15% probability, or the equivalent of a Slight Risk (level 2 of 5). For now, have no qualms with this but impacts from morning rain and thunderstorms within the strong WAA within the warm sector and how this affects the afternoon environment will need to be closely monitored. Behind the boundary within the drier airmass invading the area, strong CAA will support windy conditions with gusts nearing the 35 to 45 mph range out of the west.
Temperatures will likely falter as a result but this looks to be late enough in the day that afternoon highs will still reasonably reach the upper 50s to lower upper 60s across the area.

Tuesday and beyond...mid-level heights will be quick to fill in early Tuesday as a cutoff low at h5 takes shape across the northern High Plains. This feature will likely remain fairly stagnant through the middle of the upcoming week with a slight cooling trend in temperatures expected for the later half of the forecast period.
Latest ECMWF Extreme Forecast Index (EFI) guidance hints at a continuation of strong winds from Monday into Tuesday. Another cool front will traverse the area by late Tuesday into Wednesday, bringing about a more noteable drop in temperatures. Some low-end PoPs will also dot the forecast, favoring areas north of Interstate 80. For now, confidence is greatest in precipitation Thursday afternoon and evening. Higher percentile values from the NBM suggest the potential for wetting rains but variances in deterministic and other ensemble guidance limits confidence in this occuring thus far.
Temperatures will favor below normal values through the time frame which will include overnight lows falling to near the freezing mark.
This would line up with frost/freeze climatological median dates so potential need for headlines next week cannot be ruled out.

AVIATION /18Z TAFS THROUGH 18Z SATURDAY/
Issued at 1220 PM CDT Fri May 3 2024

Strong south winds continue this afternoon for terminals south of Highway 2. A cold front will then slide through the area this afternoon and evening, with a line of thunderstorms expected to develop along this boundary. Gusty, erratic winds and MVFR visibilities can be expected with the passage of these storms.
Additionally MVFR ceilings are expected to develop in the wake of the cold front/thunderstorms, and will persist into early tomorrow morning. Clouds will quickly clear from west to east by late tomorrow morning, with VFR expected through the end of the valid period.

LBF WATCHES/WARNINGS/ADVISORIES
None.

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