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Area forecast discussion...updated 
National Weather Service Huntsville Alabama
956 am CST Wednesday Mar 4 2015

Update...
morning update.

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Discussion...
the warm front that moved through early this morning has sunk to the
south and even washed out a bit. The only indication of a boundary is
that we are in a more southwesterly flow versus southerly to our
south. The cold front as of 15z is through Nashville and still
sitting north of our Tennessee counties and stretching down through western
MS and it is not moving very fast. Temperatures behind the front are
into the middle 40s to lower 50s in northwest Tennessee. With the SW flow...we
probably will not warm up too much more...maybe just a degree or
two...with 15z temperatures still in the lower to upper 60s.

As for the update...did not make any changes to the overnight
forecast as guidance seems to be initializing well with the current
temperatures and front location. We will need to continue to monitor the
progression/timing of the cold air as well as when the precipitation gets
here. In terms of today/this afternoon...went with scattered showers
through noon as there is not much out there right now. There are some
showers along the front and are just barely getting into far northwest
Lauderdale County. Will continue the likely probability of precipitation after 18z but went
with isolated thunder wording instead of slight chance to convey how
isolated it will be. Kept thunder in because there is some decent
instability with the frontal passage along with good deep layer shear
and dew points in the lower 60s ahead of it. Cannot rule out a couple
storms producing locally gusty winds and brief moderate to heavy
rainfall with it. As well...if that front continues moving
slowly...some of these showers could train over the same area which
is happening along the front now. This could produce some ponding of
water in low lying areas.

Line

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Aviation...
/issued 545 am CST Wednesday Mar 4 2015/
periods of showers are forecast today. Increased instability could
bring an isolated T-storm or two...with the primary threat from gusty
winds. Much colder temperatures are expected this afternoon and evening...
as an Arctic cold front moves across the region...and temperatures fall
below freezing after midnight Wednesday night and before daybreak Thursday. This
will result in rain becoming a wintry mix of freezing rain and sleet.
South-southwest winds 10-20kt at the taf start will become west...then north-northwest after
the front passes...with some gusts to 25kt. Lighter winds at the
surface this morning and stronger winds aloft will be close to wind
shear criteria.

Rsb

&&

Previous discussion...
/issued 424 am CST Wednesday Mar 4 2015/

The previously advertised warm front has now moved north of the
forecast area...with many locations in the middle to upper 60s.
Meanwhile...a cold front extends from upstate New
York...southwestward into eastern Oklahoma. Showers...and a few
thunderstorms...were observed just along this front...moving to the
northwest across western Tennessee. This front will make its slow
progression southeast today...with model consensus moving the front
into the northwestern portion of Alabama by 18z...and then slowly
moving east of the area between 00z and 03z. Prior to the front
reaching the forecast area...forecast highs into the upper 60s...and
possible lower 70s are expected. Showers and isolated thunderstorms
will also be possible along the front.

As the front moves east over the next 12 to 18 hours...a southern
stream closed upper level low over Southern California will phase
with a northern stream trough. The resulting longwave trough will
slowly progress eastward...as the upper level jet stream
intensifies. This will provide an abundance of lift behind the
front...with widespread showers expected to develop by the evening.
Meanwhile...the shallow cold air behind the front will move in fairly
quickly across the forecast area...however with temperatures rising
to near 70 degrees prior to frontal passage...it will still be after
midnight before temperatures fall below freezing. As temperatures
fall...rain will transition to freezing rain from west to east
through 12z. Many locations across the east may not see frozen precipitation
until after 12z...however have kept a chance for freezing rain mixing
in with rain...to account for the colder air possibly reaching these
locations sooner than forecast. Models are coming in with better
agreement that the Post frontal precipitation will continue through at least
18z...with more quantitative precipitation forecast then previous runs. Therefore...increased probability of precipitation
during the 12z to 18z time frame...which puts the eastern half of the
County Warning Area in frozen precipitation longer. Therefore...also made an adjustment to
the ice accumulation grids. Confidence is a little on the low side
concerning the sleet and snow mixing in with freezing rain. Forecast
soundings across the board show the warm nose remaining in place
during the heaviest precipitation...with the temperature profile going below
freezing just as the drier air filters in. However kept the chance
for moderate sleet across the northwestern portion of the area and
left the sleet/snow amounts unchanged.

Sleet/snow accumulations of up to an inch are possible...mainly
across the northwestern 3 counties in Alabama and all of southern
middle Tennessee. Ice accumulations of one to three tenths of an
inch will also be possible. Given that we will be fairly warm prior
to the onset of frozen precipitation...most of the accumulations will be on
elevated surfaces. However...the majority of the frozen precipitation is
expected to occur just before...and during...morning commute
tomorrow...which may create hazardous travel conditions.
Additionally...winds will be in the 15 to 20 knots range while precipitation
is falling...which may lead to greater ice accumulations than
forecast. Given this uncertainty...have decided to include
Madison...Morgan...and Cullman counties to the watch...given that
quantitative precipitation forecast amounts may be higher than forecast in these locations after
12z.

Precipitation will taper off by 18z...with lingering light snow flurries
possible during the afternoon. Temperatures will not warm very much
during the day...and most locations will likely stay below freezing.
Temperatures will bottom out early Friday morning...with lows dropping into
the teens. This...combined with winds staying near 10 kts...will
create frigid conditions across the forecast area. The good news is
that the forecast area will then experience a warming trend during
the extended portion of the forecast period. Model guidance beyond
Friday is inconsistent...so kept a blend of the guidance...with the
chance for light precipitation returning to the area on Monday. Temperatures will
moderate to near normal for this time of year.

73

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Hun watches/warnings/advisories...
Alabama...Winter Storm Watch from late tonight through Thursday afternoon for
alz001>007-016.

Tennessee...Winter Storm Watch from late tonight through Thursday afternoon for
tnz076-096-097.

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