Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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445 FXUS63 KDVN 220240 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 940 PM CDT Fri Jun 21 2024 .KEY MESSAGES... - Probability of storms producing heavy rain and flash flooding late tonight into Saturday morning has increased especially if storms repeat over the same areas. - Shower and storm chances over the next 24-30+ hours, peaking for many later Saturday afternoon/evening. - Potential for strong to severe storms increases Saturday PM/ evening. Damaging wind is the primary hazard, with large hail a secondary threat. Heavy rain and localized flash flooding also remains a threat. - Hot and humid once again on Saturday with PM heat index readings in the 90s to around 100 from around Hwy 30 on south. - Dry conditions return for Sunday and generally remain through much of next week, outside of another round of storms Tuesday and Tuesday night. && .UPDATE... Issued at 844 PM CDT Fri Jun 21 2024 A flood watch has been issued for the Highway 20 corridor and from Benton County east to Jackson County through 1 PM Saturday. Confidence is at 50 percent that heavy rainfall will be seen in the northern parts of the Highway 20 corridor counties. Real data shows a weak outflow boundary from earlier storms is now in the Highway 20 corridor west of Dubuque. Surface wind speeds indicate this boundary will likely remain in place through the night. 00z DVN sounding has a PWAT of 1.76 which is roughly the 95th percentile and about 0.10 from the max. Additionally, there is a very healthy tropical connection of deep moisture from Texas that runs around the west side of the high in the lower Mississippi Valley. Other short term model guidance is suggesting the development of a boundary in the Highway 20 corridor during the night and into Saturday morning. MCS tool has been slowly trending south the past 4-5 hours and now suggests storms with the potential for very heavy rain in at least the Highway 20 corridor. More disturbing are the Corfidi vectors. They have slowly become perpendicular to each other with a period of opposition prior to sunrise. If correct, storms would have a tendency to repeat over the same areas with potentially a very slow (10-15 mph) eastward speed. Some other heavy rain tools suggest the bulk of the heavier rainfall would remain well north of Highway 20. However, the conceptual picture is beginning to resemble a Maddox synoptic flash flood event with an east/west frontal boundary where the heaviest rainfall can fall along and on the warm side of the front. Given the very high PWATs, rainfall rates may approach 2 inches per hour. Flash flood guidance for the area is generally around 2 inches or just under it for both the 1 and 3 hour time frames. Rainfall totals by Saturday morning are expected to vary considerably. Some locations in the watch area may see not much rainfall. Other areas potentially could see 3-4 inches or more. && .SHORT TERM /THROUGH SATURDAY NIGHT/... Issued at 340 PM CDT Fri Jun 21 2024 Semi-zonal to WSW flow aloft dominates the region early this afternoon, with a trough over the Intermountain West and a a large ridge /heat dome/ over the south-central/southeast U.S. A frontal zone extends from a Lee low across Nebraska and northern Iowa into southern Wisconsin. This boundary continues to be the focus for showers and storms. A weak wave in the flow and diurnal heating was aiding widely scattered convection from far northern Missouri into central/southern Iowa this afternoon. As this translates east-northeast we`ll continue to see the potential for widely scattered activity develop into our service area through this evening. Weak effective shear and mid level lapse rates should preclude any severe weather potential with this activity. Then turning toward later tonight, the focus will be watching convective trends to our north in a synoptic setup/pattern that resembles Maddox conceptually with heavy rain/flash flood threat with a stalled front to our north, and an anomalously deep moisture axis with PWATs near 2 inches pooling near the boundary, and additional lift from a shortwave emerging from the Intermountain west and a ramping nocturnal LLJ 30-35+ kts. In the Maddox synoptic pattern conceptual model the heaviest rains usually occur on the warm side of the front. Despite the deterministic and CAM guidance largely keeping the heaviest rains just to our north, given this conceptually brings some concern that this heavier rain axis could develop/ shift further south and possibly into northern portions of the service area late tonight and Saturday morning with aid of outflow propagation. Confidence is not high enough to issue a Flood Watch at this time for our northern counties, but the evening shift will monitor convective trends and re-evaluate this potential. Saturday`s forecast could potentially be rather challenging with respect to high temperatures and subsequent