Area Forecast Discussion
Issued by NWS Paducah, KY

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000
FXUS63 KPAH 121721
AFDPAH

Area Forecast Discussion
National Weather Service Paducah KY
1221 PM CDT Sat Aug 12 2017

.UPDATE...
Issued at 1220 PM CDT Sat Aug 12 2017

Updated aviation discussion.

&&

.SHORT TERM...(Today through Monday night)
Issued at 359 AM CDT Sat Aug 12 2017

Early this morning, a weak frontal boundary was entering western
KY and the southernmost counties of southeastern MO, noted by a
drop in dewpoints and a light northerly wind behind it. The drier
air filtering in behind the boundary should continue to mitigate
fog formation. Meanwhile, ahead of the boundary, patchy fog will
remain quite likely, and could be dense at times where there are
no clouds. Lingering shower activity in the Pennyrile region of
KY is expected to gradually dissipate through sunrise as light
northerly low level flow takes over the entire region.

An area of surface high pressure, centered over northern IL/IN,
will dominate our region`s weather through the weekend. Deeper
moisture should stay south of us at least through then. In the
slightly cyclonic flow aloft, some model solutions suggest some
pcpn could sneak into parts of southeastern MO and the Purchase
area of KY Sun night, but the main thrust of possible shower and
tstm development for mainly the southern half of our region should
be more toward Mon night. Low level moist fetch is progged to
increase by then, and a surface warm front will approach from the
southwest, resulting in at least scattered showers and tstms
there.

Temps will continue to be below seasonable levels in the short
term, with a modest increase in overall humidity early next week.

.LONG TERM...(Tuesday through Friday)
Issued at 359 AM CDT Sat Aug 12 2017

The extended period begins with an Omega block over central Canada
with low amplitude ridging extending southward into WFO PAH forecast
area. The medium range models, especially the deterministic GFS
guidance, continues to break down this ridge faster, allowing more
precipitation chances with the approaching trough to the west. For
Tuesday and Tuesday night, adjusted from the regionally blended
model initialization closer toward the ECMWF/CMCnh solutions which
are drier and holds the mid/upper level ridge in place over the WFO
PAH Forecast area.

The cyclonic vortex, which has been parked over Alaska and the
western Canadian province/southeast Alaska coast, appears to move
eastward across Canada and into the northern Plains in the U.S. The
net effect is to turn the upper level flow from from west-
northwest to southwest from late Wednesday through early Friday.
The energy from the lows/shortwaves in this vortex are translated
into the Hudson Bay low and help to deepen a trough in the upper
Midwest on Friday.

The question beyond Friday with the model guidance is how strong and
progressive the ridging over the northern and central Plains will be
next Saturday. The ECMWF is stronger with the ridge than the GFS or
Canadian model guidance next weekend. This will impact rain chances
for next weekend leading up to the solar eclipse. At this point in
time, leaning closer to the ECMWF going into next weekend for
PoPs/Weather.

...2017 Solar Eclipse Weather Outlook-WFO PAH Forecast Area...

This is an initial outlook on what some of the medium range guidance
is leaning toward the hour of the solar eclipse across the WFO PAH
forecast area.

The Climate Prediction Center (CPC) has nearly all of the WFO PAH
forecast area in a 40% probability above normal precipitation for
the period from August 19th through August 25th.

Although some of the numerical model guidance (e.g. GFS/ECMWF) has
been available for August 21st during the past several days on the
Internet, this guidance is now be more universally available to NWS
offices beginning with this weekend. Here is some local weather
highlights based on certain numerical models:

00z Sat August 12 deterministic GFS...The GFS model guidance,
centered at 1 pm CDT August 21st, places the WFO PAH forecast area
just upstream of a low amplitude shortwave trough. This guidance
places most of forecast area (except Southwest IN and the West KY
Pennyrile) under a swath of rain, with 1000-500 mb relative humidity
(potential for cloud cover) greater than 70 percent over almost the
entire area. Although the GFS precipitation scheme is much more
robust that other guidance, the GFS has been suggesting
precipitation periodically over the area during the last few model
runs.

00z Sat August 12 deterministic ECMWF...The ECMWF model guidance, is
suggesting a shortwave ridge axis over the WFO PAH forecast area,
with no precipitation and 1000-500 mb relative humidity between 40-
50% over the area at 1 pm CDT on August 21st. This would bode well
for lesser cloud cover, but warmer temperatures.

00z Sat August 12 deterministic Canadian(CMC)...The Canadian model
guidance is suggesting a weaker and lower amplitude shortwave over
Missouri, with rain just north of the area of totality at 1pm CDT
August 21st.  The Canadian guidance has been trending closer with
the ECMWF guidance recently, with some evidence of ridging aloft
still in place with this run.

With all that said, these medium range models, with lower
resolution, can only provide a general idea of what can be expected
during the eclipse time. There are numerous variables that can
impact and change this forecast. Some smaller weather systems, known
as MCS (Mesoscale Convective Systems) can evolve on smaller time and
distance scales, but can still have an impact several hundred miles
away. In addition, we are approaching the peak time period for
tropical weather systems (e.g. tropical storms and hurricanes) to
develop. The cloud cover from these systems can cover areas hundreds
of miles away from the storms.

Stay tuned...there will be many changes in the forecast before the
actual passage of the eclipse early Monday afternoon on August 21st.

&&

.AVIATION...
Issued at 1220 PM CDT Sat Aug 12 2017

Light northerly winds and VFR conditions can be expected at the TAF
sites with possibly a few gusts at KCGI/KPAH this afternoon. Low
level moisture will be enough for some scattered cu, most
prevalent up in the KEVV/KOWB area. High clouds will also be
streaming across. Winds wilL be light tonight. More cu will be
possible late Sunday morning, mainly near KEVV/KOWB once again.
Winds will likely shift to easterly Sunday morning as well.

&&

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

$$


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