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Area forecast discussion 
National Weather Service Birmingham Alabama
1208 am CDT Sat Apr 25 2015

Update...
aviation discussion.

&&

Discussion...

Looking at the infrared satellite imagery and lightning display...the
convective activity across the arklatex is holding together as it
races to the east. Dewpoints in the arklatex are well into the
upper 60s and low 70s. Closer to home...surface dewpoints are
lagging a good bit with dewpoints generally in the 50s with a few
low 60s showing up across the southern counties. The forecast for
the remainder of the night still remains rather challenging. The
hi-res models continue to show the mesoscale convective system holding together as it
moves into western Alabama around 3-4 am. The main question now
is when will the warm front move far enough north for convection
to become surface based. These same hi-res models do not indicate
SBCAPE moving into southwestern portions of the area until around
12z. By this time...expect the mesoscale convective system will be weakening as it
approaches the I-65 corridor. Therefore...expect the main threat
will be a few damaging wind gusts across the west after 3am. Model
soundings indicate that middle-level lapse rates will be around 7
c/km and should be sufficient for marginally severe sized hail as
the storms move into West Alabama. If these storms can become
surface based before racing to the east...there will be the
potential for weak brief isolated tornados but confidence in this
is rather low.

For the update...increased probability of precipitation for tomorrow with the weakening
mesoscale convective system still somewhere near the I-65 corridor around 12z. Expect
elevated probability of precipitation for 12-15z timeframe across the eastern 2/3rd's of
the state. The question is still whether there will be re-
development in the afternoon. All the severe weather parameters
are in support of severe weather but the question becomes will
there be enough lift and moisture. Models indicate that precipitable water values
go from around 1.7-1.8 around 12z to 1.2-1.3 inches by 21z.
Therefore...there is still high uncertainty in re-development
tomorrow afternoon. The highest likelihood of severe storms
tomorrow still remains across the east and northeast.

05/Massachusetts

&&

Aviation...
06z taf discussion.

Have timed out approaching clusters of thunderstorms across northern
terminals...affecting those sites from west to east beginning at
06z. At mgm and toi...do not expect this first round of convection
to extend far enough southward...but MVFR ceilings are expected to
spread across the area by 09z. This first wave of convection is
expected to continue through the early morning hours...which a large
amount of certainty concerning redevelopment during the late morning
and afternoon. Storms early this morning will be strong to severe
with damaging winds and heavy rainfall. The possibility of
redevelopment exists areawide late Saturday morning through
afternoon and this activity will likely be severe.

14

&&

Previous discussion...
/issued 309 PM CDT Friday Apr 24 2015/

The forecast through Saturday evening is not an easy one. The
evolution of storms near the arklatex will have an impact on what
occurs downstream across our area early Saturday morning through
Saturday evening. The first feature of interest is the warm front
that will surge northward tonight while an mesoscale convective system organizes to our
west. Models indicate rich Gulf moisture moving in from the
southwest after 06z with precipitable water values ranging from 1.7-1.9 inches
by 12z. This airmass will be characterized by dewpoints in the
upper 60s to lower 70s...so there is a concern that storms
associated with the overnight activity may encounter surface-based
cape as they move into our west/northwest counties in the 3-6 am
timeframe. Shear values will be more than sufficient for a
damaging winds...and perhaps a tornado threat as instability
becomes surface-based.

The severe weather threat during the late morning into the
afternoon is very uncertain...but 500mb height falls and the
approach of the right entrance region of a jet streak at 300 mb...
may support additional thunderstorm development along boundaries
remaining in the wake of morning convection. This will only be the
case if enough surface-850 mb moisture remains in place. Perhaps the
best chance of additional development will be along and east of
I-59 where models indicate the highest precipitable water values for the
afternoon. 850 mb flow will become more westerly with time...and
capping could occur. Local breaks in the cap would allow for the
rapid development of storms with 700-500 mb lapse rates of 7 c/km.
With 0-6 km shear of 60 knots... these storms would quickly become
supercells with a potential for large hail and damaging winds. The
tornado threat will be modulated by nearly unidirectional flow...
but if storms continue into early evening hours...a strengthening
low-level jet could increase the tornado potential. Again...this
whole scenario is conditional and confidence is very low. It is
entirely possible that an early morning mesoscale convective system could delay or put an
end to any threat for the afternoon.

Conditions should quiet down Saturday night into Sunday as winds
shift to the west and northwest with a frontal passage. The dry
conditions will continue into Monday and part of Tuesday as the
area remains between weather systems. A slow-moving trough will
begin to move toward the region on Tuesday into Wednesday...but
this system is likely to remain suppressed by a large upper-level
trough near the East Coast. Rain chances are expected to increase
by Tuesday night into early Wednesday as the best upper-level
support approaches and an inverted surface trough moves across the
area. Dry weather should return as the the system passes by.

87/Grantham

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Bmx watches/warnings/advisories...
none.
&&

$$

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