Flash Flooding and Heavy Rain in Tennessee, May 18 to 19, 2017

Area Forecast Discussion
National Weather Service Nashville TN
551 AM CDT Thu May 18 2017

.UPDATE...
FOR 12Z AVIATION DISCUSSION.

&&

.SHORT TERM...

A band of increased mid to high level moisture is moving across
the Mid-State early this morning, and is expected to park itself
over our eastern areas later this morning and this afternoon.
We've already had some scattered light showers overnight, in
association with that increased moisture field, and believe once
the sun comes up, showers and thunderstorms will increase--
especially over our most eastern areas.

Temperatures early this morning were quite mild, owing largely to
the patchy clouds and a light wind that stayed up overnight. At
3 am, temperatures were mainly in the lower to middle 70s.

Still expect only a slight chance of showers and thunderstorms
over the northwest today, with a band of dry conditions in
between our northwestern and our eastern shower areas.

Things remain warm today, and between the patchy clouds and
showers we're still looking for temperatures to reach the lower
and middle 80s.

Models are consistent with developing scattered showers and
thunderstorms across our area tonight, but then lifting the
deepest moisture northeastward into Kentucky by Friday afternoon.
POPs in the 30-50% range will be common for tonight and Friday.

By Saturday, the deepest moisture is north and west of the Mid-
State, with only a slight chance for showers and thunderstorms
over the northeast and extreme west.

Cold front still on schedule for late this weekend, although there
are some small timing differences between the GFS and ECMWF. Since
both models deepen the upper trough over the Mississippi Valley
Saturday night and Sunday, believe front should be steadily
progressive and in line with the slightly faster ECMWF. Thus,
expect the front to be knocking on our northwestern door by 12z
Sunday, then sweep to the Cumberland Plateau by 00z/Mon.
Everybody should get a pretty good dousing of rain, with amounts
commonly ranging between three quarters of an inch and one and a
quarter inch during the period from 06z Sunday to 06z Monday.

SPC has placed northern Middle Tennessee in a marginal risk for
severe storms in their Day2 outlook, and our northwest in a
marginal risk for Day 3. The main threat from any strong to severe
storms that might develop will be from hail and gusty winds.

.LONG TERM...

Upper pattern undergoes some significant alterations by early next
week, as upper trough continues to dig over the easter United
States and ridging builds over the southwestern United States.
This puts us in a somewhat cooler northwesterly flow. In addition,
it allows for a more unsettled weather pattern, with shortwaves
dropping down in the northwest flow. Even though we should eek out
a dry day on Monday, another shortwave quickly rolls back in,
bringing us another chance for showers and thunderstorms, by
Tuesday, and some "cold-air" wrap-around showers on Wednesday.

The good thing about a northwest flow pattern is that our high
temperatures will settle back down to more seasonable levels in
the 70s. Readings of 80 degrees will probably be hard to find
after Saturday.

&&

.AVIATION...
12Z TAF DISCUSSION.

BNA/CKV/CSV...Surface map this morning shows essentially a
summer-like pattern with high pressure centered off the East
Coast, a warm, moist flow across Middle Tennessee, and a slow-
moving boundary to our northwest. Already, isolated showers are
occurring in parts of Middle Tennessee. Expect scattered
convection to develop along the Cumberland Plateau later this
morning owing to more abundant moisture over that region.
Elsewhere, scattered cells are likely to develop later tonight as
the aforementioned surface boundary draws closer to Middle
Tennessee.

&&

.OHX WATCHES/WARNINGS/ADVISORIES...
NONE.

&&

DISCUSSION......19
AVIATION........08

Hazardous Weather Outlook
National Weather Service Nashville TN
504 AM CDT Thu May 18 2017

TNZ005>011-023>034-056>066-075-077>080-093>095-191015-
Stewart-Montgomery-Robertson-Sumner-Macon-Clay-Pickett-Houston-
Humphreys-Dickson-Cheatham-Davidson-Wilson-Trousdale-Smith-
Jackson-Putnam-Overton-Fentress-Perry-Hickman-Lewis-Williamson-
Maury-Marshall-Rutherford-Cannon-De Kalb-White-Cumberland-Bedford-
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504 AM CDT Thu May 18 2017

This Hazardous Weather Outlook is for portions of Middle
Tennessee.

.DAY ONE...Today and Tonight

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

Scattered storms will develop on Friday in the vicinity of a
quasi-stationary surface front. Some of the storms may become
strong to severe, especially north of I-40. The primary threats
are large hail and damaging straight line winds.

The threat of strong to severe storms will persist on Saturday and
Saturday night as a cold front approaches the mid state from the
west. The best chance for severe storms will occur to the west
and north of Nashville.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed Friday and again Saturday and
Saturday night.

Rose