Bill Schintler
EF4
A period of moderate snow will affect eastern Iowa starting mid-afternoon Friday. Most areas will see a few inches of accumulation, and winds will increase to 20-25 mph after midnight. Here are forecasts for specific locations in eastern IA:
Cedar Rapids:
Snow will start at 4:30 PM CST and accumulate to between 2.5 inches (Marion and north Cedar Rapids) to 3.0 inches (Eastern IA Airport) before tapering to flurries by 6 AM Saturday morning.
Iowa City:
Snow will start at 4:00 PM and accumulate to 4.0 inches.
Marengo:
Snow will start at 3:45 PM and accumulate to 4.0 inches.
Mount Vernon/Lisbon:
Snow will start at 4:30 PM CST and accumulate to 3.0 inches.
Discussion:
UA chart shows 175 kt H3 max racing towards the Pacific NW this evening while the SRN branch jet was located along the SRN tier of states in the CONUS. H5 closed circulation which brought light snow today to parts of KS and NE had moved SE over CNTRL MO while strong upstream wave – tomorrows WX maker – is just now coming on shore and sampling of this feature by models is still somewhat poor.
This potent system in the Pacific NW will dive into the Upper-Midwest as another one of a series of clipper-type systems that we have seen lately. Overnight and early Friday, the low will move southeast out of southern British Columbia and affect the Upper-Midwest Friday afternoon through Saturday morning, with the low tracking across IA from NW to SE. The NAM has been initializing poor with respect to 850mb thermal fields over much of the area, and tonight was no exception with temperatures 2-4C too warm. The GFS also had this bias but not as much. The NAM has been fairly consistent with precipitation onset, having speed up the onset only slightly from earlier runs while weakening the SFC low evolution over MN and now IA. Each of the last three runs of the NAM has trended 70-100 miles further south with low track while QPF has been increasing and is now about twice that of the GFS. There will be some mesoscale processes with this system that the GFS is not resolving.
The GFS, which had maintained a more southerly track of the surface low in IA, also in good agreement with the ECMWF and UKMET, has been strengthening and slowing the system slightly with each newer model run. As mentioned above, upstream processes were poorly sampled in earlier runs, and only marginally samples at 00Z, and the system should verify stronger then guidance suggests. Forcing with this system should also arrive quicker then models suggest given the very strong upper jet diving into the area. A negative for snowfall accumulation will be a large region of dry air between 850mb and 650mb which will need to be overcome, and much of the initial forcing with the very strong UVV and strong positive omega fields will be used to saturate this column. A period of isentropic up-glide is also a good bet, while a narrow deformation zone will develop about 50 miles north of the low track, or roughly along I-80, which will cause a continuation of precipitation into the late evening hours Saturday. Mesoscale banding within the deformation zone should result in locally higher precipitation then is resolved by the models.
Moisture and temperature profiles in eastern IA should support snow/water ratios to 20/1 which will couple with progged QPF’s in the 0.2†to 0.25†range along with PW’s increasing to 0.3 inches to 0.4 inches. Application of the Garcia method, assuming forcing lasting six hours along with effective mixing ratios peaking to 3g/kg, yields 3 to 4 inch snowfall totals. Overall, a difficult snowfall forecast, with decent moisture below 850mb and very strong forcing offset by dry mid-levels delaying the onset of the precipitation.
- bill
Cedar Rapids:
Snow will start at 4:30 PM CST and accumulate to between 2.5 inches (Marion and north Cedar Rapids) to 3.0 inches (Eastern IA Airport) before tapering to flurries by 6 AM Saturday morning.
Iowa City:
Snow will start at 4:00 PM and accumulate to 4.0 inches.
Marengo:
Snow will start at 3:45 PM and accumulate to 4.0 inches.
Mount Vernon/Lisbon:
Snow will start at 4:30 PM CST and accumulate to 3.0 inches.
Discussion:
UA chart shows 175 kt H3 max racing towards the Pacific NW this evening while the SRN branch jet was located along the SRN tier of states in the CONUS. H5 closed circulation which brought light snow today to parts of KS and NE had moved SE over CNTRL MO while strong upstream wave – tomorrows WX maker – is just now coming on shore and sampling of this feature by models is still somewhat poor.
This potent system in the Pacific NW will dive into the Upper-Midwest as another one of a series of clipper-type systems that we have seen lately. Overnight and early Friday, the low will move southeast out of southern British Columbia and affect the Upper-Midwest Friday afternoon through Saturday morning, with the low tracking across IA from NW to SE. The NAM has been initializing poor with respect to 850mb thermal fields over much of the area, and tonight was no exception with temperatures 2-4C too warm. The GFS also had this bias but not as much. The NAM has been fairly consistent with precipitation onset, having speed up the onset only slightly from earlier runs while weakening the SFC low evolution over MN and now IA. Each of the last three runs of the NAM has trended 70-100 miles further south with low track while QPF has been increasing and is now about twice that of the GFS. There will be some mesoscale processes with this system that the GFS is not resolving.
The GFS, which had maintained a more southerly track of the surface low in IA, also in good agreement with the ECMWF and UKMET, has been strengthening and slowing the system slightly with each newer model run. As mentioned above, upstream processes were poorly sampled in earlier runs, and only marginally samples at 00Z, and the system should verify stronger then guidance suggests. Forcing with this system should also arrive quicker then models suggest given the very strong upper jet diving into the area. A negative for snowfall accumulation will be a large region of dry air between 850mb and 650mb which will need to be overcome, and much of the initial forcing with the very strong UVV and strong positive omega fields will be used to saturate this column. A period of isentropic up-glide is also a good bet, while a narrow deformation zone will develop about 50 miles north of the low track, or roughly along I-80, which will cause a continuation of precipitation into the late evening hours Saturday. Mesoscale banding within the deformation zone should result in locally higher precipitation then is resolved by the models.
Moisture and temperature profiles in eastern IA should support snow/water ratios to 20/1 which will couple with progged QPF’s in the 0.2†to 0.25†range along with PW’s increasing to 0.3 inches to 0.4 inches. Application of the Garcia method, assuming forcing lasting six hours along with effective mixing ratios peaking to 3g/kg, yields 3 to 4 inch snowfall totals. Overall, a difficult snowfall forecast, with decent moisture below 850mb and very strong forcing offset by dry mid-levels delaying the onset of the precipitation.
- bill
