Sunrise 6:44 AM   Sunset 8:36 PM Moonrise 10:53 PM   Moonset 6:58 AM

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FXUS63 KLBF 261136 AFDLBF

Area Forecast Discussion National Weather Service North Platte NE 636 AM CDT Fri Apr 26 2024

KEY MESSAGES

- Threat for heavy rain and possibly some severe thunderstorms continues Friday across portions of western and central Nebraska. A Marginal Risk for severe weather exists for areas east of Highway 183.

- After a lull in activity late Friday into early Saturday, round 2 begins to move into the area late Saturday into Sunday with renewed potential for locally heavy rain.

- Fairly active weather continues through the upcoming week with recurring chances for rain and thunderstorms.

SHORT TERM /THROUGH TONIGHT/
Issued at 431 AM CDT Fri Apr 26 2024

Widespread rainfall with embedded convection continues across much of the northern Sandhills through central Nebraska. Latest radar estimates of rainfall as of 08z (3am CDT) have ranged from around a half inch for all locations south of an Alliance to Ainsworth line to fairly widespread 1.00" south of Interstate 80 and even a few locations topping 2.00" in the area. Keep in mind, these are radar estimates so we'll wait to validate with ground truth later this morning. Morning observations show developing surface low to the south with an inverted surface trough extending north into western South Dakota. Downstream of this feature, broad warm air advection is helping to drive persistent rain and thunderstorms from central Nebraska eastward through the Missouri valley. Satellite analysis shows the broad shield of dry air wrapping around the southern periphery of the main h5 low and this is helping to erode the rain shield across the area.

Regarding the severe threat Friday...the surface low is progged to lift north and east through the morning hours, placing it in our eastern zones around midday Friday. Placement of this central low will have large implications on today's thunderstorm potential.
Ahead of this feature, southeasterly low-level flow will continue to advect moisture into the central circulation. A narrow plume of 60+ degF dew points will extend from north central Nebraska south and east through the southern Plains. While many locations behind the passing low will remain on the cooler side with northerly flow, clouds, and precipitation, the immediate vicinity of this moisture tongue will see all the ingredients come together for another threat for severe weather: steep lapse rates aloft overtop adequate surface moisture and heating, allowing instability to build. In close proximity to a deep h5 low, kinematics will certainly not be lacking as h5 flow ranges from 50 to 70 knots allowing surface to h5 BWD values to exceed 30 knots and even approach 50 knots for eastern Nebraska. Initiation looks to be tied to the h85 front meeting the surface warm front. Timing of this appears to be somewhat tricky but recent runs of various hi-res guidance, notably the 06z HRRR and NAM Nest, suggest this occurs around or shortly after midday in our north central Nebraska zones. Forecast soundings show modest clockwise curvature within the narrow warm sector with abundant 0- 3km shear as SRH values climb to around 150-250 m2/s2. With 0-3 CAPE values exceeding 200 j/kg in a few spots and fairly widespread 1000+ j/kg of MLCAPE values, am growing somewhat concerned about the prospect for rotating storms amid a low LCL environment capable of a tornado or two. To emphasize, this is all contingent of placement of surface low around midday today. The latest SPC outlook highlights areas east of Highway 183 in the Marginal Risk (1 of 5) with the Slight Risk (2 of 5) immediately east of the forecast area in Boone/Antelope Counties. Should the timing of the surface low and richer low-level moisture line up just right, as the HRRR is advertising, I believe an upgrade to Slight Risk with a further northwest expansion of the greater tornado threat will be warranted.
Higher confidence exists in the threat for large hail and damaging winds. All forecast parameters are lining up with local research on large hail events in the county warning area...h7-h5 lapse rates of 7.5-8 degC, MLCAPE of 1000-1500 j/kg, and SHIP values exceeding 2.
Limiting confidence is fairly weak inflow and limited upper-level BWD values as flow is generally uniform in the 2-7km layer. With storms likely surface based in a localized area, efficient downward mixing of stronger winds is anticipated which highlights the damaging wind gust potential. Though DCAPE values look to remain below 1000 j/kg in the local area, fairly significant delta theta-e values (-20 to even -30 degC) are enough to increase concern.

