Winter Storm FCST: 12-6-07 IA/IL/MO

[Brandon Sullivan]

A snow event as a result of Isentropic Lift and Warm Air Advection will occur over the tri-state region tommorow.

Model QPF values show a fairly broad area of 4 inches or so across the entire area.. Snow Advisories have been posted, as it does not appear to be warning criteria at this time.

 

[Jeff Miller]

There will be some isolated 6" snow amounts - so brief areas of warning criteria may indeed materialize but will not be widespread enough to merit Heavy Snow Warnings for this event, I don't believe.

 

[Brandon Sullivan]

I agree. There will be areas where a given mechanism will be maximized for a certain time, so wouldnt at all be surprised to see some 6' amounts.. But nothing widespread..

 

[Bill Schintler]

Winter WX FCST: ERN IA, 12/06/07

Cedar Rapids:
Snowfall will start at 3:30 PM CST and accumulate to 3.0 inches before ending around midnight.

Iowa City:
Snowfall will start at 3:30 PM CST and accumulate to 3.0 inches before ending around midnight.

Marengo:
Snowfall will start at 3:00 PM CST and accumulate to 3.0 inches before ending around 11PM.

Synopsis:
Both the UA subjective analysis and the WV loop indicated several pieces of SHRTWV energy in the PAC NW, and of particular importance to tomorrow’s WX maker were two pieces of energy over ID and WRN MT, which were associated with 60m height falls in the H7-H5 layer along with 4C H5 temperature falls, both over the last 12 hours. The details of these forcing mechanisms and MDL initialization to them are important for forecasting snowfall amounts Wednesday; and the NAM, GFS, and NGM all initialized well with regard to H5 details.

Discussion:
Main FCST challenge is the Thursday ULVL system and temperatures both overnight and on Thursday. First - overnight, expect cold temperatures under clear skies and a new snowpack as high pressure slides E over the area and winds diminish. Will stick with MDL guidance given dewpoints in the single digits above zero at 00Z.

MDLS are now in close agreement in SFC and ULVL features, and QPF for Thursday’s fast-moving clipper; and latest trends take the strongest forcing further S, producing lighter QPF in ECNTRL IA but higher ratios. Strong WAA will commence Thursday morning as the LLJ increases, bringing increasing moisture into the area. Primary lift mechanisms initially will be WAA and isentropic lift as frontogenetic forcing is weak. After the onset of precipitation and especially after 23Z, strong kinematic forcing will be enhanced by the left-exit region of a 110kt H3 streak. Two compact H5 VORT maximas associated with this feature will phase and provide QG-convergence in the H7-H4 layers, especially along US-34 in SERN IA as the H7 VV bulls travels to the E along the IA/MO border from I-35 at 20 Z to SERN IA at 01 Z.

Isentropic lift along 285-300K SFC’s will commence by 18Z as a 40-50kt H85 LLJ noses into ERN IA. Top-down saturation will occur through 21Z with support of PV advection and Q-vector convergence fields. Temperature profiles indicate -6 to -12C in the DGZ which will be collocated with strong lift and implied ageostrophic flow from the aforementioned ULVL features. This will result in moderate snowfall rates lasting for a period of about 4 hours. Applying a 10:1 ratio along US-34 to 14:1 along US-20 to QPF output from GFS/NAM/NGM/UKMET produces as much as 5 inches between US-34 and I-80. Similarly, application of the Garcia method with 3-4g/kG mixing ratio, 290K SFC, and 4 hours of lift gives 3-4 inches of snowfall. Lighter amounts will fall N of I-80.

- bill
11:30 PM CST, 12/05/07

 

[Brent A. Clair]

Latest runs are slowing it down a lot and moving it further south. Hate to say it, lest I jinx it every time, but heaviest corridor is lining up along the IA/MO border. 12Z NAM actually puts a spin and rotates it up northeast through W IL, but I think that is a dead giveaway on the lack of confidence of the NAM right now. For now, I will lay out the 3-5" band along US 34 and taper accordingly north and south.

... so wouldnt at all be surprised to see some 6' amounts.. ..

Brandon, 6 FEET of snow? Wow!

 

[Brandon Sullivan]

Hahah.. Oops.. 6'' .... hahaha