Area forecast discussion
National Weather Service Wilmington Ohio
347 am EDT Thursday may 23 2013
Synopsis...
as an upper level trough moves east through the Great Lakes...rain
showers will be likely through the day today. A cold front will move
south through the region tonight...with cool and dry air moving in
from the north through Friday. High pressure will build in through
Saturday.
&&
Near term /until 6 PM this evening/...
although a weak and almost indiscernible cold front has passed
into the region tonight...there remains a small amount of elevated
instability...and the atmosphere is still rather moist. With
heights still falling and weak synoptic lift over the
region...some showers (and even a couple weak thunderstorms) have
managed to keep on going overnight. Probability of precipitation were increased in the
next few hours to account for this precipitation...although the
overall trend (and diurnal timing) would argue against much
additional development through the morning.
By early afternoon...attention will turn to the stronger cold
front approaching from the north. As this front moves closer to
the iln County Warning Area...the main axis of the upper level trough will also be
pivoting overhead. This will lead to a significant decrease in
temperatures aloft...well before the surface frontal boundary
arrives. Thus...some weak instability is expected to develop.
Combined with the forcing from the trough...all indications are
for fairly widespread convective shower development by
afternoon...with the greatest coverage (and perhaps intensity) in
the northern and northeastern parts of the County Warning Area. Probability of precipitation were
increased to likely for the entire County Warning Area...and categorical in the
NE. The instability will be narrow and shallow...so the chances of
thunder may not be especially high. This forecast package will
keep thunder going in the grids for a while in the afternoon...but
this is not particularly certain...and thunder may not occur at
all in the northwest County Warning Area. Also...while these cold upper-level systems
can sometimes allow for convective showers to develop small
hail...the cape on BUFKIT soundings just does not seem to reach
high enough (or cold enough) to make that very likely either.
Maximum temperatures have a bit of a gradient today...as the surface cold
front begins to get into the northern County Warning Area by the end of the
afternoon...perhaps limiting heating by a couple hours there.
There may be less cloud cover in the southeast early in the
day...so maximum temperatures are a bit higher there. Overall the forecast
was not changed significantly from the previous grids...and no
major discrepancies were noted in the main suite of model guidance.
&&
Short term /6 PM this evening through Saturday night/...
subsidence will begin in earnest behind the cold front on Thursday
night...with a switch to northerly winds allowing for cold
advection to begin at the surface. The min temperature forecast brings
the entire County Warning Area into the lower to middle 40s...with temperatures near
40 (or even around 38-39) in the far northern counties.
Surface high pressure should be centered over the region on
Friday...and this air mass appears quite dry. 850mb temperatures will
also be at their coldest on Friday...leading to a rather cool maximum
temperature forecast that keeps the entire County Warning Area in the lower to middle
60s.
Although the surface will still be in an area of high pressure by
Friday night...the upper flow will switch to the
northwest. The surface high is forecast to weaken by
Saturday...but the impact of upstream moisture aloft looks
somewhat uncertain. Some models continue to allow for a narrow
strip of quantitative precipitation forecast by Saturday afternoon and evening...mainly in the SW
County Warning Area...but this would have to fall through a very dry air mass in
the lowest several hundred mb. In fact...at the time of
precipitation...GFS soundings show cloud bases of at least 7000
feet. Keeping the forecast dry appears the better option for
now...with the surface high and dry air arguing against much of an
impact from whatever may be coming in on the northwest flow.
&&
Long term /Sunday through Wednesday/...
GFS is still showing these 500 mb disturbances undercutting the ridge
and nosing into the region Monday/Tuesday where European is somewhat
similar with an elevated convergent layer but much further
southwest and out of County Warning Area.
Surface high moves offshore early next week with drying southeast
surface winds over the Ohio Valley. Upper level flow is showing a
broadening of the 500 mb ridge as it moves over the eastern third of the
country. Have increased the chances for middle level disturbances to
undercut the ridge at this time but keep the threat for showers low
at 20% towards the middle of next week.
Expect a gradual warmup each day...starting in the upper 60s to
around 70 Saturday...warming to near 80/low 80s by Wednesday. PM lows will
be comfortably cool in the 40s through Monday morning under a dry
airmass and then warm to the climatological norms in the middle 50s by
middle week.
&&
Aviation /07z Thursday through Monday/...
upper level low centered over the western Great Lakes this morning
will pivot southeast across our region through the day. This
feature will be followed by a more vigorous disturbance in the
northwest flow aloft and will be associated with a stronger cold
front which is forecast to pass southeast across the taf sites
late this afternoon into this evening.
Firstly...a weak cold front is poised to pass across the taf sites
between 09z and 15z this morning. Scattered showers along and
ahead of the front will likely affect the taf sites with some MVFR
visibilities. Initial aviation problem early this morning is whether
predominate stratus will form at the taf sites...especially at
kday...Kiln...kcmh...and klck. Low stratus is floating around in
this region but with showers and still some low level mixing...it
is unclear if this will form into a permanent deck. As such...have
left a scattered low cloud at these taf sites and will monitor
trends to see if this becomes permanent. If they do...ceilings
will likely be in the 6-8 hundred foot range.
From late this morning into this afternoon...large scale ascent
associated with the main upper low will combined with daytime
heating/instability to produce widespread showers. Can not rule
out some embedded thunder...but this will be too sporadic to
pinpoint in the tafs at this time so have left out. Showers should
become a little heavier out ahead of the secondary cold front this
afternoon and evening. And with moistening low levels...have gone
with predominant MVFR conditions.
For tonight...cold front will slide southeast and will result in
winds switching to the north...which will become gusty around 25
knots. Dilemma immediately behind the cold front is how low
ceilings will drop in the cold air advection pattern. Some deterministic and
ensembles suggest that ceilings could drop into IFR for a
time...especially up in the kcmh/klck area. Have kept ceilings in
the 1000-1500 range at this time and will address this again with
the next taf package at 12z. Otherwise...once the secondary
disturbance passes southeast...much drier air will filter into the
region under strong subsidence between 09z and 12z. As such...any
low level stratus should scour out and/or push southeast away from
the region...leaving mostly clear skies by 12z Friday.
Outlook...no significant weather expected.
&&
Iln watches/warnings/advisories...
Ohio...none.
Kentucky...none.
In...none.
&&
$$
Synopsis...hatzos
near term...hatzos
short term...hatzos
long term...franks
aviation...Hickman
