Eastern Iowa Winter Storm Forecast
Forecast:
Light freezing drizzle has been falling since midnight and will continue to do so throughout the day, with little or no accumulation. Snowfall will start in the Iowa City around 7 PM and Cedar Rapids around 8 PM tonight. It will be heavy between 12 AM and 6 AM, and "thundersnow" will be possible during this period, leading to some very high snowfall rates. Tomorrow, the snowfall will taper off to lighter rates until late in the afternoon at which time heavier snowfall will begin again. The snow should exit the area between 3 AM and 6 AM Thursday morning.
Snowfall amounts:
9" - 12" along and 20 mi either side of a Creston, IA, to Iowa City to Quad Cities line.
6" - 9" along and south of a Des Moines to Cedar Rapids to Clinton line.
3"-5" along and south of a Storm Lake to Waterloo line.
South of a Creston to Washington line, a mixture of sleet and freezing rain will mix in with lighter snowfall totals. Further south in MO and IL, a very significant ice storm will occur.
Discussion:
At 12Z today, a strong baroclinic zone was noted from SERN OK through SRN MO through SRN IL. Surface low pressure is advertised to develop and ride along this boundary over the next 36 hours in response to a very nice coupled upper-level jet structure as the upper-low in the SWRN states shears out and opens up. The first of a series of shortwaves will approach the upper-midwest this evening, enhancing an area of strong isentropic assent that will be in place during the period. An extended area of deformation zone precipitation will also occur late in the day Wednesday, as a second shortwave approaches. The 12Z Eta initialized well with regard to heights and temperatures in the upper-air data, as well as current precipitation fields. Total storm Eta QPF appears to be around 1.5" along and 50 mi either side of the I-80 corridor. During the first period of heavy precipitation tonight, a 6:1 snow/water ratio will be occur SE of a Muscatine to Bloomfield line with sleet and freezing rain mixing in; while an 8:1 ratio will be found S and E of a Clinton, Iowa City, to Chariton line; a 10:1 ratio will be S of a Dubuque to Des Moines line. Given pockets of elevated instability (MUCAPE), shear, and strong upward motion; convective banding should occur during this period between Des Moines and Cedar Rapids, providing locally higher snowfall rates. Late in the day Wednesday during the second round of heavier snowfall, overall ratios will be higher - on the order of 15:1 along a Clinton to Des Moines line - higher to the north and lower to the south.
- bill
