Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
Issued by NWS Quad Cities, IA IL
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287 FXUS63 KDVN 021754 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 1254 PM CDT Tue Jul 2 2024 ...18z Aviation Update... .KEY MESSAGES... - Flash flood watch is in effect for the area this afternoon into evening. Very heavy rainfall is possible from these storms. - Severe thunderstorms are possible today and tonight, with all hazards possible. - Unfortunately, Independence Day is looking to be a wet day, as a warm front brings the potential for widespread showers and storms once again. Too soon to determine the severe risk. - Moderate to Major River Flooding continues, with further changes to river forecasts expected as we transition from predicted rainfall to observed rainfall over the next 36 hrs. && .SHORT TERM /THROUGH TONIGHT/... Issued at 402 AM CDT Tue Jul 2 2024 We are sitting on the precipice of what could be a very impactful flash flooding/severe event for the area. An inverted trough located west of the area paired with a strong 50-60kt LLJ was leading to showers and storms across the area. Moisture transport with this jet was bringing large quantities of moisture to the area. The wave that induced the LLJ will move east of the area this morning as showers and storms either fester or die out. Another stronger wave is expected to move into the area this afternoon and evening. This inverted trough will become a cold front as this wave approaches and will slowly move SE across Iowa. Moisture will pool ahead of this front leading to an even more saturated effective inflow layer. As the LLJ increase with the wave, even more moisture will surge into any ongoing storms. As far as moisture amounts go, the deterministic models are all predicting over 2.5 inches this evening. This would be above record PWAT values for our area! The HREF, suggests 2 to 2.4 inch PWATs, especially across our southern CWA. We have a lot of moisture around and all we need are thunderstorms to turn this into heavy rainfall. Models have this moisture and instability stopping near the river. I`m not convinced this will happen and later shifts will need to determine if the flash flood watch will need to be extended to the east. There remains questions on how today`s event will evolve. 1st, will we see mainly re-development or festering of ongoing convection today. If it`s redevelopment, the OFB from today`s storms could wash out or move and thus the area for redevelopment would be more difficult to ascertain. If it`s festering, and this seems more likely due to no cap and weak LLJ during the day, there should be a well-defined E/W thermal boundary for storms to fire on. To make identifying that location even more difficult are the differences in guidance. The 06z HRRR says HWY 30 corridor is where this should occur. The 06z NAMnest suggest redevelopment out west and turning quickly into a line. The spectral models, except for the ECMWF which agrees mostly with the NAMnest, has the heavier rain across the HWY 20 corridor. I`m not confident where this will occur at this time. However, we should have a much clearer picture this AM when we can whats happening with the convection. We should be able to nail this down better and should message that. Now on to impacts. Severe weather is possible and flash flooding is looking better and better each time I look at it. Whatever happens as far as evolution, supercells should start out first. If they form out west, then its a line when it gets here. If they form on the E/W thermal boundary two things are possible. 1. They will stay supercells longer and 2. if they latch on to that boundary we may see an even greater tornado and flash flood risk where that occurs. Now, I want to talk about the 06z HRRR. That depicts a worst case scenario for both tornadoes and flash flooding. The 06z HRRR has supercells that train and become embedded in a line. This would favor a better tornado threat, in fact the updraft helicity tracks support this. Also, if we have rotating updrafts in any part of these storms, we can expect even higher rain rates. The dynamic pressure forces caused by the mesos induce even greater low pressure and upward motion. This, if the 06z HRRR occurs, would be a high impact event with tornadoes and flash flooding and would require a step up in messaging. It`s one model run, but we need to understand the environment is primed for that potential. Luckily the 07z HRRR is more in line with the NAMnest, however, as I said before I`m not confident on storm evolution at this juncture and until we nail that down, we must respect this environment. