Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
Issued by NWS Quad Cities, IA IL
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023 FXUS63 KDVN 241107 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 607 AM CDT Mon Jun 24 2024 .KEY MESSAGES... - Dry conditions today overall, save for a low chance of showers and storms early this morning and again late tonight, some of which could be strong to severe tonight - Heat and humidity increases today, with heat indices well into the 90s to nearly 100 this afternoon for our southwestern half of the area - Very conditional forecast is on tap for Tuesday, with another round of severe weather, hot and humid conditions, and flash flooding all dependent on how things play out tonight - Quiet conditions expected Wednesday and Thursday before another round of storms moves in for Friday and Saturday && .SHORT TERM /THROUGH TUESDAY/... Issued at 409 AM CDT Mon Jun 24 2024 An area of high pressure currently over our region will gradually shift eastward with time this morning. Southerly return flow will develop in its wake today, which will lead to increasing heat and humidity across the area later today. Early this morning, we are watching a cluster of thunderstorms that has developed over southwestern Iowa. This appears to have developed in a corridor of 850-700 mb layer warm air advection, ahead of a nocturnal low-level jet. CAMs largely keep this activity out of our forecast area, save for our far southwestern areas, so don`t be surprised if a storm or two moves through that area later this morning. Otherwise, the heat and humidity will be the main story during the daylight hours today. 850 mb temperatures look to increase pretty steadily to around 20 to 22 degrees C per the GEFS ensembles. Additionally, dew point temperatures should increase back to the upper 60s to lower 70s, resulting in heat index values into the 90s to nearly 100 degrees in our far southwest. I wouldn`t be surprised if some isolated locations see heat indices reach 100 degrees, but the more robust warm air advection doesn`t move through until Tuesday, and confidence is higher that it will only be a brief bout of ~100 degree heat indices, which precludes a Heat Advisory for today. However, we did decide to use a Special Weather Statement to draw attention to the sub-Heat Advisory level heat and humidity. Likely our entire southwestern forecast area sees 95+ degree heat indices. Attention then turns to the potential for strong to severe storms late tonight into early Tuesday morning. This is one of the more uncertain forecasts I`ve had to work in a while, as a few scenarios could play out. Looking at the big picture, we are watching a strengthening upper-level jet that will be situated along the US/Canada border, with a mid-level impulse translating over Minnesota by sunset this evening. A complex of thunderstorms, likely as an MCS-type system, appears to dive southeastward through the overnight hours along the instability gradient. The latest suite of CAMs don`t provide much clarity on how this MCS will evolve, with some keeping it just northeast of our forecast area or clipping us. Complicating the convective picture is a mid-level shortwave that will eject from the Intermountain West region, and translate towards western Iowa before sunrise Tuesday. Some guidance has another complex of storms developing in this area, which may move through our southwestern areas as well. All of this to say that there remains high uncertainty on how things will play out. A fairly strong southwesterly low-level jet is likely to be in place during the early morning hours Tuesday, with values between 35 to 45 knots per the HREF ensemble mean, so there will be support for convection in our region. We will have to keep a close eye on this activity as the latest guidance comes in. Given the uncertainty, SPC has introduced a fairly large Slight Risk (level 2 of 5) for areas along and north of Highway 34, with Marginal Risk (level 1 of 5) elsewhere. With the LLJ in place, and MUCAPE on the order of 1500 to 2500 J/kg per the HREF, damaging winds will be the main threat, along with large hail. && .