Winter weather update - Iowa
A major winter storm will bring very heavy snowfall and winds to much of Iowa starting early Tuesday morning. Snowfall will be light to moderate during the day Tuesday with a few inches of accumulation by nightfall. Very heavy snowfall rates are likely Tuesday night, with many areas experiencing thunder snow and whiteout conditions. The other weather story will be very strong winds, especially late Tuesday night into Wednesday morning, when gusts of 50 mph will be possible! Travel will likely become impossible by late afternoon Tuesday. Below are forecasts for specific locations in Iowa.
Cedar Rapids, IA (Eastern IA Airport):
Snow will start at 4 AM Tuesday and accumulate to 10.7 inches.
Iowa City, IA:
Snow will start at 3 AM Tuesday and accumulate to 9.5 inches.
North Linn Co., IA (Paris and Coggon):
Snow will start at 4:30 AM Tuesday and accumulate to 11.2 inches.
Marengo, IA:
Snow will start at 2 AM Tuesday and accumulate to 11.1 inches.
Union, IA:
Snow will start at 1 AM Tuesday and accumulate to 10.8 inches.
Discussion:
Visible loop indicates developing baroclinic leaf over the southwestern US into the Midwest. A 90kt H5 streak is currently rounding the base of a positively-tilted SWRN CONUS trough. A sharp H85 thermal gradient is noted over western TX along with dewpoints of 8-10C south of I-20 in TX and LA. Most of TX, OK, and KS remain socked in ST; another indication of the rich moisture to our south that will be transported N into the storm over time. Light snow has already spread into portions of KS, NEB, and extreme SW IA with a developing band of moderate snowfall along the KS/NEB border. Looking at model initialization and verification, the GEM and UKMET have performed the best, with the NAM and GFS continuing to suffer latent heat feedback issues until most recent run. The GFS and NAM have been trending towards a slightly more northerly track of the H85 low with the latest run, bringing it more into line with the more consistent ECMWF. The consensus takes the H85 low on a track from just N of STJ at 00Z, to just S of DVN at 06Z, and 30 miles W of MKE at 12Z Wed.
The LLJ will strengthen ahead of the system, while transporting aforementioned moisture north towards the area. WAA increases on Tuesday as the system organizes to our west, with strengthening isentropic lift and 2-4g/kg mixing ratios throughout the day. Snow will overspread the state from SW to NE between 06 and 12Z; however forcing will remain weak through 21Z. Light to moderate snow will persist through much of the day until strong Q-G forcing arrives by late afternoon as the SFC low tracks from KS into NRN MO. By Tuesday evening, an impressive deformation zone sets up across eastern IA, with snowfall amounts to 15 inches possible along a 50 mile-wide axis extending from the Quad Cities to Oskaloosa, IA. 1-2 inch/hour snowfall rates appear likely. Additionally, steep mid-level lapse rates of 7-7.5 C/km with elevated CAPE over the E and SE part of IA will support thundersnow and intense banding structures. Cross-sections of forecast EPV and theta-es indicate moderate CSI. The deformation zone will slowly shift east between 06Z and 12Z Wednesday.
All models continue to indicate in excess of 1.0 inch of storm total QPF. Cold air remains entrenched in the storm, with the H85 0C line remaining S of the IA/MO border for the duration of the storm. Snow ratios should range from 10 south of I-80 to 16:1 north of US-20. The upper-system will become increasingly negatively-tilted as a 120kt H5 streak races towards the southern Great Lakes. The SFC low bombs to nearly 975mb as it approaches the Great Lakes area, with a tightening pressure gradient on the backside. H925 winds to 45 kts are forecast at 12Z Wednesday, with a 25-30 mb SFC pressure differential across the state. Forecast soundings indicate a mixed-layer of 100mb deep by 15Z Wed. Formation of gravity waves may result in wind gusts in excess of 50 mph at the surface, with snowdrifts will approaching 6 feet by Wednesday afternoon.
- Bill
10:17 PM CDT, 12/07/09
Highest drifts I ever remember seeing around here are maybe 7-7.5 foot high drifts. Of course I was a kid, so maybe they seemed like 7-7.5 feet tall and were less. During the winter of 1978/1979 we had two major storms within several days of each other have seen pics of snowdrifts of around 10 feet high. I am too young to remember it though.
