Winter weather update - Iowa
Freezing rain will affect most of Iowa starting early Wednesday morning and continuing through Thursday morning. Locations generally south of I-80 in central Iowa will see a transition to “regular†rain during the afternoon hours Wednesday, limiting Ice accumulations there. In eastern Iowa, the heaviest icing should be along US-30, where as much as 0.4 inched of ice is possible. In addition to obvious travel problems, this amount of ice may cause power outages. Looking ahead, another system will affect the area on Saturday, with early indications suggesting moderate-to-heavy rain as the primary precipitation type. Below is time of arrival and total ice accumulation for Wednesday’s event for specific locations in central and eastern IA:
Cedar Rapids, IA (Eastern IA Airport):
Fog and freezing drizzle overnight. Freezing rain will start at 6 AM Wednesday. Total ice accumulation: 0.30 inches.
Iowa City, IA:
Fog and freezing drizzle overnight. Freezing rain will start at 5:30 AM Wednesday. Total ice accumulation: 0.33 inches.
North Linn Co., IA (Paris and Coggon):
Fog and freezing drizzle overnight. Freezing rain will start at 6:30 AM Wednesday. Total ice accumulation: 0.28 inches.
Marengo, IA:
Fog and freezing drizzle overnight. Freezing rain will start at 5:30 AM Wednesday. Total ice accumulation: 0.34 inches.
Union, IA:
Fog and freezing drizzle overnight. Freezing rain will start at 6 AM Wednesday. Total ice accumulation: 0.30 inches.
Discussion:
UA analysis indicates an H5 ridge over the Upper-Midwest bounded by a low over the NRN Great Lakes and a developing trough over WA/OR. Over time, lift will increase as a lead H5 S/WV develops over NEB/KS within the left-exit region of an 80kt H5 speed max embedded within the SRN stream. In the mid-levels, moisture transport will increase overnight as a LLJ strengthens in advance of a developing H85 trough over NEB and KS. WAA and isentropic lift will increase between 00Z and 06Z. The H85 0C isotherm will push well N of IA by 06Z, 01/20/10; with temperatures along the I-80 corridor FCST to be between 3-4C by 12Z.
Timing of precipitation onset is a challenge as guidance is depicting too little moisture trapped beneath the inversion as compared to observations, and as such timing of onset will be adjusted one to two hours earlier S of US-30. This reasoning is supported by increasing fog and ST trends across SRN IA in association with theta-e advection in the H925 layer. Along and N of US-20, however, high-pressure over the Great Lakes will maintain a dry ELY SFC fetch. Visible satellite indicates clearing over WI and NERN IA in a zone of weak subsidence. Within this area, a deep cold and relatively dry layer in combination with evaporative cooling will support sleet for the first several hours of the event on Wednesday; with FCST soundings not showing a warm nose aloft much above 2 or 3 C. MDLs consistently support 0.3-0.4 inches of storm-total QPF across CNTRL IA, with the GFS most bullish and the ECMWF lighter then previous runs. Lesser QPF is noted in the N and E where forcing is offset by the dryer SFC AMS and is blocked along the back side of the H5 ridge.
Regarding precipitation type, the NAM maintains FZRA over the entire area for the duration of the event; while the GFS advertises a change over to RA S of I-80. The NAM likely has a more accurate handle of the temperature structure of the atmosphere. Supporting this is that weakening of the SFC low over time and a FCST motion E or even slightly SE, or along I-70 in MO. This should prevent significant warm entrainment into the storm while keeping SFC wet bulb temperatures below freezing for the duration of accumulating precipitation N of I-80. Additionally, the snowpack remains deep over the entire area. Looking at FCST soundings, NAM sounding valid for CID at 00Z, 01/21/00, indicates 31-32F at the SFC with the layer extending from 3 kft to over 7 kft above freezing.
Looking ahead, light precipitation will continue though early Thursday until dry air works into the mid-levels. A saturated AMS and easterly flow will remain in place near the SFC, resulting in periods of DZ and FRDZ with little or no additional accumulation. On Friday, a weak ridge in the mid-levels overspreads the area ahead of this weekend’s storm. This should dry out things in the LLVLs; however, cloudiness should prevail in the mid-levels.
- bill schintler
8:00 PM CST, 01/19/10
