Area Forecast Discussion
Issued by NWS Oklahoma City, OK

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS64 KOUN 101708
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
1208 PM CDT Thu Aug 10 2017

.DISCUSSION...
18Z TAFs.

&&

.AVIATION...
At least a couple of rounds of thunderstorms are expected across
the area over the next 24 hours. Wave 1 is currently in southwest
Kansas and will move into northwest Oklahoma soon. Storms may
continue to move southeast and may affect central Oklahoma later
this afternoon. Another round of storms is expected to develop
later this afternoon and again spread southeast across the area.
With both rounds of storms, there will be the chance of strong
winds. A surface front will move into Oklahoma overnight with
general northeast winds expected north of the boundary. Also could
see some MVFR ceilings in the morning, especially north of the
boundary.

&&

.PREV DISCUSSION... /issued 643 AM CDT Thu Aug 10 2017/

..Aviation Discussion...

AVIATION...

For the 12z TAFs:

The remains of overnight MCS continues to decrease over south-
central Oklahoma. Showers may possible at/near LAW/SPS through
mid-morning. A remnant boundary may focus diurnal development this
afternoon, but coverage will be sparse and confidence was too low
to include at any TAF site. More organized thunderstorms will
develop by mid-late afternoon in Kansas and the Oklahoma and Texas
Panhandles. These may develop eastward to near WWR/GAG/PNC by late
afternoon, before organizing into a large complex that will
traverse the area into the night. We have prevailed thunder at
most sites, except southwest Oklahoma and western north Texas.
Outside of thunderstorms, VFR conditions should prevail.

PREV DISCUSSION... /issued 458 AM CDT Thu Aug 10 2017/

DISCUSSION...
An anomalous pattern for August continues with the potential for
several MCSs and heavy rainfall/flash flooding through the weekend
and maybe beyond. There is some risk of severe thunderstorms as
well, especially this evening and tonight. See details below.

Thunderstorms over Kansas moved into northern Oklahoma last
evening and consolidated into a small MCS. This MCS has steadily
weakened and hasn`t posed a severe weather threat since last
evening. However, low-level jet has veered and short term guidance
shows a surge of higher theta-e air into what remains of this MCS
this morning. This is supported by observational data with KAMA
00z sounding quite a bit more most near and above 850 hPa as
compared to KOUN. Also, GPS satellite-derived precipitable water
at Clinton showed a marked uptick over the last few hours.

On the western flank of this MCS, one particular thunderstorm has
displayed impressive rainfall rates (3.05" in 30 minutes at
Chickasha mesonet). Convection may continue through early-mid
morning across southeastern portions of our area given the moist
air mass, although the downward trend should continue as veering
low-level winds weaken further. Much of the late morning through
late afternoon may be convection-free across the area, although
thermodynamic profiles in short term models show CIN eroding in
pockets across the area which could support isolated thunderstorm
development. We were fairly broad-brush with low probabilities
this afternoon given uncertainty of small scale details such as
local surface moisture maxima, strongest diabatic heating, and
remnant boundary placement. Once we get a better handle on these
key details we`ll be able to speak with more confidence on which
portions of the area may see isolated diurnal convection later
today.

There is a strong signal for an MCS to evolve out of High Plains
convection and move through the area later this evening and
overnight. Forced south by a potent shortwave trough moving into
the Great Lakes region will be a cold front oriented southwest to
northeast from near the Palmer Divide eastward through the
Colorado plains into northern Kansas. Low-level theta-e surge
north/northwest toward the boundary will contribute to moderate
instability. Aloft, unseasonably strong westerlies will contribute
to storm depth shear over 50 knots to our north across eastern
Colorado and Kansas, and as high as 40-45 knots over northern
Oklahoma by evening. Numerous intense thunderstorms should form
near the boundary, and perhaps behind the southward surging
portion over eastern Colorado. Storms should quickly organized
into a forward propagating MCS with damaging wind potential. The
cold pool driving this MCS could become quite extensive.

There is some uncertainty with regards to the structure/intensity
of the low-level jet and resultant low-level wind shear values.
Some of the model guidance suggests >30 knots 0-1 km bulk shear,
for at least small area. In such a moist environment (surface dew
points >70F), a low end isolated tornado threat could materialize
depending on how storm-scale/meso-beta details evolve. Most
likely, thunderstorms would propagate forward fairly quickly
limiting the threat, but magnitude and orientation of 0-3 km bulk
shear, and >40J/kg CAPE in 0-3 km layer, may offer a fairly
optimal shear/cold pool balance and suggests there is some
potential for QLCS tornadoes. The low-level jet and shear
magnitudes don`t appear to be quite as strong as a few nights ago
when Tulsa`s area experienced QLCS tornadoes, though this setup
should be monitored closely.

Also worth noting is the hail threat which should be confined to
any supercell structures that may form. This seems more likely in
Kansas. Even though winds may be fairly weak in the mid-levels
(typical August), the storm depth shear, and in particular the
shear from within the hail growth zone upward to the equilibrium
level must be considered. Significant hail sizes were observed
with convection in southern Kansas this past evening likely due to
this wind profile favoring optimal hail growth regime. See Johnson
& Sugden (2014) publication in EJSSM for more information on
this type of setup.

A more substantial threat that could develop later in the night is
heavy rainfall amounts and flash flooding. As low-level jet veers
and strengthens some resulting in weaker backshear Cofidi vectors,
the western end of the MCS may become oriented in such a way that
training of cells could occur. Given the anomalously moist
environment (PWAT values once again near the max moving average --
RE: SPC`s sounding climatology), thunderstorms are expected to be
efficient rain producers, similar to what has been seen tonight
near Chickasha. The mesoscale details will determine which portion
of our area is most likely to see training and a localized flash
flooding risk.

Much of the day Friday may be convection free, with the exception
being near remnant boundary from departing MCS which could be
somewhere across southern Oklahoma arcing northwestward into
west-central Oklahoma. This will all depend on the strength of the
cold pool that forms from overnight convection. This could also
augment the front over Kansas and push it further south resulting
in cooler temperatures across northern Oklahoma. This happened
following the last MCS a few days ago. We have lowered
temperatures some, but potentially not enough.

The general pattern remains the same for the next few days at
least, and another overnight MCS may move southeast Friday night
bringing more potentially heavy rain lasting into Saturday.
Flooding may become more of a threat especially toward Saturday
when one or more rounds pre-condition us (i.e., saturated
ground). Models seem to suggest weaker low-level jet for
subsequent rounds after Friday night, so confidence is not
particularly high. With weaker low-level wind field, potential for
convective overturning and inability to quickly from depleted
low-level moisture may limit activity. The mesoscale details will
be very important and we will be able to more confidently assess
rainfall amounts and flooding threat once these smaller scale
details become more apparent.

We should see a decrease in activity early next week as mean
ridging to our south flattens some but more importantly persistent
troughing and embedded shortwaves over northern Plains/Midwest
ejects out and ridging builds in.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  91  70  84  71 /  30  80  30  60
Hobart OK         95  72  89  74 /  20  50  30  40
Wichita Falls TX  97  76  95  77 /  20  20  30  20
Gage OK           91  65  82  69 /  20  80  30  60
Ponca City OK     87  68  82  70 /  20  80  20  50
Durant OK         91  74  92  74 /  30  20  50  40

&&

.OUN WATCHES/WARNINGS/ADVISORIES...
OK...None.
TX...None.
&&

$$



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.