Area Forecast Discussion
Issued by NWS Oklahoma City, OK

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FXUS64 KOUN 221546
AFDOUN

Area Forecast Discussion
National Weather Service Norman OK
1046 AM CDT Fri Sep 22 2017

.DISCUSSION...
Temperatures are mostly running a few degrees less warm than
yesterday at this time. 850 mb temperatures are similarly cooler
than yesterday, so this afternoon`s high temperatures won`t be
quite as hot as yesterday. This low-level cooling, combined with
slight warming in the middle levels, should further reduce the
chance of showers and thunderstorms today, but not quite to zero.
Some short-range models do generate a few showers in the
southwestern part of our forecast area this afternoon.

The above considerations appear to be adequately handled in our
current forecast, and no changes are currently anticipated this
morning.

CmS
&&

.PREV DISCUSSION... /issued 619 AM CDT Fri Sep 22 2017/

..Aviation Discussion...

AVIATION...
For the 12z TAFs:

The only exception to VFR will be a low probability of MVFR
ceilings at airports in southwest and central Oklahoma and
western north Texas. Latest 10.3-3.9 micrometer satellite shows
low stratus moving north in central Texas. Short term model
guidance may be slightly too aggressive with bringing this stratus
northward, so we only included a brief TEMPO group for broken MVFR
at OKC/OUN/LAW/SPS. Confidence isn`t particularly high. Otherwise,
gusty south winds will continue and subside at all but western
Oklahoma airports this evening.

PREV DISCUSSION... /issued 304 AM CDT Fri Sep 22 2017/

DISCUSSION...
Gusty south-southeast winds have persisted overnight particularly
across northwest Oklahoma. Gustiness should increase area wide by
mid morning as mixing increases. Conditions shouldn`t be too
dissimilar to yesterday since the synoptic scale pattern will
change little. Deep western trough will move a little closer but
still shouldn`t have much impact on our weather. Lee surface low
to our northwest will result in continued breezy south winds
through the afternoon and into the night, particularly across
western portions of the area.

For today`s temperatures we went with the higher side of model
guidance (i.e., MAV) for persistence in a relatively unchanged large
scale pattern. This is a couple degrees warmer than the previous
forecast and was the main short term change to the forecast. A
modest increase in high clouds could temper insolation slightly, but
there really wasn`t much other reason to lower much from previous
day`s temperatures.

Convective initiation is expected to occur by mid-late afternoon in
closer proximity to the western trough, from the western side of the
Caprock into the western end of the Oklahoma Panhandle. Mean
meridional flow will carry this convection north-northeastward into
southwest Kansas and will not support eastward propagation into
our area. Most of the model forecast soundings show sufficient CIN
to prevent convective development across the entire area this
afternoon, although where deeper boundary layer moisture will be
present (south-central/southeast Oklahoma) CIN is minimal by late
afternoon and it`s not totally out of the question that an
isolated shower/storm could form there. Confidence was not high
enough to include in the forecast at this time. Day shift will
need to look at the latest observational data and short term
guidance and re-evaluate this very low probability scenario.

There is good agreement among medium range models and their
ensemble members into the weekend and this leads to a fairly
confident forecast. The earlier concern was a trend toward a
slower/deeper western trough and as a result we kept pushing rain
chances further out in time. The last handful of runs have been
much more consistent. Showers and thunderstorms will be close
northwest Oklahoma through Saturday, but the majority of the
activity will not move into our area until late Sunday and
especially Sunday night as a shortwave embedded within the
longwave trough swings through the central High Plains.

Sunday night into Monday a band of deep moisture sets up across
western Oklahoma within the core of persistent low level jet/warm
conveyor belt. Model PWAT and 850 hPa mixing ratio are at or
above the 90th percentile. Convection should be extensive and
locally heavy rainfall/potential flooding will become a
possibility.

Tuesday and beyond are less clear as medium range model differences
emerge. Agreement generally continues with northern stream
evolution in which GFS, most of its ensembles, and ECMWF all eject
western trough eastward through the northern Plains. This sends a
cold front south into our area. The differences in the models are
with upstream energy and enhancement of southern stream. The GFS
suite is more progressive while the ECMWF is more closed off and
holds an upper low back in the Southwest through mid week. The GFS
may be a little more favorable for another round of precipitation
in the Wednesday-Thursday timeframe. In this scenario, warm
conveyor belt could be lifted atop the aforementioned frontal
slope in a statically stable environment yielding a fairly
widespread warm advection precipitation event. We`ll of course
have greater confidence as the time nears.

We lowered temperatures closer to raw guidance early-mid week in
post frontal environment with probable clouds and precipitation.
These scenarios often turn out colder than even the raw guidance
indicates so a further downward trend in temperatures may be
needed.

BRB

FIRE WEATHER...
Persistent southerly winds in response to deep western trough will
continue for the next few days, peaking during the afternoon.
Temperatures will trend slightly cooler and boundary layer
moisture slightly higher and so minimum relative humidity values
won`t be quite as low as seen in previous days. Nevertheless,
elevated thresholds could be at least briefly met across western
Oklahoma and portions of western north Texas this afternoon and
again Saturday and perhaps Sunday afternoons. Some initial attack
fire activity could occur given these conditions.

BRB

&&

.PRELIMINARY POINT TEMPS/POPS...
Oklahoma City OK  90  72  90  68 /  10  10   0   0
Hobart OK         93  72  90  70 /  10  10   0  10
Wichita Falls TX  92  73  92  70 /  10  10   0   0
Gage OK           93  71  88  68 /  10  10  10  10
Ponca City OK     92  73  92  69 /  10  10   0   0
Durant OK         91  71  93  69 /  10  10   0   0

&&

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

$$

06/16



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