Area Forecast Discussion
Issued by NWS Huntsville, AL

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FXUS64 KHUN 191421 AAB
AFDHUN

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
821 AM CST Sun Feb 19 2017

.NEAR TERM...(Rest of Today)
Issued at 820 AM CST Sun Feb 19 2017

Visible imagery indicates fog was most prevalent along the Elk River
and tributaries in southern middle TN, west of the highway 231
corridor, and across the southern half of Cullman County. Fog was
also noted along the I-59 corridor and the TN River in our eastern
counties. The fog will gradually dissipate over the next 1-2 hours.
Will extend patchy fog in the gridded forecast through 15z.

.SHORT TERM...(Tonight through Tuesday Night)
Issued at 212 AM CST Sun Feb 19 2017

Excellent radiational cooling conditions should set up tonight with
light and variable winds and clear skies. So even with the warm
daily highs, lows will still be able to drop into the middle to
upper 40s. This is still around 10 degrees above normal lows.

The upper ridge continues to build over the area and move east on
Monday with the surface high shifting east enough to give us a weak
southeasterly flow at the surface. 850mb temps rise another 1-2
degrees which should push our highs into the lower to middle 70s,
despite a veil of cirrus expected to move over the region.

The western upper low in the Omega block digs into southern Texas as
of Monday afternoon, which is where the clouds are coming from. At
the same time, an upper trough is digging into the upper midwest
with a surface frontal boundary essentially connecting the two
systems. As clouds overspread and thicken, lows Monday night will
stay mild, in the lower 50s.

There are still some pretty major differences between the GFS/ECMWF
with the CMCnh acting as the middle range. The biggest difference
between the two is how quickly the model breaks down the upper ridge
to the east. The GFS breaks it down in the north fast enough where
the upper low north of the Great Lakes quickly shears off and
weakens and leaves the cut off low over S. Texas with no lift to
sustain the frontal moisture all the way to the local area. This
essentially dissipates the precip before it reaches the area.

The ECMWF 19/00z however keeps the ridge in place which allows the
upper energy and associated front/precip to reach the area before
breaking it down and both systems move off to the east. Given the
strength of this ridge, leaning more towards a wetter ECMWF
solution. Even so, I have already seen over the course of the past
two days these two models flip sides and have been extremely
inconsistent. I am more confident in higher pops on the far western
edge because even if the drier GFS is correct, we should still get
some rain making it to those areas. In terms of thunder, first of
all, we still aren`t sure if we are even going to get any rain.
Assuming though it ends up more like the ECMWF, I`m not seeing any
instability, most of the lift is to the south, lapse rates are
unimpressive, so will continue to leave thunder out for now. The only
thing going for it is a weak LLJ and a favorable upper jet pattern.
A rumble is possible but not worth a 15% mention.

The upper low will pull south and east into the Gulf by 12z
Wednesday with any precip pushing off to the east shortly after.

Even with the cloud cover and rain, highs Tuesday should still get
into the lower 70s with lows remaining warm, in the lower to middle
50s.

.LONG TERM...(Wednesday through Saturday)
Issued at 212 AM CST Sun Feb 19 2017

Light rain will be coming to an end by daybreak on Wednesday, as the
weak upper shortwave moves east of the area. Ridging will build in
through the day, as southerly flow increases in response to a
surface high off the Gulf Coast. This fetch of warm air, combined
with mid/upper level height rises will allow temps to warm back into
the low to mid 70s Wednesday afternoon. Similar conditions are
expected during the day Thursday, as the weak ridging holds and
southerly flow persists. Overnight lows in the mid 50s seem more
like our normal highs for the end of February. Although we will be
roughly 15 to 20 degrees above normal for highs and lows, the
current forecast is still a few degrees below record values.

An upper level trough will eject east of the Rockies during the day
Thursday, with surface cyclogensis occurring over the Central Plains.
Winds through the vertical column will shift to the southwest
Thursday night as the TN Valley becomes situated between the
retreating ridge and approaching trough. While the upper/surface
lows shift to the northeast and into Great Lakes region on Friday,
the cold front will move into the western portion of the Mississippi
Valley. There are still quite a few differences in terms of the
timing and strength of this system as it enters the TN Valley, with
the GFS roughly 6 to 12 hours quicker on the 19/00Z model run. Given
these discrepancies, have maintained a chance of rain during the day
Friday and through the overnight hours, with precip ending by
daybreak on Saturday. The synoptic set-up in the latest suite of
models suggests that thunderstorms are possible, however there are
still quite a few details that need to come to gather before many
specifics can be discussed, especially given the run-to-run
inconsistencies. Though it should be noted that given the set-up in
the current run, the dynamics are a bit too far north to see any
widespread thunderstorms.

&&

.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 516 AM CST Sun Feb 19 2017

A Dense Fog Advisory is in effect through 8AM so will keep 1/4SM and
fog in the terminals through that time frame. Low visibilities and
MVFR CIGS may linger for a few hours after that. Skies should be
clear by this afternoon and remain through 12Z Monday. High clouds
will probably move in after 06z tonight but with light winds
throughout the TAF, those clouds weren`t going to cause any
operational impacts and did not add a group to the TAFs.


&&

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

$$

NEAR TERM...17
SHORT TERM...LN
LONG TERM...73
AVIATION...LN


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