Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 231626 AAC

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
1126 AM CDT Fri Mar 23 2018

For 18Z TAFS.


.NEAR TERM...(Rest of Today)
Issued at 820 AM CDT Fri Mar 23 2018

Band of sprinkles and light rain was shifting south-southeast of the
TN River this morning. This will continue to exit over the next
couple of hours as forecast. Extensive mid and high level ceilings
will remain through the day for most areas. There area a few breaks
in middle TN into far northeast AL. However, the clouds will likely
suppress temperatures a few degrees, mainly in northwest AL. Thus,
have lowered highs in these areas. Otherwise, the rest of the
forecast looks good.

.SHORT TERM...(Tonight through Sunday Night)
Issued at 420 AM CDT Fri Mar 23 2018

The upper ridge starts to break down tonight as the upper low
continues to move east into the Atlantic. Another shortwave starts to
dig into the upper midwest with surface cyclogensis occurring over
CO/KS. A warm front will stretch southeast ahead of it along with a
developing LLJ from the SW. Given the strength of the mid level ridge
to our south and the LLJ from the southwest, the best lift and the
front will stay off to our north and east. For tonight, the highest
pops, chance, will be in our northern areas with slight chance
extending slightly south. As winds turn more westerly aloft after 12z
Saturday, another shortwave will move in from the west with the
surface low slinking southeast towards the AL/MS border. This will
introduce chance pops from the NW across the whole area through the
day Saturday with likely pops in the far northeastern areas, closer
to the frontal boundary. Another thing to note with that LLJ on
Saturday is that it will be quite breezy with wind gusts of 20-25mph

In terms of thunder, we really don`t have any instability to work
with on Saturday with a strongly capped atmosphere. Even when the cap
weakens late Saturday, lapse rates are meager and only a tiny bit of
MUCAPE is forecast and that will be elevated. So will continue to
leave it out of the forecast at this time.

The boundary sinks to the south, although it becomes quite diffuse,
Saturday night into Sunday. Pops lower with the front as it moves
south and the northern half of our area could be dry Sunday, maybe
with a few peaks of sun. Mid level ridging over the Gulf Sunday
night, combined with a shortwave moving west to east along the ridge,
will help push that boundary back north, giving scattered showers to
the entire area once again. With the capping inversion back on
Sunday, will leave thunder out of the forecast.

Highs on Saturday will continue to warm and be near 70 in western
areas and lower 60s in the northeast where the higher chances for
rain are. Slightly cooler temps on Sunday as a bit of a wedge of cold
air noses down from the east so highs will only be in the lower to
middle 60s. Lows will be mild; in the upper 40s to lower 50s through
Sunday night.

.LONG TERM...(Monday through Thursday)
Issued at 420 AM CDT Fri Mar 23 2018

By late Sunday night into Monday morning, the boundary stalls across
northern Georgia extending west through some portion of northern
Alabama or Southern Middle TN. Models differ significantly past this
point in time concerning how much QPF we will see near this front
through the remainder of the day. GFS holds onto more boundary layer
moisture and thus more QPF, as a strong surface high tries to push
south-southwest into the area. However, ECMWF advects much drier air
into the region courtesy of the area of high pressure sliding south.
In fact it hardly has any precipitation at all after 18Z on Monday,
where GFS keeps forecasting between .05 and .10 inches of rain. Went
with a compromise and kept scattered showers in the forecast, but
very light rainfall totals. As this high pressure builds in, it will
not only advect drier air into the region, but cooler air via
easterly/southeasterly winds. It looks very breezy with sustained
winds between 10 and 15 mph with gusts to around 20 mph. It will be
much cooler, especially east of I-65, where highs in the upper 40s to
lower 50s look reasonable (maybe even a bit cooler than that if a
few models are correct). Near the Mississippi state line it should be
warmer with highs in the mid 50s to around 60 degrees are possible.

As the the southwestern edge of the surface high continues to build
west into the region on Monday night, any showers should be very
isolated. Continued cloudy and breezy conditions (sustained southeast
winds 10-15 mph with gusts around 20 mph) should help keep low
temperatures fairly warm, as temperatures drop a bit (but only into
the mid to upper 40s).

By Tuesday, good upper level ridging remains over the area, but
the area of high pressure begins to slowly retreat to the east. This
will give us some southwesterly return flow originating from the
Gulf of Mexico. This should begin to warm up temperatures as low
level moisture advection increases and the atmosphere is able to mix
up to 925 mb where warmer temperatures are present. Still though,
highs will likely only make it into the upper 50s to mid 60s on
Tuesday (highest mid 60s near the TN/MS border). It should be dry as
strong subsidence will still be in place and deeper moisture will
still be scarce. Also, by Tuesday, the low level jet really
strengthens over the southeast ahead of a fairly strong longwave
trough axis (and associated cold front) pushing east towards the
Tennessee Valley. This will make for a breezy/windy day with
sustained winds around 10 mph with gusts to around 20 mph. This could
be a bit low and we will need to watch for gusts meeting wind
advisory criteria as we get closer to Tuesday.

Due to the strength of the ridge to our east, this front quickly
loses its momentum southeastward as winds behind it become parallel
to the frontal boundary. This effectively stalls the front somewhere
across Arkansas, southeastern Missouri, or western Tennessee Tuesday
through Tuesday night. This should keep us dry, but provide a heavy
rainfall/flooding threat further west and northwest. With such strong
return flow continuing from the Gulf of Mexico that is tapping into
the sub-tropical jet, PWATS climb impressively over the area and near
the afore-mentioned front to between 1.5 and 2.0 inches. Lows should
be pleasant only dropping into the mid 50s to around 60 degrees
given the strong moisture advection occurring.

Models eventually pushes strong forcing associated with this frontal
boundary east into northwestern Alabama late Wednesday afternoon or
evening. During the day on Wednesday, cannot rule out some isolated
showers in response to the very moist conditions and approaching lift.
However, the heavier rainfall looks to push in Wednesday night and
continue through Thursday, as an upper low moving east from the
southwestern U.S. phases with the stalled front and kicks it east.
However, not before really strengthening the lift and adding even
more moisture to the atmosphere ahead of the re-intensifying cold
front. Given the strong forcing shown and some surface based/elevated
convection, cannot rule out some isolated thunderstorms. At this
point they do not look very strong, but a marginally strong
thunderstorm risk could occur on Wednesday given the strong winds
aloft with this system and just enough surface based CAPE.
Temperatures look a good bit warmer, with 925 mb temperatures warming
to between 14 and 16 degrees. Highs in the 70 to 75 degree range
look reasonable, given the strong warm air advection and mixing
expected, despite heavier rainfall, especially south of the Tennessee
river, which may not be rain cooled until Thursday afternoon. Mostly
though just expect a heavy rainfall potential. For now including
rainfall totals of 1 to 2 inches Wednesday through Thursday in
Southern Middle Tennessee and in northwestern Alabama. However, if
the boundary is over the area long enough some models are producing 2
to 4 inches with locally higher amount over most areas near and west
of I-65 and into Southern Middle Tennessee during this period. So we
will need to monitor trends with this system as we get closer for
possible flash flooding and river flooding issues.

Drier and somewhat cooler weather returns by next Friday into next
weekend, as high pressure behind the front builds into the area.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1126 AM CDT Fri Mar 23 2018

VFR flight weather conditions are forecast during the next 24 hours.
Mid level ceiling at or above 050agl are expected through the period.
Lower clouds/ceilings at 020-030agl (MVFR) are possible by 14-15Z. At
this time, the probability of showers is below 30 percent, so have
left out of this forecast.





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