Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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581 FXUS63 KOAX 011123 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 623 AM CDT Mon Jul 1 2024 .KEY MESSAGES... - Severe thunderstorms and flash flooding will be possible late this evening and overnight tonight. The primary hazard looks to be damaging winds at this time, though all modes of severe weather will be possible. - Another round of strong to severe thunderstorms will be possible tomorrow afternoon, primarily across southeast Nebraska and southwest Iowa. All severe hazards will again be possible with the primary risk being damaging straight-line winds. - Unsettled weather continues through the week and into the weekend, with almost daily chances for thunderstorms. Another system may severe thunderstorms on the 4th of July. && .DISCUSSION... Issued at 245 AM CDT Mon Jul 1 2024 Today and Tonight... Scattered thunderstorms are already developing across portions of northeast Kansas and south-central Nebraska. This activity is expected to expand in coverage and shift northeastward over the course of the morning. Though expected to be non-severe, PWAT values between 1.0 and 1.5 inches in addition to strong moisture transport from the south will bring the potential for very heavy rain in the stronger storms. Widespread amounts of up to an inch will be possible before storms dissipate and move east of our area. The bulk of this activity will have dissipated by noon, though a lingering shower or two will be possible through 2 PM, particularly across portions of western Iowa. This convection should remain elevated and non-severe, though small hail will also be possible in the strongest storms. At the surface, a warm front is expected to stretch from central Nebraska, southeastward into east-central Kansas. Along this boundary, a narrow corridor of ample moisture return will result in a tongue of moderate instability (MLCAPE 2000-3000 J/kg) into south-central and central Nebraska by this afternoon. The prevailing upper-level pattern aloft will sustain troughing over the northern Rockies, with an embedded shortwave rounding the base expected to eject out into the northern Plains by this evening. This system will provide the forcing for ascent for thunderstorms to develop west of our area this afternoon/evening in the vicinity of southwest Nebraska/northwest Kansas. As these storms move northeast, upscale growth into an MCS is expected. Though there is still uncertainty with how far north or south this MCS will track, there is confidence in a swath of strong to severe winds stretching from central Nebraska into east-central Nebraska. The strongest segment of this MCS will be anchored to the northward extent of the instability axis, most likely to be near the US-30 corridor, though severe storms could be as far south as I-80 or as far north as US-20. Further north than this in our area, instability will likely be insufficient for severe convection. Though there is a non-zero chance for strong to severe storms to build south of I-80, the potential is very low (~10 percent) as a very strong capping inversion will be in place during the evening and overnight hours. Wherever this MCS tracks, damaging straight line winds will be the primary hazard. Some wind gusts greater than 75 mph will be possible, particularly between US-30 and I-80, and west of US-75. Just after dark, a strong low-level jet is expected to develop and enlarge boundary layer hodographs. Some CAMS suggest 0-1km SRH may exceed 500 m2/s2 by 10 pm. Where these storms are able to remain surface based, there will be the potential for spin-up tornadoes on the leading edge. Isolated instances of severe hail may also be possible as well though this will likely be a secondary hazard with mid-level lapse rates largely remaining below 7 C/km. One uncertainty that has yet to be ironed out is the eastward extent in which these storms will be severe. Storm propagation will almost certainly exceed the speed at which the surface front lifts to the north and east. As such, at some point late tomorrow evening or night storms should ride over the front and become elevated. This will likely happen somewhere near or just west of the US-75 corridor. When this happens, the tornado threat will likely be reduced to near zero, with the damaging wind potential reducing but remaining for a few hours after. All in all, the bulk of tomorrows severe weather should happen before 1 AM. Behind this line of storms, training thunderstorms may develop on the south side of the cold pool that is laid out, likely near the I-80 corridor. This will be a favored area for additional heavy rain through the night tomorrow night, with flooding becoming the primary concern after midnight. Storms may linger until 7 AM Tuesday morning. As such, some CAMs suggest a swath of 1-2 inches of rain somewhere in east-central Nebraska. Depending on how long storms are able to persist over any given area, some local amounts may be as high as 4 to 5 inches. A very low probability (5 to 10 percent) but potentially high impact scenario would be if a discrete supercell is able to develop ahead and/or southeast of this MCS tomorrow afternoon/evening. Low-level shear profiles and buoyancy southwest of the warm front will be more than sufficient to support intense supercells should one develop. Strong capping is expected to remain across this portion of the area through the afternoon and evening likely impeding storm development ahead of the MCS. Nonetheless, due