Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 241601

Area Forecast Discussion
National Weather Service Huntsville AL
1001 AM CST Fri Feb 24 2017

.NEAR TERM...(Rest of Today)
Issued at 1001 AM CST Fri Feb 24 2017

A warm start to the day across the TN Valley, as temps have already
risen into the upper 60s to lower 70s at 10 am. Southerly flow will
continue to increase through the day as a surface low moves into the
Great Lakes region and high pressure shifts to the east along the
Gulf Coast. Meanwhile, the parent upper trough axis will move into
the MS Valley by this afternoon, increasing southerly flow aloft.
This will combine to help temperatures continue to warm, with record
highs expected this afternoon.

The surface low, and associated cold front will reach the MS River
by late in the near term period, and showers and thunderstorms may
enter the area just before 00Z. Short term guidance is slightly
inconsistent in terms of the exact timing, though the higher chances
for precip will be later this evening. In addition, any threat for
severe thunderstorms (though marginal at best) will be later this
evening and information can be found in the discussion below.

.SHORT TERM...(Tonight through Sunday)
Issued at 415 AM CST Fri Feb 24 2017

The main weather impacts in the short term portion of the forecast
will occur tonight as a cold front crosses the region. The front will
be to our west at the start of the period (6 PM CST) and will cross
the area overnight, exiting NE Alabama around 3-6 AM. In the warm
sector ahead of the impending colder airmass, an elevated mixed
layer and capping inversion centered around the 700-800 mb layer will
keep any shower/storm activity limited into the afternoon. However,
as forcing for ascent is focused along the progressive frontal
boundary, and slight weakening of the cap occurs by late afternoon,
showers and thunderstorms are expected to develop to our west. A
number of hi-res models indicate a discrete cellular mode is
initially favored with a gradual transition to a broken line of
showers/storms as the activity moves across the area. Storm
organization and coverage is expected to be higher/better to our
north where forcing along/ahead of the surface front will be better
coupled with forcing due to DPVA associated with the upper trough.
Nevertheless, regional/global models depict a complicated pattern of
vort advection along the leading edge of the trough that could favor
perhaps a couple of narrow lines of showers/storms in close
proximity to each other as they cross the area. Wind shear will
weaken during the evening, but will still be substantial enough to
maintain some supercellular structures and quasi-discrete convective
elements within the line. This is even depicted in some of the latest
hi-res guidance. CAPE is still largely present above ~700 mb, with
decent lapse rates noted around the -10C to -30C layer. This, in
addition to other thermodynamic features suggests at least a marginal
threat for large hail and damaging winds with a few stronger cells. A
brief, small tornado still cannot be ruled out given low LCLs, and
supportive effective inflow helicity, although the overall risk still
appears to be rather small. The main question is the capping
inversion/CIN and the extent/coverage of eventual development this
evening and overnight, which is still somewhat uncertain.
Nevertheless, nearly all guidance has indicated an increase in
expected development with the latest 00Z guidance suite, and thus the
forecast will reflect this slowly increasing confidence.

The shower and thunderstorm activity is expected to come to an end in
the east by 6 AM as the colder airmass moves into the area. While
high temperatures will be in the 70s and 80s on Friday, only low/mid
50s are expected for most locations on Saturday afternoon as the
modified Canadian airmass moves into the region. So, what a
difference a day will make as temperatures will be about 25 degrees
cooler on Saturday!

Saturday night will probably be the coldest portion of the forecast
period as the dome of high pressure settles over the region. Under a
clear sky, temperatures are likely to fall below freezing for most
locations. As high pressure shifts to the east of the area on Sunday,
temperatures will begin the process of warming once again as winds
begin to shift from the southeast during the day. High clouds may
begin to increase during the afternoon, with highs expected to be in
the mid/upper 50s for most locations.

.LONG TERM...(Sunday night through Thursday)
Issued at 415 AM CST Fri Feb 24 2017

A mid-level trough along the WA/OR coast early Sunday evening is
forecast to dig southeastward across the western CONUS through 12Z
Tuesday and this should maintain a belt of strong west-southwesterly
flow aloft across the TN valley through this period. At the surface,
a ridge initially across the eastern Carolinas is forecast to slowly
spread offshore early next week, allowing southeasterly low-level
return flow to gradually strengthen across the region. All guidance
suggests that several lower amplitude disturbances embedded in the
southwest flow aloft will enhance precipitation chances early next
week as large scale ascent from boundary layer warm/moist advection
increases with the development of a 35-45 knot low-level jet. This
activity will likely begin prior to 12Z Monday for the west, and
spread eastward through the day -- perhaps not ending until Tuesday
morning. Although instability parameters appear too stable for
thunderstorms at the onset of this event, forecast soundings suggest
that an EML and plume of steeper lapse rates aloft will be advected
across the area Monday night providing a more favorable environment
for elevated convection.

The western CONUS longwave trough is forecast to shift eastward from
the Rockies into the upper MS valley region on Tuesday-Wednesday,
with winds aloft strengthening and backing further to the southwest
during this period. Concurrent with this, a strong low/mid-level
capping inversion will likely develop and prevent a significant
coverage of showers and storms during the day on Tuesday. Thus, we
have lowered POPs/increased temps accordingly. A fair amount of
stratocumulus may become trapped beneath the inversion aloft, and
due to this we have kept temps in the l/m 70s. If cloud cover is
less than anticipated, highs Tuesday should easily reach the
u70s/l80s. Meanwhile, a strong wave ejecting out of the central
Plains trough is expected to focus low-level cyclogenesis across the
western Great Lakes late Tuesday, with another strong cold front
expected to sweep across the region Wednesday as the deepening low
lifts into southern Quebec. Although the most recent run of the GFS
came in quite a bit faster with fropa, we will trend the forecast in
the direction of the more consistent ECMWF/GEM solutions and
advertise frontal passage during the afternoon/early evening.
Widespread convection is expected to develop by early Wednesday
morning as the low-level jet increases to around 50 knots, with
showers/storms continuing until time of frontal passage. The
combination of large scale ascent, steep lapse rates aloft,
dewpoints in the l60s, and deep-layer shear around 60 knots
certainly raises concern for a severe weather event at some point
Tuesday night or Wednesday. However, the impacts of any nocturnal
convection provide a large amount of uncertainty, as do issues with
speed of fropa.

The portion of the forecast from early Thursday morning through
Friday should be fairly uneventful as a colder mixture of north
Pacific and Canadian air spreads into the region. A reinforcing cold
front will likely push through the forecast area late Thursday
night...maintaining highs in the m50s on Friday.


.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 540 AM CST Fri Feb 24 2017

VFR conditions currently prevail at KHSV and KMSL terminals.
Scattered low clouds have developed overnight, but bases have been
above 4 kft. Winds are generally around 5-10 kts currently, but will
increase after sunrise and with the onset of mixing. Wind gusts may
reach or exceed 20 kts at times between 15Z and 04Z. A cold front
will approach the region tonight and instigate shower/storm
development. Impacts are possible by ~02Z at KMSL and ~04Z at KHSV.
Although, exact timing remains a little uncertain and impacts may
occur within +/- ~2 hours. Due to uncertainty in coverage/extent of
convective activity, TAFs only include MVFR cig/vis impacts at this
time, but certainly lower cig/vis into IFR range are possible if a
heavy shra/tsra impacts a TAF location.





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