Discussion for Iowa
A powerful and lengthy winter storm will bring snow, freezing rain, and rain to much of the area from Wednesday through Thursday. Recent trends suggest that (regular, NOT freezing) rain will be the primary precipitation type in the Cedar Rapids/Iowa City area. Nevertheless, most areas should see a tenth of an inch of freezing rain early in the event, and several inches of snow from Friday morning through the weekend. Easterly winds of 30 mph on Wednesday and Thursday may cause power outages in areas that do see significant icing, however this concern should be well north of US-20. A secondary concern, especially areas south of I-80, will be flooding of rivers and streams. Fortunately, river levels are currently low.
Cedar Rapids, IA (Eastern IA Airport):
Fog and freezing drizzle from now until Wednesday morning. Freezing rain will start at 9 AM Wednesday and change to (regular) rain at 2 PM with 0.05 inch of ice accumulation. Rain will continue through early Friday, with total (liquid) rain of 1.8 inches. Rain will change to snow at 8 AM Friday with periods of moderate snow on Friday, and light snow and snow showers continuing through the weekend. Total snow: 3 inches.
Iowa City, IA:
Fog and drizzle from now until Wednesday morning. Freezing rain will start at 8 AM Wednesday and change to (regular) rain at noon with 0.05 inch of ice accumulation. Rain will continue through early Friday, with total (liquid) rain of: 1.4 inches. Rain will change to snow at 7 AM Friday with periods of moderate snow on Friday, and light snow and snow showers continuing through the weekend. Total snow: 3.5 inches
North Linn Co., IA (Paris and Coggon):
Fog and freezing drizzle from now until Wednesday morning. Freezing rain will start at 9:30 AM and change to (regular) rain at 2:30 PM with 0.10 inch of ice accumulation. Rain will continue through early Friday, with total (liquid) rain of: 2.1 inches. Rain will change to snow at 9 AM Friday with periods of moderate snow on Friday, and light snow and snow showers continuing through the weekend. Total snow: 2.5 inches.
Marengo, IA:
Fog and drizzle from now until Wednesday morning. Freezing rain will start at 8:30 AM Wednesday and change to (regular) rain at noon with 0.05 inch of ice accumulation. Rain will continue through early Friday, with total (liquid) rain of: 1.8 inches. Flooding along the Iowa River is a concern. Rain will change to snow at 7 AM Friday with periods of moderatey snow on Friday, and light snow and snow showers continuing through the weekend. Total snow: 3 inches
Union, IA:
Fog and freezing drizzle from now until Wednesday morning. Freezing rain will start at 11 AM Wednesday and change to (regular) rain at 2 PM with 0.10 inch of ice accumulation. Rain will continue through early Friday, with total (liquid) rain of: 1.6 inches. Rain will change to snow at 5 AM Friday with periods of moderate snow on Friday, and light snow and snow showers continuing through the weekend. Total snow: 4.5 inches.
Discussion:
Lots to talk about with this complicated system. During the near term, an extended period of DZ/FRDZ is on store with the inversion in place and LLVL moisture concentrated below 1 KM AGL. This shallow moisture depth should preclude any rain or snow through Wednesday morning. On to the main event…. Wednesday through Thursday, changes are on the way as a trough deepens while a 100kt H5 speed max rounds the base over the Four Corners area. Another piece of the puzzle is phasing of another piece of energy diving SE out of BC. This second ULVL low will dive SE out of CAN, while phasing with the southern system occurring over IA. By 00Z, 12/24, MDLs show a 90-100kt H5 streak rounding base of trough over SRN NM/AZ, while a trailing 80kt speed max dives S through ID. The upper trough closes off and becomes negatively-tilted between 12Z, 12/24; and 00Z, 12/25. During the same period, strong lift develops over IA within the left-exit region of the lead jet max. Latest runs of GFS, UKMET, and GFS indicate a blocking ridge over Newfoundland. This will cause the southern branch system to track increasing NEWD with time while slowing. The ULVL system eventually occludes and stalls out over the Upper Midwest. Indeed, recent MDL trends take the SFC low on a slightly more NWRLY track which shifts warmer moisture profiles further N and W then earlier FCST. The SFC low will develop over WRN TX on Wednesday, then initially head E into OK, and then hook sharply N on a trajectory through ERN IA through Friday morning. On Friday, a triple point develops over NRN MO with CAA wrapping into there first, and then later in ERN IA.
This storm track advertises a warmer then earlier thought scenario with mixed precipitation. Snow and FZRA will change over to all RA south of US-30, while ELY flow along and N of US-20 should keep SFC temperatures a few degrees below freezing given the deep snowpack until later in the afternoon when a 45kt southerly H85 jet and WAA overcomes cold air at the SFC. Precipitation changes back to all snow Friday morning as shallow cold air surges in behind occlusion and with dynamical cooling with the H85 layer rapidly cooling. Snow should linger through Christmas day, with continued periods of light snow and flurries lasting throughout the weekend with cold air pulled behind the low.
Determining precipitation types and amounts for specific areas remains difficult, which has significant individual impacts. Precipitation-type indicators such as the SPC SREF winter precipitation FCST, and NAM and GFS FCST soundings are still inconclusive. S of I-80, FCST H85 temperatures warm to nearly 4-5C between 00Z and 12Z Thursday. 18Z H85 temperatures are already verifying several degrees higher then FCST. FCST Soundings between I-80 and US-30 agree on a warm layer in the 900 to nearly 700mb layer from Wednesday into the Thursday before the CAA works in. Top-down investigation of soundings yields above-freezing depths that are variable in both space and time. Further complicating things is QPF amounts. It appears as though one or more periods of dry-slotting will take place, reducing precipitation rates at times. Looking at pavement temperatures, most roads in ECNTRL IA are currently 32-34F.
Finally, a secondary concern of this system is flooding of area streams and rivers, especially S of I-80. Heavy ainfall will be widespread, with storm-total QPF’s in the 1-2 inch range. PWATs are running at least two standard deviations above seasonal norms. This precipitation in combination with nearly two inches of liquid equivalent snow already on the frozen ground raises hydrologic concerns, especially if SFC temperatures approach 40F Thursday night in SRN/SERN IA.
- bill s
4:56 PM CST, 12/22/09
Worried about all the holiday trailers if that pans out for eastern Kansas.
