Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 170521
AFDHUN

Area Forecast Discussion
National Weather Service Huntsville AL
1121 PM CST Mon Jan 16 2017

.UPDATE...
For 06Z TAFS.

&&

.NEAR TERM...(Tonight)
Issued at 648 PM CST Mon Jan 16 2017

Latest surface analysis shows a high centered along the Mid Atlantic
Region with a low centered near Kansas City, Missouri. A trailing
cold front from the surface low extended southward through the
Arklatex into Deep South Texas. Closer to home another warm winter
evening with temperatures currently running in the lower to middle
60s under mostly cloudy skies.

Regional radars show a rather expansive shield of showers and a few
thunderstorms extending across the Mississippi River Valley. Patchy
shower activity was noted further east from middle Tennessee across
the Shoals and into central MS. The latest HRRR and high res WRF both
indicate a s/w trough approaching from the west this evening. This
coupled with some enhancement with the nocturnal jet should help to
increase rainfall potential overnight and spread the precipitation
shield further east.

Overall, the current forecast is pretty good shape. May make some
minor tweaks to the PoP/QPF grids slightly for higher values in the
far western areas. Have already made very minor adjustments to hourly
temps as well.

.SHORT TERM...(Tuesday through Wednesday)
Issued at 132 PM CST Mon Jan 16 2017

The cold front will be right on our doorstep by daybreak on Tuesday
and then slowly move eastward through the day. By this time, the
upper level trough will weaken and become situated over the eastern
Great Lakes, with mid and upper level flow becoming nearly zonal.
There may not be much progression as it becomes oriented east to
west and parallel to the flow aloft. An abundance of moisture will
remain in place as this front moves eastward the area, thus precip
chances were kept around 60 to 70 percent through the day. As stated
above, most of the forcing will be well removed from the area,
however did keep isolated thunder in the forecast, as forecast temps
rise into the 70s and allow for some weak instability.

The front will slowly shift to the south during the overnight hours
on Tuesday, with the higher precip chances coming to an end. Kept in
slight chance to chance pops, given some uncertainty with how far
south the front will reach before it stalls. However, guidance is in
fair agreement that it will stall south of the area, giving us a
break in the rainfall on Wednesday. In addition, winds will shift to
the north behind the front, and weak CAA will bring slight cooler air
into the TN Valley. Forecast high temperatures are expected to remain
in the lower 60s on Wednesday.

.LONG TERM...(Wednesday night through Sunday)
Issued at 250 PM CST Mon Jan 16 2017

Good agreement with this frontal positions in model data, so
confidence is fairly high that the ridge will finally break
down towards the middle/end of the week and rain chances will
increase. Although the ridge looks to build back in briefly during
the day on Wednesday over the Tennessee Valley, a potent upper low
develops upstream Wednesday night into Thursday morning over
the Texas/Oklahoma area.

During this time, increasing southerly flow and fairly strong
shortwaves could produce some heavier showers Wednesday night. At
this point instability looks too meager for any thunderstorms, but
wouldn`t be shocked to hear a rumble of thunder. Rain chances look
to increase from chance to likely by Thursday morning across
northern Alabama. Isolated thunderstorms are possible, as some
elevated instability develops. At this point, they do not look very
strong though.

Through the day on Thursday, models continue to show weaker
disturbances continuing to push northeast ahead of the main upper
low, which moves little through much of Thursday. This could help to
keep instability a bit lower due to shower/storm activity/cloud
cover. However, isolated thunderstorms will remain possible.

By late in the day on Thursday, models converge on this upper low
finally kicking northeastward into Missouri and Kansas. A very
strong shortwave develops on the southeast portion of this upper low
and rotates northeastward through the Tennessee Valley Thursday
night. Although there is some potential for instability to be cut-
off to our southwest on Wednesday, do not think given the track of
the upper low/associated shortwave that this will happen with this
system. Trends are not moving this upper low more east or southeast.
Surface CAPE is not great, but could reach 300 J/KG in northern
Alabama/southern middle Tennessee. A little better elevated CAPE is
forecast (around 600 J/KG). Extremely strong upper level
forcing/vorticity is progged by all models with this system. This
strong forcing couples with a 40 kt 850 mb jet, ample moisture, and
a 60 kt 500 mb jet. Based on parameters and synoptics, a potential
for strong to severe thunderstorms continues to be a possibility on
Thursday night into Friday morning. However, believe surface based
instability is too meager for tornado development. Damaging winds
and small hail look like a decent possibility given other
ingredients. In addition, PWATS due to strengthening southerly flow
will increase to around 1.4 inches by Thursday afternoon into
Thursday night. This will allow for heavier rainfall totals with
this system, possibly between 1 and 2 inches with locally higher
amounts. Rainfall amounts may be higher given such strong forcing.
I would not be surprised to see a record high minimum temperature or
two on Friday morning (given strong southerly flow), but did not go
quite that warm at this point.

Models are fairly convergent moving this energy quickly north Friday
morning. By 10 am, we might only be cloudy with a bit of very light
rain primarily east of I-65. Models are hinting at a brief break
from cloud cover Friday morning. This might allow us to climb back
into the upper 60s or a bit higher again.

As an even stronger system moves from Oklahoma into Arkansas on
Saturday night into Sunday, rain chances increase again. However,
with a front stalled to our southeast, this may keep better moisture
and instability well south of the area through Sunday morning.
However, given again how strong this forcing is forecast to be, by
Sunday afternoon into Sunday evening, another round of strong to
severe storms may be brewing. Instability is higher with this system
by Sunday afternoon/evening, but this is still seven days out.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1120 PM CST Mon Jan 16 2017

A cold front will continue to slowly approach the region from the
west overnight and into the day on Monday. This will increase rain
chances and lower ceilings across the terminal sites over the next
12-18 hours. Ahead of the frontal boundary, VFR ceilings late this
evening will slowly lower to MVFR category overnight. Have also
included predominant rain by midday on Tuesday though precipitation
intensity should be rather light and visibilities unrestricted for
the most part. Ceilings will likely continue to lower into IFR
category during the day on Tuesday as moisture increases underneath
shallow inversion. Winds will remain southerly at 10-13 knots
through much of the period with a wind shift possible late.

&&

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

$$

NEAR TERM...15
SHORT TERM...73
LONG TERM...KTW
AVIATION...15


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