Sept. 20 Technical Forecast Discussion

Technical Forecast Discussion

Synopsis:

Clouds and rain showers will hang around as a broad area of low pressure over the upper plains cuts itself off from the main jet stream and slowly drifts southeastward. As it does so, it will continue to pull moisture in from Gulf of Mexico and Atlantic into our region. Thus, expect mostly cloudy skies and rainy conditions to continue throughout the whole week and into the weekend. The temperatures will hover around seasonal values, possibly falling a little toward the weekend as the cold front associated with the cutoff low finally sweeps through the Northeast.

Short-term (Tuesday night-Wednesday evening):

As of Tuesday evening, a strong low pressure system is currently spinning over Upper Midwest in response to a strong dip in the jet-stream. That low pressure system is forecast to lift northeastward into Ontario. However, the main focus will be another surge of 500-mb positive vorticity dropping down from central Canada later Wednesday night. As result, an upper level low will develop over central U.S.  Ahead of this new upper-level low, moisture will already be streaming across the region due to a weak low pressure system coming from the Tennessee Valley. Expect showers starting to fall late Wednesday morning and continue throughout the day. If there is sufficient surface heating during the day, there will be enough instability in the atmosphere for thunderstorm developments. The CAPE is forecast to be less than 500 J/kg on Wednesday and upper-level shear is not expected to be strong enough for widespread severe weather. Anything that develops should also stay to our east closer to where the cold front associated with the first low is located.

Mid-term (Thursday-Friday night)

As the new upper-level low approaches, on and off showers will continue on Thursday into Friday night. Most of the showers will stay on the lighter side. The showers may also exhibit diurnal characteristics as daytime surface heating will enhance atmospheric instability. The daytime highs will hover around mid- 60s to low-70s for Thursday and Friday. The nights won't be as cold as last week as clouds and rain hang around.

Long-term (Saturday-Sunday)

A temporary break from the showers looks to be possible for Saturday as the upper level low finally drifts to our west. However, the clouds will linger around for the weekend. More rains looks to fall early next week as the upper-level low spins a new surface low up the east coast. The temperatures will also fall slightly as the colder air finally works its way into the Northeast.