Flash Flood Warning 10, NWS Huntsville, AL

Area Forecast Discussion
National Weather Service Huntsville AL
1228 PM CDT Sun Apr 12 2020

.UPDATE...
For 18Z TAFS.

&&

.NEAR TERM...(Rest of Today)
Issued at 907 AM CDT Sun Apr 12 2020

No major changes to the previous forecast. SPC did expand the
moderate risk category farther north including most of NW AL as well
as the Huntsville metro area. Showers/thunderstorms over NW AL will
continue to lift north and east the morning, but given SFC
temps/dewpts in the 50s not expecting these storms to exceed severe
criteria.

Previous discussion:

Not a significant severe threat with the morning showers and
thunderstorms as the stable layer near the surface will inhibit
downdrafts from reaching the surface, and updrafts from tapping into
the low-level wind shear. As the warm front continues to slowly move
northward, however, we will see that inversion near the surface
become shallower and shallower which will slowly increase the
potential for gusty winds and hail.

During the day today, our hi-res convective allowing models still
don't seem to be in agreement on how far north the warm front gets
before it stalls. Most of these models really tend to support the
theory that the showers through the morning hours will help to hold
the warm front mostly south of the Tennessee Valley until the cold
front gets closer during the evening hours. During the day, before
around 5-7 pm, the primary threats will likely be wind and hail.

.SHORT TERM...(Tonight through Monday Night)
Issued at 345 AM CDT Sun Apr 12 2020

Now for the tonight period. Most of the convective allowing models do
actually lift the warm front north through at least Northeastern
Alabama, if not all of northern Alabama and southern Middle
Tennessee. This will be out ahead of the main cold front. The HRRR
appears to be the main outlier with probably our "worst-case"
scenario, lifting the warm front north of the AL/TN border during the
afternoon. The earlier this warm front lifts north, the more
instability the cold front will have to work with to generate
stronger storms across the Tennessee Valley. This would probably mean
some form of QLCS feature develops with bowing segments producing
damaging winds and embedded circulations possibly generating
tornadoes. The "best-case" scenario, on the other hand, doesn't
have the warm front lift north into the Tennessee Valley really at
all, which keeps most of the severe weather with the more dynamic
cold front to our south. Models showing this scenario still clip Sand
Mountain with a stronger storm or two, but leaves NW Alabama pretty
much out of the severe thunderstorm threat. We will have to watch the
warm front very closely through the day today. The likely scenario
will probably be somewhere in between.

We have a second frontal boundary that is more associated with the
base of the upper-level trough and a secondary surface low
developing along the occlusion. This may bring one final line of
showers and thunderstorms through just behind the stronger cold
front around 10 PM to 2 AM. This could possibly be another round of
severe thunderstorms. Main question with this line will be will the
environment be "tapped out" by the time it arrives? Will the strong dynamics
with the cold front aided by the upper-level trough help these storms
stay strong and overcome a lesser amount of instability? There will
still be a significant amount of low-level and bulk shear. At this
time it's hard to say what to make of this second cold front. All I
can say at this point is continue to be weather aware going into
Sunday night.

Now for winds. This dynamic system generates a fairly strong pressure
gradient out ahead of the surface low, setting up across most of
Alabama and Middle Tennessee. This will mean gusty southerly winds
are expected to develop with the strengthening pressure gradient this
afternoon through late tonight. The gradient really doesn't relax
until the base of the upper-level trough swings through, which might
not be until after midnight tonight. In Northeast Alabama, the
gradient might not relax until as late as 7 AM Monday morning. We
have a wind advisory out until midnight tonight, however this may
need to be extended in NE Alabama depending on how the low actually
tracks.

The cooler air finally arrives behind the second cold front after
midnight Monday morning. Showers and Thunderstorms will be moving off
to the east, and skies will be clearing. Northwesterly winds, while
they will be bringing in cooler air, the friction generated by wind
will help keep temperatures from plummeting Monday morning. Lows will
probably only drop into the low-to-mid 50s. Highs Monday afternoon
will probably only get up into the low-to-mid 60s under sunny skies
with northwesterly winds gradually weakening through the afternoon
and evening hours.

