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Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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602 FXUS63 KLBF 141854 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 154 PM CDT Wed Aug 14 2024 .KEY MESSAGES... - A Marginal Risk (level 1 of 5) is in place for Wednesday across much of western and central Nebraska. - A cool front will drop south into the area later this afternoon, bringing the threat for thunderstorms as well as a slight cooldown for Thursday. - Daily rain and thunderstorm chances continue through the upcoming weekend, however, PoPs remain limited to 35% or less any one day. && .SHORT TERM /THROUGH TONIGHT/... Issued at 350 AM CDT Wed Aug 14 2024 Morning satellite analysis shows a plume of high cirrus over the eastern half of the forecast area. This is originating from thunderstorm activity that moved through the area late Tuesday night and early this morning. Satellite imagery shows little to suggest deep convection is expected to redevelop in the area this morning. At the same time, radar shows broad spin across central Nebraska likely associated with an MCV tracking out of the area. Modest lift on the northern periphery of this feature was leading to an increase of scattered showers and weak convection for north central Nebraska. For Wednesday...mid-level PV anomaly continues to shift east through the area. Height falls are maximized in close proximity to this feature and these are expected to wane through the late morning hours. Weak low pressure will develop across southwest South Dakota. This is largely driven by enhanced h5 flow ejecting across the central Rockies. This will drive a cool front south into the area with broad downsloping winds pinching a warm sector east into central and eventually eastern Nebraska. This warm sector will be denoted by surface dew points in the upper 60s to low 70s. While the area will be lacking a strong EML, cooling mid-level temperatures should foster some modest lapse rates. As afternoon highs climb into the mid to upper 80s, moderate instability is expected to develop with MLCAPE values likely climbing into the 2000-3000 j/kg range. This will largely be confined to the warm sector east of Highway 83, at least initially, as dry air to the west limit both LI and instability values. Low-level convergence on the leading edge of the approaching cool front will be enough to lead to at least isolated thunderstorms across north central Nebraska. Tuesday evening HREF output shows this potential well with NAM Nest being the most aggressive individual ensemble member. Of interest, recent runs of the HRRR are fairly pessimistic to this potential likely due to strong capping in place and not as strong low-level convergence. Given some limited confidence, will limit PoPs to around 45% (Chance category). Given modest deep-layer shear, multicells are the likely storm mode and with most instability being confined to the mid- levels, isolated instances of large hail and damaging wind gusts are the main concerns. This thinking largely lines up with the latest SPC outlook however, confidence quickly wanes with westward extent. Here, the decision was made to decrease PoPs and introduce a dry forecast for portions of the western Sandhills into southwest Nebraska. This is largely due to a significant lack of low-level moisture and as a result, much weaker instability. Forcing will largely come via the cool front but in these areas, convergence is much weaker and dry air is quick to overspread the area. In fact, most synoptic scale NWP solutions depict mid-level humidity dropping below 50%. And while HREF probabilities show thunderstorms likely forming off the higher terrain, an unfavorable environment with weak shear and limited instability suggest activity won`t be able to reach our western zones easily. The most bullish solutions, the NAM Nest and FV3, limit any activity extending east of Highway 61 so have maintained a small area of 15-20% (Slight Chance) PoPs to account for this. Storms will quickly die though and with convection likely having exited central Nebraska by then, mostly dry conditions are expected for Wednesday night into early Thursday. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Issued at 350 AM CDT Wed Aug 14 2024 Thursday...Deeper troughing will settle into the Northern Plains by early Thursday. This will lead to broad height falls across central and eastern Nebraska. The main surface low will remain well north of the area and drying air advecting in northwesterly surface flow will greatly inhibit any rain or thunderstorm chances. Temperatures will be slightly cooler than what was seen Wednesday as h85 temperatures will see modest cooldown from 24 hours prior. Highs will generally be in the low to middle 80s and outside of an isolated shower or weak thunderstorm along the Highway 20 corridor, dry conditions are expected. Friday through early next week...southern Plains ridging will build through the period as h5 heights climb to around 595 dam. The ridge axis will still along the Rocky Mountains, leaving the area in northwesterly flow. NBM probabilities of measurable precipitation are quite muted with < 40% outlook for seeing greater than 0.01" of liquid any day through the weekend. Meanwhile, the EPS and GEFS solutions are not as dire but they too are limited in their optimism of noteworthy rainfall. Heights continue to build in as the high pressure aloft extends its influence into the central and southern Plains. Sometime late Monday into early Tuesday, a shortwave will round the ridge and bring about the best potential for rain and thunderstorms to the area. Latest NBM guidance shows PoPs reaching around 35% late Monday evening into Tuesday morning. This closely coincides with EPS thinking that shows western Nebraska seeing 40 to 60% probabilities of precipitation exceeding 0.10". Higher percentile values show a widespread wetting rain event, but median values show a much less impressive signal. So uncertainties remain and folks should continue to check later forecasts for more details. Overall, temperatures will peak in the upper 80s to lower 90s as highlighted by 25th and 75th percentile values. It is interesting to note that 90th percentile values are much warmer with mid to upper 90s being advertised. This is likely being driven by the GFS and its ensemble suite and discounted as a bias issue. Overall thinking is middle ground between GEFS and EPS for day-to-day temperatures is more realistic and thus produces the middle 80s to low 90s in the extended period. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 122 PM CDT Wed Aug 14 2024 VFR conditions will prevail across all of western and north central Nebraska through tonight into Thursday morning. Isolated thunderstorms cannot be ruled out this afternoon and evening, but widespread activity is not expected. Winds will gradually shift to the northwest at 5-15 kts. && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...NJ LONG TERM...NJ AVIATION...Taylor