Wednesday, November 28, 2007

Winter Storm: Dec. 1-2

A portion of Iowa got to see a hint at winter last week just before Thanksgiving, with mainly areas of snow falling over southcentral and eastern Iowa. Up to 5 inches had fell with that quick storm, all of it melting in a matter of days. This next system looks to give the entire state a shot at some impressive amounts of precipitation, worse is that it is all not likely to fall as the same type of precip. Currently the two main weather models for forecasting, the WRF & the GFS have some fairly significant differences. The WRF wants to be stubborn with the upper level wave that is likely to aid in the development of a lee of the rockies low pressure system; keeping it further west than the GFS. The difference in timing is ~6 hours and location wise, the WRF is ~150 miles further south with the strongest QPF (precip). Besides the timing and location, which are two important things obviously; the amount and locations in terms of levels within the atmosphere are going to be very important for this event.

The WRF and GFS are in agreement for some locations for the type of precip at the beginning of the system. However, the GFS is much more aggressive with the low pressure system and thus the warm air advection. This ends up transitioning the event from snow, to sleet and possibly freezing rain and even to straight rain in some areas. This is another conflict that the models are currently having and making forecasting a little trickier, although current consensus is leaning towards agreement with the GFS. Thus, the following forecast for the sections of Iowa:

Northwest: Mainly snow event, although some sleet & other types of frozen precip can't be rules out. Potential will exist for some very heavy snow amounts, current model forecasts are for 8-12". Timing looks to be from Saturday morning tapering off into Sunday evening.

North-central: Another mainly snow event, although a little better chance for some sleet by Sunday afternoon. Snow amounts between 5-9" seem possible in this area, beginning Saturday Noon and lasting into Sunday.

Northeast: Snow seems likely to begin by early afternoon on Saturday, potentially changing over to sleet and even rain by Sunday. Snow amounts between 4-7" are possible, although any rainfall could dramatically decrease that amount due to melting.

Southwest: Areas of southwest Iowa are going to be in a harsh area of winter weather in models are correct, with snow and substantial freezing rain and finishing off with cold rain. Snow forecasts are still 6-9", but lower amounts of snow and higher freezing rain amounts are likely as well. This is a very tricky area of forecasting, and will likely fluctuate up until the actual event.

South-central: Another area that will likely be along the lines of heavy snow, freezing rain and a complete turn-over to a cold rain. Snow forecasts are still indicating 4-7", but once again with a tenth or two of freezing rain also possible the snow amounts could be dramatically different. Rainfall after the snow/frz rain could also aid in actually helping the situation by melting, although it could also create an even worse environment. Once again another area that is going to be in a forecast fluctuation up until the event.

Southeast: Snowfall, freezing rain and rain all seem possible for this area of the state. Snowfall amounts within the range of 3-6" seem possible, but freezing rain also possible as well as rain over portions of the state could likely reduce that amount. Saturday afternoon snowfall with the precipitation tapering off by Sunday evening seem the likely timing.

Forecasts will likely be revived each night as the event nears, each model run could likely change the type of precipitation expected due to the 'touchy' nature of the system and the temperatures affects of precip type. By tomorrow night there may be some type of winter storm watches out for the state, it has been mentioned by the NWS offices that cover the state. With tomorrow's update I will post any information on those watches if they have been issued, or with information on when they will be issued. A tricky event that could be potentially dangerous to any travelers this weekend across the state..