.LONG TERM...(Tuesday through Saturday)
Issued at 345 AM CDT Sun Apr 12 2020

The medium range models continue to show an unseasonably cool
weather pattern early next week, followed by a modest warming trend.
Continental scale trough will continue to dominate the CONUS, with
the southeast region remaining in a west-northwest flow regime as a
result. An upper trough axis will bring mostly mid level clouds and
perhaps a few showers Tuesday Night. However, most precip will be
well southeast along the frontal system from the central Gulf Coast
through the Carolinas. Then, dry weather is foreseen through Friday
with lows in the u30s Wed-Thu mornings. Highs Wed will be in
u50s-a60, with 60s on Thu. As high pressure shifts off the southeast
coast on Friday, southerly flow will bring modest warm advection and
highs closer to normal with u60s-l70s. A fast moving frontal system
will arrive either Friday night or Saturday (variance in the models).
This will bring only very light precipitation at most for our area
with deeper moisture and convection remaining nearer to the Gulf
Coast.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1227 PM CDT Sun Apr 12 2020

Several rounds of rain and thunderstorms are expected during the TAF
period. The first round at the start of the period will bring rain
and TSRA through 20z. After a brief break in thunderstorms, a
stronger band of thunderstorms will move from SW to NE between 23z to
03z. Gusty winds up to 35 kts is possible along with CIGS and VSBY
reductions into IFR criteria. Have also included wind shear in the
TAF with 50 kts not too far above the surface. Gradual improvement
overnight with a return to VFR by sunrise.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Flash Flood Watch until midnight CDT tonight for ALZ001>010-016.

     Wind Advisory until midnight CDT tonight for ALZ001>010-016.

TN...Flash Flood Watch until midnight CDT tonight for TNZ076-096-097.

     Wind Advisory until midnight CDT tonight for TNZ076-096-097.

&&

NEAR TERM...GH/McCoy
SHORT TERM...McCoy
LONG TERM...17
AVIATION...GH

For more information please visit our website
at weather.gov/huntsville.

Hazardous Weather Outlook
National Weather Service Huntsville AL
338 AM CDT Sun Apr 12 2020

ALZ001>010-016-TNZ076-096-097-122300-
Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan-
Marshall-Jackson-DeKalb-Cullman-Moore-Lincoln-Franklin TN-
338 AM CDT Sun Apr 12 2020

This Hazardous Weather Outlook is for north Alabama and portions of
southern middle Tennessee.

.DAY ONE...Today and Tonight...

A severe weather outbreak is possible across the Tennessee Valley
region this afternoon and evening. All modes of severe weather will
be possible, including tornadoes, damaging winds and large hail.

Excessive rainfall of 2 to 3 inches, with a few areas receiving 4 to
5 inches, will result in an elevated threat of flash flooding. A
Flash Flood Watch is in effect from Noon through Midnight.

A Wind Advisory is in effect from Noon through Midnight, with
sustained winds of 15 to 25 mph and gusts of 35 to 40 mph,
particularly in the higher elevations.

.DAYS TWO THROUGH SEVEN...Monday through Saturday...

No hazardous weather is expected at this time.

Rises on area rivers, creeks, and streams may produce flooding
this week as a result of rainfall on Sunday and Sunday night.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management personnel is
expected this afternoon and evening.

Please relay any information about observed severe weather to the
National Weather Service while following all local, state, and CDC
guidelines.

Special Weather Statement
National Weather Service Huntsville AL
853 AM CDT Sun Apr 12 2020

ALZ001>003-121430-
Lauderdale AL-Franklin AL-Colbert AL-
853 AM CDT Sun Apr 12 2020

...SIGNIFICANT WEATHER ADVISORY FOR CENTRAL LAUDERDALE...COLBERT AND
NORTHWESTERN FRANKLIN COUNTIES UNTIL 930 AM CDT...

At 852 AM CDT, Doppler radar was tracking a strong thunderstorm 7
miles east of Tishomingo State Park, moving northeast at 55 mph.

