Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 181740
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1140 AM CST Sun Feb 18 2018

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 941 AM CST Sun Feb 18 2018

With the exception of a few passing high altitude clouds, a return of
nearly full sunshine is a nice way to start a new week. Seems like is
has been over a week since all of the area has experienced sunny
skies. The solar insolation and a light N-NE wind, courtesy of
surface high pressure sprawled across the Ohio Valley, was helping
to raise area temperatures up into the mid 40s to lower 50s.

For those who enjoy the sun, it unfortunately will be a short-time
affair, as clouds will return in earnest later this morning and this
afternoon. Along with the clouds, rain chances - yes, more rain is in
the forecast for the later afternoon into the evening.

The 0.64um channel "red or plain visible" satellite view from GOES-16
indicated more stratus just south of the greater Huntsville forecast
area. These clouds were south of a line approximately from the Golden
Triangle of eastern Mississippi, to Blount county, and just south of
the I-20 corridor along the AL/GA border. Short term forecasts from
the HRRR/RAP/NAM were in pretty good agreement, returning this
stratus from SW-NE during the course of the day. Along with the
clouds, rain chances also return, thanks to deeper Gulf moisture
returning inland, and strong upglide in the 295-300K region.

So will do some adjustments to the grids, mainly to reflect latest
trends from the shorter-term models. This will result in clouds on
the increase faster than we have now, so most of the forecast area
will be mostly cloudy to overcast by the mid afternoon. Will delay
somewhat the start of showers until later in the afternoon - moving
in from SW-NE with the clouds. The sun and winds becoming southerly
could support high temperatures a degree or two warmer, especially
north of the Tennessee River (where the sun will be more direct for a
longer period). Updates will be out soon.

.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 18Z TAFS through 18Z Monday afternoon)
Issued at 1140 AM CST Sun Feb 18 2018

VFR flying weather across the Tennessee Valley, will deteriorate in
the mid/late afternoon and evening, as lower level stratus returns
from the south. The northern edge of those clouds extended west-east
from north of Columbus AFB MS to north of BHM and near I20 and the
AL/GA border. MVFR CIGs under this deck were 10-20 nmi south of
leading edge of the stratus. The northward movement of these clouds
should result in MVFR ~2000-3000 ft AGL over KMSL and KHSV by the
late afternoon. Along with the clouds, VCSH are also possible, mainly
in the late afternoon and evening. Lower MVFR CIGS ~1000-2000` AGL
are forecast for late tonight and Mon AM. SE winds should also
increase into the 5-15 kt range by late Mon AM.

&&

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

$$

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


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