Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 220244
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
944 PM CDT Fri Apr 21 2017

.UPDATE...
Issued at 943 PM CDT Fri Apr 21 2017

As of 02z, the 1 hour deterministic GFS and the 12z Canadian
(CMCnh), and to a letter degree the ESRL HRRR and 4km NAM-WRF
(ARW version, seem to be getting a better handle on the timing and
intensity of the convective activity for the remainder of the
first period (tonight).

A Mesoscale Vorticity Circulation (MVC) that moved across
Southern Missouri earlier this afternoon enhanced lift across the
WSW-ENE 850 mb frontal boundary stretching from north of Poplar
Bluff to near Owensboro, producing moderate rain and sharpening
the surface pressure gradient across the northeast part of the WFO
PAH forecast area.

The 850mb flow gradually increases for the rest of this evening
over Arkansas into Southern Missouri, gradually working northeast
after midnight. Sharper lift will be realized over Southeast
Missouri with a gradient of thunderstorm development along and
east of the inverted trough (currently along the western border of
Arkansas).

Anticipate that Southeast Missouri should see the greatest
increase in QPF (rainfall amounts) from 1 am to 5 am CDT. Given
the antecedent (past) rainfall, the potential for flooding should
spike during this time period. This area still looks to have the
best chance for flooding overnight.

Modified the coverage of the thunderstorm activity to coincide
with transit of the surface low/inverted trough overnight.

Am somewhat concerned that the zone of higher QPF may shift
southward into West Kentucky around daybreak. However, given the
eastward speed of the system expected to increase slightly, there
will be a delicate balance between residence time of higher
rainfall amounts at any one location in West Kentucky. Allowing
for the lack of significant antecedent rainfall over southern
sections of West Kentucky the past two days, am doubtful that any
extension of a flood watch into Kentucky will be needed.



UPDATE Issued at 750 PM CDT Fri Apr 21 2017

Updated aviation discussion.

.SHORT TERM...(Tonight through Saturday night)
Issued at 240 PM CDT Fri Apr 21 2017

Main concern through Saturday is heavy rainfall potential.
A moist southwest flow aloft has been overrunning the cool low
levels, resulting in persistent moderate rains across parts of the
area. The heaviest rainfall has been in southeast Missouri, where
1 to 3 inches have fallen.

A Flood Watch has been issued for most of se Missouri and srn
Illinois through Saturday. Antecedent rainfall and higher soil
moisture has made those areas more favorable for quick runoff. In
addition, the 850 mb warm front is forecast to remain quasi-
stationary across those areas through Saturday morning. As a well-
developed 500 mb shortwave moves east toward our region, forcing
for ascent will increase along the 850 front tonight into early
Saturday. The models indicate the low-level southerly flow aloft
will increase to 30-40 knots south of the 850 mb front tonight.
All of this looks like a good setup for heavy rainfall, especially
with increasing elevated instability later tonight.

WPC and model qpf is in fair agreement that the axis of heaviest
rain will be near the 850 mb warm front. The actual values are not
especially high, generally 1.5 to 3 inches in the watch area, but
all guidance underforecast amounts that fell last night. In
addition, there is a chance thunderstorms will increase overnight
as the low-level jet strengthens and the mid levels cool with the
approach of the 500 mb shortwave. This could result in isolated
higher amounts than indicated by the models or WPC guidance.

Rainfall intensity will diminish quickly Saturday afternoon once
the 850 mb low passes across the Lower Ohio Valley. However,
numerous areas of light rain will occur in the moist deep cyclonic
flow into Saturday evening. The system is forecast to slow down
over the Tennessee Valley Saturday night. Nuisance-type areas of
light rain/drizzle will persist much of the night. Temps will show
very little change through Saturday night, generally stuck in the
upper 40s to mid 50s.

.LONG TERM...(Sunday through Friday)
Issued at 240 PM CDT Fri Apr 21 2017

Sunday looks to be the the beginning of improving weather
conditions, especially over the northwest half of the region as high
pressure presses slowly southeast from the Midwest toward the Ohio
Valley. Will need to leave some small lingering POPS in the morning
over the southern Pennyrile of wrn KY, and pesky wrap around clouds
may hang on most of the day along and south of the OH River.
However, most of se MO and srn IL should have a much prettier day
with highs in the mid 60s.

High pressure will take over for the first part of next week. In
addition to the return to ample sunshine, a warming trend will get
underway, especially Tuesday as the sfc high moves off to the east
and winds turn southerly. Afternoon temperatures by Tuesday
afternoon should end up very close to 80 at most locations.

