Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Huntsville Alabama
746 am CST Monday Dec 22 2014
to add patchy drizzle this morning.
widespread low ceilings is producing patchy drizzle across the Tennessee Valley.
Visibilities have dropped to at or below 3sm in some locations due to dz and br.
The rest of the forecast remains valid with no further changes
expected. Shower activity will increase in advance of the next in a
series of shortwaves. Radar mosaic indicates rain showers in eastern MS and southwestern
Alabama which may move our way by midday into early afternoon.
/issued 637 am CST Monday Dec 22 2014/
for 12z tafs...poor aviation weather conditions will persist through the
duration of this taf cycle...with low MVFR/IFR ceilings expected to
remain intact. Coverage of -dz will continue to expand through the
morning and have placed this as prevailing weather conditions for both terminals
by 22/15z...with a larger area of rain expected to shift northeastward and
impact the airports beginning around 18z. Despite this...prevailing
ceilings should still manage to stay at or slightly above 1000 feet with
only minor visibility reductions 6sm -- given strength of southeast winds. The
area of rain will likely shift northeastward by 23/01z...with IFR ceilings
developing in its wake. A separate round of scattered rain showers/thunderstorms and rain will
develop along an eastward moving cold front...and impact mean sea level by 06z and
hsv by 09z.
/issued 529 am CST Monday Dec 22 2014/
Current water vapor imagery shows an upper level low across the
northern plains...with upper level ridging just off the southeast
coast. Winds have begun to veer more towards the south...and with
extensive cloud cover...temperatures have risen by a degree or two
over the last couple of hours. This trend will continue as warm air advection ramps
up in advance of the next system that will move through the forecast
area. A weak upper level shortwave is expected to round the base of
the broad upper level trough...which is currently situated over much
of the central Continental U.S.. as this shortwave moves northeast over the
area...scattered light rain will develop...with coverage increasing
during the afternoon and evening hours.
The main upper level trough will continue to amplify and dig southward
into the arklatex by 06z on Tuesday...as a strong upper ridge begins
to push east onto the West Coast. A cold front will move into the
Mississippi Valley by daybreak on Tuesday...before stalling just to
our west. Southerly flow will continue to push in deep Gulf
moisture...along with the warmer temperatures. This will lead to the
development of showers and thunderstorms late Monday night...continuing
The upper level trough will continue to deepen on Tuesday...with a
strong cut off low developing over Oklahoma. Meanwhile...surface
cyclogenesis will occur along the Louisiana Gulf Coast along the
stalled surface cold front. This low will move to the northeast
overnight Tuesday...almost parallel to the Interstate 59
corridor...with a warm front that is currently along the Gulf
Coast...pushing to the north as well. As the low races north
Wednesday morning...the surface cold front will exit to the
east...with much of the precipitation coming to an end Wednesday
The severe threat still appears to be very minimal across the
forecast area. Although extensive cloud cover could help keep the
instability low across the area...the strong warm air advection may be enough to
offset the cloud cover and precipitation. Fortunately...current
guidance suggests that the warm front will not quite make it this far
north...before the low exits the area. Middle level lapse rates will be
decent...thus the continued mention of thunderstorms...and there
could be the potential for small hail and subsevere wind gusts. The
bigger impact may be the amount of rainfall expected. Storm total
amounts could reach above 2 inches in localized areas...though this
looks to occur over the course of about 24 hours. However...with
moistening ground conditions ahead of the convection Tuesday
evening...there is the potential for streams to reach bankfull by
Wednesday morning. However...the flooding threat appears to be too
low to mention anything in the severe weather potential statement at this point.
The upper level trough will finally move across the forecast area
Wednesday evening through Thursday morning...with lingering moisture
enough to warrant scattered light showers across our northeastern
counties. The entire system looks to be slightly slower than
previous guidance suggested...with high temperatures reaching into
the upper 50s Wednesday afternoon. Cloud cover will remain in place
across the area...therefore overnight lows do not look to drop as
much as previously forecast. Forecast soundings suggest that much of
the moisture occurs while temperatures and dewpoints are above freezing.
Therefore...took out mention of any frozen precipitation.
Christmas day will be pleasant across the region...as high pressure
builds briefly into the area. Did not make too many changes to the
extended...given the general disagreement among guidance.
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