Nice end to summer.. unsettled start to fall

It was a beautiful end to summer over southern MB with sunny skies, balmy southeast breezes and temperatures in the 25C range. Fall officially arrives Monday morning at 10:44 am, and will be greeted with some unsettled weather in the form of showers and possible thunderstorms. The unsettled weather will come in two waves. The first wave of precipitation is expected overnight as a band of showers and scattered thunderstorms over North Dakota moves northward into the Red River valley. These showers may bring 5 to 15 mm of rain to Winnipeg overnight, although heavier amounts of 25-35 mm are possible in thunderstorm activity over portions of southern MB by early Monday morning. There should be a break in the shower activity through midday Monday which will allow temperatures to warm into the 20s along with dewpoints in the midteens. This will set the stage for another round of showers and thunderstorms to develop by late afternoon over southern MB ahead of a cold front. This line of activity may be accompanied by severe thunderstorms late Monday into Monday evening with heavy downpours (giving local rainfall amounts of 30-50 mm), strong winds, and possibly large hail. There may even be the threat of isolated tornadoes with some supercell storms early in the evening. Stay tuned as Mother Nature may be ushering in fall on a stormy note.

Late summer weather before fall arrives Monday

Some nice weather is on tap for Southern MB over the next couple of days as Mother Nature gives us some late summer weather before the official start of fall on Monday. Thursday will see brisk southerly winds of 40 to 60 km/h developing in the Red River valley as a warm front moves through southern MB. Afternoon temperatures will climb into the 24-27C range along with plenty of sunshine. Friday will see a weak cool front push through from the Interlake, but temperatures should remain above normal Friday with highs of 25C or so before cooling off in the afternoon or evening as northerly winds develop. Saturday will likely see cooler temperatures as we get northeast outflow winds from a cool high pressure area over northern MB. Current advertised highs of 25C are likely over-optimistic for Saturday, with highs of 17-20c more likely in the cooler airmass. There could also be considerable cloudiness around along with areas of dense fog as that frontal boundary remains stalled over southern MB separating warm air to our south from cooler air to the north. (in fact, Saturday's weather may be much like today's.. overcast and cool) By Sunday we should be getting back into south or southeast winds which would boost temperatures back into the low to mid 20s (current advertised high of 27C still likely about 5c too high for Winnipeg and areas to the north and east) So although the bulk of the weekend is expected to remain dry, I don't think it's going to be as warm or as sunny as current forecasts indicate.

Fall officially arrives at 10:44 am Monday morning but current models are suggesting we'll be staying with above normal temperatures in the low to mid 20s through Tuesday before more fall-like temperatures return by mid to late week.

Ike damage

Hurricane Ike has brought incredible devastation on the Bolivar Peninsula just east of Galveston along the Gulf Of Mexico. This area was the hardest hit being on the east side of Ike's eyewall. Communities of Crystal Beach and Gilchrist have been wiped off the map by Ike's winds and storm surge which likely topped 15 feet over this unprotected peninsula.The image on the left shows a single home standing in what used to be the beachside town of Gilchrist. Satellite images show the extensive damage brought to the Texas Gulf coast, including the destruction of Crystal Beach.

Lake effect season is here

This morning is providing a good example of a common phenomenon in southern MB in the fall - lake effect precipitation. Lake effect precipitation develops when you have cool air aloft moving over the relatively warm lake waters, creating narrow bands of instability precipitation to the lee of the lakes. You need at least a 13C difference between the lake water and the temperature at 850 mb, and winds need to be well aligned in the lowest 10000 feet (i.e. little directional shear) to form these narrow bands. This morning we have 850 temperatures of +2C over 18C lake waters, along with low level winds aligned in a 330-340 degree orientation giving lake precipitation to the southeast of Lakes Manitoba and Winnipeg. Typically these lake effect bands form in the cool air behind a cyclonic storm system and will dissipate as subsidence moves in with an approaching ridge of high pressure.

Early in the season, these precipitation bands are in the form of rainshowers. As we get further into the fall with colder temperatures aloft (i.e. 850 temps of -5C or colder), these lake effect bands will produce narrow swaths of snow to the lee of the lakes.. sometimes quite heavy. See an example from October 11, 2006 for an early season lake effect snow event over southern MB.. notice that the lake effect bands from that case are oriented virtually identical to those this morning (except it's rain today, not snow!) Lake effect bands off Lake Manitoba commonly set up between Portage and Elie, but can come into Winnipeg if the low level flow is in the right direction, from around 300 degrees, as seen in this example from last September.

Some rain on the way

A weak low pressure system over North Dakota will be tracking slowly east over the next 24 hours, bringing an area of rain to southern MB. The leading edge of the rain shield is just spreading up to the MB border from the south as of midday, and should continue edging north this afternoon likely reaching Winnipeg by mid to late afternoon. Rain amounts are expected to be in the 5 to 15 mm range, most of it falling south of Winnipeg closer to the international border. Here in Winnipeg, look for about 5 mm from late afternoon through tonight. Cool unsettled weather is expected behind this system for Friday through the weekend.