severe weather threat/location pending convective and cloud cover trends through the first half of the day. One scenario possible is if there is more coverage of storms and cloud cover throughout Saturday AM across our north, then the severe weather threat would likely shift into our southern service area and further south. Highs would be held down in the upper 70s and 80s north, with 90s south of I-80. The other scenario would be one in which there`s limited coverage of showers and storms with the majority of the activity staying to our north through early Saturday afternoon. This would allow for more heating and instability to occur ahead of a cold front, which in turn would lead to a more widespread severe weather threat for the mid afternoon through evening hours ahead of the cold front. In terms of severe weather, modest deep layer shear of around 30 kts combined with at least moderate instability with MUCAPE over 1500-2000+ j/kg would be sufficient for some organized severe weather potential. Warm cloud depths and anomalously high moisture content with PWATs over 1.8 to near 2 inches owes to lower confidence in large hail, and higher confidence in strong winds being the primary risk via precip loading and cool pools. LCLs will be very low with the increase in moisture, and so can`t rule out a tornado particularly with any storm-boundary interaction. The anomalously high moisture will also bring the threat for heavy rain, and some concern for localized flash flooding given the veering deep layer and backward propagating shear vectors. However, storm motion may be just enough to limit this threat and perhaps focus more into urban areas. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Issued at 340 PM CDT Fri Jun 21 2024 After the front moves through late Saturday night, a much more pleasant day is expected Sunday, with lots of sunshine and more seasonal temperatures and humidity. Sunday is definitely the pick day of the weekend! In fact, the large-scale pattern appears to support a period of mostly dry conditions through Thursday, thanks to upper-level ridging developing over the Intermountain West states and surface high pressure settling into the upper Midwest. The only other opportunity for showers and storms appears to be for Tuesday and Tuesday night, as yet another compact upper-level trough and attendant surface cold front crosses the area. The timing of the front per the latest global models does appear to be advantageous for more strong to severe storms, especially with the CSU ML severe probs suggesting strong to severe convection with a hot and humid air mass again in place, so we will need to watch Tuesday as well. After this front moves through the midweek period looks to quiet down. However, by the end of next week the models show high pressure moving off and return flow / warm, moist advection ramping up across the Plains and Midwest for another chance of showers and storms for the area. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 634 PM CDT Fri Jun 21 2024 Satellite shows a pronounced east/west boundary across northern Iowa where the current SHRA/TSRA are located. A TSRA complex will develop roughly along the MN/IA border tonight and move into southern Wisconsin. Thus VFR conditions are expected through 06z/22 with isolated SHRA/TSRA. After 06z/22 potential for TSRA to impact parts of northeast IA and far northern Illinois with MVFR/IFR conditions. Areas generally south of I-80 should remain VFR with isolated at best SHRA/TSRA. && .HYDROLOGY...
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Issued at 926 PM CDT Fri Jun 21 2024 Widespread 2 to 4 inches of rain is forecast near the headwaters of the Wapsipinicon, Cedar, and Iowa river basins in the next 24 hours. Some localized higher amounts over 5 inches is not out of the question as well. Much of this rain is expected in a short amount of time overnight tonight allowing for substantial runoff. As a result, new forecasts this evening have several sites along the Cedar going into flood in the next 3 to 5 days. Confidence in the placement of heaviest QPF remains very low and since it is still 3+ days out have decided to hold issuing any flood watches for all sites except for Vinton on the Cedar River. On the Mainstem Mississippi, heavy rainfall amounts between 4 and 9 inches in the past 2 weeks in southern MN and western WI, will result in rises through the rest of June and into July. Went ahead and issued flood watches for portions of the river this evening with more likely in the coming days.
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&& .DVN WATCHES/WARNINGS/ADVISORIES...
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IA...Flood Watch through Saturday afternoon for IAZ040>042-051>054. IL...Flood Watch through Saturday afternoon for ILZ001-002. MO...None.
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&& $$ UPDATE...08 SHORT TERM...McClure LONG TERM...McClure AVIATION...08 HYDROLOGY...Gross