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Issued at 402 AM CDT Tue Jul 2 2024 Previous discussion still looks good and is attached below. A brief break in the wet weather is expected Wednesday, but the emphasis is on brief, as a surface warm front passes north into the area as early as late Wednesday night/early Thursday AM with convection potential increasing (40-70%) south of I-80. Then, the shower and storm potential increases over the rest of the area on the 4th of July into Thursday evening, as a vigorous shortwave trough evolves into a closed low while traversing near the international border to the north. Ensembles point to PWAT values of 2.00+ inches again pooling near and south of the boundary supporting a heavy rain threat. Where this warm front sets up will be critical to where this heavy rain threat and potential severe weather threat eventually sets up, and will bear watching in the coming days. Next weekend the pattern looks to transition to a mean trough over the north central U.S. Thus, the active pattern looks to persist with additional shower and storm chances with lower predictability on timing given the lower amplitude shortwaves. Many periods have PoPs Saturday-Monday, but there will likely be plenty of dry hours. Temperatures look to be near normal. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1245 PM CDT Tue Jul 2 2024 Potentially a stormy and turbulent TAF period later this afternoon into the evening, with a frontal system pushing into a warm and humid airmass. VFR CIGs with patchy MVFR areas will give way to incoming sctrd strong to severe thunderstorms moving in from the west this afternoon and evening, and tried to time the more optimum windows with TEMPO groups. Passing bouts of MVFR to IFR conditions with VSBYs reduced by torrential rainfall and lowering CIGs. Variable high wind gusts/storm outflow of 40 KTS or higher with the stronger storms as well, may be possibly up to 60 KTs just ahead of a severe storm line. After the storms clear off to the east and southeast, some lingering stratiform rain with embedded thunder for a few hours, and possible sub-1000 FT AGL CIGs post-frontal late tonight. Gusty south to southwesterly sfc winds this afternoon ahead of the storms to become light southwest later this evening, with an eventual veer to the west and northwest behind the front late tonight into Wed morning. && .HYDROLOGY... Issued at 425 PM CDT Mon Jul 1 2024 The river forecasts this morning included 48 hours of QPF in them instead of the normal 24. This was done to minimize any significant jumps in the crest forecasts later this week. Tonight`s and the Tuesday morning river forecasts will have 36 and 24 hours of QPF in them respectively. The flood crest on the Cedar River is near Conesville, IA and will enter the Iowa basin Tuesday morning. Discussion... We are looking at a potential heavy rain event across the area Tuesday into Wednesday with a bulk of the rainfall occurring Tuesday night. There is some uncertainty where the heaviest rainfall will occur. The moisture is being supplied from the remnants of tropical storm Chris.2 However, at the same time, there will be a dry layer of Saharan Dust closely following the tropical moisture. Given the data available it appears that a general 1 to 1.5 inch rainfall will occur with pockets of 2-4 inches across eastern Iowa. If the rainfall is lighter/heavier than forecast, this will result in the eventual crest forecasts being lower/higher later this week. As a result of the 48 hour QPF used in the morning river forecasts, the Wapsipinicon River near Dewitt is now forecast to reach major flood stage late this weekend. Much of the Mississippi River will reach major flood stage late this week into next week. Additional rises are expected on the Iowa, Cedar, English, Wapsipinicon and Maquoketa basins. Manchester on the Maquoketa River is under a flood watch based on the 48 hour QPF. Additional flood watches are possible over the next 24 hours for parts of the Cedar, English and Wapsipinicon Rivers if the overall forecasted QPF remains fairly consistent. Outlook... As expected the MJO has turned more active over the Indian Ocean and is currently in a phase 3 and is forecast to go into a phase 4 as it moves into the Maritime Continent. Phase 3 has a high correlation with above normal precipitation across the upper Midwest and mid-Mississippi Valley. While the correlation is not as high, phase 4 does support a signal of above normal precipitation across the mid- Mississippi Valley. The current Climate Prediction Center outlook for the second week of July has a 33 percent chance favoring above normal precipitation. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...Flood Watch through late tonight for IAZ040>042-051>054- 063>068-076>078-087>089-098-099. IL...Flood Watch through late tonight for ILZ001-002-007-009-015- 016-024>026-034-035. MO...Flood Watch through late tonight for MOZ009-010. && $$ SHORT TERM...Gibbs LONG TERM...McClure AVIATION...12 HYDROLOGY...08