Bill Schintler
EF4
Another in a series of fast-moving clipper systems will affect the upper Midwest tonight. The heaviest accumulations will be well to our north, with several inches falling in the Twin Cities area and WI. Accumulations will be light in eastern IA, amounting to less then an inch in most locations; however, winds will once again pick up out of the northwest in the storm’s wake and result in some blowing and drifting snow. Looking ahead, another system will bring an inch or two Sunday night. Finally, long-range indications suggest the possibility of a major winter storm followed by an Arctic blast during the third week of December; however, forecast confidence is still low at this point. Below are forecasts for specific locations in eastern IA:
Cedar Rapids (Easter Iowa Airport)
Light snow will start at 10:00 PM and accumulate to 0.9 inches
Iowa City:
Light snow will start at 10:15 PM and accumulate to 0.7 inches
North Linn Co. (Paris and Coggon):
Light snow will start at 9:45 PM and accumulate to 1.2 inches
Marengo:
Light snow will start at 10:10 PM and accumulate to 0.6 inches.
Discussion:
The Upper Midwest will remain locked in a NWLY UA pattern for the foreseeable future with below-normal temperatures and a shot at an inch or two of snowfall every few days as clippers translate through the area. The current snow depth chart shows snow cover extending S to nearly I-70 in MO. Over time, the snow pack will increase which will reinforce the cold AMS to our north and the baroclinic zone to the S. Over the coming weeks, this temperature gradient should serve to intensify winter storms as the pattern finally shifts to SWLY flow and Gulf moisture is pulled NWD. Pressure falls of 1.5mb/hr are noted over the ERN Dakotas and are somewhat higher then forecast earlier by models, suggesting that the storm is verifying stronger.
During the day Friday, SFC winds will shift back to the SW between retreating high pressure and a seasonably strong clipper approaching the upper Midwest. A strong pressure gradient across ERN IA into MN develops as an H85 LLJ increases to nearly 50 kts. At the SFC, winds will gust to 30-35 mph with weak LLVL lapse rates owing to the snow pack. A deck of mid-level clouds, associated with WAA, will push SE across IA late in the day and early evening. From Friday afternoon through midnight, Saturday, precipitation will break out in MN and later in WI through ERN IA in response to isentropic upglide and subsequent top-down saturation along 270-284K SFCS. Later, mid-level UVV will increase to between -3 to -6ub/s in response to a vort max diving SWD out of MN.
Regarding precipitation amounts, snow-liquid ratios will be higher then with the previous system, perhaps in the 16:1 to 18:1 range; however, limited moisture is available with PWT FCST at between 0.1 and 0.2 inches. The ECMWF and GFS have initialized well and will be used for primary guidance. Negative feedback with dry snow and the shallow Arctic AMS is a challenge.
- bill, KD0DJG
1:45 PM CST, 12/05/08
Cedar Rapids (Easter Iowa Airport)
Light snow will start at 10:00 PM and accumulate to 0.9 inches
Iowa City:
Light snow will start at 10:15 PM and accumulate to 0.7 inches
North Linn Co. (Paris and Coggon):
Light snow will start at 9:45 PM and accumulate to 1.2 inches
Marengo:
Light snow will start at 10:10 PM and accumulate to 0.6 inches.
Discussion:
The Upper Midwest will remain locked in a NWLY UA pattern for the foreseeable future with below-normal temperatures and a shot at an inch or two of snowfall every few days as clippers translate through the area. The current snow depth chart shows snow cover extending S to nearly I-70 in MO. Over time, the snow pack will increase which will reinforce the cold AMS to our north and the baroclinic zone to the S. Over the coming weeks, this temperature gradient should serve to intensify winter storms as the pattern finally shifts to SWLY flow and Gulf moisture is pulled NWD. Pressure falls of 1.5mb/hr are noted over the ERN Dakotas and are somewhat higher then forecast earlier by models, suggesting that the storm is verifying stronger.
During the day Friday, SFC winds will shift back to the SW between retreating high pressure and a seasonably strong clipper approaching the upper Midwest. A strong pressure gradient across ERN IA into MN develops as an H85 LLJ increases to nearly 50 kts. At the SFC, winds will gust to 30-35 mph with weak LLVL lapse rates owing to the snow pack. A deck of mid-level clouds, associated with WAA, will push SE across IA late in the day and early evening. From Friday afternoon through midnight, Saturday, precipitation will break out in MN and later in WI through ERN IA in response to isentropic upglide and subsequent top-down saturation along 270-284K SFCS. Later, mid-level UVV will increase to between -3 to -6ub/s in response to a vort max diving SWD out of MN.
Regarding precipitation amounts, snow-liquid ratios will be higher then with the previous system, perhaps in the 16:1 to 18:1 range; however, limited moisture is available with PWT FCST at between 0.1 and 0.2 inches. The ECMWF and GFS have initialized well and will be used for primary guidance. Negative feedback with dry snow and the shallow Arctic AMS is a challenge.
- bill, KD0DJG
1:45 PM CST, 12/05/08
