NEW AFD FROM NWS BMX
.SHORT TERM...
/Updated at 0410 AM CDT Wed Mar 17 2021/
Through Thursday.
A back door front has pushed westward to near the I-65 corridor
early this morning. Surface dewpoints were in the 50s across east
Alabama with lower to middle 60s across west Alabama. An east to
west band of showers that was located just south of I-20 has been
pushing slowly northward and this trend will continue this morning.
The better rain chances through mid-morning will be north of I-20 as
the boundary pushes northward. Since the activity will be moving
into a more stable air mass, thunderstorms will be limited. The
exception will be across northwest counties where the warm front was
located and surface based CAPE was already near 500 J/kg. Despite
clouds and fog expected through mid morning, forecast surface based
CAPE from the RAP shows values reaching 1500-2000 J/kg by 14z across
the southwest counties. Models show a considerable amount of
convection developing this morning in the warm sector (areas mainly
west of I-65 and south of I-20), so an isolated severe storm
possible. Southerly flow begins to kick in by mid morning across the
southern counties and across a majority of the northern counties
this afternoon. Hi-res models indicate surface based CAPE values
will increase to 2500-3500 J/kg this afternoon across areas south of
line from Marion County to Lee County. Latest HRRR model runs are
showing higher surface based CAPE and more discrete cells during the
afternoon. Potential is increasing for the development of rotating
supercells this afternoon. STP and EHI severe parameters are high
enough for strong tornadoes. Earlier model output indicated a
possible lull in the activity between the diurnal storms and the
overnight QLCS, but this scenario is looking less likely as the QLCS
may be arriving into west Alabama earlier. Even though instability
will be less when the QLCS moves through Central Alabama, increasing
0-6km bulk shear and low level wind profiles will likely overcome
decreasing CAPE. Better overlap of shear and instability will be
across west Alabama, and therefore this area has been elevated to a
high risk for severe storms. Even though the forecast risk outlook
is less across east Alabama overnight, the potential will remain
high for severe storms including strong long track tornadoes. The
QLCS will be exiting the southeast counties around sunrise Thursday,
and the rain should be out of east Alabama by mid morning.
A tight pressure gradient on the back side of the exiting surface
low will produce brisk west to southwest winds 10 to 20 mph on
Thursday. Strong cold air advection will cause temperatures to fall
across areas west of I-65 during the day Thursday, with many areas
west of I-65 experiencing highs during the early morning hours.
Areas east of I-65 and south of I-20 will get more hours of sunshine
than other areas, but wrap around low clouds will quickly advect
eastward and overspread most of central Alabama by sunset.
58/rose
.SHORT TERM...
/Updated at 0410 AM CDT Wed Mar 17 2021/
Through Thursday.
A back door front has pushed westward to near the I-65 corridor
early this morning. Surface dewpoints were in the 50s across east
Alabama with lower to middle 60s across west Alabama. An east to
west band of showers that was located just south of I-20 has been
pushing slowly northward and this trend will continue this morning.
The better rain chances through mid-morning will be north of I-20 as
the boundary pushes northward. Since the activity will be moving
into a more stable air mass, thunderstorms will be limited. The
exception will be across northwest counties where the warm front was
located and surface based CAPE was already near 500 J/kg. Despite
clouds and fog expected through mid morning, forecast surface based
CAPE from the RAP shows values reaching 1500-2000 J/kg by 14z across
the southwest counties. Models show a considerable amount of
convection developing this morning in the warm sector (areas mainly
west of I-65 and south of I-20), so an isolated severe storm
possible. Southerly flow begins to kick in by mid morning across the
southern counties and across a majority of the northern counties
this afternoon. Hi-res models indicate surface based CAPE values
will increase to 2500-3500 J/kg this afternoon across areas south of
line from Marion County to Lee County. Latest HRRR model runs are
showing higher surface based CAPE and more discrete cells during the
afternoon. Potential is increasing for the development of rotating
supercells this afternoon. STP and EHI severe parameters are high
enough for strong tornadoes. Earlier model output indicated a
possible lull in the activity between the diurnal storms and the
overnight QLCS, but this scenario is looking less likely as the QLCS
may be arriving into west Alabama earlier. Even though instability
will be less when the QLCS moves through Central Alabama, increasing
0-6km bulk shear and low level wind profiles will likely overcome
decreasing CAPE. Better overlap of shear and instability will be
across west Alabama, and therefore this area has been elevated to a
high risk for severe storms. Even though the forecast risk outlook
is less across east Alabama overnight, the potential will remain
high for severe storms including strong long track tornadoes. The
QLCS will be exiting the southeast counties around sunrise Thursday,
and the rain should be out of east Alabama by mid morning.
A tight pressure gradient on the back side of the exiting surface
low will produce brisk west to southwest winds 10 to 20 mph on
Thursday. Strong cold air advection will cause temperatures to fall
across areas west of I-65 during the day Thursday, with many areas
west of I-65 experiencing highs during the early morning hours.
Areas east of I-65 and south of I-20 will get more hours of sunshine
than other areas, but wrap around low clouds will quickly advect
eastward and overspread most of central Alabama by sunset.
58/rose