Next weather maker will come with the approach of a cold front from
the northwest on Wednesday. There looks to be ample instability by
that time to support thunderstorms, so will keep that mention in the
forecast. There is some indication the front may stall very close to
the region Wednesday night, then retreat back north as a warm front
Thursday. This will all depend on how quickly upper ridging off the
Southeast Coast builds during the period. If the upper ridge builds
as quickly as the GFS indicates, we would be quickly back into a
warm sector regime with warm temps and little chance of precip. The
12Z ECMWF is not so quick to build the ridge, and actually pushes
another H50 short wave toward the mid MS River Valley. The jury is
still out on this one, at least for now.

&&

.SHORT TERM...(Tonight through Saturday night)
Issued at 240 PM CDT Fri Apr 21 2017

Main concern through Saturday is heavy rainfall potential.
A moist southwest flow aloft has been overrunning the cool low
levels, resulting in persistent moderate rains across parts of the
area. The heaviest rainfall has been in southeast Missouri, where
1 to 3 inches have fallen.

A Flood Watch has been issued for most of se Missouri and srn
Illinois through Saturday. Antecedent rainfall and higher soil
moisture has made those areas more favorable for quick runoff. In
addition, the 850 mb warm front is forecast to remain quasi-
stationary across those areas through Saturday morning. As a well-
developed 500 mb shortwave moves east toward our region, forcing
for ascent will increase along the 850 front tonight into early
Saturday. The models indicate the low-level southerly flow aloft
will increase to 30-40 knots south of the 850 mb front tonight.
All of this looks like a good setup for heavy rainfall, especially
with increasing elevated instability later tonight.

WPC and model qpf is in fair agreement that the axis of heaviest
rain will be near the 850 mb warm front. The actual values are not
especially high, generally 1.5 to 3 inches in the watch area, but
all guidance underforecast amounts that fell last night. In
addition, there is a chance thunderstorms will increase overnight
as the low-level jet strengthens and the mid levels cool with the
approach of the 500 mb shortwave. This could result in isolated
higher amounts than indicated by the models or WPC guidance.

Rainfall intensity will diminish quickly Saturday afternoon once
the 850 mb low passes across the Lower Ohio Valley. However,
numerous areas of light rain will occur in the moist deep cyclonic
flow into Saturday evening. The system is forecast to slow down
over the Tennessee Valley Saturday night. Nuisance-type areas of
light rain/drizzle will persist much of the night. Temps will show
very little change through Saturday night, generally stuck in the
upper 40s to mid 50s.

.LONG TERM...(Sunday through Friday)
Issued at 240 PM CDT Fri Apr 21 2017

Sunday looks to be the the beginning of improving weather
conditions, especially over the northwest half of the region as high
pressure presses slowly southeast from the Midwest toward the Ohio
Valley. Will need to leave some small lingering POPS in the morning
over the southern Pennyrile of wrn KY, and pesky wrap around clouds
may hang on most of the day along and south of the OH River.
However, most of se MO and srn IL should have a much prettier day
with highs in the mid 60s.

High pressure will take over for the first part of next week. In
addition to the return to ample sunshine, a warming trend will get
underway, especially Tuesday as the sfc high moves off to the east
and winds turn southerly. Afternoon temperatures by Tuesday
afternoon should end up very close to 80 at most locations.

Next weather maker will come with the approach of a cold front from
the northwest on Wednesday. There looks to be ample instability by
that time to support thunderstorms, so will keep that mention in the
forecast. There is some indication the front may stall very close to
the region Wednesday night, then retreat back north as a warm front
Thursday. This will all depend on how quickly upper ridging off the
Southeast Coast builds during the period. If the upper ridge builds
as quickly as the GFS indicates, we would be quickly back into a
warm sector regime with warm temps and little chance of precip. The
12Z ECMWF is not so quick to build the ridge, and actually pushes
another H50 short wave toward the mid MS River Valley. The jury is
still out on this one, at least for now.

&&

.AVIATION...
Issued at 750 PM CDT Fri Apr 21 2017

A complex convective event will continue across the region
throughout most of the 00Z TAF period. An west-to-east warm front
around 4-5 kft off the ground will continue to bisect the region,
resulting in a wind shift from northerly to southwesterly with
increasing height. This boundary will also continue to be focus for
increased pcpn as mid level impulses move through. Overnight, an
inverted surface trough is progged to move into the area from AR,
increasing gustiness of the northeasterly surface winds in some
locations. In the west, cigs should be intermittently MVFR this
evening, but should drop again overnight, eventually to IFR levels.
In the west, cigs should become predominantly MVFR with time. Vsbys
may go briefly IFR in bursts of heavier rainfall. Pcpn is expected
to begin to let up toward the end of the TAF period.


&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through Saturday afternoon for ILZ081>094.

MO...Flood Watch through Saturday afternoon for MOZ076-086-087-100-
     107>111.

IN...None.
KY...None.
&&

$$

UPDATE...Smith
SHORT TERM...MY
LONG TERM...GM
AVIATION...DB



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