Looking at the system affecting much of central to southern ON and into Atlantic Canada will start with 500 mb vorticity. As the system develops we can see there are two bits of energy to watch, 1 coming up from the Gulf tracking up the eastern U.S. and the second from the upper low dropping south from the Canadian Prairies. Looking at the ECMWF, GLB, and GFS for Sunday morning (when I suspect the worst will be) can see that they have pretty good agreement with how they handle the dynamic energy. The GLB/GFS are a little faster pushing the secondary low little further east but overall nothing to different. The uncertainty will be how the two will interact but for the most part the left overs of the initial low from impulse 1 and how that will interact with impulse 2. But at this point the northeastern U.S. and Atlantic Canada will see the worst as impulse 1 will strengthen as it tacks northeast and absorb impulse 2.

Looking at the surface for overnight Saturday into Sunday morning the ECMWF/GFS show the decaying low tracking through the Niagara region with the new low redeveloping along the triple point (intersect of occlusion, cold front and warm front) along the east coast. With the GFS being the more aggressive model with the decaying low and the ECMWF keeping little further south. The GLB at this point has dissolved the initial low into a surface trough but this is quite drastic from the previous run which was quite similar the 12Z GFS shown below, thus making me a little suspect of the GLB at this point. The other stand out feature is the ECMWF shows a low over Lake Superior as a result from the second vorticity center as pointed out before. The GLB shows an extended but pretty strong trough with closed surface low in the northern thumb of Michigan and the GFS showing an elongated trough with the wind field (not shown) indicating a cyclonic circulation. These features will ultimately have great impact on how this system will play out. The issue now becomes where the decaying low will tracking. The GFS will have the greatest potential for off lake enhancement off Lake Ontario for the GTA, Golden Horseshoe, and along the QEW corridor to St. Catharines with winds down the axis of Lake Ontario. The ECMWF does show a brief period of some lake enhancement but not as strong as the GFS and the GLB has none with on a trough pushing through. To add insult to injury the GLB is now beginning to hint at a dry slot scenario for parts of southern Ontario, mainly for the Lake Erie and Lake Ontario regions which would result is very little snow. As the secondary impulse swings around the secondary low spins up a little more which will result in a little more snow for eastern Ontario and along the 401 east of Kingston for the day on Sunday.

In terms of snowfall ECMWF/GLB still have some agreement but that is because of the lower QPF amounts shown by the ECMWF. Across much of southern Ontario general 5-15 cm is possible from Saturday evening through to Sunday midday. Below I show the GFS 12 hour snow forecast which is on the extreme end showing 15-30 cm across southern Ontario and 5-10 cm for central Ontario. Obviously the GFS will be higher given the trend it shows above. Thus for now a blend between the ECMWF/GFS looks reasonable and my current thoughts are as follows:

Along and south of 401 from London through to Ottawa 10-20 cm are possible with Golden Horseshoe into the higher end of the range.

Remainder of province into the 5-15 cm range depending on how the second low form northern Ontario pans out.

This is just my initial guess and likely adjust some as the forecast evolves. There is the risk of a dry slot scenario as well which if pans out heavy accumulations will be pushes northwest.

An interesting weekend is in store for much of Ontario as a large autumn Colorado Low is forecast to push into the province. For the most part, forecast models are struggling to resolve some of the details at this point in time.

Thursday morning, the initial warm front from the large developing Colorado Low will push in with an elongated frontal boundary extending back into the mid-western United States and Rockies. The warm front will bring the first wave of showers and thunderstorms as it pushes through southern Ontario. Instability parameters are not too high along and behind the front so nothing severe is expected in terms of winds, but some heavy rain is likely which could meet the severe criteria in terms of rainfall. This is depicted in the image below.

 The real fun will start on Friday when the large upper trough crests the Rockies which will cause the low to organize and deepen (strengthen) as depicted in the image below. From the get go we can see there is a great deal of disagreement between forecast models as to where exactly the low’s center will be located and where the associated frontal boundaries will lie.

Compared below are the NAM (first image) and GFS (second image) forecast models and it looks as though a hybrid of the two should give an appropriate solution for the forecast this weekend. Given the position and strength of the upper trough, the location of the low as per the NAM seems reasonable but with the ridge of high pressure building through Manitoba and northwestern Ontario, it seems as though the GFS has the better solution with where the warm front should be positioned. Ultimately, the bottom line here is those along and south of the warm front will see periods of rain and thundershowers but have well above seasonal temperatures and even humidity.

