SEVERE STORMS TO END THE WEEK
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ALL ABOARD, THE THERMAL TRAIN IS RUNNING,,,
We are now into that time of year when the thermal train is running wild. The remnants of winter clashing with the growing warmth of spring sets the stage for major temperature swings, sometimes measured more by the season they resemble than the actual thermometer. We're in the thick of such a period now with the GFS in the Quad Cities indicating 2 major warm-ups surrounded by 3 distinct cool-downs over the next 9 days.
The roller coaster is set to start climbing soon as you can see below in the time stamped temperature departures for Thursday and Friday. Thursday morning starts cold (below freezing) but by Friday afternoon we're pushing 70 in the south.
This initial temperature bump is fostered by a strong storm ejecting out of the Rockies Friday. Along with a day of much warmer weather will come the threat of the seasons first widespread severe weather event. Here you can see the energy as it dumbbells' into the Midwest in the form of a closed 500mb circulation.
A closer inspection shows a 100mph jet max surging into SE Iowa late Friday afternoon.
It's transporting warm moist air into a surface low over northeast Iowa. By late afternoon strong to severe thunderstorms are expected to develop from the triple point down the spine of a cold front extending through eastern Iowa. Note the radar simulation below.
In light of the increasing severe weather parameters the Storm Prediction Center has issued a moderate (level 4) risk of severe weather Friday afternoon. A level 4 risk indicates widespread severe thunderstorms are likely including long lived intense thunderstorms.
Initially thunderstorms that develop will be discrete in a high shear environment allowing supercells, (rotating thunderstorms). The 3k NAM supercell composite index is quite high over SE Iowa around 6:00pm.
Updraft helicity at 0-6 km is 90kts in spots which indicates significant shear to create rotation. That allows a robust tornado index up to 7 in SE Iowa.
The updraft helicity over time Friday afternoon and evening shows the tracks of individual rotating thunderstorms. There's a number of them and some are long tracked.
The 3k NAM even shows numerous virtual PDS tornado soundings over SE Iowa. PDS standing for particularly dangerous sounding. Here's one near Iowa City around 5:00 Friday evening. Hodographs (top right corner) show impressive shear profiles with 0-3 shear near 50 kts.
At least for now, it does appear the potential for supercells and perhaps tornadoes is a real threat. Early supercells will have the ability to produce damaging winds and tornadoes. The big question is how long can cells remain discrete after they develop? Storms will be fast movers and it may not take long for them to congeal into a line (what's known as a QLCS). At that point wind becomes the dominant type of severe weather but tornado spin ups would remain possible as the line races east. These tend to be weaker rotations and usually don't last long as their inflow quickly gets choked off. These tornadoes can still be problematic reaching EF1 to EF2 status.
Despite the impressive severe weather parameters, heating and instability will be the ultimate key to the magnitude of whatever severe weather occurs. If festering showers and clouds exist into Friday afternoon, that limits instability and reduces the severe potential. If not, it's game on and we face the fact strong storms could be a force going into Friday afternoon or evening.
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MORE SNOWFLAKES BY SATURDAY MORNING
The cold front responsible for the storms is going to race east at speeds up to 50 mph meaning storms won't be in one spot long and the severe weather threat is over in all areas by sunset Friday, even earlier west of the Mississippi. The next phase of the storm is strong northerly winds, sharply colder temperatures, and snow showers later Friday night and early Saturday. The 3k NAM shows an inch of snow as far south as Cedar Rapids and Dubuque.
Here's the surface depiction of the snow late Friday night being driven by NW winds gusting to 40 mph. Ugh.
Wind chills Saturday morning start in the teens.
After highs in the 60s to near 70 Friday, Saturday will remain in the mid to upper 30s north to mid 40s far south. That's a good 20-25 degrees cooler than 24 hours earlier.
BOUNCE BACK LATER IN THE WEEKEND.
As poor as the weekend starts out, it ends on a high note with readings soaring back into the range of 60-65 Sunday. That introduces us to our next significant storm next week which could bring the the threat of more thunderstorms (possibly strong) and heavy rain. Lots of details to work our there. Meantime, we'll deal with the coming system first. Have a strong day and roll weather...TS