Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
Issued by NWS Quad Cities, IA IL
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
944 FXUS63 KDVN 012155 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 455 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Generally two rounds of convection are expected from later tonight into Wednesday morning accompanied by a threat for heavy rain. There is potential for flash flooding, initially this threat is low but may increase Tuesday night. Severe weather potential also exists, mainly late Tuesday PM through evening. - 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 320 PM CDT Mon Jul 1 2024 A dry easterly fetch off high pressure over the Great Lakes and Ohio Valley is providing another fabulous day with low humidity and below normal temperatures around 70 to the mid 70s. A mid level ridge extends from the Upper Midwest to the Lower Mississippi Valley. A low amplitude wave is shifting east from the Missouri River Valley early this afternoon, but rain has been dissipating as it moves toward E Iowa and encounters the mid level ridging and low level dry air. Showers and even a few storms were noted further back into W Iowa and W Missouri within the moisture gradient and main PWAT reservoir of 1.75 to 2 inches. Initially late this afternoon/evening we`ll get some weak lift to brush especially north/west portions of the service area. The dry air sub-cloud will continue to chew away, but eventually enough saturation may occur to where some sprinkles would be possible with a very low potential (<20%) for measurable. Otherwise, the focus will revert back to our west tonight where additional showers and storms are anticipated to develop over portions of the Northern and Central Plains ahead of an ejecting shortwave from the Rockies. This approaching wave and a veering low level jet with strong moisture advection (PWATs ramping to 1.5 to 2 inches) will bring increasing chances for showers and storms later tonight into Tuesday AM, with the greatest chances (60-80%) north/west of the Quad Cities. Severe threat appears very low with the primary instability axis residing further west. Heavy rain is possible with this activity, but the threat for excessive rainfall appears low with the convection anticipated to be more scattered in nature. Nonetheless, will have to keep an eye out given the burgeoning tropospheric moisture content, and a favorable SW component to the LLJ for possibly repeated convection. Can`t totally rule out some localized excessive rain/isolated flash flood risk. Lows tonight look to be notably warmer with aid of a SE breeze, clouds and gradual increasing moisture. && .LONG TERM /TUESDAY THROUGH MONDAY/... Issued at 320 PM CDT Mon Jul 1 2024 Another round of showers and storms is expected Tuesday late afternoon into Wednesday, as stronger synoptic forcing aided by a shortwave aloft and the right entrance region of a 100-120 kt 250 hPa jet steak overspread the Upper Midwest, while at the surface a cold front moves through. This will be especially true through the nighttime hours, which appear to have the best chance for widespread heavy rainfall. The primary concern for this period remains heavy rain and flash flooding. NAEFS and ENS continue to support PWATs climbing to 1.75 to around 2.00 inches by 00z Wednesday, which is 2 to 3 standard deviations above climatology and approaches the 99.5 percentile or is near max of their respective climatologies. NBM guidance shows high probabilities of at least 1.00 inch from Tuesday evening through Wednesday AM over the west 2/3rds of the service area, and an area of modest chances (40-50%) for at least 2 inches of rain focusing over northwest portions of the service area. Given the convective nature and magnitude of the synoptic ascent doubling the PWAT would give a general idea of the higher limit, which in this case with PWATs around 2.00 inches would yield potentially higher amounts of around 4.00 inches. 12z HREF 24 hr probability matched mean QPF depicts this potential as well with swaths of 3-5 inches by 12z Wednesday. With rivers rising, we will be watching this heavy rain potential closely, as this will lead to fluctuations in the river forecast. In addition, the significant moisture advection (IVT) and a southwesterly LLJ of 40 kt could contribute to backbuilding and an elongated MCS, resulting in a risk of flash flooding. This flash flood threat may be highest especially across east-central and northeast Iowa, into northwest Illinois, where rainfall in the past week has accumulated 125-200+ percent of normal and soils are saturated, and where the signal has been pretty consistent in the deterministic QPF and ensemble probabilistic QPF guidance. The potential though extends also into southeast Iowa, northeast Missouri and west central Illinois. Once the convection organizes with low level flow parallel and nearly equal to the deep layer flow the Corfidi Vectors are slow, so some training/backbuilding potential seemingly exists. However, the uncertainty lies with the potential for cold pool development which if occurs could push the convection along and limit the flash flood potential. As convective trends become better defined over the next 12-18 hrs we may need to consider Flash Flood headlines. Severe weather potential will exist by Tuesday PM/evening, as deep layer shear 0-6km increases to 35-45+ kts aided by the approaching shortwave trough. Models try to build in the instability during this time, especially across SE Iowa and NE Missouri with MLCAPE increasing to 1000-2000+ j/kg. Initial supercell potential will exist before upscale growth into a line of storms, transitioning to a wind threat. SPC continues with an Enhanced Risk (level 3/5) for parts of SE Iowa and NE Missouri where this better instability is progged. A Slight Risk (level 2/5) extends across much of the rest of E Iowa and a portion of WC Illinois. Main threat again looks to be damaging wind, and a secondary lower threat for a tornado with favorably low LCLs. Warm cloud depths over 12-14kft AGL should limit the hail threat. 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 TUESDAY/... Issued at 1157 AM CDT Mon Jul 1 2024 VFR conditions expected to persist through this evening. There is a slight chance (10-20%) for a brief shower late this afternoon and early evening at KCID and KDBQ, however there is a copious wedge of dry air sub-cloud and so it may be more sprinkles than anything with no impacts. By later tonight and Tuesday morning (after 06z through 15z), the shower and storm chances will be increasing with a veering SW low level jet and abundant moisture advection with the greatest chances at KCID and KDBQ requiring prevailing SHRA/TSRA mention, with lower chances PROB30 at KMLI while potential appears too low for mention at KBRL right now. MVFR to local IFR conditions (mainly for visibility) will be found in the storms and heavier showers. Winds will be predominantly from the E/SE around 10-15 kt and then occasionally gust 20-25 kt. LLWS is possible at KCID and KDBQ late tonight/Tuesday morning, but too much uncertainty as winds may not decouple completely tonight and marginal criteria precludes mention with the 18z TAFs. && .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. 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...None. IL...None. MO...None. && $$ SHORT TERM...McClure LONG TERM...Gunkel AVIATION...McClure HYDROLOGY...08