Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Wilmington Ohio
408 PM EDT Tuesday Jul 29 2014
unseasonably cool temperatures will continue through the week
although some slow moderation is likely each day as a large trough
of low pressure over the eastern United States remains the primary
feature driving the daily weather. In fact...the trough will
retrograde westward and weaken during the remainder of the week
which eventually will lead to increased warmth and humidity by the
end of week...but by July standards it will still feel quite
comfortable through Friday. Several disturbances rotating through
the trough will bring threats for afternoon and early evening
showers and isolated thunderstorms...tonight...tomorrow and
Thursday. Wednesday will see the highest probabilities of
rainfall. While temperatures will remain below normal into the
weekend...they will warm into the lower 80s with a continued
threat for showers and thunderstorms...especially on Saturday.
Near term /until 6 am Wednesday morning/...
500mb negative anomaly /to 4 Standard deviations below late July
climatology/ remains centered over the eastern United States in the form
of an unseasonably deep/strong longwave trough. The -4 sigma
height anomaly is actually centered in the Carolinas /with respect
to climatology/ and with 850mb temperatures subsequently 2 to 3 Standard
deviations from climatology across Ohio/KY...it is no surprise that
record low maximum temperatures for Tuesday July 29th are being threatened. A
renewed burst of middle level cold advection owing to a weak
shortwave trough is driving a weak surface trough and scattered-broken line
of convection across Southern Lower Michigan/northern Indiana at the present /20z/.
This is tied very strongly to the diurnal cycle despite some
meager middle/upper level support...and hi-res guidance /29.00z sseo/ is
in good agreement that rain showers/isolated thunderstorms and rain will begin to diminish past
00z as it moves into southeastern ind/western Ohio. Until then...it should
maintain into at minimum west central Ohio by 23z. Objective
analysis indicates MLCAPE to 500 j/kg just to the west where
activity is located and weak deep shear /20kts/ thus think activity is
just about at maximum or will be so in next hour as it crosses eastern
ind before it starts to cycle down through the early/middle evening.
For low temperatures tonight went a few degrees warmer than last night
but still well below normal. Daily records for the 30th /min temperature/
should be safe at all climate sites.
Short term /6 am Wednesday morning through Thursday night/...
decent jet streak /seen nicely on 2.0 pv surface/ digging down the
west side of the deep eastern Continental U.S. Longwave trough will quickly
translate through the Great Lakes on Wednesday. This is already
responsible for storms in northern Minnesota/Western Lake Superior and is
enhancing/steepening the deeper layer lapse rates amidst
increasing moisture. Think arrival of the left exit region of this
jet streak over the area around late morning/afternoon will initiate a
more /relatively/ concentrated area of showers and storms...again
with diurnal enhancement. Forecast soundings /29.12z NAM and GFS/
suggest MLCAPE should range from 300-700 j/kg across the area with
a rather steep but relatively moist lapse rate - should be
efficient for generating rain showers/storms given the upper level
support. Considered widespread rain chances in peak heating /per
29.09z sref probabilities/ but dialed back a little based on
29.12z hires-arw WRF which is more refined and tracks several
small clusters of storms across the area. With instability...cold middle
level temperatures /-16c at 500mb and low wbz heights/ and what should be
some modest diabatic heating before cumulus erupts...some small
hail certainly seems likely in the stronger cores Wednesday afternoon. Deeper
shear still weak /20-25kts of unidirectional northwesterly flow/...so while
a rogue severe storm possible think instability will limit this
potential. It does bear mentioning that 29.12z NAM soundings were
by far the most unstable of any previous run or any sref/GFS
member or run in the past few days...so if that bears true /MLCAPE
in excess of 1000 j/kg over northern County Warning Area/ we may have a few more strong
cores than currently thinking. But as of now throwing out that
more unstable thinking based on lack of agreement in other data.
Did increase rain chances quite a bit...and may need further
enhancement in the northern County Warning Area where jet influence and diurnal
instability will be maximized.
Activity should dwindle quickly as available instability is turned over
and planetary boundary layer decouples Wednesday evening. Another cool night in the 50s...but
warmer over previous nights. Thursday...a similar story...as
longwave trough axis retrogrades slowly westward as Atlantic
ridging bulges north and increases. This should keep minimal
instability /though much lower than Wednesday/ and potential for weak
disturbances rotating through the longwave trough to combine for
small diurnally driven shower/storm chances. As heights slowly
rise within the trough...adding a few degrees onto each days high
temperature for Wednesday and Thursday...getting US back toward 80 degrees by
the end of the work week.
Long term /Friday through Tuesday/...
an upper level trough will be in place across the area from Friday
through the weekend. Multiple disturbances will bring chances for
showers and thunderstorms off and on through the weekend. The
greatest chance for precipitation will be during the day on
Saturday. Below normal high temperatures can be expected through
the weekend with this trough in place. This feature will begin to
move east of the area on Monday allowing for day conditions. A weak
disturbance will move across northern portions of the forecast area on Tuesday
allowing for a chance of thunderstorms across the north.
Aviation /20z Tuesday through Sunday/...
a large scale upper level trough will extend from southeast Canada
to the eastern Continental U.S. Through the taf period. Diurnal instability
and embedded features within a northwest flow aloft will bring a
threat for some showers and perhaps some thunderstorms as well.
Through early this evening...a weak surface trough is forecast to push
southeast into the northern parts of the forecast area. This area
may see a few showers late in the day into the evening hours. By
late evening...showers should wane and the surface trough weaker...leaving
partly cloudy skies overnight for all locations. Light winds and
some pockets of mostly clear skies may bring some MVFR visibilities to
the normally fog prone locations of kluk...Kiln and klck.
On Wednesday...looks like an embedded disturbance aloft will
approach the region toward peak heating. It will become partly
cloudy with a considerable cumulus field...along with scattered
showers and perhaps some embedded thunder during the peak heating.
Best coverage appears slated for the northern taf sites of kday...
klck. Best coverage should be after 18z.
Outlook...thunderstorms possible Friday and Saturday.