Area Forecast Discussion
Issued by NWS Paducah, KY

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FXUS63 KPAH 190522
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1222 AM CDT Sat May 19 2018

.UPDATE...
Issued at 1222 AM CDT Sat May 19 2018

Updated aviation discussion.

&&

UPDATE...
Issued at 916 PM CDT Fri May 18 2018

Cleaned up the forecast a bit to catch up to the most recent
observational trends. The thunder/heavy rain threat is over, as
most of the instability over the region has been expended. Isolated
thunderstorms will remain possible over portions of the region for
the remainder of the night, as the upper low moves through the
region. They should be short-lived with just sporadic lightning
and a brief heavy downpour.

The other concern is fog development which is already underway
over much of the region. At this time much of southern Illinois,
west Kentucky and southeast Missouri have MVFR level fog with KCGI
already to IFR levels. Most of the area is near saturation already
this evening, as winds remain nearly calm, and some clearing
develops, dense fog is not out of the question. Added areas of
fog to the forecast, mainly over southeast Missouri where the more
persistent clearing is likely. We will continue to monitor the
dense fog potential for the remainder of the night.

&&

.SHORT TERM...(Tonight through Sunday night)
Issued at 339 PM CDT Fri May 18 2018

The negatively tilted closed low centered just northwest of St.
Louis Missouri, with an axis stretching down into the Missouri
Bootheel at 230 pm CDT, will continue to slowly drift northeast this
evening and overnight. The highest rain chances will continue to the
east of the low/trough axis, where enhanced lift in the moist
adiabatic environment will produce effective rainfall amounts with a
minimum of 3/4 of an inch or higher per hour. Given the extremely
isolated nature of the pockets of heavy rainfall, will just utilize
hydrological advisories for local drainage/minor flooding issues.

With the sharper vorticity gradients aloft, non-supercell based
landspouts will be possible again, especially over southeast
Missouri this afternoon and this evening. We have been carrying
statements the last couple of days hinting at that potential.

The 13km RAP and 12km NAM-WRF continue to perform rather will in
placement and general intensity of afternoon convection within the
favorable isentropic lift zone.

With the loss of daytime heating, any convection should be
limited to the very narrow zones of enhanced lift aloft overnight.
With near calm and saturated condition at the surface, added a
mention of fog for the western sections of the WFO PAH forecast
area (mainly southeast MO and southwest IL) late this evening and
overnight.

A very narrow ridge will continue to move in behind the closed low
tonight and Saturday, capping the coverage of convective activity
across most of the WFO PAH forecast area on Saturday and Saturday
evening. Worked to keep as much of the area dry as possible,
limiting any spurious model generated pockets of rain chances.

Rain chances will initially develop late Saturday night and early
Sunday along an impressed surface (warm frontal) boundary along
the interstate 64 corridor, then expand southward as diurnal
heating generates scattered convection over the remainder of the
area ahead of channeled vorticity in the southwest flow ahead of
a closed low in eastern Nebraska and western Iowa.

.LONG TERM...(Monday through Friday)
Issued at 339 PM CDT Fri May 18 2018

Have rather low forecast confidence in the coverage and timing of
the precipitation through the work week next week.

The previously mentioned closed low over Nebraska and Iowa is
forecast to move little on Monday, with minor ridging possible
early in the day. The placement of the PoPs/Weather seem a little
too far west into the WFO PAH forecast area, but for
collaborative and regionally blended model guidance initialization
purposes, left a low chance PoP across most of the area.

There is a disparity among the ECMWF, GFS, and the Canadian medium
range guidance on how dominant the ridge building will be along and
west of the WFO PAH forecast area. The GFS and Canadian generate a
low over the southeast U.S., southeast of the central U.S. ridge.
With the significant uncertainty associated with this forecast low,
leaned toward lesser PoPs/Weather for the latter half of the week.

Temperatures were a blend of the regional model initialization,
the national blend of models, and the Canadian guidance.

&&

.AVIATION...
Issued at 1222 AM CDT Sat May 19 2018

As an upper low centered just to our northwest migrates east-
northeastward, and a weak surface low over our eastern sections
moves generally north overnight, shower and tstm activity will be
sparse initially. However, better mid level vorticity will be
concentrated in the eastern half of the region late in the night,
and this may enable re-ignition of shower and tstm activity there,
lingering well into the morning hours. As a result, there should be
an accompanying decrease in cig heights to MVFR/IFR mainly east of
the MS River. Light winds overnight will be conducive to some fog
formation, with intermittent cloud cover a limiting factor.
Noticeably improved conditions will be noticed by late morning/
afternoon Sat.

&&

.PAH WATCHES/WARNINGS/ADVISORIES...
IL...None.
MO...None.
IN...None.
KY...None.
&&

$$

UPDATE...DRS
SHORT TERM...Smith
LONG TERM...Smith
AVIATION...DB



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