On the backside of the tracking low pressure, mid-level convergence will increase which will lead to the development of a north to south oriented precipitation shield near if not just west of the Highway 61 corridor. While precipitable water values will remain fairly modest, slow movement of moderate intensity rain and thunderstorms should lead to localized areas of heavy rainfall again through midday. Values will likely range from 0.25-0.75" but short term guidance hints at a few locations potentially seeing 1"+. This will occur in a span of about 6 hours, so no flooding is anticipated.
This area of convergence will gradually wane, following the surface low north through the afternoon. As the low lifts north, precipitation chances will follow and decrease through the latter half of the day. Save for areas north of Highway 2 overnight Friday into Saturday, many locations will see a notable break ahead of the second round expected to come Saturday. Daytime highs Friday will range from middle 50s across the Sandhills to middle 60s south and east. Lows tonight will only fall into the low to middle 40s under partly to mostly cloudy skies and with a steady northwest breeze.

LONG TERM /SATURDAY THROUGH THURSDAY/
Issued at 431 AM CDT Fri Apr 26 2024

Saturday and Sunday...Round 2 of rainfall will arrive during the daytime Saturday. Recent trends have been to slow arrival time slightly, so PoPs have been decreased for the daytime hours with peak values arriving during the evening hours. This is tied to a slowing of the h5 low ejection and greatest mid-level height falls which will favor the overnight hours in the local area. Though modest decreases in PWATs are expected between system 1 and system 2, easterly to northeasterly flow on the back side of the departing system will help sustain moisture with Gulf Moisture streaming north into a stalled warm front stretching from northwest Kansas into eastern Nebraska. It's this boundary that should keep the threat for severe weather south and east of the forecast area but any wavering in placement of this boundary may reintroduce the threat closer to home so stay tuned. Even so, steep mid-level lapse rates may yield a strong storm or two locally though shear appears to be significant enough that storms may have a hard time persisting. The SPC maintains a Marginal Risk (level 1 of 5) across our southern 5 counties. Should any storm threat materialize, it would be elevated in nature and likely only pose a threat of hail. Will need to monitor trends regarding this threat with later forecasts. Daytime highs will range from the upper 40s in the north to middle 60s in the south. As forcing increases in the evening, precipitation will again become fairly widespread for Saturday night into Sunday. Have fairly widespread Likely and Definite categories (55%+ and 75%+ respectively) with potential for another 0.50-1.00" of QPF. Even lower percentile values from the NBM suggest another round of wetting rains, so confidence is fairly high in moderate to heavy rainfall amounts. Of course, being convective in nature, locally higher amounts are possible. Though Flash Flooding appears unlikely, antecedent wet grounds from the first round of rain may creep this concern up just slightly. More concerning would be low-land flooding in localized areas with the expected rainfall amounts. Precipitation looks to depart the area late in the day Sunday, with dry conditions returning for much of the area by Sunday evening. Even with rain ending, clouds will linger and cooler air will remain in place. This should hold temperatures to well below normal values as highs only reach the 40s and 50s.

Monday and beyond...as system 2 continues to lift north into southern Canada on Monday, mid-level heights will fill in and a more zonal patter will become established. This will favor moderating temperatures early in the week. Multiple northern stream disturbances will track west to east out of the Pacific Northwest and onto the Northern Plains which will remain north but drop frontal boundaries into the region. This will allow for recurring low-end potential for additional rain and thunderstorms. PoPs in the extended are generally limited to Slight Chance or Chance (35% or less) so confidence in finer details is low in any individual event at this time. NBM temperature guidance shows temperatures recovering to the 70s by Tuesday and likely lingering through the following weekend but with probabilities of seeing > 0.01" QPF no greater than a 50/50 shot any day, the threat for any additional noteworthy rain events appears quite low.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/
Issued at 635 AM CDT Fri Apr 26 2024

A low pressure system will continue to provide forcing for showers and thunderstorms across western and north central Nebraska today.
Conditions likely to remain MVFR to IFR at both terminals through most of the afternoon and evening, with KLBF returning to VFR tonight. With the low passing, winds will shift and become northerly throughout the day and gusty this afternoon. However, winds may become erratic in and around thunderstorms. With the low in the vicinity, ceilings remain quite challenging to forecast, and amendments may be needed if conditions changes more rapidly than expected. Thunderstorm timing also remains a challenge. Have omitted thunderstorms from KLBF, and covered threat at KVTN with VCTS.



LBF WATCHES/WARNINGS/ADVISORIES
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