Temperatures are really beginning to heat up in sections of Southern and Eastern Ontario as an upper level ridge builds into the East allowing for a strong Southeasterly flow around the high.
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This surge of heat is occurring in advance of a cold front that will move through in the afternoon on monday. The front will manage to produce much needed rain for many areas in Ontario that have been drying out lately. I think TWN is a little early on when the rain starts. They have sunday afternoon with rain for Toronto but I see no model support for this. You can see the very dry conditions that have developed especially in areas SouthWest of the GTA.
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The front will leave a very humid air mass behind it and won't really do much with regards to cooling the air temperature. It will feel hot, sticky and the atmosphere will become very unsettled due to high humidity levels an therefore we will see some showers and thunderstorms popping up on tuesday behind the front. More heat, humidity and thunderstorms will start to build up in the middle of next week until a front moves through thursday afternoon which will probably produce a severe weather threat for Southern and Eastern Ontario as well as Quebec. This front will usher in the trough that has been mentioned several times now. The stronger the trough the more severe weather we will see because of more clashing between the hot and soupy airmass we are about to see and the cool air mass to follow.
144 hour GFS doesn't show that much action along the front but I believe that the higher resolution models will pick up on it as we get closer to the event.
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Trough digs in on the 23rd and sticks around all though its weakening going into the beginning of July. I believe that a beyond the 28th or 29th we will not see much of a cooling effect although we won't see any major heat surges. Near normal temperatures to slightly above normal(which are warm at this time of year) are expected from the 28th and beyond, with more heat building towards the beginning of July. From the 23rd to the 28th southern and eastern ontario can expect temps a few degrees below normal.
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By the 28th we can see some cooling is still being produced by the trough although just near normal "cooling". From the 28th to the first few days of july we will be about normal to above normal until a heat wave build in as we get past the 4th of july.
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You can see the weakening as we go later into the period although the trough stays put near the great lakes region, ridging will be returning soon after this(temps are already above normal at this time):
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The euro has the trough farther east but begins to pull it back west with the same results. I feel pretty confident with this forecast although I feel that the models might be a little bit underdone with regards to the trough strength as we get to the 28th and beyond and the cooling effect that it might have. To get a better handle on this I took a look at the EPO forecast values. The GFS is keeping it negative into the beginning of july which would argue that the trough will stay put over the lakes. The euro on the other hand has the EPO going neutral and then positive towards the end of june which I believe is the correct solution along with the idea that the trough will be further east.
EPO goes positive and look where the trough goes:
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Trough east, ridge builds and will push out trough later in the period (around the 29th but not the 3rd which the GFS is saying):
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The trough position further east will allow a stronger ridge to build into the rockies which will push the trough out of the lakes sooner. The typhoon that is recurving near Japan would argue that the trough will be stronger from the 23rd to the 28th so watch out for that. I believe the models are underdone right now and i think a significant cool shot will occur. to sum up the trough will be stronger but will move out around the 29th and not the 3rd or beyond.:
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Taking a look at the MJO beyond this period we can see the warmth begins to really build around the 5th because we get into a really warm phase(weak three and then into the origin) and a heat wave begins to sustain itself for about 7-10 days. I believe this will occur form the 4th of july onwards.
Heres the MJO which stays in a cool phase until about the beginning of july. We won't see to much heat build until about the 4th or 5th because of the previous trough that left cooling. Beyond this point I believe the SOI values will continue to plummet and more frequent cool shots will be coming. I believe this will be the largest heatwave of the year and possibly the only one. We may have a chance at 3-6 day periods of heat but nothing like we saw last year.
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