Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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872 FXUS63 KLBF 190935 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 435 AM CDT Thu Sep 19 2024 .KEY MESSAGES... - Dry conditions with above normal temperatures in the 80s today and Friday. - Elevated to near-critical fire weather concerns exist on Friday across western Nebraska due to low relative humidity and windy conditions. - A much welcomed pattern shift to cool and wet conditions likely for the weekend with the potential for widespread, accumulating rain across portions of the area, especially southwest Nebraska (>60% chance for total rainfall of 0.25"+). && .SYNOPSIS... Issued at 425 AM CDT Thu Sep 19 2024 Recent GOES-16 WV imagery and RAP 500-mb analysis showed three potent troughs of low pressure across the CONUS. Our weekend system was apparent across the western US with a deep upper-level trough of low pressure digging southward along the coast of California. Further to the northeast of this feature, a negatively tilted trough was exiting northeast out of the northern Plains into southern Manitoba. The third upper-level trough of interest was noted across the South Atlantic with an upper-level ridge just to the north, extending northwestward into the Hudson Bay. Another area of high pressure was centered across northern Mexico into the southern Plains. All of these features combined has resulted in southwesterly flow aloft across Nebraska. At the surface, an occluding low pressure system was located over the Montana/North Dakota/Canada border with an attendant cold front extending southwest into central Nebraska. At 4 AM CT, temperatures ranged from 54 degrees at Gordon and Ogallala behind the front to 69 degrees at O`Neill ahead of the front. && .SHORT TERM /THROUGH FRIDAY NIGHT/... Issued at 425 AM CDT Thu Sep 19 2024 Today and Tonight...Quiet and dry conditions will be felt across western and north central Nebraska as surface high pressure builds into the region. High temperatures will remain in the above normal range from the low to mid-80s with abundant sunshine. Due to being on the southern periphery of the surface low, west-northwest winds will be breezy this afternoon across northern Nebraska, gusting up to 25 mph. Although afternoon humidity falls into the 25 to 30 percent range, these weaker winds will preclude critical conditions across northern Nebraska. Skies will remain clear heading into the night with low temperatures falling into the mid-40s across the Sandhills to the mid-50s across southwest Nebraska. Friday and Friday Night...The upper-level ridge will build in across the region, ahead of the strong upper-level trough pushing ashore across southern California. This will cause temperatures to climb into the upper-80s to low-90s which is 10 to 15 degrees above normal for the middle of September. These anomalously warm temperatures will support afternoon humidity values to bottom out in the 15 to 20% range across the Sandhills. A surface trough approaching from the west and surface high pressure continuing across the Midwest will result in a tightened surface pressure gradient (SPG) across the area. As a result, southerly winds will strengthen Friday afternoon with widespread gusts up to 25 mph, peaking up to 30 mph across the Sandhills. Although minimum humidity values meet critical thresholds, limited overlap of critical values with the strongest winds will likely inhibit widespread Red Flag conditions being met. Despite this, elevated to near-critical conditions are anticipated across portions of the Sandhills and briefly critical conditions cannot be ruled out midday across Fire Weather Zone 204. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... Issued at 425 AM CDT Thu Sep 19 2024 Saturday through Monday...The upper-level ridge will be shunted south as the southern California trough tracks into the Four Corners, approaching the central Rockies on Saturday. Southerly flow will bring an influx of moisture across the area with PWAT values forecast to exceed 1" across the area, meeting or exceeding the 90th percentile as highlighted by both the NAEFS and ENS ensemble guidance. As height falls overspread the area and the previously mentioned trough exits northeastward into the central Plains, forcing for ascent will increase. There continues to be significant run-to-run difference between each operational run and the NBM has begun to trend down across the Sandhills, favoring areas to the south and east for widespread wetting rain. When looking at precipitation totals, NBM Probabilities for 48 hour (12Z Sat through 12Z Mon) 0.25"+ liquid equivalent precipitation ranges from 20 to 40% across the Sandhills to an overwhelming 60 to 80% across southwest Nebraska. All hope is not yet lost for the Sandhills, as rainfall amounts could increase a bit further north should a more northern track play out. The ECMWF EFI (Extreme Forecast Index) continues to show increased confidence of an anomalous event with this system, such that the SoT (Shift of Tails) is greater than 0 across the entire forecast area with a SoT greater than 1 across southwest into central Nebraska. Depending how the system evolves and all is said and done, total rain amounts ranging from a few tenths of an inch to 1" or greater appears probable with the greatest probabilities for 1"+ currently residing across southwest Nebraska. Outside of precipitation, north-northeasterly surface flow will strengthen with increasing pressure rises on the backside of the surface low to our southeast resulting in gusty winds this weekend, especially during the day on Saturday. At this time, widespread gusts of 20 to 25 mph appears probable with the strongest gusts up to 30 mph across northwest Nebraska where the pressure rises will be strongest. One thing confidence continues to increase on is temperatures falling back into the at or below normal range this weekend, continuing into early next week. The coolest day appears to be Sunday with highs ranging from the upper-50s to low-70s which is 10 to 15 degrees below seasonal values. We may even see our first night of temperatures falling below 40 degrees Sunday night into Monday morning. With height rises behind the departing system, skies will begin to clear out and winds will become weak, allowing for efficient cooling. In fact, NBM Probabilities indicates a 40 to 70% chance of Min Temperature below 40 degrees for areas generally west of Highway 83. High temperatures moderate back into the 60s and 70s on Monday with additional low-end precipitation chances Monday evening as a shortwave swings through the area. Tuesday and beyond...Evolution of the upper-level pattern remains uncertain during this period. Models are in general "consensus" that northwesterly flow will continue across the region through mid-week with the upper-level ridge building back in across the western US. However, another large scale trough appears to move onshore over Pacific Northwest mid-week, nudging the upper-level ridge further east into the Plains. Although questions remain, temperatures trending back into the above normal range and dry conditions appears in the cards for the region. This is in line with CPC`s 6-10 Day Temperature and Precipitation Outlooks favoring above normal temperatures and below normal precipitation chances returning for Nebraska. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1232 AM CDT Thu Sep 19 2024 VFR conditions are expected outside of patchy low-level fog across central Nebraska which may encroach upon LBF for a few hours early in the period. Currently, southerly flow is noted ahead of a surging frontal boundary that is fairly pronounced on radar. Current location as of 19/0530z is an ANW to TIF to OGA line. This feature is quickly shifting southeast and timing of this into LBF is around 19/08-09z. Ahead of this, visibilities are already dropping modestly but the arrival of this boundary will lead to a wind shift to the west-northwest and drier low-level air and the expected end to any fog threat. Additional concern with this boundary is the threat for isolated to widespread convection across north central Nebraska. This should remain east of Valentine which is already post-frontal but uncertainty with southward extent and whether impacts are felt at LBF remains medium at best. For now, will omit mention and monitor going forward. Outside of some patchy cloud cover early this morning, skies will remain clear with even the most aggressive NWP guidance suggesting the same. Increased high- level clouds will approach from the south near the end of the period but arrival may fall just beyond the valid period. && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Viken SHORT TERM...Viken LONG TERM...Viken AVIATION...NMJ