If you’ve been outside in the past few days, you’ve undoubtedly thought that spring is on the way. High temperatures have been in the 50s and 60s, although somewhat hampered by gusty winds around 30 MPH… but nonetheless, nice weather. This looks to change back to a classic Iowa winter event as we head into the early part of the work week.
As far as atmospherics go, an upper-level trough will be shifting northeastward from the Southern Rockies into the Plains and Mississippi Valley on Monday, developing into a closed area of low pressure, before exiting out of the region on Tuesday. At the surface, this will be accompanied by a low in eastern South Dakota and western Minnesota, with an occluded front draped to the southeast into eastern Iowa and Illinois. Showers and thunderstorms will begin forming by tonight, predominately in southern Iowa as warm air advection and forcing increases along the southeasterly-oriented low-level jet. Dry air should keep things mum in the northern section until the system and adjacent front approaches the state, which will make rain more widespread by morning.
In northwest Iowa, the concern for a changeover to snow exists in the early morning hours as cold air wraps south and east around the system. Further to the east, it becomes more tricky as surface temperatures will be at or above freezing throughout the day. This will require more cooling processes aloft in the form of diabatic cooling and strong height falls. All-in-all, snow should switch over after the lunch hour, with the heaviest precipitation arriving by afternoon with precipitable water values almost to an inch courtesy of the aforementioned front. It’s important to note that while general consistency has been seen in the last several model runs, some natural uncertainty exists, primarily in the regard of when and where the exact snow transitions will take place in north central and northeast Iowa. In addition, the change over from rain to snow should be fairly quick, but a period of light icing or light sleet is still possible.
Another issue to point out is the fact that instability (CAPE) values will be high enough to support some risk for lightning and thunder, which could cause some locally enhanced snowfall rates. This will be during a short period, but can cause snowfall rates of up to an inch at times. Putting these issues aside, four-to-six inches of snow is expected in northeast Iowa, perhaps up to eight inches. Two-to-four inches is expected outside of that towards northwest Iowa with a steep cutoff generally along Highway 20. If there’s any good news, it’s the fact that a) the snow will fall during the day time and b) ground temperatures are well above freezing. These two factors support the idea that a lot of the snow will be able to melt. The actual snow that sits on the ground will probably be less than what’s graphically-predicted.
Outside of the snowfall amounts, it’s imperative to note that blowing snow will be an issue, regardless of whether it sticks – winds will shift northwesterly and sustain around 25 MPH, gusting to 35 MPH. While the bulk of the snowfall should end by Monday night, some light snowfall and continued blowing snow will continue to impact the morning commute Tuesday. By this point, snow will have accumulated and will blow around fairly easily, considering much of these parts are rural. A Winter Weather Advisory has been posted running from and until various times for these concerns. Please see our Active Alerts page for details. Farther south, rainfall amounts up to an inch are possible. This may cause some localized flooding.
Stay tuned to the Iowa Weather Network for the latest information!
Forecast written on the evening of March 4, 2018 by Henry Luker, in collaboration with Alan Shriver and Jeff Wilcox.