02/16/07 - 02/17/07 FCST: Midwest (WINTER PRECIP)

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 0.2â€￾ to 0.25â€￾ 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
 
My analysis of the models causes me to believe that most of west central and northwest Illinois will see 2"-4" snowfall amounts. I think the rather quick speed of the system coupled with the somewhat restricted supply of moisture levels will keep snowfall totals somewhat suppressed. The bigger concern will be the winds, which look to be sustained at at least 15-20 mph with higher gusts.
 
Forecast Track of the Low would take it from around an Omaha NE, to Kirskville MO to Quincy IL. NAM is definitely the wettest of all the models shoowing PWAT and .39 While GFS says .11 and WRF saying .05 Cant say as the NAM amounts seem reasonable ATTM. As gulf pretty well cut off.. I would guess 2-4 inches 50 miles either side of a galesburg to Daventprt IA line.. More model runs and better moisture analysis will be crucial.
 
This is a very tricky little system to forecast. The models have underestimated the strength of it up to this point, and it'll be interesting to see what really evolves out of it. The main problem with this little storm is the lack of moisture, which of course is common with a clipper like this.

A few things I do like about this storm is the relatively slow movement, making for a longer duration event than normal for a clipper like this; and the strong dynamics and lift associated with it.

Snow to water ratios will be pretty high, like 20:1 or so as mentioned above. I believe the models may still be underestimating QPF. I would expect a band of 3-5" snows from near Burlington IA through the Springfield IL area towards southern IN by tomorrow afternoon. Due to the very strong lift, and relative longevity with this storm, I would not be too surprised to see a few isolated areas report 6 or 7".

One interesting note, there's currently a band of heavy snow plowing right into the very dry air in extreme eastern IA and about to enter IL. It will be interesting to see how this all evolves over the next several hours...
 
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