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Area Forecast Discussion
Issued by NWS Omaha/Valley, NE
Issued by NWS Omaha/Valley, NE
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860 FXUS63 KOAX 140436 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1136 PM CDT Tue Aug 13 2024 .KEY MESSAGES... - There is a 60-80% chance of thunderstorms tonight into early Wednesday. Greatest threat will be localized flooding, but hail and damaging winds will also be possible (5-15%). - Severe storms are possible Wednesday afternoon and evening (10-20% chance). Large hail, damaging winds, and localized heavy rain and flooding will be the primary hazards. - Continued small shower and storm chances will persist into the Thursday (15-20% chance), but the overall trend will be toward drier, seasonably warm weather over the weekend. && .DISCUSSION... Issued at 322 PM CDT Tue Aug 13 2024 We have another cloud day today, with temperatures only peaking in the mid-to-upper 70s this afternoon. Clouds will slowly try to clear, with the best chance for clearing being over northeast Nebraska. There is an axis of moisture along a convergence boundary stretching from Seward southeast to Falls City which will make clouds difficult to clear with low (15-20%) chances for light rain through this afternoon along this boundary. We will see clouds try and clear along this boundary later this afternoon, but will want to continue to monitor its location as it likely will become a focus for overnight storms and potential heavy rainfall (training storms). CAMs have been trending farther north for the potential axis of heavy rainfall, bringing it into northeast Nebraska. Our "best guess" for placement is probably somewhere between I-80 north through around Yankton, SD. Within this axis we`ll probably see swaths of 1 to 2 inches of rainfall with locally higher amounts possible if storms consistently train over one area. The dynamics of this system show a shortwave trough moving through, triggering storm development in a region with strong moisture advection from the low-level jet. This region will see vertical motions enhanced by an area of strong divergence aloft associated with the curvature of the Jet Stream. This is a nearly ideal scenario for heavy rainfall production, with the only limiting factor being the orientation of the LLJ isn`t completely perpendicular to the surface boundary. This could help the system be more progressive in nature with less storm training, but locally heavy rainfall will still be possible. Timing for this system is looking to be later, with storms moving into our area generally after midnight continuing through around 10 AM Wednesday. Depending on how long it takes for the rain to clear, we will likely see cloud cover hang around again through much of the day again on Wednesday. We might see clouds clear across far southeast Nebraska southwest Iowa, where temperatures could reach near 90 degrees tomorrow afternoon. Areas that remain under cloud cover most of the day likely won`t see high temperatures get out of the upper 70s to low 80s. Wednesday Severe Weather: Daytime conditions on Wednesday will play a big part in how well our environment can charge ahead of another shortwave that will move through Wednesday afternoon and evening. The best severe weather potential will likely be limited to southeast Nebraska/southwest Iowa where we see some clearing and temperatures get into the upper 80s to near 90 during the afternoon. The environment will be too stable to the north where cloud cover is slow to clear. We do see a dry line move across southeast Nebraska which will provide the lifting mechanism for storms to develop in a region with ample moisture ahead of the boundary and dew points in the upper 60s to low 70s. With ample instability and moisture in place, the next thing to look at is shear. Model soundings in southeast Nebraska show weak 0-1km shear, but strong 0-3km SRH indicating the potential for strong, rotating updrafts. Because the 0-1km shear is low and LCL heights are up around 1000 meters or higher, tornadoes won`t be a big risk for tomorrow afternoon/evening, but do remain possible. The bigger threats will be damaging winds and large hail. Hail size will be dependent on the amount of instability we can achieve, with the potential for up to 2" hail possible. Severe weather should mostly clear the area by midnight. Thursday - Friday: We`ll see a trend toward warmer temperatures going into the weekend as we finally see the cloud cover clearing by Thursday afternoon, and mostly clear skies on Friday. Temperatures should get up into the mid-to-upper 80s each afternoon. We could see a couple showers/storms develop Thursday afternoon as well as ridging starts to build into eastern Nebraska. This boundary could kick off a few storms, but chances are around 15-20%. Our southern counties could get clipped by an MCS moving across eastern Kansas into western Missouri Friday night, but severe potential is low. Saturday - Monday: We`ll remain seasonably warm through the weekend with highs in the mid-to-upper 80s. There is some uncertainty in the dry forecast for this weekend as the latest GFS not included in the latest NBM forecast from this morning does show the potential for an MCS developing and moving through Saturday night into early Sunday, but it appears to be an outlier at this time with other models showing high pressure centered over the region. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1129 PM CDT Tue Aug 13 2024 A line of thunderstorms currently ongoing across southeast Nebraska will move into northeast Nebraska (KOFK), east-central Nebraska (KOMA) and southwest Iowa tonight after 07Z. These storms will have the potential for heavy rain, wind gusts up to 60 knots, and hail up to the size of quarters. As of now, the highest chances for thunderstorms appears to be just north of KLNK, so have refrained from a TEMPO TSRA group for now. There is still a 30 to 40 percent chance of thunderstorms in Lincoln, so will monitor to see if TAFs need to be amended to include this. Otherwise, a deck of low stratus has developed across eastern Nebraska and southwest Iowa (all TAF sites). Ceilings are lower than what current guidance has suggested, so have manually brought ceilings down in the TAFs. A scattered deck of IFR ceilings is expected to build in tonight. Have kept these out of TAFs for now, but if observations shows more robust IFR cloud cover moving into terminal areas, will amend the TAFs accordingly. Lastly, there will be another chance for scattered strong to severe thunderstorms (KLNK and KOMA) tomorrow evening. Have denoted the more likely time frame to see precipitation with a SHRA FM group. After the round of thunderstorms tonight, will likely have a TEMPO TSRA group included in the KLNK and KOMA TAFs to start narrowing down the timing for this second round of storms. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ DISCUSSION...McCoy AVIATION...Darrah