A SOGGY SPRING STORM...
The sun was back with us Wednesday, but despite its late April power, the region remained very much on the cool side with highs of 55-60. Thursday morning promises to be rather crisp as well, with lows in all but the far south expected to dip into the 30s. Over the NE, readings will approach freezing and frost advisories and freeze warnings are in effect until 8:00am.
I'm hopeful this will be the last time this spring we see a freezing temperature. On average, the last freeze in my area typically ranges from mid-May north to late April south. In the Quad Cities, the earliest last freeze was March 28th in 1878, while the latest was nearly 2 months later May 21st, 2002. Take a look.
Following the chilly start Thursday, the day ends with partly sunny skies and temperatures of 60-65, making for some fine spring weather. Enjoy it, a potent weather pattern promises severe weather and heavy rain this weekend. More in that part of the weather story below.
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A TROUBLED WEATHER PATTERN THIS WEEKEND...
This weekend, the central Midwest will be impacted by two significant waves. Both take similar paths, thanks to a strong western trough that only slowly migrates east. The initial system late Thursday night and Friday spins through NW Iowa. In doing so, it brings a surge of warm moist air advection, generating showers and active thunderstorms. The initial wave of storms early Friday are generally looking to be elevated. As such, the primary concerns from this convection are some hailers and bands of heavy rain. The speed of the warm front will determine the amount of heating and instability. Today's data indicates the warm front will be several hours slower and will take much of the day to pass I-80, setting up a rather cool damp day, especially in the north. Look at the temperature range the EURO suggests Friday.
With the warm front slowly advancing NE, rain and storms should diminish and end in the south Friday afternoon. Elsewhere, they will continue roaming the north once arriving around midday. They are still possible in the north through evening until the warm front finally clears. At that point, a quiet period seems likely in all areas after midnight. Temperatures should hold steady or even rise.
Overall, the primary concern from this first batch of energy is heavy rain with only spotty severe weather, mainly in the form of hail. At this time, SPC shows most of my region with a marginal risk of severe storms. The far west has a slightly better chance due to somewhat higher instability. SW Iowa is very much under the gun, with an enhanced level 3 risk in place into NW Missouri.
Saturday, the second storm quickly emerges. By then, a nearly stationary front is draped across the NW half of Iowa the remainder of the weekend. It becomes the focus for additional showers and storms. The big difference for my area from Friday is the fact we have entered the warm sector of the system where dew points in the 60s and temperatures in the 70s, (perhaps pushing 80 in the far south Saturday). That creates CAPE (convective allowing potential energy). Essentially, CAPE measures instability, and there should be plenty of that both Saturday and Sunday.
Below you can see the 500mb flow Saturday afternoon, this is where things get interesting. That major SW trough is pumping warmth, deep moisture, and rounds of energy into the Midwest. Water vapor cracks 1.50 inches, which is in the 90th percentile for late April. Additionally, there's diffluence, confluence, shear, and a potent low level jet. With CAPE in place, the gun is loaded for severe weather.
Just because the gun is loaded, it doesn't mean it goes off for us. That depends on the tripper being pulled and where all that energy is aimed. That and the timing will become better defined in the next 24-48 hours.
As it stands now, Saturday should start dry but see the potential for storms to pop later in the day. If these are discrete with the shear available, all modes of severe weather are possible. As Saturday night arrives, the low level jet increases and may generate a cluster of storms locally known as an MCS. These are notorious for producing heavy rain, sometimes exceeding 2.00 inches. This needs to be monitored.
Sunday should start fairly quiet, but by afternoon instability builds again. A strong speed max ahead of the approaching trough should pop another round of strong storms later Sunday afternoon or evening. Again, all modes of severe weather are on the table. For now, SPC has slight risk outlooks in place both Saturday and Sunday, but these could be increased in coming days. These risk outlooks don't come down until late night, so I won't have any potential upgrades included in this post.
I also need to mention the heavy rain threat appears to be genuine. Multiple rounds of thunderstorms can really add up in conditions like this. For the time being, the Weather Prediction Center has these outlooks for excessive rain.
They also show 5 day rainfall outlooks that point to a high potential of 1-3 plus rainfall.
Here are what models are suggesting for rainfall Friday through Monday. As you can clearly see, all are pushing the idea of heavy rain hard.
The EURO
The GFS
The Weather Prediction Center
The National Blend of Models
The GEM
In a nutshell, the weekend certainly has the makings of an action packed period for forecasters around the central Midwest. Stay tuned. In the meantime, enjoy what appears to be one final fine looking day Thursday. Roll weather...TS