to the strong forcing for ascent associated with the shortwave ejecting out into the Plains, it is within the realm of possibility that a storm or two may be able to break through the cap. The most likely area for these to develop will be west of US-77 and south of US-30. Tomorrow and Wednesday... By tomorrow morning, an advancing cold front will enter the region from the northwest. This front is expected to push southeast over the course of the day. South of this front, airmass destabilization will be universal and happen much earlier in the day than today. As such, scattered showers and thunderstorms will develop along this front as early as noon. By this time, the front will likely extend along a line from Hebron to Onawa. The earliness of convective development will likely temper the magnitude of destabilization, at least across southeast Nebraska and southwest Iowa. Most CAMs suggest MLCAPE will likely be below 1500 J/kg. Nonetheless, through the afternoon hours, widespread development along the front should consolidate into a line, or a series of bowing segments. Again, damaging winds will be the primary hazard, increasing in probability with south and eastward extent. A brief tornado or two and isolated instances of severe hail will also be possible. Individual storm elements will track to the east-southeast, with the front pushing more squarely to the southeast. The severe weather potential will likely be south and east of our area by 7 PM. A more widespread concern tomorrow will be heavy rain yet again. An extremely moist airmass will be positioned over our area, with PWATs likely exceeding 2 inches. The prolonged period of potential showers and thunderstorms will yield again large swaths of precip up to an inch, with isolated instances up to 3 inches. The greatest concern for flooding will be along and north of I-80 where soils will already be saturated from the several rounds of thunderstorms expected today. Isolated instances of flash flooding cannot be ruled out south of I-80 tomorrow evening however. Wednesday is expected to be dry during the day, with a high pressure expected to develop and move across the central Plains from west to east. Wednesday night, convection from the High Plains may reach eastern Nebraska after midnight in the form of a decaying MCS. Severe weather is not expected at this time, though gusty winds and brief heavy downpours will be possible. Independence Day... Medium range guidance is in agreement of an amplifying shortwave trough deepening and ejecting out into the central and northern Plains on Thursday. This system looks to be a stronger piece of energy than the one responsible for today and tomorrow`s storms, with stronger height falls and PVA overspreading our region Thursday afternoon. At the surface, a warm front will likely extend across central Iowa into northeast Nebraska. Further west, a cold front will approach the region by the afternoon, with the most likely position of the front being between US-77 and US-81 by the early afternoon hours. Moisture in the warm sector will be abundant, with guidance universally depicting boundary layer dewpoints above 70 degrees. This will result in moderate to extreme instability, with SBCAPE likely around 3000 J/kg. Some guidance suggests SBCAPE may exceed 5000 J/kg across southeast Nebraska and southwest Iowa. The capping inversion will likely inhibit convection through mid afternoon with CIN values exceeding 50 J/kg. By the early evening, strong forcing for ascent and height falls will help erode the cap resulting in convection developing across portions of eastern Nebraska and southwest Iowa by early evening. Though the details on exact timing, extent of coverage and exact location of frontal boundaries is unclear at this time, current guidance favors the development of strong to severe thunderstorms capable of all modes of severe weather. This system has the potential to be a more significant severe weather outbreak, especially with it coinciding with July 4th, so keep a very careful eye on the forecast for this day as the details get ironed out over the next few forecast cycles. Friday through Sunday... Behind the Fourth of July system, northwesterly flow seems likely aloft over the Plains with a high pressure system establishing itself at the surface by the end of the week. The quiet weather may only last a couple of days, as medium range guidance has fairly persistently shown a deepening trough diving southeast across the northern Plains sometime over the weekend or early next week. Differences in timing are too great at this time to pin down any given day as most likely to see thunderstorms. Regardless, the general trend is to have another storm system move through our region by the end of this forecast period. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 623 AM CDT Mon Jul 1 2024 An area of showers and thunderstorms will overspread the terminal locations within the next hour with precipitation lasting generally two to three hours before ending. Latest model data continue to indicate lowering ceilings to MVFR and potentially IFR as the precipitation moves through, and that notion is supported by current observations in central NE. Another wave of more intense thunderstorms is forecast to move through the area tonight with locally strong wind gusts possible. Otherwise, southeast winds of 14-19 kt with gusts of 23-30 kt are expected to persist through much of the forecast period. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...Darrah AVIATION...Mead