Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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681 FXUS63 KLBF 181127 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 627 AM CDT Wed Sep 18 2024 .KEY MESSAGES... - Invading dry air behind frontal boundary will lead to elevated and critical fire weather conditions mainly for areas west of Highway 83 where Red Flag Warnings are now in effect. - Low-end chance (< 20%) of showers and thunderstorms across far north central Nebraska Wednesday afternoon. - Turning much cooler with a slow-moving system bringing a long duration rain event to the area early next week. - More seasonable temperatures are forecast for next week with potential but confidence in precipitation potential remains low at this time. && .SHORT TERM /THROUGH TONIGHT/... Issued at 443 AM CDT Wed Sep 18 2024 Morning satellite analysis depicts a deepening low across eastern Montana with a broad plume of moisture streaming north into this feature. At the surface, a frontal boundary has stalled out across our western zones but is expected to make a renewed push east through the day as the disturbance to the northwest continues to occlude and the dry conveyor grazes our western zones. This is setting the stage for fire weather concerns with details found in the following Fire Weather discussion. The dynamic system northwest of the local area will continue to lift north today, following the impressive upper-level dynamics to include strong h5 height falls, upper-level divergence, and strong DCVA. While surface features will continue to shuffle along with the parent low across the Northern Plains, limited cross-front flow will allow the stalled boundary to waver only slightly today. Steady southwest flow ahead of this will maintain middle 60s dew points to the east but a surge of dry air coincident with the dry conveyor will infiltrate our western zones. This will promote a large discrepancy from east to west where it`ll be warm and humid to warm and dry respectively. Forecast highs remain largely unchanged with afternoon peak values of the low to middle 80s. Winds will favor behind the front (west), coincident with the strongest push of dry air and deepest mixing. NWP forecast soundings show efficient momentum transfer with gusts likely to reach 35 mph for the northwest Sandhills. Further east, have introduced a limited chance (< 20%) of showers and thunderstorms for far north central Nebraska. This is in response to increasing low-level convergence in an area of modest instability as MLCAPE values climb to the 1000-1500 j/kg range. That said, capping looks to remain strong so it`s believed that most location will remain dry. HREF ensemble max QPF shows increased amounts early afternoon but signals are greatest across eastern South Dakota. Should anything develop, severe weather is not anticipated though a stronger core could still produce sub-severe wind gusts and small hail. Any activity will quickly lift northeast and return dry conditions to the area. The surface boundary will make a small retreat late tonight but the lack of any upper-level support will help maintain a dry forecast. Pooling moisture following the recent rains main introduce some patchy fog to the area but confidence is too low for inclusion at this time. Low temperatures will fall into the 50s for most and low 60s east of Highway 281. && .LONG TERM /THURSDAY THROUGH TUESDAY/... Issued at 443 AM CDT Wed Sep 18 2024 Thursday and Friday...dry conditions are expected for each day. Strengthening northwest flow will usher the frontal boundary to the east and dry air will promote tranquil weather. Modest high pressure will nose in front the east and track through the area by early Friday morning. Temperatures will generally favor the 80s each day which will remain on the warmer side of climatology. Moisture return later Friday on the backside of departing high pressure will be the first signs of an increasingly active weekend and early week. Saturday through Monday...Southern Plains ridging will retreat south as the next disturbance digs through the Desert Southwest and begins to approach the central Rockies. Southerly flow will increase further on Saturday and bolster PWATs which are progged to surpass 1.00" yet again, or exceed the 90th percentile. The ECWMF Extreme Forecast Index (EFI) has steadily increased its signal for Saturday and Sunday and recently has begun showing values of 0.6-0.8 for southern Nebraska with SoT values of 1-2. This is suggesting a fairly anomalous precipitation event across at least the southern half of Nebraska. Forcing increase late Saturday as height falls overspread the area and the main h5 trough exits Colorado and moves onto the Central Plains. Forcing for ascent maximizes Sunday which closely matches the EFI peak signal. Ensemble solutions show mean QPF values exceeding 1.00" across much of the area as a deepening surface low lifts northeast from western Kansas into eastern Nebraska. This puts western Nebraska squarely under the main deformation band with a vertically stacked system largely remaining immediately to the southeast. Barring any late shifts in forecast track, this should also keep any dry air surge to the south. The expected long duration rainfall event should bring beneficial rainfall to portions of the area while also