HIGH IMPACT RAIN EVENT

SIGNIFICANT FLOOD POTENTIAL FOR MUCH OF EASTERN IOWA AND NW ILLINOIS

UPPER MIDWEST

NATIONAL HYDROMETEOROLOGICAL DISCUSSION...FRIDAY NIGHT

The NHD indicates considerable flash, urban, and riverine flooding impacts are likely this weekend into early next week as QPF guidance suggests 2 consecutive days of heavy rainfall day 2 (Sat) through day 3 (Sun). (2–4"+ each day, totals could approach 5–10") across eastern IA, far northern MO, northwest IL, and southern WI.

The best potential for overlap of consecutive days of heavy rain looks to be across eastern IOWA where basins are already vulnerable to additional rainfall (and there is ongoing elevated flows/river flooding). Streamflows are running above normal (USGS) and top-layer soils remain moist after yesterday's rainfall, therefore this additional QPF will be enough to elicit renewed and new river flooding by early next week. Significant river flooding is possible.

The GFS and NBM-forced National Water Model (NWM) forecasts show an increase in both areal coverage and magnitude of response across eastern IA into southwest WI and northwest IL in comparison to signals from yesterday. The number of basins showing potential for rapid onset flooding has increased, and there are now some indications for higher magnitude flows in the 10 - 20% AEP range. This in conjunction with a consistent placement/increasing magnitude QPF forecast from WPC provides increased confidence in potential for significant flooding impacts late this weekend into early next week.

The above statement, generated by meteorologists at the National Meteorological Center Friday, indicates an unusually high risk of excessive rainfall and flooding exists near my area later Saturday into early next week. Slow moving thunderstorms will be capable of producing rainfall rates of 1–3 inches per hour. Widespread rain amounts of 2-5 inches are possible in close proximity to my area.

WPC indicates flash floods are possible, especially in the moderate risk areas highlighted in red, encompassing my area.

The heavier rains are expected this weekend across eastern Iowa and NW Illinois, especially during the evening and overnight hours Saturday into early Sunday morning. If you live in a flood prone location, plan ahead by having multiple ways to receive warnings, supplies ready, and evacuation routes reviewed. A flash flood watch is in effect for the balance of my area from Saturday afternoon through Monday afternoon.

To give you some perspective on what models are implying for rain, here's what the GFS suggests for rain totals through next Tuesday. It hammers the NW half of my region. Despite what is shown, it's only one model and amounts can still vary considerably the next 24 hours, so what the GFS indicates is far from reality at this point. Still, it's a disturbing run.

The 10K GEM shows rains of up to 8 inches in my NW counties.

The first wave of rain and storms is due to race southeast early Saturday along a cool front that slips into NE Iowa. The storms may clip my NW counties around daybreak and should weaken and dissipate as they outrun the front, which becomes stationary from central Iowa into SW Wisconsin. Deep tropical moisture pools along the stalled front, where it interacts with afternoon heating following the morning storms. Instability will be exceptionally high south and east of the front. Where and when the front stalls will be when it will be at its strongest, and able to support explosive development of storms. The cold pools they produce will generate additional storm development.

Guidance is in general agreement that the storms will initiate to the north in Wisconsin Saturday, then back-build down the front towards the southwest. WPC, in its heavy rain discussion, indicates the predominant flow will be southwesterly, parallel to the front, with Corfidi vectors perpendicular to the flow, out of the northwest. Thus, some training of storms is expected as they move towards the northeast. Since the forcing front will be stalling out, the steady advection of additional Gulf moisture and instability on the low level jet will support additional convective development towards the south and west, with those storms then tracking east. This will result in areas where there are multiple hours of heavy rain over individual areas. While the exact location of those bands is very difficult to pinpoint much before the event begins, the Moderate Risk areas highlighted above are where the greatest density of locations subject to multiple hours of heavy rain will be. Previous heavy rainfall over Iowa has led to rivers in the area already higher than normal, so this significant addition of rain will greatly increase stream, creek, and river levels all across the area. WPC indicates a moderate risk of flash flooding and urban flooding is possible, resulting from 1-3 inch per hour rainfall rates with the stronger storms, widespread rainfall totals of 2-5" inches plus. As is typical of these events, the greatest coverage and intensity of storms is likely to occur overnight Saturday night into Sunday, as the heaviest rains push through eastern Iowa. Most likely, 2–3 rounds of storms will occur over the heaviest impacted spots, (location still uncertain) which appears to be centered over parts of eastern Iowa and SW Wisconsin. ARIs (average rainfall occurrence levels) are shown to be near the 25-year recurrence intervals along the Mississippi River, with up to a 70% chance of exceeding flash flood guidance over much of eastern Iowa.

Another concern Saturday afternoon is the severe storm potential. How this plays out will be tied to the amount of CAPE (instability) that can blossom before storms redevelop late in the afternoon or evening. The EURO is robust with CAPE of 3,500 to 4,500 j/kg in spots.

For now, SPC shows my entire area under a slight level 2 of 5 risk for severe weather.

If the 3k NAM is on track, it shows much of the area along and west of the Mississippi with a significant supercell composite index of 10 to 25 around 6:00pm. The most likely time for severe storms would be in the 5:00 to 10:00pm window from north to south if trends hold. High winds would be the greatest threat, but shear may be strong enough for a couple of tornadoes. We will know more by midday how things are setting up.

Additional rain is still on the table Sunday and Monday, but until we know the mesocale details of outflow boundaries and cold pool placement from early storms, it's tough to do much more than broadbrush the fact more rain, possibly heavy in some locations, remains on the table.

Not much to do now but watch things unfold as Saturday progresses and the event evolves. My hope is we can keep totals on the low end of the spectrum, which is the 2-4 inch range. That is possible and is what I would consider the low end of the potential. The worst case scenario would be to double those amounts. Not to say it will happen, but there is evidence to support bands as heavy as that. Needless to say, Saturday afternoon and night is a period to keep a close eye on for impactful rain. Roll weather...TS