Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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056 FXUS63 KOAX 220915 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 415 AM CDT Sat Jun 22 2024 .KEY MESSAGES... - Main focus will be centered around severe thunderstorms tonight and the potential for continued heavy rain across our north. - Flood Watch for northeast Nebraska and along the entire Missouri River. - At least small storm chances nearly every day through the middle of next week. Severe weather will be possible at times. - Gradually warming temperatures through the weekend into early next week. Heat indices approaching 100 to 110 on Monday and possibly Tuesday. && .DISCUSSION... Issued at 340 AM CDT Sat Jun 22 2024 Current Scenario... Showers with a few embedded thunderstorms have been generally decreasing in coverage and intensity since 2 AM, but do continue to drop moderate rain in parts of east central NE as we approach 4 AM. Rainfall estimates suggest that 3-4 inches or so have fallen in parts of western Butler County, while OLU measured 2.72" through 3:40 AM. The outflow from this system has pushed well south of the ongoing convection while the LLJ has veered slightly and lifted its most convergent region to the northeast, disassociated with the low level outflow boundary and amidst weakening instability. All of this suggests that current activity will continue to diminish and lift to the east northeast over the next couple of hours. It is also worth noting that a healthy elevated supercell produced intense winds in the North Platte area this past hour, but is pushing into an increasingly hostile environment with eastward extent, and appears likely to dissipate prior to reaching the OAX forecast area. The flood watch for northeast Nebraska (not including the Missouri River Watch) has been cancelled. A Note On River Flooding... Another round of heavy rain fell over the already flooded areas of southeast SD and northwest IA into southern MN. The James, Vermillion, Big Sioux, Floyd, and Little Sioux Rivers all of forecast points in major flood stage with several already exceeding or forecast to exceed record flood stage. All of this water is headed into the Missouri River below Gavins Point Dam, and thus uncontrolled by the system of dams. The timing of flow in these relatively flat river basins is complex, and observations are of key importance in modeling downstream flows. This is likely to cause flooding along the Missouri for several days. Current forecasts indicate that the river will not reach flood stage at or below Decatur until at least Monday, and the crest will not reach the Omaha area until June 28 or 29. Given the extent of upstream flooding, and the amount of additional rain on Friday night, it appears very likely that the Missouri will reach flood stage, and anticipate upgrading the existing watch to a warning with the issuance of the morning Missouri River Mainstem forecast. Even still, there will be some level of uncertainty with the specific forecast magnitude until the water from those upstream rivers gets closer to...or into...the Missouri and flows are observed. We should also note that the Ponca Creek at Verdel is in Moderate flood stage this morning, but forecast to improve over the coming day. The Elkhorn upstream of Norfolk had quite a bit of rain fall around Oneill in the past 48 hours, and is currently forecast to reach action stage, but will be monitoring closely for flood potential. Storm Potential Today... The effective cold front is located just northwest of the forecast area early Saturday morning, but a short wave trough moving across western SD will progress steadily east over the course of the day and push the cold front southeast in its wake. By noon, the front should be approaching I-80 and by 6 PM should be southeast of the forecast area. Forecast soundings in the warm sector are initially capped, but show some cooling of the cap as the front approaches, perhaps eroding inhibition enough to allow early afternoon convection along the front. One ingredient that seems to be largely missing in this equation is that boundary layer convergence along the front is pretty weak, and most CAMs are rather limited in convective initiation... keeping storms weak or non-existent until later in the day and only in far southeastern parts of the forecast area. However, with that in mind, there is a fair potential for earlier initiation along the front so long as the cap does effectively erode, and if convergence or other boundary layer lifting mechanisms are sufficient to support it. If sustained updrafts develop, deep layer shear would likely be sufficient for right moving supercells to organize, with some potential for hail and damaging wind. As it stands, the threat is low, and hinges on whether or not sustained updrafts will exist in our forecast area. Middle to Long Term Period... Sunday will be mainly dry, although a small chance for a late day storm west as southerly flow returns on the back side of high pressure. Upper level ridging builds in for Monday, and lingers into Tuesday with hot conditions expected. The near- surface airmass will also be quite humid, and heat index values are likely to exceed 100...possibly coming close to 110. While precip chances are quite low, this could be a sneaky chance for isolated severe storms late Monday, and again late Tuesday as a short wave trough over the Northern Plains drops a front into the area. Model agreement is then quite good regarding a strong Northern Plains short wave trough moving through Thursday into Friday, possibly bringing a decent setup for severe weather and heavy rainfall. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 631 PM CDT Fri Jun 21 2024 Rain and embedded TS continue to move across the region, likely to continue for a few hours at OFK, and will come very close to impacting LNK and OMA. At this time, believe OMA will have a couple hours of TS likelihood, while LNK may just have it in the vicinity. Both LNK/OMA will also have an outflow boundary move through with a wind shift out of the northwest, which will last for a couple hours before turning back out of the south. LLWS will continue at all sites tonight. A cold front will move through all site during the day on Saturday, with lower MVFR cigs at OFK after the front passes, but probably in the VFR category at OMA/LNK as it passes a bit later in the day. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Barjenbruch AVIATION...Barjenbruch