Sunday, December 30, 2012

New Year Rain

Today has been a beautiful day with a high temperature of 38. A surface high pressure has settled in the area underneath upper level ridging. Aloft, the ridge is rapidly breaking down and moving east because of a very deep positively tilted trough over Southern California. This trough has very impressive winds downstream which will be intensifying to upwards of 160 knots into the week as it traverses east. These strong winds and significant positive vorticity advection at 500 mb indicates substantial upper level support for areas downstream of this trough, which is currently the central and southern Great Plains. The timing for precipitation for our area is looking most promising Tuesday morning, which corresponds to the highest 700mb vertical velocity values over the Ohio River valley. Because of the stated dynamics aloft, the surface high will quickly track east overnight, producing a return flow over the area for tomorrow, bringing Gulf moisture and slightly warmer air into our area. A majority of the moisture will reside to the south and west of our region, since our area is near the upper extent of advection as seen below. This will lead to lower precipitation values for the event.

12Z (6 AM) 1/1/13 Tuesday NAM 850 mb dewpoints (deg. C), winds (knots), and heights. Notice the high pressure in the Atlantic east of Florida pumping in Gulf moisture across the gulf coastal states.

For Tuesday, there are two surface features coming into play. There is a weakening cold front from the NW crossing our area, and there is also a weak surface low pressure system that will stay along the Louisiana gulf coast. This gulf low will help to pump additional moisture into the southeast and north into our area, where the cold front will act as the lifting mechanism. The timing of the front will be around noon. Expect slight warm air advection associated with the return flow Monday afternoon into Tuesday morning. The freezing line will be slightly to our north, but with the presence of warm air advection, any precipitation should fall as rain along the frontal boundary.
NCEP GFS mean sea-level pressure, 6 hour precipitation (inches), 1000-500mb thickness at noon Tuesday 1/1/13. 

Notice the GFS output above indicating most precipitation associated with the frontal passage and gulf low to the south and west of our area with the 540 thickness line (a rough indicator for the rain/snow boundary) to the north and west of our area. Behind the front into Wednesday morning, expect skies to clear and temperatures to drop. Wednesday should be nice, but cool with a surface front entering the area from the northwest on Thursday. This feature will need further monitoring, but is not showing significant strength or availability of moisture.

Days at a Glance:

Monday (New Years Eve):  High: 41  Low: 27  Precip: 30% chance of rain  Wind: S 5-9 mph

Tuesday (New Years Day): High: 36  Low: 29 Precip: 80% chance of rain  Wind: S 7-10 mph switching to NW 8-10 mph late. Precipitation will range .20-.40 in total for Monday night into Tuesday depending on the amount of moisture that makes it to our area and strength of the weakening cold front.

Wednesday: High: 33  Low: 21  Precip: 0% chance  Wind: NW 5-8 mph

Forecaster: Austin Boys

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