keeping temperatures on the cooler side. Forecast highs fall to upper 60s to upper 70s Saturday but will struggle to climb out of the low 60s Sunday. Looking at lower NBM percentile values, a sharp gradient in expected QPF is appearing likely and potential exists for northern Nebraska to miss out on wetting rain potential. For now, will maintain inherited PoPs which top out in the 40 to 75% range from north to south. Total QPFs of 0.50-1.50" will be possible but refinements with later forecasts will likely shift where the heaviest amounts fall so folks should continue to monitor subsequent forecasts for the latest. Precipitation will likely wane through the day Monday with a return to dry conditions likely by the evening for most if not all locations. Temperatures will moderate slightly as well, but only into the upper 60s to lower 70s. Tuesday and beyond...evolution of the upper-air pattern in the long range remains somewhat in question. The overall idea is from amplifying northwesterly flow towards the middle of next week. How the departing trough evolves across the Great Lakes will be a big thing to monitor with implications on the forecast going forward. NWP guidance is hinting at another large scale trough moving onshore the Pacific Northwest around the same time but progression and placement Wednesday and beyond is uncertain at best. For now, believe precipitation chances around the middle of next week will be limited until we approach the following weekend when the system encroaches on the Northern Plains. NBM probabilities for precipitation don`t appear overly robust, however, so a continuation of slightly above normal temperatures and dry conditions appears probable. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 621 AM CDT Wed Sep 18 2024 An area of stratus has begun to blossom across central Nebraska and will continue to expand through the morning, impacting KLBF. This will result in a period of MVFR CIGs this morning with a quick return to VFR conditions across all of western and north central Nebraska this afternoon. Southerly winds remain light for most at 10 to 15 kts today, though increasing this afternoon near the Pine Ridge, with gusts out of the southwest of 20 to 25 kts. && .FIRE WEATHER... Issued at 443 AM CDT Wed Sep 18 2024 Red Flag Warnings have been issued for Fire Weather Zones 204 and 208 due to the combination of strong winds and low afternoon humidity values. A surface boundary will creep east today before settling across the central and eastern Sandhills this afternoon. This will be the dividing line between much richer low-level moisture characterized by middle 60s dew points to the east and much drier air to the west. This drier air will be enhanced by strong downslope flow as the dry conveyor belt from a deep surface low to the northwest grazes the area. This will lead to a large discrepancy across the area with afternoon humidity minimums ranging from around 50% in north central Nebraska to near 15% in the far northwest. This will be most apparent in Zone 208 where forecast humidity minimums will range from ~15% at Nenzel to 48% at Butte. Opted to include 208 in the Red Flag Warning given favorable overlap of the lowest humidity and strongest winds west of Highway 83. Forecast soundings show deep mixing, nearing 3km in the vicinity of the Pine Ridge, which will support efficient momentum transfer of winds through unidirectional flow to the surface. This will likely tap into stronger flow around h7-h6 layer where speeds will approach 30-35 knots. This should be more than enough to yield 35 mph gusts at the surface with a few gusts nearing 40 mph on the higher terrain around the Pine Ridge possible. MRMS observations show a few select locations managed to see 0.10-0.20" of rainfall Tuesday night, but with plentiful sunshine and the strong winds forecast for today this is likely to not be overly beneficial. Recent hi-res guidance, notably the HRRR/RAP and deterministic NAM, all depict sub 20% humidities in these areas. NBM deterministic guidance shows fairly widespread 15-20% afternoon humidity minimums. Using a blend of all guidance, went with the 50th Percentile value which produced sub 20% humidity northwest of a Chappell to Valentine line and ~15% humidity northwest of a Nenzel to Alliance line. Winds should peak around early afternoon and remain strong into the early evening before the boundary layer begins to stabilize with the loss of insolation. Was on the fence about including Fire Weather Zone 206 but felt the limited spatial coverage north of Highway 2 and west of the timezone line was not enough to warrant inclusion. Areas near and just south of McKelvie should also exercise caution as conditions would warrant the rapid spread and erratic behavior of any fire starts. Elsewhere, humidity above 30% and winds below 20 mph limit overall concern. Wetting rains appear likely early next week beginning as early as Saturday but confidence in precise QPF is low. For now, greatest potential of seeing wetting rains is generally south of Highway 92 with decreasing confidence with northward extent. && .LBF WATCHES/WARNINGS/ADVISORIES... Red Flag Warning from 1 PM CDT /noon MDT/ this afternoon to 9 PM CDT /8 PM MDT/ this evening for NEZ204-208. && $$ SHORT TERM...NMJ LONG TERM...NMJ AVIATION...Viken FIRE WEATHER...NMJ