Saturday doesn’t get any easier to forecast, either. As depicted in the images below, the NAM (first image) pushes the warm front well into Quebec where the GFS (second image) keeps it through the Ottawa Valley. Here is another tough call as the ECMWF and GLB models agree with the GFS but the NAM model isn’t totally out to lunch either. The high in the Atlantic Ocean will help reinforce the southerly flow behind the warm front but at the same time the ridge to the north and northwest will act against it. Furthermore, the Ottawa Valley tends to hold on to the cooler air longer so once again it would seem that a hybrid solution of the models is needed. The main message to grab from this analysis is that an unsettled pattern continues with scattered showers and thunderstorms for southern Ontario on Saturday.

On Sunday, the model disagreement continues lending again to the uncertainty with when the associated cold front will push through. This will be depicted once again in the images below. Looking at the temperature forecast for Sunday evening, it can be seen that the GFS (first image) model is much faster than the GLB (second image) model. As these lows develop and the cold air surges south to the southeast, it becomes rather difficult to slow the progression of the cold air down. Taking this into account, it would be wise to lean more with the GFS solution here. It is somewhat worrisome when the GLB and ECMWF forecast models do agree a little better. None the less the cold front’s passing will bring more rain and likely usher in some gusty winds as well. The main issue here is whether this will occur Sunday evening through the night or on Monday.  

A broad trough currently located along the Ontario/Manitoba border this morning will push eastward through the day today. Associated with this trough is a surface low pressure which is currently located near the Lake Nipigon region. Associated with this low are several waves, with the leading warm front pushing through this morning which is bringing an area of showers and thundershowers to southwestern Ontario this morning. These showers/thundershowers are expected to clear out through the morning hours.

In terms of timing with the frontal passage, models are in pretty good agreement with a late afternoon/evening event. A few convective models do paint a little bit of a different picture. The WRF does indicate a pre-frontal line of storms mid-afternoon which does eventually give way to a line of storms. The reason why I am pointing out this particularly model solution is the fact that it hints art what could be a few isolated storms before a large line develops. With low level shear in the 20-30 kt range at this time, these storms could have the potential to develop a brief tornado. But this scenario looks to be the outlier at this point in time but can’t be ignored as it has initialized very well with the morning scenario.

Most models look to be trending toward a line of storms breaking out along the front from the Nickel Belt region straight through to extreme SW’rn ON. I believe this is the more likely scenario as shear vectors are just about parallel to the cold front which will favour more of a linear storm mode and not so much isolated supercells. But again with low level shear forecast to be 20-30 kt, surface winds from the south to even slightly southeast, and lake breeze interactions, cannot rule out the risk of a few brief, weak tornadoes. Thus thinking some sort of QLCS (Quasi-Linear Convective System) type spin up could be possible, very similar to landspout type tornadoes. Instability parameters are not ideal with SBCAPE values forecast to be between 1000-1500 J/kg from the southern tip of Georgian Bay-London-Windsor region, hence the very minimal risk of brief/weak landspout type tornadoes.

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.   

Amongst this relatively quiet pattern there appears to be some active weather to be discussed about in Alberta. The pattern has been in place for a while but Saturday afternoon looks like there could be a little more pop. Aloft (500 mb flow) we can see that there is are large upper lows over northern ON and in Alaska (in red) with the ridge (in blue) in between, this is referred to as an omega block. This is what is responsible for this relative stagnant weather pattern. Looking ahead to the 500 mb forecast for Saturday afternoon (second panel) we can see not much has changed, except that the upper low in ON has expanded some and the ridge in the north central part of the country has broken down some.

In response to this pattern at the surface is essentially a general area of high pressure over Saskatchewan in response to the upper ridge and a weak broad area of low pressure in BC in response to the upper low pushing into the Oregon/Washington states. Looking at the 925 mb forecast (I chose this as it is a little “cleaner” to look at) we can see general outflow from the high in SK which generates southeasterly flow into AB, a textbook setup for upslope lift and thus resulting in storms across the foothills.

This pattern has been ongoing for a few days now but Saturday afternoon looks to have some greater potential but also has some issues as well. Working in tomorrow’s favour is the fact that there is much more instability forecasted (SBCAPE forecast from GEM/NAM are both in 2000-3000+ J/kg) than what has been recently and a little bit more moisture to work with. That being said, 850 mb winds are also forecast to ramp up in the evening and early overnight thus increasing low level shear. On top of that hodographs do show some veering with height particularly within the lowest levels. On the down side we can see there is some ridging aloft which will hinder lift some adding some capping issues into the mix. There is also very weak flow aloft which may lead to storms chocking on their own outflow as well. Thus, it seems possible that weak, slow moving supercells are possible tomorrow, with steep lapse rates in place, looks like large hail would be the main risk tomorrow. IF AND I MEAN IF a storm is able to really root itself and tap into the more unstable air, a brief tornado could be possible, but there is very little chance with this type of setup. Looks like storms will stay mostly confined to the foothills with the best action between Hwy 1 and 16, but there is the slight risk of a few stray storms popping up along the QE2 corridor from Calgary to Edmonton, see last panel for surface analysis and risk area outline.