A large upper low that has been nearly stationary along the Pacific coast is forecast to slowly push its way eastward over the next couple of days which is set to bring rather wet and unsettled conditions to southern British Columbia and Alberta Thursday/Friday, and possibly Saskatchewan on Friday. It does appear as though there is equal amounts of pros and cons that come along with the setup over next couple of days.

Thursday

Below shows the 250 mb winds which shows a brand of the polar jet stream nosing into the southern part of BC/AB which will help supply favourable upper level divergence with a difluent region over southwestern AB.

Next looking at the 500 mb vorticity field there the first of two disturbances pushing through. This shortwave will help to induce falling pressures over the Rockies resulting is a southeasterly to easterly flow on the lee side of the Rockies.  Note will not spend too much time on these upper air charts as model agreement is rather good with these features.

At the surface can see the resulting easterly flow which will result end a good setup for upslope winds into the foothills and Rockies. This lift will be further enhanced into the evening hours as the 850 mb LLJ steadily increasing in intensity and result in increasing low level shear. Bulk shear trough the sfc-500 mb layer is not horrible but is a little on the low side between 25-30 knots. As the upper toruh pushes eastward it will also bring a plume of Pacific moisture and lowering T’s aloft giving suitable instability to fuel storms as well, models are forecast SBCAPE values between 1000-2500 J/kg but do have rather low MLCAPE values. Secondly, CAP will also be an issue as well but near/along the foothills with persistent upslope cooler T’s CAP should be less of an issue. Thus all factors considered it would appear as though the area outlined by Red Deer-Castlegar-Lethbridge looks to be the area of concern for stronger storms through Thursday afternoon/evening. Parameters right along the foothills (foecast hodogrpahs do show veering with height) will be favourable for rotating updrafts and a few supercell type storms, but looks like limited shear and moistures will maintain a minimal tornado risk. Through the evening and overnight system is expected to congeal into a larger scale system, with persistent upslope bringing a significant rainfall for the foothills region and into west-central AB.

Friday

 Quick peak at 250 mb winds and 500 mb vorticity can see things have shifted slightly eastward with the jet streak nosing into southern AB ans southwestern SK with the next impulse at 500 mb pushing into the Rockies.

At the surface things become a little more complex with a developing surface low and a relatively open wave. Models do show quite a bit of disagreement with this situation but it appears that the GFS/ECMWF/GLB favour the low developing near the Medicine Hat region with a slight less open wave. The NAM solution looks to be slightly north with the low with a more open wave. Can’t be ignored none but the less will reject it for now. Thus with convergence along the warm front, Sfc-500 mb bulk shear values pushing into the 30-50 kt range, and good instability (1000-2000 J/Kg). There will be the risk of a few strong storms, possibly supercells for the southeast corner of AB and southwest corner of SK. But the limiting factor here will be amount of cloud cover from morning precipitation which may limit instability and the presence of a pretty good CAP. Thus storms, if they manage to fire, will likely be in the late afternoon or evening hours.