Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 241025

Area Forecast Discussion
National Weather Service Huntsville AL
525 AM CDT Sat Mar 24 2018

.NEAR TERM...(Today)
Issued at 525 AM CDT Sat Mar 24 2018

North-northwest flow aloft will continue to back to more of a west-
northwesterly direction this morning, as a 500-mb shortwave trough
crossing the upper MO Valley weakens an amplified ridge to its east.
At the surface, low-level flow will gradually veer to the south and
strengthen as a low related to the mid-level wave shifts eastward
through KS and into western MO. The combination of increasing
southerly flow and nearly saturated profiles above the 7-8 kft layer
indicates that lows have likely already been achieved, with temps
expected to slowly rise for the remainder of the morning.

The mid-level wave to our northwest is predicted to weaken as it
drifts into the upper Midwest over the course of the day, and becomes
embedded within the circulation around a stronger upper low dropping
southward though the eastern Great Lakes. As the system`s weakening
surface low begins to turn southeastward toward the lower OH Valley,
we anticipate gusty southwest winds to overspread the TN Valley--
especially as deeper mixing begins around 14Z. Favorable lower-
tropospheric WAA within a moist environment will support an
increasing coverage of showers, although the greatest threat for
measurable precipitation will come during the afternoon hours as low-
level dewpoints recover into the mid 50s. A few thunderstorms will
also be possible with any deeper updrafts late this afternoon,
although much of the CAPE evident in forecast soundings will be
located above a layer of more stable air in the mid-levels. In spite
of an increasing coverage of clouds throughout the afternoon,
southwest flow should force max temps to rise into the u60s/l70s for
all but the higher terrain of southern TN/northeast AL.

.SHORT TERM...(Tonight through Monday Night)
Issued at 525 AM CDT Sat Mar 24 2018

Short term model guidance suggests that the shearing mid-level wave
and associated surface low will shift southeastward through the OH
Valley and into the southern Appalachians overnight. The trailing
cold front is expected to push southeastward across northern
AL/southern TN late this evening and early Sunday morning,
accompanied by a band of more widespread convection. Neutral to
slightly positive height changes aloft imply that low-level
convergence along the front will be the primary forcing mechanism for
the development of any convection, but it does appear that this will
be sufficient to support a solid band of moderate-heavy showers and
a few thunderstorms.

The cold front is expected to push to the southeast of the local
area by 12Z Sunday and eventually stall, with its proximity to the
southeastern portion of the CWFA warranting a low chance POP for
light showers on Sunday. To the north of the front, light northerly
winds will advect a slightly cooler airmass into the region, and with
a very slow dissipation of postfrontal stratus clouds expected by
afternoon, high temperatures will be around 5 degrees cooler compared
to today.

Southeasterly low-level winds will begin to strengthen considerably
from SE-to-NW beginning Sunday evening, due to the combination of an
approaching longwave trough initiating cyclogenesis across the
southern high Plains and strengthening CAD in the lee of the
Appalachians. A thick deck of stratus clouds will build
northwestward with the onset of the southeasterly flow, and the
combined effects of boundary layer mixing and an overcast cloud deck
around 2000 ft AGL should keep lows a few degrees warmer than
guidance indicates. These conditions will largely persist Monday and
Monday night, and although not explicitly noted in the current grids
a Wind Advisory may eventually be warranted for higher elevations to
the north/east of Huntsville. Given that the cloud base is expected
to be fairly close to the ground, a low chance of light showers will
also exist throughout the period. However, with dry advection
occurring near the surface, we will keep POPs in the 20-40% range.
Max temps on Monday will suffer due to the clouds, with highs only
reaching the 50s. However, lows Monday night/Tuesday morning should
be even a bit more mild due to the breeze and overcast skies.

.LONG TERM...(Tuesday through Friday)
Issued at 525 AM CDT Sat Mar 24 2018

The extended forecast will be soggy but have mild temperatures. On
Tuesday, a deep trough with a closed low will be in the intermountain
west and a sfc low will lift into the Great Lakes, extending a cold
front into the MS and OH Valleys. Sfc high pressure extending down
the east coast will keep the front to our northwest for now. The
damming pattern that set up on Monday will weaken but linger into
Tuesday. The drier, cooler air filtering in from the northeast will
provide a temp and dewpoint gradient from east to west. Highs will be
in the upper 50s/lower 60s in NE AL and in the upper 60s over in NW
AL. The drier air along with the upper-level ridge across the region
will limit precip on Tuesday.

As the upper-level trough moves into the Southern Plains and becomes
an open wave, it pushes the ridge off to the east. Models do agree
on this overall pattern, along with a sfc low developing and lifting
through the TN Valley on Thursday. There is more disagreement on the
sfc high that weakens and moves southeast. The placement of the sfc
high off the eastern coast is affecting the models timing of the

Rain will start to move in on Wednesday. Depending on the model it
will make little to modest progress moving into the forecast area due
to the sfc high set up. Regardless, there will be deep moisture from
the Gulf in place with PWATs reaching 1.5". Showers and isolated
thunderstorms will become more widespread Wednesday night and
Thursday. With deep layer winds parallel to the front, the slow
movement of the front and training, flooding is a concern. Precip
chances will start to taper off from west to east Thursday night with
showers exiting by Friday morning. Temps will be a little cooler
Friday afternoon with highs reaching the lower 60s. Total rainfall
amounts will range from 3" in NE AL to 5" in NW AL. Stuck close to
the trends from the blended models for this forecast package.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1259 AM CDT Sat Mar 24 2018

Bkn-ovc cigs in the 8-10 kft range are currently observed at both
terminals, along with a lgt SE wind. These conds should prevail thru
at least 10Z, before low-level flow begins to veer more to the SSW
and strengthen downstream from a sfc low crossing the central Plains.
Winds will increase further after the onset of deeper mixing arnd
14Z, with 10G18 kts common at both terminals thru late aftn. Although
the stronger SW flow and moisture advection will lead to an
increasing coverage of both clouds and showers, we will maintain VFR
cigs and only include VCSH during this period until timing becomes
more certain. Low-lvl flow will gradually veer to the WSW and
diminish by the end of the TAF period, as a cold front approaches
from the north. The threat for showers will continue through tomorrow
evening, with cloud bases forecast to fall well into the MVFR
category by 25/03Z.





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