CENTRAL IOWA WINTER STORM WATCH: FRIDAY INTO SATURDAY

OVERVIEW/:

The third winter storm of the season is likely to impact much of West-Central Iowa this upcoming Friday through Saturday morning. This isn't anything unprecedented or something we would not expect this time of year, but do expect difficult driving conditions from snow-covered roads and reduced visibility from blowing snow. The worst conditions are expected to occur from midday Friday through predawn Saturday followed by a reinforcing shot of cold air with temperatures hovering in the teens and low 20s during the day and single digits to near 0F by Saturday night.

NWS ALERTS:

A Winter Storm WATCH has been issued for all counties in Central Iowa. This will likely be upgraded to a warning at some point tomorrow.

STORM SETUP AND METEOROLOGICAL DISCUSSION:

This upcoming system is quite unique because it will feature "clipper-like" characteristics but likely produce snowfall totals that we would expect to see from a Colorado low. In addition to the Alberta clipper portion of this upcoming storm, there is also a well-developed shortwave trough moving into the Pacific Northwest late Wednesday night which contains moisture from the Pacific Ocean and Subtropical jet stream. Eventually, the output from the deterministic models have the Pacific NW shortwave and the Alberta Clipper phasing early Friday over the Central Plains into one larger upper level storm system that will have more moisture--although still somewhat limited-- and more mid/upper level dynamics to support heavier snowfall than what typically occurs in a NW flow regime. With a more curved look to the mid-upper level height field, this indicates that there will be more vorticity or spin, and as that is advected to the east/southeast over the Missouri River Valley, it will induce upward motion allowing for the formation of clouds and widespread precipitation.

Snow will be the dominant precipitation type for the duration of this system for much of Central and Western Iowa. SW portions of the state may be above freezing during the morning-early afternoon hours of Friday--which may lend to a period of light rain showers. Evaporative cooling into a dry layer and strong cold air advection from the north will allow for temperatures to cool steadily through the day. Snow-liquid ratios will initially start out in the 8:1-10:1 range through before increasing to 12:1-16:1 range late Friday evening into predawn Saturday morning. Essentially, this means that snow will start heavier and wet before becoming powdery, fluffy and drier. Snow will end from north to south Saturday morning as high pressure builds in behind the departing system.

Total snowfall amounts will range from as little as 1-2" across far Eastern Iowa to upwards of 8-10" across Northwest Iowa. Much of Central Iowa, including the immediate Ames/Des Moines area is likely to see 6-8". There will be periods of heavy snowfall rates upwards of 1" per hour, especially late Friday afternoon into the evening rush hour. The most likely zone for heavy snowfall rates will be near or just west of I-35. Current thinking is that our northwest, and northern counties, particularly north of HWY 20, and west of I-35 stand the best chance at seeing some of the greatest accumulations as this is where temperatures will be coldest most of Friday AND snowfall is expected to occur with largest snow-liquid ratios. Therefore, snow will be more efficient at accumulating very quickly in these zones.

TIMING, IMPACTS:

Outdoor and travel conditions will deteriorate from northwest to southeast during the day on Friday. Parts of Northwest Iowa will likely start to see snow as early as sunrise. In the immediate Des Moines/Ames areas, snow will begin between 9AM-12PM. For our eastern/southeast counties, snow will likely not begin until between 12-3PM.

Snow/ice covered roads are expected with this event, especially by late Friday afternoon well into Saturday until plows get a chance to clear the roads. Reduced visibility <1/2 mile will also be an issue for a majority of this event. For the first half, this reduced visibility will be induced by moderate to heavy snowfall rates, but later on as winds pick up, and snowfall becomes more light/powdery, expect blowing snow to become the dominant factor in reducing visibility. This will especially be true for large, open, and tree-less areas where snow can easily blow without being obstructed. While winds will increase, at this time, it looks like we will not reach blizzard criteria.