Thursday, January 3, 2013

Slight Chance of Wintery Precipitation

Currently, we are underneath the left entrance region of a downstream progressing jet streak, which implies convergence aloft and divergence at the surface.  Hence, a surface high pressure is nosing into our region producing westerly and then northerly winds to the south, keeping the Gulf moisture to the south of our area. This is partially why the cold front to our NW is weakening and will remain dry, but will provide slightly cooler air for Friday as it passes the area. A stationary front, marking the moisture boundary, is draped across the coastal southeast showing a broad swath of precipitation from coastal Carolina to southeast Louisiana. This is underneath the right entrance region of the jet streak, which implies divergence aloft. The jet streak will help to lift out the positively tilted trough that has been present throughout the past week. In the process, it will form a cutoff low over the mountain west. Over the Rocky Mountains, there is a split flow to the jet stream, which is converging over the lower Ohio River valley. This will create a more zonal flow until mid to late Saturday when the earlier stated cutoff low will become reabsorbed in the flow, creating divergence over the Ohio River valley. This divergence will be enhanced by the coupling of two jet streaks Saturday afternoon into the evening seen below.
21Z (3 PM) Saturday NAM 250 mb Heights and Wind (kt) over 50 knots. Notice the coupled jet streaks over the central Appalachians and the New England Atlantic coast.

At the surface, the high pressure will move east and quickly break down as a surface low sweeps across the lower Great Lakes, with an expected cold frontal passage around 6 AM Sunday. On Saturday, expect a slight warming trend ahead of the front. This is looking to be the main focus for precipitation for the forecast period, supported by the strongest 700 mb vertical velocity returns. Even so, moisture advection will be very slight and brief before the frontal passage. The significant nose of moisture is staying just south of south-central Kentucky shown below.
00Z (6 PM Saturdy) Sunday GFS 850 mb Mixing Ratio and Wind (vector). Notice the broad, but weak advection of moisture with the major nose only extending into northern Alabama.

The freezing line will be bisecting and to the south of the area, so any precipitation that falls could be mixed or in the form of snow. With the lack of moisture, I will keep the forecast dry. After the cold frontal passage, ridging aloft will take over and a surface high pressure will settle into the area giving way to a warming trend into the next week.

Forecast at a Glance:
Today: High: 41  Low: 21  Precip: 0%  Wind: NW to N at 4-7 mph
Friday: High: 39  Low: 22  Precip: 0%  Wind: NW to W at 6-8 mph
Saturday: High: 45 Low: 28 Precip: 20% chance of mixed Wind: SW at 5-10 mph
Sunday: High: 38  Low: 23 Precip: 30% chance of mixed/snow  Wind: SW at 5-10 mph switching to NW at 10-13 mph then to 4-7 mph late.

Forecaster: Austin Boys

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