http://www.hpc.ncep.noaa.gov/basicwx/93fwbgus.gif
http://www.hpc.ncep.noaa.gov/basicwx/94fwbgus.gif
Looking at what is anticipated to happen a area of low pressure is going to be dropping down out of central Canada with a cold front and this cold front is going to drop across the northern tier region. This cold front and area of low pressure will be the trigger to produce severe weather across the area's highlighted..
In the northeast we have more of an isolated low risk area because this area has less in the way of severe parameters and it is dependent upon a warm front as the trigger which we never are to enthused with warm fronts being a severe weather trigger.
Tonight's severe zones are based on more of a blend of the NAM/GFS and no use of the SREF because it was not available for the 21 Z run.
So lets look at the Parameters that will be in place..
TEMPERATURES
Temperatures are going to be on the warm side from the 70 degree range up towards 90 degrees. Coolest severe region will be the Northeast where temperatures will mainly be in the 70-80 degree range.
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_TMPF_2_M_ABOVE_GROUND.png
DEWPOINTS
Dewpoints in the Northeast are actually only going to be in the 50s but pretty much in the 2nd severe zone we are looking at humid conditions with dews in the 60s-70s range..
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_DPTF_2_M_ABOVE_GROUND.png
CAPE or INSTABILITY..
As expected in the NE we are looking at lower CAPE as compared to the northern tier severe zone. In the Northeast we are looking at about 500-1000 . In the northern tier zone we are looking at 750 to about 3500.
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_CAPE_SURFACE.png
LIFT INDEX
Lift index is a little lower then what we have been use to seeing lately in terms of severe weather with ranges from about 0 to roughly - 6 depending on where you are located.
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_PLI_30_0_MB_ABOVE_GROUND.png
SHEAR
Shear is also not as impressive as what we have seen in prior severe weather events with only about 30-40 knots across the region with the highest up in Canada where it is 50 knots
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_SHRM_500_MB.png
ENERGY HELICITY INDEX
When you look at the EHI you might think ..well supercells are definitely likely and Tornado's are definitely a possibility. We do have some pretty impressive EHI in place with ranges at 3.00 to 5.00
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_EHI_3000_M.png
HELICITY
You will see that the helicity is mainly at 300 and under. Generally for tornadic supercells you would want the HELICITY to be at 400 and above. So even though we have EHI that is supportive of a tornado threat ..the HELICITY is less supportive.
http://www.twisterdata.com/data/models/nam/221/maps/2012/06/08/00/NAM_221_2012060800_F24_HLCY_3000_M_ABOVE_GROUND_0_M_ABOVE_GROUND.png
LAPSE RATES
Lapse rates are generally supportive of severe weather. With a range around 6.5.
If the NAM is correct on the dewpoints being in the upper 60s and lower 70s then we could be looking at tornado potential from extreme western Upper MI back into Wisconsin , Minnesota and the Dakotas. For right now we will place that region in a 2-5 % chance. As these storms move east southeast with the cold front they will be going into less favorable parameters and would weaken...This is why we are not taking the threat any further east then extreme western Upper MI.
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