Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Huntsville Alabama
1228 PM CDT Sat Mar 15 2014
for 18z tafs
/issued 1011 am CDT Sat Mar 15 2014/
The forecast impacts for today mainly comprise a transition from warm
and dry weather associated with a surface high pressure system, to
increasing moisture with an approaching middle-level shortwave trough
currently over Texas. The approaching shortwave trough is spreading high
cs across the southeast which could mitigate daytime highs today
despite antecedent warm air advection in place. Forecast soundings indicate an
inverted v dewpoint/temperature environment between the surface and 850 mb
and a noticeable cap at 750 mb. In addition, rap/NAM model output
indicate a subtle 400-750 mb shear axis moving northward from the
Gulf Coast. This may bring just enough lift to overcome the cap over
the SW portions of the forecast area. Though lapse rates are weak,
saturation in the ice nucleation zones, weak instability, and enough
lift to overcome the cap could mean isolated pulse-like thunderstorms
this afternoon. So, have retained the mention of thunderstorms in the
forecast mainly for northwest Alabama and Cullman County this afternoon. Did
adjust the timing of thunderstorms to late afternoon (21-00z) with
peak daytime heating and timing of shear axis crossing the Tennessee Valley.
The primary threat is gusty winds up to 30 miles per hour. More substantial
lift (warm front and middle-level pva) is expected later this evening
into tonight as the wave moves across the MS River Valley into the
Minor adjustments were made to the daytime high temperatures to
account for low clouds (3.5-5.0 kft) that have developed over Tennessee.
Also, with low clouds building north from central MS and high clouds
expected to prevail across the region today, have increased cloud
cover this afternoon/evening.
Otherwise, the rest of the forecast looks fine.
for 18z tafs...
VFR conditions should prevail for the next 18 hours with changes in
category possible due to thunderstorms and rain development as a disturbance move
across the southeast. Due to initial isolate nature of thunderstorms and rain between 01-05z,
have included thunderstorms in the vicinity during that time period. Widespread rain showers and
stronger thunderstorms and rain activity is possible as a warm front lifts northward
between 05-12z and have included a tempo group between 08-12z to account
for a drop in category to MVFR. MVFR conditions and increased
southerly winds (12g22kts) should then prevail after 12z.
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