Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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867 FXUS63 KOAX 010425 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1125 PM CDT Sun Jun 30 2024 .KEY MESSAGES... - Flooding continues on the Missouri River below Decatur this week. With multiple rounds of storms and heavy rain expected tonight through Tuesday, additional river flooding or flash flooding will be possible. - There is a small chance for a few very intense storms to develop Monday between 5 and 9 PM, mainly southwest of a Columbus to Lincoln to Beatrice line. If these storms develop, they will be capable of producing very large hail, damaging winds, and tornadoes. - Widespread thunderstorms will move into the area from the west late Monday evening and overnight. These storms could produce very heavy rainfall, isolated large hail, as well as intense damaging winds and a few tornadoes mainly south of a Neligh to Blair to Clarinda line. - Widespread storms are likely again on Tuesday. The greatest severe potential will be south of a Hebron to Fremont to Onawa line. Heavy rain, hail, damaging winds, and perhaps a tornado will once again be possible with the strongest storms. - Wednesday night into Independence Day features yet another round of storms. Currently, the main focus appears to be from 10 PM Wednesday to 8 PM Thursday. && .DISCUSSION... Issued at 225 PM CDT Sun Jun 30 2024 It`s a beautiful Sunday afternoon as the forecast area finds itself on the west side of departing high pressure. Temperatures are in the 70s with an east southeast breeze. As advertised, there are a few light sprinkles sneaking across northeast NE this afternoon, but any drops reaching the ground are short lived. There is also a slightly more organized area of showers over south central NE and central KS associated with a mid level short wave trough. This activity should continue as the system moves east this evening, but continued dry easterly low level flow will limit overall precip. Southern parts of the area may see light rain or sprinkles through this evening, but again, it shouldn`t add up to much. Low pressure all along the lee of the Rockies has increased the pressure gradient over the High Plains with southeasterly flow from west TX up through western NE and into Montana. Seeing some moisture return on that southerly flow this afternoon, and expect a healthy low level jet to develop overnight in advance of the next short wave trough moving into the local area by the early morning hours. While the surface moisture will be a bit slower to return with vigor, the deep moisture profile will be favorable for some heavy rainfall rates as PWAT exceeds 2" for several hours around sunrise. Expect widespread rainshowers and thunderstorms to develop, with elevated parcels likely realizing up to 1500 J/kg of MUCAPE. The effective shear will not be particularly strong, but could potentially support marginal elevated supercell structures if able to root the inflow layer low enough into the LLJ to lengthen the shear vector. All-in- all, the potential for hazardous weather tonight is quite low, but will need to watch for some low-end potential for severe hail, and while peak rainfall rates will be impressive, it also appears that those heavier rates will not last particularly long at any given location so 0.5 to 1.5 inch rain totals will be common with limited potential for locally higher amounts. Highly Conditional, Low-Probability-High-Impact Severe Potential late Monday : After morning storms move east (exiting the forecast area by around 10 AM), much of the day will be dry other than some potential for additional shower/storm activity in far northern NE. Southeast winds will be gusty, and it will become notably more humid over the course of the day. Expect 70+ dewpoints to surge as far north as a Neligh to Omaha to Clarinda line. One of the biggest questions will be just how much heating occurs within that axis of deep surface-based moisture. A substantial capping inversion will be in place through the day, but some height falls in the afternoon through evening hours will weaken the cap a bit. However, it appears that some substantial heating into at least the mid 80s or warmer may be needed to allow surface based thunderstorms to develop in the late afternoon and early evening. I am unsure if this will happen, but it will be close, and at least a few models suggest total erosion of the cap by 6 PM, mainly west of an Albion to Columbus to Beatrice line. This will be a very important part of the forecast because if deep convection is able to develop and sustain itself in the narrow mobile warm sector from north central KS into south central NE, and progress east into the aforementioned area before 9 PM or so, the storms would be likely to become intense supercells with very favorable low level shear profiles for tornadoes. Again, IF one of these storms can develop, we should be prepared for the potential for tornadoes to become quite intense. That said, it currently looks like only a 10-20% chance that storms develop in this