Area Forecast Discussion
Issued by NWS Huntsville, AL

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422
FXUS64 KHUN 180909
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
309 AM CST Sun Feb 18 2018

.NEAR TERM...(Today)
Issued at 309 AM CST Sun Feb 18 2018

A weak frontal boundary has dropped south of the region and into Central
Alabama, leaving some lingering low stratus around 2kft in its wake
across the Tennessee Valley. High pressure at the surface is
attempting to build into the region from the north and west. This has
helped to scour out this low stratus across much of Western and
Middle Tennessee which has allowed the temperatures to drop and fog
to form due to the moist boundary layer.

While the clouds have hung on for most of the night so far across
the Tennessee Valley, some clearing was noted on GOES-16 Nighttime
Microphysics RGB. This clearing as started to seep into a few areas
in far Northwest Alabama and Southern Middle Tennessee as of 09z.
Expect to see at least some partial clearing in the 09-12z in areas
north of the Tennessee River. This clearing should allow temperature
to slip into the upper 30s to lower 40s around sunrise and lessen the
dewpoint depressions to 1-2 degrees in these areas. Thus the
potential for some patchy fog will exist across most of the region
early this morning due to these moist boundary layer conditions.
However, given the limited window before the sun rises and heating
ensues, think that the potential for dense fog is still low and would
generally be very localized. However, will continue to monitor
trends for fog formation in the predawn hours.

Drier air will attempt to erode cloud cover from north to south this
morning as the aforementioned surface high briefly attempts to build
in from the north. Think that by 15z, at least the northern half of
the region will be mostly sunny, with partly sunny conditions
developing otherwise through 18-21z. However, the boundary that moved
through the region yesterday will move back north late Sunday
afternoon/evening as a broad shortwave lifts out of the Mississippi
Delta into the Mid-South. Thus, another round of scattered light
showers will develop and move north through the area Sunday evening,
before tapering off after midnight. Do think coverage will a bit more
limited than yesterday and have favored a scattered solution in this
forecast package. The sunshine should allow us to warm into lower
60s today, but cooler east to ESE flow will prevent temperatures from
going any higher.

.SHORT TERM...(Tonight through Tuesday)
Issued at 309 AM CST Sun Feb 18 2018

A bermuda high will amplify early next week and gradually build into
the Southern Appalachians and the Tennessee Valley. In response,
winds at the surface and aloft will veer to the south on Monday and
eventually the southwest by Tuesday. This will begin to allow Gulf
moisture to filter back into the Tennessee Valley as dewpoints rise
back into the upper 50s to lower 60s. Additionally, with the
amplifying upper-ridge building northwest, height anomalies will be
quite high (with 850 mb temperatures warming to as high as 14C by
18-00z Tuesday). As a result, high temperatures will jump
significantly -- first into the lower 70s Monday and into the upper
70s (and perhaps to the 80 degree mark) on Tuesday.

Cloud cover would be the only mitigating factor for temperatures
both days. Model soundings continue to indicate a fairly stout
inversion, trapping moisture in the lower boundary layer through the
morning and into the early afternoon. However, the magnitude of the
building ridge and increasing southerly flow should help to overcome
this initial cloud cover. Also, soundings also show a good bit of dry
air aloft which should be able to mix down and erode some of the
clouds, especially on Tuesday. Records for Tuesday are 79 at HSV set
in 1986 and 79 at MSL set in 1917 and am currently forecasting highs
of 80 degrees at both sites which would break those records. At the
very least, am fairly confident we will either come with in a degree
or two of tying and breaking these records. In other words, it`s
going to feel like a warm, Spring day.

As mentioned, soundings suggest a fairly strong capping version
around 800 mb that will likely help reinforce a more stable air mass
in place for most of the day on both days. However, some breaks in
the clouds in the afternoon would allow for some heating and modest
destabilization. With the boundary well north of the region there
will not be an appreciable lifting mechanism nearby. Thus, have
trended lower on PoPs and have kept thunder out of the forecast as
think the capping will mostly win out. As a result, felt confident
enough that diurnal convection would support a mention isolated
showers both days. But, as one would expect, most areas will remain
dry.

.LONG TERM...(Tuesday night through Saturday)
Issued at 309 AM CST Sun Feb 18 2018

For the long term period it`s going to be a question of what wins out
between an upper level ridge to our east off the SC/GA/FL coasts and
multiple waves ejecting E/NE out of a western trough. Models have
come into a little better agreement in showing that the strength of
the ridge may keep the heavy rain axis/frontal boundary between these
two features off to the W/NW on Wednesday. There is still some
indication that as main western trough finally kicks east that the
heavy rain/frontal boundary may shift to just over the TN Valley
Wednesday evening into Thursday. But while that is happening the
ridge to the east begins to flatten and the center shifts westward.
So, there is potential that the furthest east the frontal boundary
makes it is into NW AL before being sent back to the north on Friday.
Still a good deal of uncertainty in latest forecast guidance for the
latter half of the week. Now with all of that said, the forcing
along the front and instability (upwards of 500-700 J/kg), could see
a situation where outflow from any storms that develop may end up
driving the frontal boundary across the TN Valley Wednesday into
Thursday. After Thursday have kept PoPs at chance values with the
potential for a few isolated to scattered showers developing each day
as we head into the weekend. Surprisingly there is some consistency
in guidance over the weekend with the front pushing across the TN
Valley. This is probably our highest chances to see thunderstorms all
week.

With the slower progression of the front noted above, have increased
temps and slowed the arrival of higher PoPs into the area from
previous forecasts. There is some guidance that keeps the upper 70s
to around 80 degree temps in place for Wednesday but not confident
enough to go quite that warm due to the close proximity to the
frontal boundary. Upper level height anomalies do support near record
values. In fact record highs on Wednesday are 76 for both HSV and
MSL with the current forecast puts us at tying those records. The
warmer weather looks to continue as we head into March as models
continue to show troughs in the west and ridging over the
southeastern CONUS.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1035 PM CST Sat Feb 17 2018

Winds remain between 3 and 6 knots at both terminals, but are
becoming lighter as a surface high pushes into northern MS and
western TN. A low stratus deck (MVFR) between 1000 and 2000 feet
remains in place over both TAF sites and into northeastern MS. Expect
drier air to push into the area and low stratus to break up after
07Z at KMSL and 09Z at KHSV. As winds continue to decrease towards
daybreak to around 3 or 4 knots, fog could reduce visibilities to
MVFR criteria at times between 09Z and 12Z. Winds could limit this
somewhat though if they remain closer to 5 knots. Then expect winds
to shift to the southeast during the day tomorrow with some gusts
around 15 knots possible.


&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

NEAR TERM...AMP.24
SHORT TERM...AMP.24
LONG TERM...Stumpf
AVIATION...KTW


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