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... Issued at 409 AM CDT Mon Jun 24 2024 Tuesday will be the day to watch for several hazards, as a surface cold front slowly sinks southward behind the departing MCS system. The nocturnal LLJ will gradually veer with time and dissipate, with the flow becoming more west to east. Forcing for ascent will maintain along the baroclinic zone/convergence area, as well as additional shortwave energy moving through. With the enhanced convergence, chances of thunderstorms will continue throughout the day Tuesday. Additionally, any outflow boundaries that develop from previous convection will play a role in convective initiation location, which these details won`t be ironed out until after we see how the storms behave late tonight. Heat and humidity will stay with us, with highs likely into the lower to middle 90s, and heat indices in the upper 90s to lower 100s for several locations. This should result in oodles of CAPE, with both the GEFS and ECMWF ensemble probabilities of 3000 J/kg or greater around 50 to 80% for Tuesday afternoon ahead of the front. Additionally, effective shear is expected to increase as the mid-level jet sinks southward, resulting in values around 30 to 40 knots. This convective parameter space should support the chance of storms becoming strong to severe, with damaging winds and large hail being the main threats. PWATs will also once again be quite high, with values progged around 1.8 to 2.1 inches. If PWAT values exceed 2 inches for our evening balloon launch Tuesday evening, that would be a top 15 highest PWAT value for the month of June! One major concern I have is the threat for flash flooding, given these very high PWAT values, along with the Corfidi forward propagation vectors oriented from west to east, parallel to the boundary, which would support training convection. With all of this said, how things play out will be highly dependent on how the late-tonight storms evolve with time, since a more active early Tuesday morning could alter the environment to affect how things play on later on Tuesday. This would not only have impact on severe potential, but rainfall/flash flooding and heat index values too, so again a highly conditional forecast here, with lots of uncertainty. In any case, SPC has our entire region highlighted in a Slight Risk (level 2 of 5) for severe weather, while WPC has much of our region highlighted in a Slight Risk for heavy rainfall. This activity will lingering into the overnight hours, so things won`t end after sunset Tuesday! As we go into the day Wednesday, any lingering showers or storms should diminish from increased subsidence from an area of high pressure moving into the area. This high pressure system will remain in control through the day Thursday, resulting in more seasonal temperatures for both Wednesday and Thursday, with highs in the lower to middle 80s. Friday and Saturday could be active, with more chances of showers and storms. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 607 AM CDT Mon Jun 24 2024 VFR conditions are expected throughout the TAF period. The only exception might be late tonight for DBQ, when a low chance for a thunderstorm is possible, which could result in brief MVFR conditions. Otherwise, BKN to SCT mid-level clouds will stick around over the next several hours. Southerly winds generally around 10 to 15 knots will increase late this morning into the afternoon, with gusts between 20 to 25 knots expected at CID. A period of low-level wind shear will develop around midnight tonight as a 40 to 50 knot low-level jet develops. && .HYDROLOGY... Issued at 929 PM CDT Sun Jun 23 2024 No significant changes for the Cedar River basin this evening with the new forecasts. The rise on the Cedar River at Cedar Rapids has been slowed some and still may reach 14 ft or Moderate flood stage early Friday. For the Mississippi, flood watches have been converted to flood warnings for Dubuque Lock and Dam, Camanche, and Rock Island Lock and Dam sites this evening. Additional warnings can be expected in the coming days. .Previous Discussion. The Mississippi River is expected to reach flood stage at multiple locations, including the Rock Island gage, by the middle of this week. Long term we expect the Mississippi to reach Moderate and/or Major flood stages the week of the Fourth of July, with the crest at the Rock Island gage possibly reaching just over 18 feet early on the Fourth of July. However, a larger margin of error should be taken into consideration when forecasting precipitation amounts and locations in the 7-10 day timescale. Watches are currently out for most of the Mississippi River and warnings will follow in the next few as a clearer picture is drawn on what precipitation and routed water we have coming downstream. In the latest forecast package, we have opted to upgrade Gladstone and Burlington to a River Flood Warning, aided by the recent rainfall north of the area. The Rock River is also expected to reach flood stage in Joslin, with an increase noted into Action Stage at Moline. A River Flood Warning has been issued for Joslin in this latest forecast package, which is forecast to hit flood stage tomorrow afternoon. We will continue to see additional rainfall through this coming week, which may lead to further fluctuations in the river levels. If heavy rainfall is seen, then increases can be expected. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...None. IL...None. MO...None. && $$ SHORT TERM...Schultz LONG TERM...Schultz AVIATION...Schultz HYDROLOGY...Gross