area. Even with that low probability, it will be very important to watch how the environment unfolds tomorrow afternoon. Higher Probability Severe/Flooding Scenario Late Monday Into Monday Night: Regardless of what happens in the afternoon and early evening, there is a high likelihood for thunderstorm development farther west near the surface low in the evening. This convection will organize and build east into an area with ample wind shear and widespread MUCAPE 2000-3500 J/kg. It seems to be a good chance for these storms to organize into a forward propagating MCS. While mixed layer parcels become increasingly stable over time, it appears that an organized cold pool may be sufficient to advance east across much of the forecast area before weakening. If the cold pool is balanced, there would be some potential for widespread wind damage with a focus in east central and southeast NE, perhaps weakening with eastward extent. Given ample low level shear, a balanced cold pool would also be favorable for mesovortex spin-ups with enhance zones of wind damage or tornadoes. This potential too would also seem to decrease with eastward progression. Heavy rain ingredients will also be in place with PWAT 2-2.5 inches, a deep warm cloud layer, and strong deep moisture convergence. This should lead to very intense rainfall rates in the strongest convection. The strongest storms look likely to be progressive, but some re- development and training is possible overnight as the front lays out across the forecast area and interacts with the LLJ. Right now, it appears that a fairly large area will receive at least an inch of rain with some areas probably exceeding 3" on a more localized basis. Tuesday High Probability of Thunderstorms, Some Severe, Some Heavy Rain: The front stalls from SW to NE bisecting the forecast area on Tuesday. May see some showers and storms along the front over the course of the day, but the warm sector is also expected to destabilize substantially by mid afternoon. Meanwhile, the main trough axis will move east across Nebraska into Iowa by late Tuesday with height falls and enhanced boundary layer convergence leading to widespread thunderstorm development in the frontal zone. Low level shear will not be as impressive as Monday, but the deep shear will still be plenty strong to organize supercell structures especially early on. With the widespread initiation, do expect upscale growth fairly quickly, and while deep wind fields are front-parallel, expect to conjoin cold pools and progress the effective boundary to the east and out of the forecast area over the course of several hours. This should limit the overall flooding threat, but with another environment favorable for heavy rain rates, it may not take much to cause isolated flooding issues. Initial strong storms may feature a hail/wind/tornado potential, probably evolving into more of a wind and isolated hail potential. Independence Day: Much of Wednesday will be nice, but a deepening, strong, quick- moving short wave trough approaches from the northwest by late Wednesday. This drives the front back north with elevated thunderstorms again likely over much of the region Wednesday night into early Thursday. Availability of sunshine will again be key to increasing instability on Thursday, but given an abundance of low level moisture and cool advection aloft, it may not take too much heating to favor strong thunderstorms along the advancing cold front. One of the big questions, given the holiday, will be timing. The current indications may favor the front and storms pushing east of the forecast area by 8 PM or so, but this is still 5 days away and minor changes in system timing could have significant implications on holiday plans. It will be a good day to have a backup plan for any outdoor activities, just in case. Northwesterly flow will take over for Friday into Saturday. Expect cooler and drier conditions, although with a few showers possible especially northeast NE and west central IA on Friday. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1124 PM CDT Sun Jun 30 2024 Storms remain on track to impact all TAF sites early Monday morning, but trends have been slightly later than previously forecast. Expect 2-3 hours of TSRA at each site with perhaps some lingering -SHRA after. Following storms, expect IFR to low MVFR ceilings for at least a few hours. Another round of storms is expected to move through late in the period, with potential for some of these storms to be severe. There is also a small chance (20%) some severe storms develop ahead of this line, from around 22Z-01Z, with LNK seeing the highest threat, but confidence in these developing, much less hitting a TAF site are quite low. Otherwise, generally expect southeasterly to southerly winds outside of storms, with gusts of 20 to 30 kts developing by Monday morning. Can`t completely rule out some low level wind shear Monday evening ahead of the storms, but for now, thinking surface winds will stay strong enough to preclude mention. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Barjenbruch AVIATION...CA