Monday, October 13, 2014

Flash Flooding & Strong Winds Possible Wednesday

10/13 4pm
An upper level low spinning across the Missouri Valley is slowly crawling its way east-north-east.  This will be our big weather maker this week bringing the potential for heavy rains and some severe weather.

First things first, high pressure off the New England coast is keeping us in a bit of a wedge.  Warm, moist air overriding the cooler air at the surface will keep things cool with lots of cloud cover and a few showers around through the rest of the day into early tomorrow.  The wedge will finally break during the later part of the day Tuesday as a warm front lifts through the area.  This will allow warm, very moist air to work into the area in advance of the cold front.  The squall line currently over the Mississippi Valley will arrive during the day Wednesday bringing the potential for some severe weather and flash flooding.

Severe Threat
All of the cloud cover and showers out ahead of the main squall line will actually help in regards to decreasing our severe weather threat.  This will help limit the amount of energy available for thunderstorm development.  With that being said, this storm has impressive wind dynamics aloft.  The low level jet at 850 mb (about 5,000 feet up) will be cranking at around 40-50 knots (46-57mph) as the squall line passes through.  Any heavy thunderstorm that does develop will be capable of bringing some of these stronger winds all the way down to the surface.
850 mb (5,000 feet) winds
Flash Flooding Threat
An abundant amount of available moisture + a slowly moving system points towards flash flooding being our bigger threat. Dew-points in the low to mid 60s and precipitable water values climbing into the 1.5-2.0" range will both be unseasonably high for this time of year.  A very heavy 4-6 hour period of rain can be expected with the squall line, likely occurring during the middle part of the day Wednesday (although models have been waffling a bit on timing over the past few days).  Total rainfall will likely exceed 1.0-2.0", enough to cause some flash flooding concerns as most of that will fall in the 4-6 hour period mentioned above.
High Resolution Nam Wednesday Afternoon
With heavy rains and strong winds bringing down a lot of leaves, fall foliage will likely take a bit of a hit.  It might be a good idea to bring in or tie down any loose items outside and keep an eye on rising streams if you live in a flood zone.

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