Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Huntsville Alabama
529 PM CST Thursday Dec 12 2013
for 00z tafs
/issued 237 PM CST Thursday Dec 12 2013/
a high pressure ridge extended from Ohio southwestward through eastern Texas. This was
producing a north-northeasterly flow in our area with cold and very dry air
advecting into the region. Despite full sunshine, temperatures are about
5-10 degrees colder than this time yesterday. Dew points were
plummeting into the single digits and lower teens. Another cold
night is ahead as the high moves east directly over the Tennessee Valley.
Lows should again drop into the l-m20s in most valleys, and perhaps a
bit colder in more sheltered valleys.
The upper level low over socal is still prognosticated to eject rapidly eastward
followed closely behind by the system dropping southeastward toward the northern
California coast. The initial wave will quickly produce S-southwesterly flow aloft and
increase thermal and moisture advection fields significantly on
Friday into Friday night. Our low level airmass will remain very dry
as low level east-southeasterly flow prevents a significant moisture tap. With
the initial wave of strong 8-7h warm air/moisture advection during
the afternoon, virga and perhaps some light precipitation will likely move
into northwestern Alabama during the late afternoon. Forecast wetbulb temperature soundings
from the NAM and GFS indicate that precipitation that does not sublimate may
fall in the form of sprinkles or very light sleet during the late
afternoon or early evening hours before surface-800 mb warm/moist advection can
change precipitation to all rain. Light precipitation intensity and surface air temperatures
well above freezing will prevent any accumulation and no travel
problems are expected.
With time, a S-southwesterly 40-60kt 800 mb low level jet core will induce an elevated mesoscale convective system
with embedded thunderstorms along the lower MS valley which will
rapidly track eastward through northern Alabama and southern Tennessee from 09-15z. The NAM even
suggests some possible wake low development despite a decent
inversion layer, so will have watch for gusty winds behind the
initial pressure wave associated with the convection. We have
included gusty winds up to 40 miles per hour in the latest severe weather potential statement. The good news is
that the rapid progression of this system will end or largely
diminish rainfall during the middle-late morning hours of Saturday.
Some stratiform light rainfall could linger through the day, and while
the cold front arrives during the early evening hours. There is a
limited risk of a few flurries early Sunday morning, but will leave
out due to low confidence in this occurring given a shallow moisture
and cloud layer temperatures of -5c or greater.
Few changes were made to the extended forecast. Medium range forecasts
continue to indicate west-northwesterly flow aloft. A modified cp airmass arrives
on late Tuesday into Wed, followed by a slow warming trend late in the
week as flow becomes more zonal with dry weather foreseen through day 7.
for 00z tafs...VFR conditions will persist with high clouds gradually
increasing through the period. Light/variable winds will shift to become
more southerly by 12z tomorrow and remain below 10kts through the
evening hours. Winds/llws/precip may increase right at the end of the
period as our next weather system approaches from the west...with the
potential for some sleet mixed with light rain as precipitation begins.
Expect precipitation to hold off until after 00z so will not include this in
the tafs just yet.
For more information please visit our website