SIGNIFICANT DAMAGING WIND THREAT TOMORROW, SEVERE T-STORMS, TORNADO POSSIBLE

OVERVIEW:

A powerful storm system will bring near record high temperatures (near 70F), widespread possibly damaging non-thunderstorm wind gusts, and strong to severe storms to much of Central Iowa on Wednesday.

WATCHES AND WARNINGS (As of 11:30PM Tuesday):

-HIGH WIND WARNING--ALL COUNTIES IN CENTRAL IOWA UNTIL 12AM THURSDAY

-NO Convective Watches yet as these are issued shorter term.

METEOROLOGY AND STORM SETUP:

Medium range models remain in strong agreement for a very powerful, compact shortwave trof (neg.tilt, 130+KT JET MAX) to eject northeast toward

the Missouri River Valley by mid afternoon Wednesday. This will lead to rapid surface cyclone formation and deepening as the diverging air aloft and very high

values of PVA generate strong upward motion in the low levels. A very stout LLJ on the order of 70-80kts will also be in place adding to the intense low level speed and directional shear and also aid in moisture/heat transport from the south. Simply put, the kinematics will be in place for what has the potential to be a fairly significant late-season severe weather episode. At the surface, a deep surface low is expected to move just to our northwest with a bulging dry line and cold front trailing behind the dry line. The clash of the desert-like dry air with a modifying unstable air mass to the east will provide a focus of lift to induce t-storm development. Storms will be capable of producing enhanced damaging winds (up to 80mph), especially as updrafts tap into stronger winds aloft in the low level jet and mid-upper level jet and transport that energy to the surface via downdrafts. There is also a tornado risk given the strong increase in vertical wind shear in height and some veering winds as well. Currently, I think the greatest threat for tornadoes will be from roughly Des Moines and Ames metro and points northwest toward Northern Iowa where surface winds are backed southeasterly ahead of the dry line bulge. If there are any tornadoes, we are not expecting long track or violent tornadoes like the ones that struck Kentucky, Tennessee, Arkansas, Illinois, and Missouri over the weekend.

Below is a graphic explaining the threat and timing for severe weather.

In addition to the severe threat, also concerned about the potential for widespread damaging wind gusts Wednesday afternoon into Thursday morning. These will be NON-TSTORM RELATED, but rather pressure gradient/ mixing of low level jet influenced. Sustained winds of 25-40mph are expected, but gusts could easily exceed 70mph.

IMPACTS/PREPAREDNESS ACTIONS:

Winds could topple over weak trees from previous storms (aka the Derecho). Also downed powerline and minor structural damage is likely to occur as well. Prepare for the potential of widespread power outages as well. Secure any loose items, Christmas decorations and other trash to prevent them from flying away.

Also avoid high profile vehicle travel in semis or large trucks as these could also easily get flipped over in these type of conditions.

Know your severe weather preparedness plan and be prepared to act QUICKLY if a warning is issued for your area. Storms will be moving at 70-80mph due to strong steering winds present in the atmosphere.