Upper-Midwest Winter Storm Forecast
Snowfall amounts:
St. Cloud, MN: 5â€-6â€
Minneapolis/St. Paul: 5â€-6â€
Rochester, MN: 6â€-9â€
Waterloo, IA: 2â€-3â€
Cedar Rapids, IA: 2â€, With some sleet mixing in with the snow early
Iowa City, IA: 1â€, With some freezing rain mixing in with the snow early
Timing – onset of precipitation.
St. Cloud, MN: 10 AM CDT, Friday
Minneapolis/St. Paul: 11 AM
Rochester, MN: 12 PM
Waterloo, IA: 2 PM
Cedar Rapids, IA: 4 PM
Iowa City, IA: 5 PM
Discussion:
00Z analysis showed a longwave ridge in the WRN CONUS and trough in the east. Two S/WV’s were embedded in this upper flow – one entering the PAC NW/BC CAN, and the other diving S tough Alberta, CAN. At the surface, a sharp baroclinic zone was noted along from WRN ND through WRN IA to CNTRL IL; with developing low pressure in SERN Alberta. The sharpest pressure falls were occurring in ERN MT, where 3-hr falls of –2.0MB were noted. The 00Z Eta has initialized well to the surface and UA features, and takes a surface solution roughly 40 mi S of the 12Z Eta solution. This is still slightly NWRD and faster then the (12Z) GFS, NGM and UKMET solutions. Eta and GFS are remarkably close with regard to QPF, and I will use a blend of those two for surface and QPF.
Surface low pressure will continue to develop and move to the SE along the baroclinic zone, primarily under the influence of the first S/WV now lifting through SERN BC. It will track through SWRN MN then along the IA/MN border through 00Z Sat. The second S/WV will dive SSE through CAN into MN and then IA early on Saturday. This will serve to enhance QG forcing and UVV’s through SERN MN though 18Z Saturday.
Two methods used for snowfall forecast: PQF and Garcia Method. Use 10:1 snow/water ratio along I-80 for the duration of the event – in fact, there will be a region between 900MB and 850MB above freezing through 06Z, roughly along I-380 in ERN IA. Use 1:15 along the IA/MN border through 00Z, increasing to 1:20 through 06Z, then 1:25 by 12Z Saturday as 850 MB temperatures drop from between –2C and –4C to –20C at the end of the period; and surface tempertures drop from 20-25F to 5F-10F early Saturday. Temperatures between -12 to -16°C through the 900MB – 750MB layer, along with high RH values, are strongly correlated with heavy snowfall – even though amounts in excess of 6†are unusual for clipper systems in the upper-Midwest. Crystal growth rates and aggregation are maximized in these dendrite growth zones. Upward vertical motion in regions of high RH replenishes the supply of supercooled liquid water droplets and thus enables rapid dendrite growth to continue. Finally, Application of the Garcia Method (295K surface, and using 6G/Kg S and 3G/kG N) yields a similar snowfall amount for this storm.
- bill
