RIPE FOR RAIN IN THE SOUTH
June is about to leave us, a month that is known for being wet and muggy. So far, the month has not lived up to its reputation but at least for now, there are signs we are priming the pump for rain in the coming pattern. especially over my southern counties. The big question is when and how much? Before we get to that, here's some rain facts regarding June in my area.
If you look under the column PCPN on the above left, you will see the average June rainfall for Burlington (BRL), Cedar Rapids (CID), Davenport (DVN), Dubuque (DBQ), Iowa City (IOW), and Moline (MLI). It's generally in the range of 4.75 to 5.50 inches. On average, June is the wettest month of the year areawide. Typically, we see about 11-12 days a month with measurable rain, 7-8 with at least a 1/10th, 3-4 with 1/2 inch and 1.5 with amounts greater than an inch. Through Wednesday evening, only 5 days have seen measurable rain at the NWS office in the Quad Cities, far below the normal of 13.3. The 1.07 inch total is way below the typical 4.73 inches that results annually.
Here's some of the June rainfall totals with 2 days to go. A pretty grim picture, especially from far SE Iowa into central Illinois.
Going forward, rain chances hinge on the areas proximity to a heat dome to our south. On the northern flank of the heat with highs of 90-100 over northern Missouri, a boundary is laid out over southern Iowa. That position keeps the intense heat from reaching my area. However, very warm humid air pools along the boundary where it generates some healthy instability. Thursday that manifests itself in CAPE (convective available potential energy) that is shown exceeding 6,000 j/kg in in central Illinois...that's extreme!
At first glance you would think that's the zone (with the highest CAPE) that would be under the gun for strong to severe storms. However, south of the Iowa border, the air is very warm aloft. That creates a capping inversion which thwarts thunderstorm development. However, as you move near and just north of the boundary. the warm humid air to the south rides up and over it providing the lift to break the cap in that area. Thunderstorms are likely to form, especially when the nocturnal jet sets up after sunset. Throw in a minor disturbance to enhance the set-up and thunderstorm clusters known as mesoscale convective systems (MCS) are likely to form. These are very difficult to see more than 12 hours ahead of time and thus difficult to forecast beyond a broad brush approach. In other words we have a pretty good idea of a region that's susceptible (southern Iowa/WC Illinois) but defining the timing and precise location is challenging until the storms go up. Nowcasting off satellite, radar, and real-time data is often better than the data available from models. In the end, it's a bit of it all that gets the job done.
Other factors that make forecasting these situations difficult are outflow boundaries and capping. Outflow boundaries are mini cold fronts caused by the outflow from cold pools contained in towering storms. Downdrafts push the cold air out ahead of the storms where they can create new storms. Modeling often has a real tough time seeing these small but critical developments. That's a problem as they can alter the location of instability causing new storms to form in locations the model did not foresee. Getting to the point, we have a good idea there will be a ring of fire where storms exist, but its a day by day process to determine where. From the looks of things storm chances will exist at least into Saturday with the southern half of my area most susceptible to thunderstorm development which could produce heavy rain and even severe weather. Then there's the issue of capping which as I mentioned is warm air aloft that makes it hard for updrafts to develop. You can have all the instability in the world but if you can't set it free, nothing happens. Outflow and the cap will be big drivers in what happens the next few days.
By the way, with this pattern it doesn't rain all the time. There are plenty of dry hours where warm muggy conditions occur. But when it rains, water vapor is high and big downpours can dump 1-2" in a short period of time. Seeing those spots in advance is the hardest part of this forecast. For several days now models have been consistently focused on the area near and south of I-80 for the heavy rain threat Thursday night through at least Saturday night. I could see a swath of 1-4 inch rains over the 72 hour period somewhere in this region.
It's interesting to see that the Weather Prediction Center sees a slight risk of heavy rain in the week 2 period all the way from my area to the Gulf of Mexico.
Already storms festered over scattered parts of my area Wednesday. New development is possible by Thursday morning as a short wave ripples southeast out of Minnesota. Better moisture is in place for this round and some parts of the region could pick up scattered storms early Thursday but forcing is weak and I don't expect much.
Thursday brings more challenges as what's left of any overnight convection dissipates in the morning. After that a humid air mass begins to heat up building instability across the region, especially south of I-80. Much of the day some capping from daytime heating could keep storms at a minimum. The focus then turns to the afternoon and evening when shear and CAPE are favorable for active storms. If they initiate rapid storm development is likely. Again, this is all contingent on when or even if the CAP breaks. SPC likes the set-up and has issued an enhanced outlook for severe storms for much of the region along and south of I-80.
Rain and storm chances will exist again Friday and Saturday and there's no sense getting into the details yet as they will likely be influenced by some of the factors discussed above. What I am beginning to see more and more of is a trend for the bulk of the rains to stay out of my northern counties. It looks like the light for heavy rains will shine on my southern counties the next 72 hours. You can see it in the rainfall forecasts which suggest totals like this.
The Canadian 10k GEM
The 12k NAM
The Weather Prediction Center outlook
The National Blend of Models.
My far southern counties are in a marginal risk of excessive rains Thursday.
The excessive rain risk advances slightly further north Friday.
It's interesting to see that the Weather Prediction Center also sees a slight risk of heavy rain in the week 2 period as well from the central Midwest to the Gulf of Mexico. We'll see about that, I'm not ready to bite just yet.
I didn't focus much on temperatures but smoke is expected to be less of a problem Thursday and with heat and humidity poking in from the south it looks like highs will be summery with readings in the mid to upper 80s, perhaps 90 in the south through Saturday. With dew points pushing 70 the heat index Thursday could reach the upper 90s in the far south. The GFS indicates values like this.
Last but not least, Canada is seeing its worst fire season on record as hundreds of blazes rage across the country – with more than 250 burning “out of control,” according to the Canadian Interagency Forest Fire Centre. More than one third of the US population is under air quality alerts, covering more than a dozen states from the Midwest to the East Coast as smoke from Canadian wildfires sweeps in from the north. In northern Michigan, the Mackinac Bridge was shrouded in smoke Tuesday. Motorists were asked to drive extra slow and use caution due to the reduced visibility on the bridge. Look at this amazing shot.
While the impacts of fires should diminish in many spots locally, issues will continue in my northern counties. Here's what the HRRR indicates for near surface smoke Thursday afternoon around 1:00pm.
Well, that's it for now. Should be an interesting few days ahead. Roll weather...TS
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