Pea size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Florence, Muscle Shoals, Sheffield, Tuscumbia, Killen, Cherokee, St.
Florian, Underwood-Petersville, Malone and Posey Loop.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike several miles away from a thunderstorm. Seek a
safe shelter inside a building or vehicle.

&&

LAT...LON 3480 8810 3501 8766 3500 8764 3476 8746
      3455 8780 3448 8794 3446 8815
TIME...MOT...LOC 1352Z 241DEG 49KT 3465 8806

RSB

Special Weather Statement
National Weather Service Huntsville AL
922 AM CDT Sun Apr 12 2020

ALZ001-002-005-121445-
Lauderdale AL-Limestone AL-Colbert AL-
922 AM CDT Sun Apr 12 2020

...SIGNIFICANT WEATHER ADVISORY FOR EASTERN LAUDERDALE...NORTHWESTERN
LIMESTONE AND EAST CENTRAL COLBERT COUNTIES UNTIL 945 AM CDT...

At 921 AM CDT, Doppler radar was tracking a strong thunderstorm over
St. Florian, or near Florence, moving northeast at 55 mph.

Penny size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Florence, Muscle Shoals, Sheffield, Tuscumbia, Rogersville, Killen,
Lexington, St. Florian, Underwood-Petersville and Anderson.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Strong gusty winds will be capable of knocking down small tree limbs
along with other small unsecured items. Seek safe shelter until this
storm has passed.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike several miles away from a thunderstorm. Seek a
safe shelter inside a building or vehicle.

&&

LAT...LON 3501 8755 3500 8708 3498 8703 3480 8736
      3480 8742 3478 8743 3477 8743 3466 8763
      3488 8784
TIME...MOT...LOC 1421Z 238DEG 48KT 3484 8763

RSB

Special Weather Statement
National Weather Service Huntsville AL
946 AM CDT Sun Apr 12 2020

ALZ001-002-004-005-121515-
Lawrence AL-Lauderdale AL-Limestone AL-Colbert AL-
946 AM CDT Sun Apr 12 2020

...SIGNIFICANT WEATHER ADVISORY FOR LAUDERDALE...NORTHWESTERN
LIMESTONE...CENTRAL COLBERT AND NORTHWESTERN LAWRENCE COUNTIES UNTIL
1015 AM CDT...

At 945 AM CDT, Doppler radar was tracking a strong thunderstorm near
Killen, or 14 miles east of Florence, moving northeast at 45 mph.

Pea size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Florence, Muscle Shoals, Sheffield, Tuscumbia, Rogersville, Killen,
Town Creek, Cherokee, Lexington and Leighton.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Very heavy rainfall is occurring with this storm, and may lead to
localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is also occurring with this storm.
Lightning can strike several miles away from a thunderstorm. Seek a
safe shelter inside a building or vehicle.

&&

LAT...LON 3499 8694 3495 8691 3475 8725 3472 8731
      3467 8742 3468 8809 3495 8810 3500 8738
TIME...MOT...LOC 1445Z 238DEG 39KT 3486 8743

RSB

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Special Weather Statement
National Weather Service Huntsville AL
133 PM CDT Sun Apr 12 2020

ALZ001-002-004-005-121900-
Lawrence AL-Lauderdale AL-Limestone AL-Colbert AL-
133 PM CDT Sun Apr 12 2020

...SIGNIFICANT WEATHER ADVISORY FOR LAUDERDALE...NORTHWESTERN
LIMESTONE...SOUTHEASTERN COLBERT AND NORTHERN LAWRENCE COUNTIES UNTIL
200 PM CDT...

At 133 PM CDT, Doppler radar was tracking a strong thunderstorm near
Tuscumbia, moving northeast at 55 mph.

Winds in excess of 30 mph will be possible with this storm.

Locations impacted include...
Florence, Muscle Shoals, Sheffield, Tuscumbia, Rogersville, Killen,
Town Creek, Littleville, Lexington and Leighton.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Very heavy rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike several miles away from a thunderstorm. Seek a
safe shelter inside a building or vehicle.

&&

LAT...LON 3500 8768 3500 8724 3499 8698 3462 8717
      3460 8801
TIME...MOT...LOC 1833Z 219DEG 49KT 3466 8775

GH