A 300mb upper level low is expected to eject out of the four corners region into western Missouri by Monday afternoon with a surface low parallel underneath this. This continues the active pattern of a long term trough over the western half of the US that has provided several shortwaves in the past few days to support unsettled weather across the state. The surface low mentioned will be the stage for a significant winter storm that will provide a variety of precipitation types across the area, from snow to sleet and freezing rain and even rain.
With this surface low being the stage for this storm, the location, track, and timing of it is crucial for the subsequent forecast. This matters incredibly as it could mean the difference between a major ice storm of over an half inch to a major winter storm of over eight inches. This is why we have held off on releasing a forecast for the past couple of days to allow the computer numerical weather models to get into a better agreement of the aforementioned variables. Things have gotten into a better agreement over the last 12 to 24 hours or so, albeit with several wild cards still existing. Again, it will all matter with the placement of the low. The American GFS and Canadian GEM have this surface low tracking through central Illinois while the European ECMWF and American NAM have this low in eastern Iowa. While this may seem like a small difference, this will play a big impact on temperatures aloft and at the surface (NAM with a warmer solution, GFS with a colder solution).
Our current forecast utilizes a blend between all of the models mentioned, with the surface low tracking from roughly Centerville to Dubuque. This was decided alongside with the notion that things have been moreso on a northwesterly trend as of late and we revolved our forecast around that baseline. Along with this, high southerly mid-level winds will be a fast-moving conveyer belt for unseasonably, although not unexpected (El Niño), moisture to flood the state and provide the pipeline for precipitation across the state. This ample supply of moisture will lead to potentially significant accumulations of ice and snow across the area.
As far as snowfall accumulations go, the highest accumulations are expected basically west of a line from Charles City to Des Moines to Bedford, where six-to-eight inches are expected. Keep in mind that this could easily be upwards of nine or ten inches locally. Beyond this, four-to-six inches are expected west of a line from Prairie Du Chien to Lamoni, and two-to-four inches from Dubuque to Bloomfield, and a trace to two inches in the far east. Ice is a bit more of a trickier issue. The highest accumulations are expected southeast of a line from Dubuque to Knoxville to Lamoni, where a quarter to a half inch of ice will be possible. Lighter amounts will be possible outside of that.
Another area of concern will be the strong winds to impact the area. As the weather system approaches, the pressure gradient will tighten and produce winds of 25-30 MPH sustained with gusts to 40 MPH. These winds are a significant piece to all of this as a: if we do get ice, this will greatly make an already bad situation even worse by aiding in taking the final blow on taxed tress and power lines, b: cause blowing and near-blizzard conditions in areas where heavy snow will fall, creating significant travel problems, and c: keeping temperatures below freezing in the east and southeast during the day and keeping the freezing rain threat ongoing.
A Winter Storm Watch has been issued for much of the state from late Sunday night until early Tuesday morning. These will likely get upgraded to a variety of hazards from Winter Storm Warnings in the west (potentially Blizzard Warnings) to Ice Storm Warnings or Freezing Rain Advisories in the east. These will likely be upgraded sometime today, so stay tuned to the Iowa Weather network for the latest information!
