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FXUS63 KLBF 271130 AFDLBF

Area Forecast Discussion National Weather Service North Platte NE 630 AM CDT Sat Apr 27 2024

KEY MESSAGES

- Precipitation chances will increase through the day Saturday, peaking around 85-95% Saturday evening into early Sunday. A few storms in the evening may be strong to briefly severe with isolated instances of large hail the main concern.

- Locally heavy rainfall is again likely tonight through early Sunday with QPF amounts of up to 1.00" and localized amounts closer to 1.5" possible.

- Wet snow may mix with rain early Sunday across the western Sandhills but no accumulations are expected.

- Fairly active weather continues into next week with recurring chances for precipitation with PoPs generally 50% or less any one day.

SHORT TERM /THROUGH SUNDAY NIGHT/
Issued at 402 AM CDT Sat Apr 27 2024

Round 2 of active weather will begin to encroach on the area later today. This will include the threat for locally heavy rain and the possibility of strong to severe thunderstorms favoring areas south of Interstate 80.

Morning surface and satellite analysis depicts the area between two distinct systems. The first of which produced the significant severe weather across eastern Nebraska Friday, currently situated near the northern Mississippi Valley. Northwesterly flow behind this feature has propelled much cooler and stable air into the local area. To our south, the next system is beginning to take shape across far southeast Colorado into southwest Kansas. The trailing cool front from the first system has settled south of I-70 in southwest and central Kansas with broad southerly flow across the southern Plains helping promote deep moisture advection which will support another significant severe event across southeastern Kansas into eastern Oklahoma. The developing surface low to our south will track along the baroclinic zone settling across central Kansas. With converging flow in the immediate vicinity of this, a warm front will sharpen and gradually lift north as the surface low begins to shift east by early afternoon today. Winds locally will veer to the northeast with surface high pressure settling into the northern Plains. This will help keep the local area on the cooler side north of the effective boundary. As a result, high temperatures today will only range from the upper 40s across the western Sandhills to near 60 in our far south and east which falls about 5 to 15 degF short of climatological highs for late April. Aloft, general southerly h5 and h7 flow will advect another modest EML which will lead to steepening lapse rates in the local area. With the warm humid airmass remaining south of the area, potentially even south of the NE/KS state line for areas west of Highway 183, am a little wary about our potential for surface based convection. HRRR probabilities of > 500 j/kg of SBCAPE are zero for areas north of McCook. Forecast soundings show elevated instability, generally rooted around h75 by mid to late afternoon. Shear remains fairly strong in this layer, with 2-7km BWD values of 35 to 45 knots. Believe this plays to at least a threat for isolated large hail. Storms look to initiate across eastern Colorado around midday and within southerly steering flow will track north into southwest Nebraska by early evening. As they track north, they will encounter more stable surface air and limited elevated instability so the expectation is for this activity to weaken with time. HRRR storm-scale guidance shows fairly high probabilities of notable mid-level updraft helicity, nearly 90% around I-70 west of GLD, but rapidly decreasing to less than 30% once over McCook and zero by the time storms arrive to I-80. Similarly, HRRR guidance shows fairly notable weakening with storms once crossing into far southwest Nebraska as storms congeal into a larger arc of moderate precipitation. The latest SPC SWO Day 1 outlook grazes Frontier County with a Slight Risk (level 2 of 5) and a Marginal Risk (level 1 of 5) generally along and south of a Grant to Oconto line.
Confidence in seeing severe weather locally is low enough that I feel the Slight Risk can be removed from the local area. Heavy rain will continue to be the greatest concern for western Nebraska tonight through early Sunday. On the north side of the slow tracking surface low, warm air advection will increase with a sharpening gradient leading to strong fgen around h85 but extending up to h7.
With modest elevated instability remaining in place, general showers and thunderstorms are expected as PoPs climb to 85-95% for much of the area during the overnight. Even with high PoPs and convective activity, rainfall amounts will be slightly limited due to fairly average PWATs around 0.50-0.75". So slow and steady rain will again be favored which will limit the overall flooding threat. Overall, rainfall amounts of 0.50-1.00" with locally higher amounts closer to 1.50" possible. This generally lines up with HRRR Probability- Matched Mean values and the majority of this rain will fall in areas typically more immune to flash flooding threats such as the Sandhills but also areas that missed out on the heavier rainfall Thursday night.

Turning to Sunday morning, cooler air will continue to filter in from the north. This will promote low temperatures falling into the middle 30s for the western Sandhills. With marginal h85 temperatures around the freezing mark and low-level wet bulb temperatures maxing out only a degree or so above 0 degC, will have to watch for a rain/snow mix in our western zones. Antecedent warm conditions have helped allow the ground temperatures to warm nicely across the area so not expecting any wintry impacts outside maybe some marginal visibility restrictions in areas that see the snow. Through midday, the surface low to our south will lift north and east into the middle Missouri Valley. As it does, it'll weaken slightly which will allow the pressure gradient to relax and forcing for ascent to diminish. The combination of this and the focus lifting north into South Dakota will help lead to decreasing precipitation chances through the day. While low-end PoPs (< 30%) linger across north central Nebraska late into the evening, believe most if not the entire forecast area will be dry by mid-evening. Sunday night will be the coldest night of the week with lows falling to around the freezing mark. With the 50th Percentile last Spring freeze date still a week or two away, no Frost headlines are expected at this time.

LONG TERM /MONDAY THROUGH FRIDAY/
Issued at 402 AM CDT Sat Apr 27 2024

Quieter conditions arrive for Monday as mid-level heights build in and flow aloft favors a more zonal pattern. Continued southwesterly flow will support increasing h85 temperatures and as a result, moderating temperatures. Temperatures look to return to seasonable values by Monday and Tuesday, with highs in the 60s and 70s. A cutoff disturbance across the northern Rockies will gradually translate east with a few stronger PV anomalies rotating on the southern periphery of the main feature. This will push a few weaker systems across the northern Plains which will drop a few cool fronts south into the area during the week. For now, Tuesday and Thursday look to be the best chances for these fronts to have the greatest affect on sensible weather with not only a brief temperature drop late Tuesday through Thursday but also recurring rain and thunderstorm chances. Confidence in day-to-day details remains fairly low given spatial and temporal discrepancies between deterministic and ensemble solutions, but the overall pattern with enhanced mid-level flow and a warm, humid airmass in the region supports at least some potential for severe weather. More precise details will likely not become apparent until we get to Monday so continue to check later forecasts for the latest information.

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/
Issued at 629 AM CDT Sat Apr 27 2024

Satellite imagery shows large areas of stratus over most of western and north central Nebraska. Lowest ceilings have been over north central Nebraska, however forecast sounding bring low ceilings in to southwest Nebraska quite rapidly this morning. Conditions are expected to quickly diminish at KLBF this morning from VFR to MVFR, dropping to IFR this evening. As for KVTN, MVFR conditions quickly become IFR this morning and remain IFR for most of the TAF period, with conditions down to LIFR this evening. Though thunderstorms are expected across the region tonight, confidence still remains low on timing and location, so will continue to omit thunder from TAFs for the time being.

LBF WATCHES/WARNINGS/ADVISORIES
None.

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