If ifs and buts were candy and nuts, we'd all have a merry Christmas. Many times we see things in life (and weather) that look to have promise and potential but in the end fail to meet it, hence the saying above. I sincerely hope that's the case Saturday because on paper we have the makings of what could be a very serious severe weather outbreak. I would highly advise those in the risk area to pay extra attention to any watches and warnings that are issued. A significant tornado threat is a real possibility.

In it's latest outlook the Storm Prediction Center continues with a moderate risk assessment for the areas in red including my SE counties. An enhanced risk includes the rest of my counties near and south of HWY 20 in orange.

Within the moderate risk area, there is a 15 percent chance of a tornado within 25 miles of any given point. Those are very strong odds.

The risk assessments are derived from probability forecasts of tornadoes, large hail, and damaging winds produced by the Storm Prediction Center. Here is the definition of the 3 highest risk categories issued by SPC.

  • 3-ENH (orange) - Enhanced risk - An area of greater (relative to Slight risk) severe storm coverage with varying levels of intensity.

  • 4-MDT (red) - Moderate risk - An area where widespread severe weather with several tornadoes and/or numerous severe thunderstorms is likely, some of which should be intense. This risk is usually reserved for days with several supercells producing intense tornadoes and/or very large hail, or an intense squall line with widespread damaging winds.

  • 5-HIGH (magenta) - High risk - An area where a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events).

The Storm Prediction Center just issued this statement and ensuing discussion regarding Saturday's setup in its 1:00 am update. Notice at the end there is a reference to a possible upgrade to high risk at some point Saturday. High risk outlooks are reserved for extreme situations that involve violent long tracked tornadoes and damaging hail and winds. Let's hope it doesn't get to that! The SPC discussion.

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM EASTERN IOWA INTO CENTRAL AND NORTHERN ILLINOIS SATURDAY... ...SUMMARY... A significant severe weather outbreak is expected for portions of the Midwest this afternoon and evening. Threats include tornadoes, a few which may be significant, large to very large hail, and severe wind gusts.

Synopsis... Upper trough now situated over the southern Rockies will eject northeast today, reaching the middle MS Valley and Midwest region later this afternoon. Surface low in southwest KS will deepen as it develops northeast within exit region of a strong upper jet, and should reach southwest IA by early this afternoon. A warm front will extend eastward from the low through northern IL into northern portions of the OH Valley. Trailing cold front initially from eastern KS through eastern TX will advance through the MS, TN and OH Valleys. By the end of the period this front should extend from a surface low in WI southeast through the OH Valley, TN Valley and central portion of the Gulf Coast states. ...Midwest through Ohio Valley region... Areas of elevated thunderstorms will be ongoing north of warm front from northern MO, IA, IL and eastward into the OH Valley and Great Lakes. This activity will gradually shift northward during the day with a gradual destablization of the boundary layer promoted by the northward advection of mid 60s F dewpoints through the warm sector. Widespread multi-layer clouds will slow the destabiliztion process somewhat. However, back edge of cirrus associated with subtropical moisture plume should shift east of this region and partial cloud breaks will contribute to at least modest diabatic warming of the surface layer. MLCAPE from 1500-2000 J/kg is expected across the Midwest by middle to late afternoon. The process of destabilization and deeper forcing for ascent within the upper jet exit region will likely contribute to development of storms along warm conveyor belt from northeast MO, eastern IA into western IL. Vertical wind profiles with 50+ kt effective bulk shear will support supercells. These storms will move northeast through central and northern IL during the late afternoon and evening within an environment characterized by a strengthening low-level jet which will promote large 0-1 km hodographs (300-500 m2/s2 storm-relative helicity). This parameter space along with low LCLs will support potential for supercells capable of producing long-track, strong tornadoes and very large hail. Assuming sufficient destabilization occurs, other supercells might develop in an arcing band within zone of ascent accompaying the primary vorticity maximum and just east of the surface low across central IA. These storms will also pose a risk for large hail and tornadoes. An upgrade to high risk will be considered in later day 1 updates for a portion of this region.

The NWS in the Quad Cities put out a nice discussion on the threat late Friday and here it is. As you will notice when reading, there are still some important concerns that need to be overcome for this to reach worst case potential. At least some hope that things fall apart at the last hour.

Saturday afternoon...Closed upper low deepening close to 540 dam over northwest IA by evening, but models still don`t suggest it to negatively tilt. Still a dynamic synoptic scale scenario as 150 KT upper jet rounds the bend and "exit region noses" right acrs the DVN CWA by late afternoon. Much will depend on the northward extent the warm front makes it to allow portion of warm moist sector with low to mid 60 sfc DPTs to skirt acrs the southeast half or more of the fcst area(how far northwest will it make it?), and timing of southwest LLVL flow surge from approaching dry slot with dry-line like effects on it`s advancing edge, to fuel a developing line of broken generally north to south oriented storm cells/supercells somewhere acrs the CWA. How far east this occurs, how far north the warm front makes it, all questions and lingering uncertainty. Deep mainly southwest speed shear profiles of 70-90+ KTs are troubling, but back LLVLs for tornadic or funnel development to the sfc may not be there except when encountering the warm front or other discontinuity boundaries. With such strong flow profiles,the storms will be very fast moving over 50 KTs and thus short residence windows when encountering boundaries. This may make for more short- lived tornadoes, as opposed to long trackers. If there are too many cells in a broken line, see the scenario where community outflow boundaries merge and act as inflow interrupters, decreasing more lengthy severe threats. But more discrete and separated cells will still be trouble, it not for a tornado still able to produce damaging wind gusts and large hail with rear dry slot interaction. One other question, looking at fcst vertical profiles and soundings there appears some lack of more robust CAPE and depth to fuel such high shear profiles. Have scene another similar scenario several years ago in a high risk window where the shear was too much/not enough deep CAPE and tore storms apart before they could better organize. Many questions and uncertainty ongoing, but still have to respect the rare Moderate risk in portions of our area tomorrow. Trusted old fashion 12z run MCS tool with it`s tornadic shear profile is targeting SPC`s Moderate bullseye really well.

The NWS in the Quad Cities has this situation report in effect.

After the storms blow out of the area early Saturday evening the transition to windy weather will take place. By Sunday westerly winds of 45 to perhaps 50 mph will push drier and cooler air into the Midwest for the remainder of the weekend.

Once again, the potential for strong storms and even tornadoes is real Saturday afternoon, especially in EC and SE Iowa along with WC Illinois. I'll have more on how the threat is unfolding in a mid-day update.Keep a watchful eye to the sky, it's very likely that much of my area will go under tornado watches at some time Saturday. Hopefully the warnings are limited, but if they are issued for your area don't take them lightly. Roll weather...TS

© 2020 Terry Swails