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10-Day Outlook: Summer vs Fall -- Redux

Josh Larson @ 11:00 PM

Today's Weather

A strong cold front will roll through the region today sparking off very widely scattered showers during the first half of the day; chance/coverage of measurable rain <20%. Expect clouds giving way to some late day clearing with breezy conditions. Highs will in the mid 70s. Clearing and quite cold tonight with lows ranging from near 40 in the coolest suburbs (perhaps even 38 or 39 in a spot or two!) to near 50 downtown.

Pattern Overview/Watch the Tropics!

A highly changeable atmospheric pattern aloft over the next 3-5 days with a strong trough building overhead today into Friday (with temperatures, especially nighttime lows, below normal), and then a ridge returning by this weekend, which will build and expand by early next week. This ridge will be highly amplified, covering most of the eastern United States (see image at right), and will bring a return to above-normal temperatures next week along with, more importantly, the highest risk this entire season for tropical systems/hurricanes to target the East Coast; essentially, the pattern will be such that any system that develops in the southwest Atlantic is likely to head straight for either the Southeast or Mid-Atlantic coast.

The next 5 days (Sept 29 - Oct 3)
Forecast high/low: 76/54 (normal = 74/56)
Forecast precip: below normal

After Thursday's changeable weather, Friday will feature abundant sunshine along with cooler than normal temperatures and a pleasant breeze. Expect highs near 70 and another cold night with overnight lows ranging from the low 40s in coolest suburbs to the low 50s downtown. Saturday will also feature abundant sunshine, but with milder temperatures near 76/56. Continued mostly sunny on Sunday but a touch warmer and more mild, with temperatures rebounding to above-normal levels; near 80/58. Same deal on Monday, with mostly sunny skies and warm temperatures near 80/59.

The following 5 days (Oct 4-8)
Forecast high/low: 79/58 (normal = 72/53)
Forecast precip: normal??

A challenging forecast for the following five days with lower than usual confidence towards the end of the period. As mentioned above, a huge ridge of high pressure centered over the northwestern Atlantic will likely allow not only for above-average temperatures, but also for an increase the threat for landfalling tropical cyclones for the Mid-Atlantic. That notwithstanding, Tuesday and Wednesday of next week should feature sunny, dry weather with well above normal temperatures from 80-85 for highs and in the upper 50s to mid 60s for lows. The rest of the week is tricky, with some models indicating and inverted trough off the East Coast which may direct an area of low pressure (and attendant precipitation) westwards into our neck of the woods. I expect temperatures to return to highs in the mid to upper 70s with overnight lows in the 50s for Thursday through Saturday, but the Tropics must be watched....

Verification of Last Week's Outlook

Note, in general, that models, and meteorologists, have a particularly tough time during seasonal transitions -- the atmospheric "fight" between summer and fall has been going on for several weeks now, and still continues -- as the position of the jet stream is more changeable and the patterns can be more progressive (fast-moving) and amplified.

That being said, last week's 10-day outlook fared MUCH better than the previous week's bust; I'm relatively content that my forecast high temperature was off by an average of only 2.4 degrees, which nets me a solid B based on's grading scale. The biggest hit to my score was the warmer than expected temperatures Sunday through Tuesday of this week, which were generally 80-85 as opposed to my forecast near 75-80.

Pictured: Low water at Great Falls yesterday. By photographer Kevin Ambrose

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