IMPORTANT SEVERE WEATHER UPDATE

 Even though a week ago it was said that this may be a significant severe weather event, it does not look to be that way now. There has been a change in this. Yesterday, it did not look like we would even get a severe weather event. But, it turns out the NCEP models were having problems handling the development of this system, and thus its dynamics. Here is what is likely to happen. Saturday morning, looks to start off clear and sunny across most of the area with some mid level and high level clouds likely across the area, from dying upstream convection. By this time tomorrow, the strong upper level wave that is currently located over South west Canada will likely be located over western Dakota. An ongoing MCS will likely be located over western Minnesota. During the day, as a surge of hot, dry air from the desert, rides up and over the warm, moist air at low levels, strong convective instability will set up. Dew p

oints are expected to rise into the low to mid 70's according to the 06z NAM. By late Saturday afternoon, a few isolated showers and thunderstorms will likely erupt as the cold front approaches the region. By early to mid evening (7 p.m. - 9 p.m.) the cold front will be close enough to the region to allow for the development of numerous thunderstorms to our west, which will likely organize into an MCS on the nose of a rapidly strengthening low level jet. This will likely be our only chance for severe weather if this happens. If the MCS does set up, it will not move very quickly given weak to moderate upper level winds. Though deep layer shear will be increasing, the most important factor will be low level shear. This could allow for damaging winds, especially if the low level winds align in the same direction. Strong upper level divergence from the developing jet streak over the Midwest will likely allow for the development of additional showers and thunderstorms during the overnight hours. Most thunderstorm activity should end by early Sunday morning, giving way to clearing skies and warming temperatures.

Have a great day!

Model Disagreement Continues

There is still considerable disagreement among computer models with the upcoming storm system and associated cold front.

This strong wave which is forecasted to track along the Canadian border will cause a jet streak to form over the Midwest. As I have said before, a jet streak is an area in the jet stream of higher wind speeds. The first thing that this causes is considerably higher vertical sh

ear. Since the jet streak will likely not be located over the area until after the cold front moves through, I would expect that this stronger vertical shear will only have an effect on storms that develop over Wisconsin and Iowa on Saturday afternoon. Second, since jet streaks are associated with winds that are not in geostrophic or gradient wind balance, they will have associated areas of diverging winds, which will destabilize the atmosphere over which the jet is located. An effect that it will have on our area is the fact that it could cause a strengthening of the low level jet, which transports heat and momentum into storms at low levels. If this jet is enhanced, severe storms will be more likely, given stronger low level shear, and continued destabilization of the atmosphere through the overnight hours.

Because of these factors, I have still determined that the main severe weather threat will be damaging winds, due to the fact that MCS development is likely ahead of the cold front in the form of a squall line, with embedded bow echoes.

This will be posted on the blog and I should have an update by afternoon.

Have a great day!

Storms threatening on the Horizon.

1. Morning Update:
   
   Good morning everyone! After a nice morning rain with a few rumbles of thunder (from a dying convective system in association with the upper level disturbance), I expect that the mid level cold pool responsible for the system will allow for moderate instability to develop this afternoon, as solar heating commences. With moisture pooling and convergence along the boundary, dew points will increase into the upper 60's and lower 70's mainly south of Route 6 this afternoon. CAPE values will rise in the range of 1500-2500 across a good portion of the area. This, in combination with the associated cold front and shortwave, will allow for the development of scattered strong thunderstorms this afternoon. The main threats will be heavy rain and gusty winds in the strongest storms. Upper level divergence in association with a developing upper level jet streak will also help prepare the atmosphere for thunderstorm development, and at the same time providing a source of bulk shear, for downdraft/updraft separation. This could allow for some localized severe weather in some areas.
   
   As for timing, I expect that most of the thunderstorm activity will develop around and after 3 this afternoon. It will likely organize into multicell clusters at first, organizing into possibly an MCS south of the area by the late evening hours.
   
   As for the long-range forecast, a weak shortwave could push a weak cold front into the region by Thursday and Friday, causing isolated showers and thunderstorms, but with a surface anticyclone residing from the last cold front, surface moisture will be lacking, limiting storm development. By Sunday, we have bigger issues. Models continue to forecast the movement of a strong shortwave and associated storm system along the Canadian border by Saturday and Sunday. Ahead of the system, a surge of warm air from the Desert Southwest will cause temperatures to jump into the lower and middle 90's on Saturday, and possibly Sunday. A surge of moisture from the Gulf of Mexico will also make the air sticky and humid. The moisture will cause the atmosphere to become unstable by Sunday, with CAPE values as high as 4000 J/kg. With higher shear values, severe weather looks to be a good possibility. The GFS continues to vary considerably with the amount of instability, with the 00z run forecasting 5000 J/kg of instability, while the 06z run is now forecasting 3000 J/kg or less of instability across the area. I will have to continue to monitor the situation, but to me it looks as if another significant severe weather outbreak may be possible.
   
   Have a great day!
   
   Joshua Steiner
   Senior meteorologist