Area Forecast Discussion
Issued by NWS Louisville, KY

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FXUS63 KLMK 311119

619 AM EST Sat Jan 31 2015

...Updated Aviation Discussion...

.SHORT TERM (Now through Sunday)...
Issued at 305 AM EST Sat Jan 31 2015

A strong disturbance diving south across the northern Rockies will
combine with energy ejecting from a southern stream closed low over
the desert southwest to form a significant winter storm that will
ultimately stretch from Iowa to southern New England.

For the Lower Ohio Valley, guidance continues to trend farther north
with the forecast path of the surface low, allowing increasingly
warm air as far north as southern Indiana. Significant snow
accumulations are looking less likely on Sunday.

Both the GFS and NAM predict low pressure to develop over Missouri
early Sunday, and deepening it as it moves eastwards right over
Louisville late Sunday.

Later today, skies will become cloudy by afternoon. Highs will range
from the upper 30s to around 40 after a chilly morning. By evening,
light precipitation associated with return flow across an elevated
warm front will develop across southern Illinois and Indiana.
Measurable precipitation will likely be confined to locations north
of Interstate 64. Initially, snow will fall into dry air below, and
precipitation may start out as light snow, or a rain snow mix across
southern Indiana. Some light accumulations may develop across our
very northern tier of counties before this initial pulse of
precipitation tapers off towards dawn. Overnight lows through early
Sunday may initially fall off toward freezing across our far north,
then remain steady or slowly rise towards dawn Sunday. Farther
south, lows will fall towards the middle 30s.

Southerly winds will bring mild temperatures Sunday as far north as
southern Indiana. Rain will overspread the area from west to east
during the day Sunday, with widespread rains likely Sunday evening.

Temperatures Sunday will warm into the lower 50s across our south,
and could reach 50 as far north as Interstate 64.

.LONG TERM (Sunday night through Friday)...
Issued at 300 AM EST Sat Jan 31 2015

By Sunday evening, the storm system to impact the Ohio Valley is
expected to be centered across southern Indiana as widespread rain
along and ahead of the system move through central Kentucky into
southern Indiana. Once the low lifts northeast of the local area,
temperatures will begin to fall, likely to not occur until mid
evening in the west and midnight or after across the eastern areas.
However, soundings show we really lose our saturation aloft quickly
and surface temperatures likely won`t fall below freezing until
after midnight. Precipitation is expected to become spotty showers
or drizzle until the deeper cold air arrives and ice crystals are
able to form.

The wrap around moisture with this system increases early Monday
morning as soundings show saturation to around 850 mb. Most of this
lies within the -8 to -12C range and combined with enhanced low
level lift, thinking that there will be scattered snow showers
during the early to mid morning hours, including the Monday morning
commute. This could a scenario where a quick burst of snow drops a
few tenths to 1/2 inch during the morning commute. It will also be
quite blustery and north winds could be gusting 20 to 25 mph. This
will make for a cold start to the work/school day and wind chills
are likely to be in the 0 to +10 range.

Precipitation ends west to east late Monday morning and look for
improving conditions in the afternoon as the storm system lifts
further northeast and is replaced by Canadian high pressure. Plan on
highs only in the lower to middle 20s along the Ohio River, to
around 30 degrees near the TN border. A cold night still looks on
tap Monday night when readings bottom out in the mid teens to mid
20s under clear skies and light winds. Fair weather continues into
Tuesday as the surface high pushes east, giving way to light
southerly return flow.

A northern stream shortwave is expected to dive out of the northern
Plains Wednesday into Thursday, bringing a front through the local
area. We`ll be watching a southern stream piece of energy as well
and the forecast challenge the past few days is how much phasing may
occur and whether this enhances precipitation across the Ohio
Valley. 31.00z GFS shows slightly more phasing, which would bring
some light precipitation back northwest across the southern and
eastern forecast area. A time-trend analysis doesn`t add a lot of
confidence to this idea as most of the runs suggest the system
largely stays across the southeast US. The remainder of the 31.00z
deterministic and ensemble guidance keeps the system well to the
southeast. For now, a model consensus of slight chances across the
area seems appropriate. Ahead of this system, Wednesday`s highs may
push 50 degrees in most places. So, if precipitation develops it
would initially be rain until Wednesday night, possibly going over
to snow.

In the wake of the midweek system, the upper level flow looks to
become northwesterly bringing generally fair weather and seasonably
cold conditions. Highs in the 30s and lows in the teens.


.AVIATION (12Z TAF Update)...
Issued at 610 AM EST Sat Jan 31 2015

Clouds will increase and lower by late afternoon as a disturbance
Although ceilings will lower, they will remain VFR through 00z
Sunday morning.

Ceilings will remain VFR at BWG through the entire TAF period with
no precipitation. However, light rain or a light rain/snow mix may
develop after 00z Sunday across southern Indiana and the northern
Bluegrass. This area of precipitation may stay north of SDF and LEX
through 06z Monday. Will put VCSH in TAF for SDF after 00z to
account for possibility of light precipitation. Ceilings at SDF and
LEX will most likely stay VFR through 06z Sunday.

Rain and MVFR ceilings will develop at SDF by 18z Sunday.

South winds under 5kt through late morning will stay southerly at
around 5 to 8kt through Saturday evening.




Short Term........JSD
Long Term.........ZBT
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