Friday, September 23, 2016

Another hot weekend, then FALL arrives!

Tired of the heat? Yeah, me too.  If you can hold out another couple days, I have great news!

Despite fall officially starting on Thursday morning (you didn't try balancing an egg on end at 9:21am did you?), summer weather continues for a few more days. Thursday's high of 95° came within a degree of the record and we'll threaten the record high of 96° again today, as well as the 95° records both Saturday and Sunday. High pressure ridging aloft more typical of mid-summer is to blame. I'm over it...

Mid-afternoon temperatures on Friday are well above normal for many areas east of the Rockies under the dominance of high pressure aloft. (
But by Monday, we'll start to see the major pattern shift as a massive upper-level trough over the western U.S. this weekend, upstream from our large ridge of high pressure, begins shifting east. It'll flatten out some as the main low pressure area goes by well to our north. However, it will be enough to push a cold front slowly south through the region Monday and Tuesday, resulting in a chance of showers and a "seepage" of more autumn-like air into the Mid-South.

The European model shows the upper level pattern (500 mb/18,000 feet) on Saturday morning dominated by a large trough of low pressure in the west and huge ridge of strong high pressure in the east, centered over the Mid-South. Weather systems moving across the U.S. roughly follow the black line. (

According to the European model (and GFS, not shown), as the low in the image above lifts northeast into southeast Canada by Tuesday morning, the eastern U.S. ridge flattens, allowing cooler air to drop south. Meanwhile, a ridge builds back to the west. As it shifts east later next week, temperatures will warm back up, but likely not to the levels we see this weekend. (
The rain chances with this front, despite there being a fairly large temperature gradient across it, will be fairly low thanks to most of the moisture remaining to our west in the southern plains and the dynamics associated with low pressure heading well to our north. Severe weather is not on the docket. The best rain chances will be Monday into Monday evening (and even then only about 30%). We could use some rain, so the little we might get will likely just set the stage for continued dryness next week as cooler high pressure builds in.

The Short-Range Ensemble Forecast (SREF) model data shows probabilities, including this map of probabilities of 0.10" of rain through Monday afternoon. The best rain chances over the next few days occur to our west. The SREF indicates only about a 30% chance we'll see 0.10" of rain with Monday's front. (WeatherBell)
Behind the front, we'll see temperatures more akin to "normal" for late September - cool mornings in the 50s (especially outside the city) and highs in the upper 70s to near 80. Mid-range model data has some disagreement in the strength of the cool air though. If the American GFS model wins out, outlying areas COULD see some lows in the upper 40s by mid-week. For now, that seem a bit of a stretch.

The GFS Ensemble Forecast System (GEFS) consists of multiple iterations of the American GFS model. Above, the average high and low temperatures from the GEFS members show a very welcome shift from summerlike temperatures to fall early next week. Note that its European counterpart (not shown) is similar but not as cool especially for morning lows, next week. (WeatherBell)
In the long-term, it appears this current streak of 90s may be our last for this year, but above normal temperatures are expected to return by early October.  The good news is that "above normal" for that time of year puts us in the 80s and humidity values are expected to remain tolerable. Here's to the arrival of fall next week!

From NOAA's Climate Prediction Center, there is a high probability that temperatures return to "above average" for the first week in October for most of the country east of the Rockies. (NOAA/CPC)

Erik Proseus
MWN Meteorologist

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