FXUS64 KBMX 171723
Area Forecast Discussion
National Weather Service Birmingham AL
1123 AM CST Sun Dec 17 2017
Midday Update and 18Z Aviation.
Light rain with a few pockets of moderate rain continues to
overspread the area as a weak surface boundary sets up across the
southern half of the state. The rain should gradually end from
west to east through the late afternoon and evening hours as the
boundary weakens and stalls to the south. It is likely that the
steady light rain will become more showery and drizzle through the
overnight as low clouds and some patchy dense fog develops.
Expect this pattern to continue until the next potent shortwave
ejects from the west early Wednesday.
Monday through Sunday:
Southwesterly anti-cyclonic flow will be in place on Monday around
deep layer ridging centered between Florida and Cuba. A split flow
pattern will remain in place with a southern cutoff low over the
Desert Southwest and northern stream troughing over Canada. A
precipitation-reinforced quasi-stationary front will extend from
the Louisiana Gulf Coast northeastward toward southeast Alabama.
The eastern portion of this boundary may drift southward during
the morning following passage of a weak meso-low. Precipitation
forecast remains challenging due to weak/broad forcing associated
with weak waves in the southwest flow, and how quickly a residual
cold pool/area of dry air erodes in the wake of today's activity.
Showers and thunderstorms will develop late tonight/early tomorrow
morning along the frontal boundary in the vicinity of the
Louisiana Gulf Coast in a broad area of upper-level lift
associated with the right entrance region of an upper-level jet
streak and lift to the east-northeast. Generally favor a non-GFS
solution with a further south position of the front due to outflow
from today's activity and highest rain chances closer to the Gulf
Coast with some activity bleeding over into our southern
counties. A separate area of mid-level frontogenesis and moisture
will located further north closer to the I-20 corridor, which
could allow for some rain to fall there as well if low-level dry
air can be overcome. So, kept a mention of scattered showers in as
far north as Birmingham with lower confidence. Weak elevated
instability will be in place across the southern counties which
could allow for a rumble or two of thunder, but any surface-based
instability and potential for any stronger storms should remain
south of the forecast area. Also not expecting any heavy rain
during this time. Temperatures will hinge on precipitation and
lingering cool air. Will indicate highs in the upper 50s to low
60s north and mid to upper 60s south (low 70s possible far
southeast), but confidence is low as some high-res guidance keeps
temperatures across the north in the low 50s.
Some showers may linger near the I-20 corridor through the
evening. Focus for showers shifts to the northwest after midnight
as low-level isentropic lift strengthens ahead of the cutoff low
ejecting out of West Texas, and as the front lifts northward as a
warm front. This also results in a non-diurnal temperature trend
with temperatures steady/rising slightly after midnight.
Models indicate a west to east band of moderate to heavy rainfall
developing along and north of the warm front as unseasonably high
PWATs intersect the boundary. This area of heavy rain has been
trending northward, so the main threat of flooding seems to be
shifting north of the forecast area, but will continue to
monitor. The strong shortwave will move through the area late
Tuesday night and Wednesday morning while getting squeezed between
the subtropical ridge and troughing over the northeastern CONUS.
Models seem to be converging on the forecast area being in the
warm sector of a surface low moving across northern Mississippi
and middle Tennessee with low to mid 60s dewpoints and a few
hundred J/kg of CAPE ahead of a cold front/dry-line like feature.
There are still some key differences regarding the strength of the
low and associated backing of the low-level winds, and how soon
the low-level winds will veer out relative to the instability
arriving. If trends continue, a threat for isolated tornadoes late
Tuesday night and Wednesday morning could be added to the HWO in
later issuances given favorable wind profiles for mini-supercells.
Thursday through Sunday:
One dry day is expected on Thursday between systems with the only
forecast challenge being a possible wedge briefly trying to build
in. Focus then shifts to the next southern-stream trough ejecting
out of the western CONUS, and eventually phasing with a deep
northern stream trough engulfing almost the entire CONUS. Another
cold front looks to move into the area during the Friday/Friday
night timeframe. Any threat for severe weather will depend on the
degree of moisture return in the wake of Wednesday's system.
Models and ensembles have had many varying solutions for this
weekend regarding whether the front moves through, stalls, or
retreats off to the northwest, and whether the expected outbreak
of arctic air catches up to precipitation along the front. It's
certainly too early to use any deterministic model runs at this
point. General consensus continues to support only mentioning
liquid precipitation through the current forecast period at this
18Z TAF Discussion.
All sites should remain in the MVFR or IFR cat through the period
as light rain...lower vis...and low cigs spread across the state.
As the lift wains through the overnight...the light rain will
likely give way to lower cigs and drizzle until nearly the end of
the period. Some patchy dense fog is likely across the area
overnight as the surface boundary stalls over the southern half
of the state with little to no dry air mixing in from the north.
This pattern may become more regular through mid week until the
next potent short wave ejects eastward early Wednesday.
Rain showers will begin to move into the western areas this
morning and then spread eastward through the day. Activity will
also end from west to east by this evening. Less coverage on
Monday, with additional coverage on Wednesday.
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