Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 210227

Area Forecast Discussion
National Weather Service Huntsville AL
927 PM CDT Fri Oct 20 2017

.NEAR TERM...(Rest of tonight)
Issued at 927 PM CDT Fri Oct 20 2017

Another cool evening is taking shape across the TN Valley, with
temperatures in the low to mid 50s as of 9 pm. High pressure centered
over the Atlantic coast will keep a light easterly flow across the
region through the overnight hours. Meanwhile, high clouds continue
to stream eastward across the MS and TN Valleys as a shortwave moves
into the Central Plains. These high clouds will temper some of the
radiational cooling across the area and lows will generally bottom
out in the upper 40s to lower 50s. Much like last night, fog will
once again be possible near bodies of water and in valley locations,
though may not be as widespread as we saw this morning, given the
high clouds moving in. Previous forecast is on track so no major
changes were made.

.SHORT TERM...(Saturday through saturday night)
Issued at 245 PM CDT Fri Oct 20 2017

A trough over the Pacific Northwest will deepen across the
Intermountain West and Plains states as it impacts an amplified
ridge entrenched over the eastern half of the CONUS. As this occurs,
due to the strength of the ridge, the southern flank of this trough
is depicted by the med range models as cutting off from the
prevailing flow and moving across the MS River Valley later this
weekend. Before this occurs, anticipating cloud cover to increase as
deep southerly flow is induced by the approaching trough. The
primary impacts on Saturday with this setup will be an increase in
cloud cover from southwest to northeast.

The aforementioned cold air damming may keep dry/slightly cooler air
over northeast Alabama on Saturday.  Even then, temperatures should
be around 80 degrees for daytime highs except northeast
Alabama/southern middle TN where mid 70s will prevail. With the
increase in humidity due to the southerly flow, sfc dewpoints will
begin to rise on Saturday afternoon/evening into the 50s. However,
with that cold air damming still in place, models tend to struggle
with how quickly the dry air mass will be advected northeast. As a
result, was more pessimistic with increasing the dewpoints and sided
with a slower timing of the LLJ advecting higher dewpoint airmass
northeastward. This could also impact the onset timing of
precipitation later in the forecast. Despite this, overnight lows on
Saturday night should be warmer in the upper 50s to low 60s with the
increased cloud cover and higher dewpoints.

.LONG TERM...(Sunday through Friday)
Issued at 245 PM CDT Fri Oct 20 2017

Models continue to move the main longwave trough axis eastward into
south central Canada on Sunday morning, with an associated strong
cold front extending south through Illinois and Arkansas. Models keep
upper level ridging (although shifting eastward off the southeastern
coast) in place over the Tennessee Valley. At the same time, models
continue to develop a cutoff low as the southern portion of this
longwave trough axis deepens. This should help slow down the eastward
progression of the cold front a bit and keep even isolated chances
of showers and isolated thunderstorms in Mississippi and further west
through the morning hours on Sunday. Despite cloud cover, mixing
below 925 mb and some warm air advection should allow temperatures
to climb to around 80 degrees by the afternoon hours. After 3 or 4
pm, though scattered showers and isolated thunderstorms look
possible. At this point, since models have been fairly consistent
developing the cut-off low so far south, the better instability
should remain near and south of Birmingham, AL. Models forecast
little surface based CAPE as a result over northern Alabama. They do
develop a little bit of low level helicity and 30 to 35 knots of bulk
shear, but the stable airmass should prevent any damaging winds or
tornado threat from developing. Strengthening southerly flow through
a good portion of the atmosphere advects deep moisture (PWAT values
around 1.7 inches) northward into the area by Sunday

As the front stalls and the upper level low deepens over Arkansas on
late Sunday night, fairly robust cyclogenesis is forecast by models
as a surface low develops and deepens to around 1012 mb as it pushes
into northwestern Alabama. The very moist flow from the Gulf of
Mexico is forecast to continue north into the area as a result of
the surface low and approaching cold front. Since this low will be
cutoff from the main jetstream, it will be slow to move east. This
should be a classic setup for heavier rainfall through Monday. At
this point, given the combination of the very strong forcing with
this deepening low and a very moist atmosphere for this time of year,
think 2 to 3 inches of rainfall is a fairly good bet Sunday night
through Monday. Although instability will increase on Monday, it
still should be very meager (<500 J/KG). Although helicity increases
on Monday to between 200 and 300 m2/s2, the very low instability
should keep any tornado threat at bay. Given decent 0-2 km bulk shear
between 30 and 40 knots along with 0-6 bulk shear between 40 and 50
knots, some strong storms could occur producing winds around 50 mph.
Couldn`t absolutely rule out an isolated severe storm, but the
orientation of shear vectors seem to indicate that any severe threat
(even with straight-line thunderstorm winds) will be very remote. The
good news is that we can likely take between 2 and 3 inches of
rainfall given current flash flood guidance. However, would not be
surprised to see some isolated higher than 3 inch amounts (around 4
inches). So this period will need to be monitored for minor flooding

Luckily by Monday afternoon, a second longwave trough axis,
approaching cold front will push quickly southeast from the Upper
Midwest and phase with this stalled front/surface low and pull it
northeast toward Virginia. This should lower our rain chances
significantly by Monday night. If this second front does not arrive
as quickly, flooding may become more of an issue. Kept scattered to
likely (west to northeast) in the forecast to account for lingering
forcing. However, drier air punching in behind the surface low should
keep precipitation at this point much lighter. Maybe another quarter
of an inch or so is expected, but not too much more than that. As
the surface low phases with the second front over the Ohio valley
late Monday night, the low intensifies as its pressure drops to below
1000 mb. This will create very strong cold air advection behind the
frontal boundary that should push through the area on Tuesday.

This will bring much colder air to the region. Highs on Tuesday
should only make it into the upper 50s to around 65 degrees. This may
be conservative given the strong cold air advection and source region
of this Canadian airmass. It will be quite breezy on Tuesday with
sustained wind between 15 and 20 mph with gusts to around 30 mph.

As winds begin to become lighter on Wednesday into Thursday, lows
could drop into the mid to upper 30s. Thus, our first true frost of
the season may occur toward the middle of next week.


.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 607 PM CDT Fri Oct 20 2017

VFR conditions will prevail at the KMSL and KHSV terminals through
the next 24 hours. Winds will be light and variable overnight and
then become southeasterly around 10kts after 15Z. Other than passing
high clouds, skies will remain mostly clear.





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